A Report on:

Tse’Biinaholts’a Yalti

-or-

Curved Rock that Speaks

An Outdoor Public Performance TheaterAt Chaco Canyon, New Mexico

byRichard W. Loose

Organ Mountain Research

November 4, 2001 & Revised April 24, 2005

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Aerial view of Tse’Biinaholts’a Yalti.

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Table of Contents

Title Page

Introduction 4

The Participants 5

Place Name of Tse’Biinaholts’a Yalti 5

Notes on the ‘discovery’ 6

Beginning the Amphitheater Project 6

Some notes on the term ‘Amphitheater’ 6

Archeoacoustics 7

Dimensions of the Amphitheater 8

The One-Foot Contour Map and Cliff Face Profiles 11

The 3D Model of the Amphitheater 12

Acoustic Measurements 13

Discussion 15

Conclusions 17

Appendix 20Acoustics and the Amphitheater 20The Artificial Hill 28Equipment and Software 30

Selected References 31

The Survey Team 32

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List of Figures

Title Page

Figure 1. Amphitheater, view looking north 4

Figure 2. Vertical air photo view of the Amphitheater 5

Figure 3. Greek outdoor performance theater 7

Figure 4. A circle superimposed on the arc of the Amphitheater 9

Figure 5. The quarried area of the cliff face 10

Figure 6. ‘Scalloped’ areas and scaffold support holes 11

Figure 7. Laser range finder generated contour map 12

Figure 8. Center of curvature for the cliff overhang 12

Figure 9. A 3D perspective idealized view of the Amphitheater 13

Figure 10. Murex shell trumpets and wooden flutes 18

Figure A1. Time history and frequency analysis 22

Figure A2. Sonogram of a swept sine wave and an echo 23

Figure A3. 3D acoustical ray trace of the Amphitheater 24

Figure A4. 3D ray trace of multiple reflections 25

Figure A5. Light being focussed in a coffee cup 26

Figure A6. View along the Amphitheater wall showing overhang 27

Figure A7. Air photo of the Artificial Hill 29

Figure 11. The Survey Team 32

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Introduction

This is a current research summary regarding a location that the investigating team has named ‘The Amphitheater’. (See Fig. 1)

Figure 1. Amphitheater, view looking north.

This feature is located along the cliff face between the ruins of PuebloBonito and Chetro Ketl. (See Fig. 2) The research described herein is the result of a serendipitous discovery while reevaluating previous workconducted in the area known as the ‘Chetro Ketl Field’. The National Park Service has published the results of the remote sensing experimentsconducted by NPS Chaco Center in and around the ‘field’. (See Loose and Lyons, 1974). As of this writing, I don’t believe we know much more about the true nature and purpose of the CK field than we did 25 years ago. Gwinn Vivian has certainly documented some masonry features in andnear the ‘field’ area that could be interpreted as headgates for use in the control of water distribution. They are quite small, however, and could not be reasonably expected to handle major flash flood runoff without sustaining catastrophic damage. Such small features would need to be supplied from a controlled source of hydraulic head such as a dam

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with an adjustable release gate. To my knowledge, no such features havebeen recorded in the side canyons near the CK ‘field’. Of course such afeature might have been missed by the ground survey crews and the remotesensing staff.

Figure 2. Vertical Air Photo view of the Amphitheater between the ruinsof Pueblo Bonito and Chetro Ketl.

The Participants

Investigators include John Stein, Rich Friedman, Taft Blackhorse, Jay Williams, Patricia Long, Terry Nichols, Dabney Ford, and myself. John, Rich F., Taft, and Jay have provided a lot of assistance on test setups,moving equipment around, packing gear up late at night, starting and stopping acoustic transmitters and recorders, and offering many helpful ideas on how to proceed. Rich and Taft used a theodolite, laser range finder, and GPS system to record contour data of the Amphitheater cliff face. Rich F. used the data to produce a 1-foot contour interval map ofthe vertical cliff face and a 3D CAD rendering as well. This graphic imagery proved to be invaluable in understanding how the acoustics of the Amphitheater operate. Rich F. also provided a vertical air photo ofthe Amphitheater that shows how closely the arc in the cliff face follows a circular curve. (See the cover illustration)

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Place Name of Tse’Biinaholts’a Yalti

Interviews conducted by Taft Blackhorse have revealed that the Amphitheater has a Navajo place name that basically means ‘curved rock that speaks’. It is featured in a story about Dine’ siblings who receive special instructions concerning chants and tones. Mr. Blackhorse is continuing to gather information from Navajo people concerning this particular place in Chaco Canyon.

Notes on the ‘discovery’

While we were revisiting the field, Rich Friedman was doing some fine scale mapping of the area between Pueblo Bonito and Chetro Ketl. He mentioned that while John Stein and I had been walking around, he noticed some strange acoustical effects. When he was at particular points, he could hear our conversations perfectly, as if we were standing right next to him. This effect seemed confined to fairly smallareas. If he moved just a few feet, the effect went away. This reminded me of stories that I had heard from Park Rangers at Chaco over 25 years earlier: that there were strange acoustical properties between Pueblo Bonito and Chetro Ketl.

As we were discussing the sounds and old stories, I glanced toward the cliff face between Bonito and Chetro Ketl. I instantly realized that I was looking at a giant acoustical mirror, not unlike the one behind the stage at the famous Hollywood Bowl Concert Theater. This was one of those flashes of insight that you don’t get to experience often. Luckily, John Steins’ staff from the Chaco Site Protection Office was open to the idea that the area had been used for pre-Columbian ceremonies. John suggested looking into it in a bit more detail.

