[Aztlan] Plaster and the general geology of the YukatanPeninsula

[David Lubman]

Dave:

Wayne Van Kirk (the dreamer) believes you meant to direct your posting to me 
(the acoustician).

It is true that half-vaults can reflect sound well. Especially if comprised 
of smooth stone and coated with non-porous plaster.

You compared standing half-vaults with the parabolic sound reflectors we've 
seen in museums. There is some justification for that comparison.

In cross section, A half-vault is a two line-segment approximation to a 
parabola. Not a very good approximation. Not even as good as a decent 
bandshell. But I do agree standing vaults tend to be retroreflectors that 
contribute a modest amount of acoustical "gain". I estimated their combined 
gains as 2 to 4 dB.

Interestingly that modest gain is insufficient to explain the whispering 
gallery at the Great Ball Court (GBC). Something else is going on. It's been 
a mystery.

Listening to Wayne Van Kirk's voice on the other end of the whispering 
gallery, I guessed his voice level was around 45 dBA. That's more than 20 dB 
stronger than expected for ordinary inverse square law spreading.

In its present state, I estimate that the GBC whispering gallery provides 
about 20 dB of acoustic gain. That's a whole lot!

If this much gain was present originally the GBC comprises an amplified 
arena. One that could insonify thousands of people arrayed in the playing 
field. If it was present, do you think the Maya would have noticed it? Do 
you think that feature could be handy for a king or priest?

Modern technology would almost certainly rely on electronic amplification.

Below I provide details of the simple calculation of how loud a raised voice 
would be heard at a distance of 140 m.

Assume a talker and a listener stand near the lips of opposite temples, 140 
m apart.

Assume the talker speaks with a "raised voice level" of 67 dBA @ 1 m 
(typical of a lecturer).

Assuming inverse square law spreading, and ignoring both ground effects and 
air attenuation, a loss of 43 dB is calculated.  Thus, raised voice effort 
speech would be received at about 24 dBA (67 dBA - 43 dBA). That level would 
probably be inaudible under typical field conditions. What  heard seemed 
more like 45 dBA to me. I recognize that the background noise level is low.

I'll stop here for David's (or anyone else's) response so far.

David Lubman, FASA
Acoustical Consultant
-------------------------


----- Original Message ----- 
From: "David Hixson" <chunchucmil at yahoo.com>
To: <Aztlan at lists.famsi.org>; "Wayne Van Kirk" <wvk at swbell.net>
Sent: Thursday, August 21, 2008 12:15 PM
Subject: Re: [Aztlan] Plaster and the general geology of the 
YukatanPeninsula


> While I have appreciated and enjoyed Wayne's posts for many years, I must 
> reiterate my own counter-points to his views on Maya acoustics --  
> especially as it figures into the architecture of the Cross Group at 
> Palenque and the Great Ball Court of Chichen Itza.
>
> In both of these situations, it is not the decayed stucco or plaster that 
> may have changed the acoustical properties of these venues.  It is the 
> architecture itself.  In both cases, at least one of the opposing 
> structures has half-fallen to become a half-vaulted reflecting shell.  In 
> the case of the Great Ball Court, both ends of this performance gallery 
> have fallen to become what amounts to a parabolic reflection chamber (just 
> remember those science museums you visited as a child, and how the two 
> parabolic reflectors allowed you to whisper to your classmate from yards 
> away).
>
> I hope the following text appears as it should on-list:
>
>          xxxxxx
>         x   oooo
>        x    ooooo
>       x     oooooo
>      x      ooooooo
>     x       oooooooo
>     x       oooooooo
>     x       oooooooo
>     x       oooooooo
>     x       oooooooo
>     x       oooooooo
>     x       oooooooo
>     xxxxxxxxxxxxxxxx
>
> The "X" blocks remain, the "O" blocks have fallen.  A mirror opposite of 
> this half-fallen structure stands at the other end of the Great Ball 
> Court.  The result is an unintentional whispering chamber using two 
> opposing reflectors.
>
> In fact, at Chichen Itza it is unlikely that very much plaster was ever 
> used to coat the (now fallen) facades.  So, the acoustical significance of 
> plaster in such situations is moot.  To follow Wayne's train of thought, 
> archaeologists would have to rebuild the fallen halves of the north and 
> south temples (not just replaster them) in order to recreate the proper 
> acoustical effects.
>
> That being said, it's been a while since we've heard Wayne's latest news, 
> so I would be more than happy to see any new developments in his Maya 
> acoustical studies.  I've often thought that Wayne and Greg Sandor (fellow 
> listeros) should link up to conduct some VR simulations of acoustical 
> properties (e.g., shooting a laser or other item with a known trajectory 
> to evaluate its reflective properties depending upon the location of the 
> FPS).  Greg could easily add or subtract the now-fallen exterior walls to 
> determine the effect upon acoustical vectors.
>
> Love to hear more.
>
> -Dave
>
>
>
>
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