Let me get this straight...

You want acoustics information on an open-air non-ban-shelled stage that sits in 'pile of rocks' that only pretty much hosts amplified performances?

non-bandshelled*

this song is not a rebel song, this song is 'Sunday, Bloody Sunday'!

Uxbridge sounds kind of negative, but don't worry; acoustical information should be totally findable (or at least testable)! I don't know any specific sources off the top of my head, but I'd recommend asking the caretakers of the park if they've had acoustical testing done (or looking through the Journal of the Acoustical Society of America to see if they have any information of testing on outdoor amphitheaters). That said, 'acoustical qualities' could mean any number of things, so it would be helpful to know more information about what you are aiming for with this project.

Without knowing more, I'd respond thusly: The premise behind this particular 'amphitheater' seems to be that the rocks form a system of reflectors/diffusers for the sound, and the surrounding area is (presumably) quiet enough where there isn't too much outside noise to interfere with whatever performance you're giving. (Caveat: I've never been here myself, so I'm making assumptions about this particular place.) Without having seen or heard the area or knowing much about what you're looking for, these are probably the two most important qualities (and therefore studies) off the top of my head that you'd be looking for.

Regarding the first: when you design auditoria or concert halls, from an acoustical standpoint, there are several important design goals that the architecture can achieve. Without going into too much detail here, an important goal is to have early sound reflections 'envelop' you in a particular way, timing, and proportion to the direct sound from the source; a second goal is to have an appropriate level, timing, and composition of late reverberation (in proportion to the direct sound and to the early reflections). These characteristics can all be measured and documented, regardless of whether or not you are building the site or 'finding' one in nature. If you're interested in this kind of thing, I'd recommend looking up a book on architectural acoustics, seeing the different kind of criteria that are used, and using them as a guideline for the kind of information you'd ask/search for.

'Quiet' should be a little more straightforward, by comparison ;-)

Again, it'd be helpful to know what the end goal is; it's easier to search when you know what it is you are looking for!

I wasn't being negative particularly.

But other than the rocks... the thing that Red Rocks has that most amphitheaters don't have is cool clean crisp low-humidity air.

The seating arena also has what would be an 'illegal' grade (30%) to it being very steep over the course of the amphitheater. That means that every person in the amphitheater has a clean line of sight to the stage. In amplified theater arrangements, being in visible sight if the center stage means you also receive direct sound from the speakers produced.

So, the acoustics of the performances really depend on the quality of the sound equipment.

The stone monoliths, however, affect sound quality but only minimally. Neither stone surface is parallel to each other and neither stone is perpendicular to the floor. Also lacking is the roof which would further lead to either fluttering and reverb given the hard walls. That essentially allows for a single-bounce echo and a minimal reverberation avoiding both 'empty room,' 'hallway' and 'canyon-like' qualities.

The venue also lacks anything that would otherwise interfere with sound other than some small trees-- there's no curtains, upholstery, carpeting, railings et cetera. That means the sound from the stage travels relatively uninterrupted without becoming muffled, attenuated or otherwise distorted.

That being said... 'architectural acoustics' are absolute hogwash when you have amplified performances unless you happen to be performing inside of a grain silo, a submarine, at the bottom of Bryce Canyon or in an upholstered sanatorium cell.

boom

Thanks for the input! Mixmaster + Uxbridge were definitely helpful. To be fair, I'm really looking for any information on the venue that could be seen as enhancing its sounds. So everything from the crisp air to the formation of the rocks is perfect info...
I don't know much at all about acoustics, so I enrolled in an Arch Acoustics class in my grad school's M.E. Dept. This is pretty much an "anything goes" presentation. So any info is good info.

Since sound is a physical force and physical forces tend to travel in linear directions, I would diagram the site in different sections and elevations with particular regards to the stone monoliths and the seating area.

You can approximate how sound will travel in the same way you can approximate how a tennis ball or a marble will travel when you throw it sans gravity.

http://www.ndt-ed.org/EducationResources/HighSchool/Sound/reflection.htm

This is probably overly simplistic but it gives the idea.

From this, you could do some kickass illustrations and diagrams demonstrating where sound would and could go in the amphitheater. This would allow you calculate or estimate where reverberant and diffraction areas might be and show where optimal seating might be found.

Of course, this would depend on you also calculating the decay of sound. the decay of sound (sound pressure) is calculated by inverse square. I failed physics so I cannot help you with this. I'd only advise that you should use gradients!

Re: Uxbridge - tone is always a little tough to tell online, so my apologies for reading too much into it ;-)

It's pretty correct that the 'sound quality' part of acoustics is mostly handled through speakers and sound engineers when you're talking about amplified performances. In that case, the job of the room you'd design is to not interfere with the amplified performance (think 'subway announcer PA system' - an acoustically live room can have a very noticeable effect on the target quality, for example). Speaker-room combinations are actually a fairly important subject for acoustics; if you are good at both and can design the speaker system in tandem with the room, you'll do well.

Wherever you are, there will still be an 'acoustic fingerprint' left by the architecture (or rock forms, or whatever) that will color the sound. It's one of the things that may make some areas more favored for amphitheaters than others (as opposed to just 'any quiet bowl-shaped depression in the hills'). From the Wikipedia article I looked up on the place, it seems that this site was used for performances well before the era of amplification; the 'coloration' may have had some subtle effect (and still does, even with amplification). From the Google Earth image of the site, it seems like there are two side-rock formations about 500 feet apart, and then a small rise at the bottom of the 'bowl' that may have provided some early reflections if the performers were nearby. (Though, truthfully, 'sound quality' is very much a matter of cultural subjectivity - it's not like there's one requirement for "perfect acoustics" in all instances. I think Barry Blesser used to hang out on these forums - he wrote a book on this sort of thing, which I'd recommend reading.)

For the diagramming, you'll probably make the best-looking and most compelling graphics in the class if you're taking acoustics in an ME program (note: that's an interesting place to take architectural acoustics). I'd go either with Uxbridge's suggestion or maybe ray-tracing diagrams that show where the sound originates and then goes/reflects, or some combination of the two.

Also, is it true that there's a race track a few miles away and on the other side of a ridge from this thing? Those are usually pretty loud, so it kind of made me chuckle a little to see those things relatively close together.

FOH engineers generally hate red rocks: the rock formations have a habit of funneling the wind around in odd ways, meaning you can have everything dialed in perfectly and it'll sound like ass 90% of the night just because the wind is pushing the sound around.