Beginning the Amphitheater Project

By coincidence, I had been doing some personal recording of outdoor sounds as a hobbyist, and had my equipment with me. This started our investigations into the outdoor Performance Theater at Chaco- the ‘Amphitheater’. To date there have been a total of five field trips beginning in January of year 2000, with the last trip being on June 21, of 2001. Recordings were made at night for two main reasons. We did not want to intrude on the Park Visitor’s experience of Chaco, and recording conditions are better at night. There is no distracting noisefrom vehicles and usually the air becomes still right around sunset. Three trips were badly hampered by night winds. Wind affects sound

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transmission strongly, causes unacceptable noise in microphones, and a lot of unwanted noise is produced as the wind rushes through vegetation or along the surfaces of the canyon’s cliffs. A total of nearly 5 hoursof experimental recordings have been obtained so far. The analog tapes delivered with this report have been edited to about 3 hours and 47 minutes.

Some notes on the term ‘Amphitheater’

In the strictest sense, especially in Europe, the term Amphitheater is used for the oval outdoor theaters constructed by the Romans for athletic or gladitorial contests, and chariot races. Some of these wereof wooden construction and some of stone masonry. The movie ‘Ben Hur’ should come to mind. The prefix Amphi is Greek for ‘on both sides’ and refers to the fact that the audience could sit on either side to view the event. The classic outdoor Greek and Roman dramatic theaters were seimi-circular in nature. The audience sat in the semi-circular tiers of seats that rose above the performance stage at the center of the circle. (See Fig. 3)

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Figure 3. Greek outdoor performance theater, Epidauros, Greece

In the US, the popular usage of ‘Amphitheater’ is for any outdoor performance area, with or without a raised tier of seats. These 'Amphitheaters' are most often used for musical performances, but sometimes for dramatic theater as well. So at the risk of offending architectural purists who study Old World archeology, I will use the term ‘Amphitheater’ for a formal outdoor performance theater with special acoustical characteristics that enhance the quality of the show.

Archeoacoustics

Library research and Internet searches revealed that there have been some previous studies of the acoustical characteristics at a few archeological sites. The earliest example I could find was that of Morley and Stokowski in 1931. Stokowski served as the conductor at the Hollywood Bowl in the 1930s and had become interested in the acoustics of the large Ball Court at Chichen Itza. He and Morley played classicalmusic in the Ball Court on many occasions but never figured out why the acoustics there were so amenable to musical performances. Lubman has revisited Chichen Itza and reported odd sound effects from the temple ofKulkulcan. There have also been some investigations of the ‘Passage Graves’ in Europe and at some stone circles, such as Stonehenge. W.R. Corliss gives an excellent summary of acoustics in archeology (See Corliss, 2001, and the attached bibliography for a full set of references).

After reading through the references that I could find, I decided that Archeoacoustics could be defined as: “The study of how sound was used innatural or man-made settings by ancient cultures”. It is highly likely that the sound of singing/chanting and musical instruments was an important part of ritual in ancient societies. If architecture or natural landscapes (or a combination) were built or modified to enhance ritual acoustical performances, the term ‘Ritual Soundscape’ might be applied as well as the term ‘Ritual Landscape’.

A quote from Corliss (2001, p. 313) is very appropriate to this subject.

“During rituals at today’s ceremonial sites—churches, synagogues, mosques, etc.—one customarily hears music, chanting, and sundry acoustical devices. These enhance the rituals. In fact, churches and other ceremonial sites are often specifically designed to

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create various acoustical effects. It should come as no surprise, and then, to discover that Neolithic humans also appear to have enlisted sound in their rituals and, to this end, they chose and modified their sites accordingly. Of course, we can never be absolutely certain that the ancient acoustical engineers understood how to achieve standing waves, reverberations, and echoes. All we know is that our modern scientific instruments record curious and interesting effects at some ancient sites.”

Corliss was describing acoustic effects at late Stone Age sites in Europe, some perhaps 4,000 years older the Anasazi ruins in Chaco Canyon.

Dimensions of the Amphitheater

The shape of the Amphitheater can be described as part of a torus. A doughnut or tire innertube is an example of a toric shape. Picture a portion of an innertube that has been cut in half and then buried halfway. You now have a surface that is curved in two directions. The long curved axis is horizontal and the short curved axis is vertical. The deepest part of both curves is at the ground level. These curves are both arcs of a circle. The curved surfaces have the property of bringing sound to a focus at particular points. These characteristics will be discussed in more detail later in this report.

The horizontal dimension of the curved wall is approximately 500 feet (152 meters). The vertical curved wall is as much as 84 feet (26 meters) high. The average height of the cliff face in the Amphitheater is about 75 feet (23 meters). The horizontal radius of curvature is 557feet (170 meters) and the vertical radius of curvature is 150 feet (46 meters). This means that the cliff wall is more steeply curved vertically than it is horizontally.

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Figure 4. A circle superimposed on the arc of the Amphitheater. North-south lines, the central axis of the canyon, and one of Pueblo Bonito’s walls are shown in red. The second southern wall orientation of Pueblo Bonito is shown in yellow. (Image courtesy of Rich Friedman)

There is an area inside the Amphitheater where a great deal of sandstonehas been removed. This ‘quarried out’ area is fairly symmetrical about the north/south axis of symmetry of the Amphitheater wall. (See Fig. 5) The dimensions of this removed material are roughly 60 meters horizontally, 3 meters vertically, and 2 meters back into the cliff (197feet by 10 feet by 6.5 feet). The volume of this material is about 360 cubic meters (12,805 cubic feet). Assuming an average density for sandstone of 2.22 grams per cubic centimeter, one cubic meter would weigh 2220 kilograms. The removed material would weigh 800,000 kilograms (1,760,000 pounds). This would be the equivalent of about 58 modern dump truck loads.

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Figure 5. The quarried area of the cliff face.

Evidence for quarrying is substantial. The orthogonal faces and symmetry of the missing sandstone is suspicious. The edges left where sandstone was removed are still fairly sharp, indicating they have not been exposed to weathering as long as the rest of the cliff face. If this were natural slumping/spalling, there should be quite a pile of talus/detritus at the foot of the cliff. The cliff base between Pueblo Bonito and Chetro Ketl is curiously devoid of any talus or alluvial material.

There are no quarrying marks visible, but I have visited historic rock quarries that didn’t really show marks from quarrying tools. Along someof the sharp stone edges inside the quarried area, there are several series of small incised ‘tick marks’ that could be tally marks for keeping track of the number of loads carried away. There are also some very odd-looking 'scalloped’ areas that may have been used by laborers to dress up stone masonry hammers. (See Fig. 6)

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Some of these scallops are too high to be reached without a ladder or scaffolding. The width of the scallops would be about right for lapping or dressing the edge of a stone maul. There are also numerous man-made holes in the cliff face that could have anchored ladders or scaffolding.

Just above the deepest portion of the quarried area, there are three fairly evenly spaced large holes in the sandstone (See Fig. 5). These holes appear to have been natural cavities in the sandstone that have been enlarged to diameters of 30 to 60 centimeters. The hole on the northwestern side of the Amphitheater is actually large enough for a person to crawl into. After a short tunnel, it is enlarged into a smalldome shaped room large enough to hold several people. The purpose of these features is not clear, but they certainly appear to be the result of human activity.

Figure 6. “Scalloped” areas and possible support holes for scaffolding.

The One-Foot Contour Map and Cliff Face Profiles

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Rich Friedman provided a one-foot contour map of the Amphitheater cliff face. This was produced with a laser range finder, a GPS unit, and a theodolite. The map was sent to me electronically as a .dxf file, whichI converted into an AutoCAD file. (See Fig. 7)

AutoCAD was used to generate a contour map. This map was invaluable in making accurate profiles of the cliff face possible. These profiles were used to estimate the radius of curvature for the horizontal and vertical curves of the Amphitheater’s toric shape. Although somewhat idealized, the horizontal radius was estimated to be about 550 feet (170meters), while the vertical radius was about 150 feet (46 meters). These numbers suggest that there will be a line focus somewhere between 75 feet and 100 feet from the cliff wall. It appears that the horizontal and vertical focus may interfere with each other in some interesting ways after the focused sound has reflected off the ground. These effects would be more pronounced if the area in front of the Amphitheater were a hard-packed surface. (See Fig. 8)

Figure 7. Laser range finder generated contour map of the Amphitheater wall with two profiles imposed. (Image courtesy of Rich Friedman. Profiles by Rich Loose)

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Figure 8. Center of curvature for average shape of cliff overhang.

The 3D Model of the Amphitheater

Using data from the contour map and the profiles, a 3D model of the Amphitheater was produced using TurboCAD. This image is provided courtesy of Jack Crouch, W.J. Schafer Associates. (See Fig. 9)

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Figure 9. A 3D perspective idealized view of the Amphitheater wall. The quarried center section is shown to scale.

Acoustic Measurements:

Several different acoustic measurements have been made in the Amphitheater and between the Amphitheater and Casa Rinconada. (Refer tothe Appendix for additional information, ray traces, spectrograms, and arecording time history).

1. A loud speaker was placed on a small hill just to the southeast of Casa Rinconada. This hill lies on the major north-south axis of symmetry that spans the canyon from Pueblo Alto to Tsin Kletsin. Itis the consensus of the investigative team that this hill is artificial. It does not appear to be an erosional remnant of the shale that outcrops behind Casa Rinconada. It is composed of very poorly sorted angular sandstone fragments and sand. It does not appear to contain charcoal or ceramics, and is not likely to be a trash mound.

The loud speaker was set to broadcast a set of continuous audio frequency tones (80 dBa). Once the unit was turned on, Taft and I proceeded to walk across the canyon toward the Amphitheater, which was about 600 meters away. The tones became fainter as we walked away from the source. By the time we got to the Chaco Wash, I couldno longer hear the tones. Taft said they were faint but audible.

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As we approached the center of the Amphitheater’s focus, the tones again became audible, being loudest between 100 and 200 feet from the cliff face. At less than about 50 feet from the cliff, the tones were inaudible again. This is good evidence that the curved surfaces of the Amphitheater were bringing the distant sound to a focus and concentrating its’ amplitude.

2. A 50 watt powered loud speaker with a four foot tubular baffle was set up at the center of the radius of curvature of the horizontal curve in the cliff face. This point is approximately 550 feet from the cliff. This point is near the eastern end the Pueblo Bonito parking lot. By chance, there is a small concrete pad with some rusty threaded rods (stanchions) sticking out that is near the center point, and we have used it as a reference for repeated recording sessions. The sound source consisted of the 50 watt audioamplifier driving a 10 inch speaker. The speaker was placed at one end of a 10 inch diameter cardboard tube (Sonotube made for casting concrete pillars). This tube was four feet long and was placed on two tripods such that it would direct sound to the center of the Amphitheater wall. A small survey laser was used to aim the highly directional speaker at the intersection of the cliff's vertical and horizontal axes.

Sine waves swept at different rates were beamed at the Amphitheater and the resultant reflected sound was recorded. Pulsed tones of various frequencies in the human voice range were also used. Several different kinds of music were played to evaluate how they sounded in the Amphitheater. This music was played on a portable CDplayer and amplified through the tubular baffle. The recordings were analyzed using spectral analysis software and a laptop computer.

Everyone agreed that the echo and reverberation in the Amphitheater enhanced the music’s effect. The Amphitheater seemed to be filled with sound. If you walked across the canyon floor between the speaker and the cliff face, you felt enveloped by sound.

3.A similar experiment was set up with two 25 watt amplifiers driving two Jensen triaxial speaker baffles separated by about 20 feet. Although the same recordings were played again, the effect was not nearly as spectacular. Full play volume was noticeably lower with this setup, probably due to driving 6 speakers vs. just one. The inefficiency of each speaker was additive, and peak driver voltages were lower in the twin 25 watt configuration.

4.Experiment number 1. (above) was essentially reversed. We placed a recorder on top of the ‘artificial hill’ and transmitted a swept

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sine wave (full 50 watts) toward the Amphitheater face from the concrete stanchion pad. The original signal and echo were then recorded as the sounds arrived at the hilltop 600 meters away. We noticed that the echo was louder than the original sound, again indicating that the focal properties of the Amphitheater were concentrating the sound on the hill. We also noticed that the echo appeared to be frequency shifted up when compared to the original swept sine. Because the echo at the hill was louder than the original sweep, the higher frequency portions of the sweep could be heard, thus creating this illusion. (See Fig. A2).

5.Recordings were also made of Human voice, and Native American Flute played from the center of the horizontal radius of curvature. A concert flute was played from a point intermediate between the horizontal center of curvature and the horizontal focus. Patricia Long played three classical flute selections on the evening of May 23, 2001. During this recording session a possible ‘flutter echo’ was detected. A spectral analysis of the echo showed it to be the exactly same frequency as the source note. There was someone reportedly playing an ocarina in the Pueblo Bonito parking lot, so I’m not 100% sure that Patricia caused this echo. A future recording session under more controlled conditions could resolve thesource of the unusual echo.

A conch shell trumpet was played from the vertical curve’s focal point. Spectral analysis showed the Amphitheater reverb time to be over one second. An echo from across the canyon was heard with about a 3.5 second delay. The echo lasted longer than the original pulse, suggesting that reverberation was taking place on the opposite side of the canyon as well.

Discussion

Archeological work has been conducted for over 100 years in Chaco Canyon. Since I’m writing this report primarily for John Stein and Dabney Ford, I don’t feel it is necessary to review the extensive literature. They are both far more knowledgeable in this regard than I am.

At some point, almost every investigator has attempted to explain why the unusual concentration of monumental architecture at Chaco Canyon is located in such a dry and basically hostile location. Explanations are usually based on economics, trade, MesoAmerican influence, or considering Chaco as a regional center for complex ceremonialism. If ceremonialism is part of the explanation, then

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Chaco must have been the host to rituals performed by large groups for large audiences. This assumes that regionally based congregations of people occurred periodically. Ritual, or the re-enactment of myth, is the basis of most traditional ceremonies. Ritual is universally accompanied by chanting, singing, dancing, andplaying musical instruments. Even the plazas of the largest buildings at Chaco may not have been big enough to hold the crowd from a regional gathering of the pre-Columbian population of the SanJuan basin. The naturally good acoustics of the Amphitheater may have long been a drawing point for large-scale performances for verylarge audiences.

Architects as long ago as Caesar’s Rome carefully considered the acoustics of public architecture. Vitruvius gives an excellent description of how sound generated by a speaker is propagated in a spherical wavefront. It is likely that this was old knowledge even then. Good acoustic performance in concert halls and public places requires a few specific attributes. These include some reverberation, but not too much or too little. Reverberation duration of one to two seconds gives a pleasing richness to a singer’s voice or to music. Many churches have reverberation times as long as 3 seconds. Above that, the effect begins to make the sound seem ‘muddy’, garbled or distorted. Below about one half second, the sound seems uninteresting or ‘dry’. The reverberation times measured in the Chaco Amphitheater ranged from 1.2 to 1.8 seconds.

Another requirement is a lack of echoes. Multiple echoes can lead to a confusion of sound as they pile on top of new sounds being generated. The Amphitheater does have one echo, but only one, and it gives rise to an interesting effect. When combined with another effect usually considered undesirable, namely focus, the echo forms a ‘virtual sound image’. This is analogous to the virtual image seen in a mirror. There appears to be a scene on the other side of the mirror. Echoes in the Amphitheater sound as if they were generated from inside the cliff face.

Uniform sound distribution is another desirable feature of a concerthall. Although the Amphitheater does exhibit a focus, the vertical and horizontal curves work together to produce a line focus, rather than a spot. The rough surface of the sandstone and deviations froma perfect curve serve to diffuse the focus to some extent. Also consider that the Amphitheater is huge, thus allowing both performers and audience to gather within the area of special effects. It is, in essence, a giant musical instrument that allows the orchestra and audience to get inside and experience the performance in ‘surround sound’.

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Some specific special effects were noted during our recording sessions:

1. We noticed an effect similar to wearing stereo headphones, with an accompanying difficulty in determining where the sound was coming from. This sound effect was described as seeming to comefrom ‘inside your own head’.

2. During music with a strong consistent beat and repetitive chorus, the echoes produced a counter beat that seemed to emanate from inside the cliff face. This is the ‘virtual sound image’ effect.

3. Some times harmonics were perceived above or below a long sustained note.

4. Echoes were often perceived as frequency shifted to a higher pitch. The frequency shifted echo from the Amphitheater was actually louder than the original tone when recorded on the ‘artificial hill’ 600 meters distant. Lubman noted similar effects at the Temple of Kulkulcan at Chichen Itza.

5. We may have heard a ‘flutter echo’. This phenomenon if fairly rare.

6. A trilled tone was recorded near the cliff face during the rapidly swept sine wave broadcast.

7. The team experienced standing waves while moving about in the Amphitheater during the recording sessions.

8. Tones played from the ‘Artificial Mound’ (dubbed Broadcast Hill by the research team) had an ethereal timbre and the source could not be located by hearers in the Amphitheater.

9. Speech from the mound could be heard in the Amphitheater, but itwas somewhat garbled and again, the source’s direction was indeterminate. This gave the effect of ‘hearing voices’ but youcould not quite understand what they were saying.

10. A jet aircraft passing overhead fills the Amphitheater with a sustained low roaring noise. We did not have the opportunity toexperience the reverberations from a thunderclap, but I’m sure the effect would be spectacular.

11. There were many places in the Amphitheater where two people could carry on a conversation at normal voice levels though theywere far apart.

12. There seems to be a ‘volume threshold’ required to stimulate thevarious special sound effects. It would have required a coordinated effort by a large group of people to produce these effects.

There is some additional evidence that the Amphitheater was a special place. There are no Anasazi structures in the area, which is within thedensest concentration of architecture in the Anasazi world. The cliff

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face in the Amphitheater is devoid of talus or colluvium. The task of removing this material would have required a great deal of labor. In some of the Lindberg aerial photos and in the photo supplied by Rich Friedman, there appears to be two berms or walls projecting from the cliff and forming a delineated plaza. (See Fig. 2) A plaza with a hard-packed surface would have enhanced the reverberation already noted.

There are remains of what may have been a curtain wall at the southeast end of the ‘quarried out’ area. It is possible that the ‘quarried out’portion of the cliff was covered with a wall. This could have provided a ‘back stage’ area where performers could have prepared for a ceremony and seemingly emerged from the cliff. There are underground passages inthe floor of Casa Rinconada that could have been put to similar use.

The ‘quarried out’ area, if not walled up, may have been a purposeful effort to enhance the reflected sound at the ground level. The Anasazi may have created a smaller version of the Amphitheater within the largercurve. The exposed ‘stepped’ rock faces within the quarried zone contribute to phase delays in the reflected sound, and make it richer and more interesting. The area between the ceiling and floor of the quarried area may also contribute to the long resonance after an initialsound has been stopped.

The three large holes in the ‘quarried area’ and numerous smaller holes are not typical of the cliffs in the canyon. The enlargement of the western-most hole may have also been used to supply ‘special effects’. It may have acted as a Helmholtz resonator and may be the source of someof the harmonics recorded during the long swept sine wave experiment. Future acoustic tests at the entrance to the western hole could verify if this is possible.

The Amphitheater and the ‘artificial mound’ both lie on the main north-south axis of symmetry of all the major architecture in the central canyon. The canyon to the south of the hill may also have been part of the entire Amphitheater system. If so, the Amphitheater may have been an acoustical device over 1.5 kilometers long. The hill may have servedto elevate performers above the ground induced turbulence layer and madespecial effects more noticeable during windy conditions. The smooth elliptical shape of the mound may have aided in producing a laminar air flow at the hill top, which would also have made performances during mildly windy conditions possible.

The Amphitheater also lies on the major east-west axis of symmetry of the canyon layout. The intersection of the two main axes of symmetry occurs near the major focus of the Amphitheater’s two curves. It is almost as if the canyon was designed and laid out with the Amphitheater as the center.

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Conclusions

Clearly, the Amphitheater exhibits unique acoustical effects. A great deal of labor was invested in removing talus and alluvium from the area in front of the curved cliff wall. Looking at vertical air photos, I’m almost tempted to suggest that the entire cliff wall was modified in theportion that forms a circular arc. This cliff wall has a different appearance than the rest of the cliff faces along both sides of the canyon from Penasco Blanco to the ruins of Una Vida and Kin Nahasbas.

The removal of material from the ‘quarried area’ was a monumental task. At least 360 cubic meters of material was removed and hauled away. Assuming an average sandstone density of 2.2 grams per cubic centimeter,this would be 1,742,400 pounds (792,000 kilograms), or as mentioned earlier, about 58 average dump truck loads.

Musical instruments including wooden flutes and shell trumpets (Genus Strombus and Murex) were found in Pueblo Bonito only a few hundred feet away. Considering the unique acoustic features of the Amphitheater, it is highly likely that these instruments were played there. (See Fig. 10).

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Figure 10. These Murex shell trumpets and wooden flutes from Pueblo Bonito are currently housed at the American Museum of Natural History inNew York. Photos courtesy of Terry Nichols.

I would like to conclude with a quote from Aaron Watson regarding acoustic effects at the ruin of Stonehenge in England.

“Rituals at Stonehenge must have been awesome spectacles, enclosed within the enormous circle of soaring stones, the open sky overhead. And the sounds of Stonehenge ceremonies likely were just as awesome, for Europe’s most famous megalithic site could play exotic games with sound.

Stonehenge, the most impressive of the late Neolithic/Bronze Age stone circles inEurope, has been the subject of countless theories and endless research. It was in use for around 1500 years and rebuilt many times during that period. The Stonehenge we see today is the final and most sophisticated phase.”

“This was an awesome undertaking. The stones, most weighing 25 tons, were dragged 30 kilometers (18 miles) to the site and individually pounded into shape. Many of the stones were placed to align with astronomical events.

The nature of the ceremonies that took place remains a mystery. Our research, however, suggests the mysterious effects of sound may have been exploited within the dramatic setting.”

“The interior of the monument, however, seems to have been configured to direct sound in particular ways. Ancient people most likely approached Stonehenge along an avenue defined by banks and ditches, which directed them to enter the circle at

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a specific point. A horseshoe of enormous stones within the monument reflects sound sothat it is focused on that very spot, where the select passed into the circle. This thresholdwould be marked by a sudden and unexpected burst of sound that hit just as the visual spectacle of the interior was revealed.

Some evidence suggests the stones were deliberately shaped to enhance these acoustic effects. The sides that face the center are wide and seem to be either flat or concave, making them more effective at bouncing sound back into the interior.

The designers of Stonehenge apparently knew how to put on a show.” (A. Watson, 2000)

Apparently, the Chaco Anasazi took advantage of this same sort of technical knowledge to produce impressive ritual performances at the very center of Chaco Canyon.

Rich LooseOrgan, New MexicoNovember 4, 2001(Edited, expanded, and updated April 24, 2005)

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Appendix

Acoustics and the Amphitheater

The term acoustics comes from the Greek word akoustikos, which means ‘of hearing’. In modern usage, acoustics is the science of sound, includinggeneration, transmission, and reception. There are major subcategories of acoustics: (1) Physical acoustics; (2) architectural acoustics; (3) psychological acoustics; (4) physiological acoustics, and (5) electroacoustics.

Physical acoustics deals with the propagation of longitudinal waves through gases, liquids, and solids. Sound travels as waves of compression and rarefaction through matter. It behaves in ways analogous to waves on the surface of water, or as waves of light. When a sound is made in a uniform unobstructed medium, the waves will radiatein an expanding spherical shell or wavefront. The wave will have a characteristic frequency, i.e., the number of compression/rarefaction pairs that pass a given point in a given time. This is normally countedas cycles per second (CPS) or Hertz (Hz) in honor of Heinrich R. Hertz, a pioneer in acoustics. [The founder of modern acoustics is generally recognized as the German Physicist, Ernst Chladni (1756-1827). Chladni discovered acoustic effects such as standing waves and measured the speed of sound in different gasses.]

A subdiscipline of Physical acoustics, known as geometric acoustics, deals with tracing the paths that sound can travel and is analogous to geometric optics as applied to light waves. The human ear is generally considered capable of hearing from 20 to 20,000 Hz. A good stereo system will have this range of frequency response. Sounds below this frequency range are termed infrasound and sounds above this range are called ultrasonic.

Architectural acoustics deals with the distribution of useful sounds to humans in a ‘built environment’. As an applied science, it is concernedwith the reduction or exclusion of unwanted or distorted sounds. This includes the study of Concert Hall design and performance, and is relevant to the characteristics of the Amphitheater at Chaco. Although much of the Amphitheater is natural, it appears to have been modified bythe Anasazi. Man made improvements to the acoustical nature of the

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Amphitheater may have been deliberate or the serendipitous result of modifications made for other reasons.

Psychological acoustics is the study of the ways in which the human brain processes and interprets sound. This includes the human (and animal) mental and emotional reactions to various sounds. The question of which sounds are intolerable, tolerable, acceptable, or even pleasingto most people is considered part of psychological acoustics. It also includes the study of how background noise can interfere with effective speech communication.

Physiological acoustics deals with the mechanism of the ear for normal or impaired hearing. This includes the voice mechanism and the physicaleffects of sound on human (and animal) bodies.

Electroacoustics involves the electronic measurement and reproduction ofsound. It also includes sonic transducers; devices that can convert sound to electrical energy, or in reverse, convert electrical energy into sound. This is the realm of sound amplifiers, speakers and recorders.

I use the term decibel (dB) when describing the strength (power) of sounds. This is a unit (1/10 of a Bel) devised by Alexander Graham Bell. This is a logarithmic scale. By convention the lowest threshold of human hearing is between 0 and 4 dB, 30 dB is 1,000 times stronger, 60 dB is amillion times stronger, 90 dB is a billion times stronger, etc. At about 120 dB humans start to experience pain, and permanent damage to the ears will result if sustained for long. This is different than loudness as perceived by humans. The human ear is ‘non-linear’ in its’ response to sound, and loudness is subject to judgement by each individual. There is a marked decrease in sensitivity below 200 Hz, andthis difference becomes progressively greater at even lower frequencies.In other words, at frequencies as low as 50 Hz, a true sound level of 43dB will just barely be perceived as if it were the 4 dB limit. For purposes of standardization, most acoustical measurements with electroacoustic equipment are done at 1,000 Hz if a comparison with human perception is desired. The human ear actually gets more sensitiveto sound pressure levels around 3500 Hz, and then begins another loss insensitivity above 6,000 Hz. These characteristics are significant in the design of lecture and music halls, and in high fidelity audio equipment.

Acoustic Scintillation is the sound analog of a star twinkling in the night sky. Irregular fluctuations in the received intensity of sounds as they move through the atmosphere are caused by non-homogenous structure in the air along the propagation path. Wind turbulence, variations in temperature, and moisture content all contribute to this

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effect. These atmospheric inconsistencies cause aberrations of sound and include sonic refraction (bending), absorption, dispersion (different amounts of bending for different frequencies), diffraction (an orderly form of bending and disbursing light), and random scattering. These all result in distortion and attenuation of the original sound.

An echo is a sonic wave that has been reflected and returned with enoughstrength to be perceived as distinct from the original sound.

A flutter echo is a rapid succession of reflected pulses resulting from a single initial pulse.

A reverberation is the persistence of sound after the original signal has stopped. This occurs in enclosed or semi-enclosed spaces. It is the result of multiple reflections whose return times are too short to be easily discerned by the listener. The term rate of decay will often be used in conjunction with reverberation. The standard for measuring the decay is how long it takes the sound to fall to 60 dB less than its’ original level. (See Fig. A1)

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Figure A1. This is a time history and frequency analysis of a Conch shell trumpet blast about 50 feet from the Amphitheater wall. The reverberation is over 1 second long. The 440 Hz trumpet tone is still seen in the frequency analysis over 1 second after the source sound is shut off. It is still about 40 dB above the recording’s noise floor.

Spectral Analysis or Spectrum Analysis is the study of the frequency components of a complex sound. Audio spectrum analysis equipment used to be heavy, bulky, have major power requirements, and was very expensive ($50,000+) just a few years ago. Today there are many types of software packages, that when combined with a state-of-the-art personal computer can achieve frequency analysis that would have been formerly out of reach for archeological field research. These programs use an algorithm called a Fast Fourier Transform or FFT. The FFT breaksa complex sound down into the original frequency components. This type of analysis is quite useful for investigating the interaction of human voice (voices) or instruments within a particular performance space. The spectral information can be presented as a graph of power vs. frequency (spectrogram) or as a time history of frequency vs. time with the intensity shown in false color (sonogram). (See Fig. A2) This spectrogram is from a recording made on the Artificial Hill. The sourceis a speaker near the center of horizontal curvature of the Amphitheater, about 500 meters away. The echo is apparently longer and louder than the original sound. This effect demonstrates that the reflected sound energy is being concentrated on the hill by the large curved Amphitheater wall. The sound coming to the hill directly from the original sound source has been diminished at the inverse square law rate. The swept echo seems longer that the original sweep because it islouder and the high frequency part of the sweep is less attenuated. Reverberation in Amphitheater “cavity” may also increase the duration ofthe echo.

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Figure A2. Sonogram of a swept sine wave and an echo.

A sine wave is a uniformly undulating wave that can be described mathematically as a sine (or cosine) function. The FFT algorithms all assume that any wave function can be constructed from combinations of various sine waves of multiple amplitudes and frequencies. The visual image of smooth waves radiating from a pebble dropped into a still pond is a close analog to a pure acoustic sine wave. Pure sine waves of a single frequency (usually 1000 Hz) are often used in acoustic investigations when a sound is being transmitted. We rarely hear pure sine waves in natural settings from natural sources.

A Ray Trace is a theoretical map of the paths that sound will take afterbeing reflected from a surface or set of surfaces. This technique is based on the principle that a wave will be reflected from a surface at the same absolute angle as the arrival angle. This angle is usually measured from a line normal to the surface of reflection. ‘The angle ofincidence is equal to the angle of reflection’. This technique does notconsider non-geometric effects such as diffraction. Designers of

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optical systems use this same technique. It is valuable in determining where curved surfaces will focus an impinging wavefront. When considering curved surfaces, the normal will be taken from a line tangent to the curve where the angle is being measured. (See Figs. A3 and A4)

Figure A3. Three dimensional ray trace using ZEMAX. Source at Artificial Hill 600 meters from the Amphitheater wall. Courtesy of DickHorton, ad hoc Optics. This shows the line focus in front of the wall.

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Figure A4. Ray Trace of multiple reflections near the wall produced with ZEMAX. Courtesy of Matt Heino, Boeing Aircraft Corporation. The disk and line show the central axis of the toroidal curve.

A Standing Wave forms when two wave fronts collide. At the points wherewave peaks coincide, the sound is reinforced and a Standing Wave is formed. During the slowly swept sine wave experiments, standing waves were frequently encountered. As the frequency changed, these standing wave fronts would slowly change position.

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A focus is a point where a lens or mirror concentrates waves. In the case of a cylinder or torus, the focus will be a line. (See Fig. A5)

Figure A5. Light being focussed by the cylindrical side of a coffee cup. This is analogous to sound waves being focussed by the Amphitheater’s curved wall.

A torus is a curved surface that is all or part (a toroid) of a “doughnut” shaped object. Consider a portion of a cylindrical tube thathas been bent as an example. (See Fig. A6)

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Figure A6. View along the Amphitheater wall showing the overhang, whichproduces the toroidal curve. Note the person on the cliff top for scale.

Hertz are units of frequency. One Hertz (Hz) is one cycle per second.

Decibels are units of sound intensity. A decibel (dB) is 0.1 Bels. Decibels are scaled such that a change of 20 decibels is a factor of 100, 30 decibels a factor of 1000, 60 decibels is a factor of one mullion etc.

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The Artificial Hill

This elongated hill covers about 60 meters by 30 meters and is approximately 4 meters high. Although some modern Navajo Practitioners consider the hill to be part of Tse’Biinaholts’a Yalti, it has been a subject of informal academic debate for many years. A UNM Field School may have trenched the hill in the 1940s. The scars left by the trenchescan be seen in air photos. (See Fig. A7). Since the Field School excavators found no cultural artifacts in the trenches, they concluded that the hill was natural. There are some reasons to suspect that the hill is man made:

1. It is a regular ellipsoid in plan view. This is not typical of a natural hill. To my knowledge, there are no others like it on the canyon’s alluvial flood plain.

2. There appears to be an almost circular, well-defined ring around it.It looks somewhat like a giant anthill that has the classic cleared area surrounding the mound.

3. The hill consists of a very poorly sorted mixture of sand and chunksof sandstone. This is very atypical of canyon floor fill, which would have a much higher clay content and would lack the variously sized pieces of sandstone.

4. If it were an erosional remnant of the south cliff face, it should be made of the shale that exists at the same elevation in the nearbycliff.

5. The orientation of the long axis is not consistent with being an erosional remnant created by the Chaco Wash. The long axis is almost at right angles to the flow direction of the Chaco.

6. The drainage headers from the local side canyons are not long enoughto create an erosional remnant of this size.

7. The hill is located exactly on the north-south axis of symmetry thatpasses from Tsin Kletsin ruin, through the center of the Amphitheater, and on to Pueblo Alto. This line is within a few arc seconds of celestial north. The sound energy being transmitted to the Amphitheater’s line focus, or received from the Amphitheater at the hill is least attenuated or distorted on this line.

8. The elevation of the hill above the canyon floor reduces sound attenuation due to ground induced air turbulence or stratified air refraction effects. This result may have been achieved intentionally on the part of the designers/builders.(See “Acoustic Scintillation” above).

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Figure A7. The Artificial Hill near Casa Rinconada lies directly on the major north-south axis of symmetry at the central Chaco architectural layout.

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Equipment and Software

Equipment Used

Recorders/accessories used include:

1. Sony TCD-5M LED controlled servo motor cassette tape recorder.Frequency Response 20-19,000 Hz. Wow/Flutter 0.06%

2. Sony MZ-R 30 digital minidisk audio recorder. Frequency response20-20,000 Hz. Wow/Flutter not measurable. Used with Sony stereo microphone ECM-MS9079 (100-15,000 Hz.)

3. Radio Shack Dual Display Digital Multimeter with frequency meter function

4. Radio Shack Digital Sound Level meter (50-126 dB +/- 2 dB

5. Radio Shack 10" bass loudspeaker (Free air resonance 33 Hz.)

Amplifiers used include:

1. Optimus 50 watt stereo audio amplifier (25 watts/channel)Frequency response 20-20,000 Hz.

2. Audiovox 100 watt stereo audio amplifier (50 watts/channel)

Software Used

1. SpectraPlus Studio quality FFT Spectral Analysis System by Sound Technology http://www.soundtechnology.com

2. CoolEditPro Stereo audio editing and FFT Analysis by Syntrillium Software http://www.syntrillium.com

3. Spectrogram R.S. Horne Shareware http://www.visualizatinsoftware.com/gram.html This is an outstandingstereo spectral analysis program

4. AutoCAD Lt. 98 by Autodesk http://www.autodesk.com

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5. Paint Shop Pro Photo Editing Software by Jasc http://www.jasc.com

6. ZEMAX Optical Design Program by ZEMAX Development Corporationhttp://www.zemax.com

7. TurboCAD by IMSI http://www.turbocad.com

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Selected References

Loose,R.W., and T.R. Lyons1977 The Chetro Ketl Field: A Planned Water Control System in Chaco

Canyon. In Publication Number One of the Chaco Center, National Park Service and UNM, Albuquerque.

Devereux, P., and R. G. Jahn1996 Cognitive Considerations of the Acoustical Resonances of Selected

Archeological Sites. Antiquity Vol. 70.

Watson, A., and D. Keating1999 Architecture and Sound: An Acoustic Analysis of Megalithic

Monuments in Pre-historic Britain. Antiquity Vol. 73.

Watson, A2000 Sounds of the Spirit World: Ancient Monuments Wrap Their Mysteries

in Eerie Sound Effects. In Discovering Archeology (from Scientific American Magazine), February, 2000.

Brunhouse, R.L.1971 Sylvanus G. Morley and the world of the Ancient Mayas. University

of Oklahoma Press, Norman. (Numerous passages on the Chichen Itza Ballcourt)

Jahn, R.G., Devereux, P., and Michael Ibison1996 Acoustic Resonances of Assorted Ancient Structures. J. Acoust.

Soc. Am. 99(2):649-658. February 1996.

Lubman, D.1997 An Archeological Study of Chirped Echo from the Mayan Pyramid of

Kulkulkan at Chichen Itza. Paper presented at the October, 1998 meeting of the Acoustical Society of America, Norfolk Virginia.

Helmholtz, H.1885 On the Sensations of Tone. Original Ellis translation

republished by Dover, 1954. Dover Publications, New York. ISBN 0-486-60753-4 (This book is a must-read for anyone interested in the human perception of sound)

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The Survey Team

Figure 11. Left to Right: Rich Friedman, Taft Blackhorse, John Stein, and Jay Williams

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