Half a croissant, on a plate, with a sign in front of it saying '50c'
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Custom Echo Wall

Record your voice in stone
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I came up with this idea while visiting Chichen Itza on my honeymoon. The tourguide demonstrated to us that when you stand in front of the Temple of Kukulcan (the large stepped pyramid) and clap, the returning echo sounds like a chirp as it reflects off the many stone steps. I realized that by varying the height of the steps, you could arbitrarily shape the returning echo to your liking.

I propose a massive wall with vertical panels instead of horizontal steps, each segment angled to face a particular spot, where the observer will stand. The area of each segment determines how much sound energy will be reflected back to the observer at that particular time step. The input sound should be as brief yet as loud as possible to minimize distortion, so instead of clapping, the listener will probably pull a lever that sends two giant steel spheres crashing into each other, like an enormous Newton's Cradle.

Designing the wall would be almost trivial: digitally record the sound you want to produce, and the apparent area of each segment 'n' should be proportional to the amplitude of sample 'n' in your PCM waveform. You would need to work out the right panel spacing for the sample rate you're using.

The tricky part is the scale of the thing. If you want to record a brief 1 second message, the wall would need to be 1/2 of one sound-second long, or 172 meters. Also, note that above I said, "apparent area"-- more distant segments will appear smaller to the observer, due to perspective, so unless you want your message to fade as it plays back, the panels will need to get bigger and bigger as they go (subject to modulation.) Finally, you will probably be deafened by the input sound, so you'll need to wear earplugs, and pull them out right after the initial "CRACK!"

Still, the challenges pale in comparison to the promise of having a sample of your voice set in a giant stone wall for all eternity. I just need to figure out what 1 second of speech would be worth recording-- "Anthony rules!"?

lankybits, Feb 21 2007

How to make a filter with ducting http://www.ketterin...ustics/Filters.html
A low pass filter is a duct with an expansion chamber in it. [Ling, Feb 25 2007]

Acoustic mirror http://math.univ-ly.../Nachbin-slides.pdf
Time reversal, electronics required. [csea, Feb 26 2007]

SciAm Acoustic reversal http://www.phschool...on_the_rebound.html
Put two of these together and reverse the reversal? [csea, Feb 26 2007]

Natural Echo Wall http://www.vivanewm...om/ghosts/echo.html
Natural weather-created echo wall [csea, Feb 26 2007]

Recording in Clay http://itre.cis.upe...rchives/002875.html
Somewhat tongue-in-cheek! [csea, Feb 26 2007]

[link]






       You do rule, this is an ace idea. Especially if it can be made to work from a single human clap.   

       <clap>   

       <Croissant!!>
theleopard, Feb 21 2007
  

       Excellent - although the costs would be enormous and you'd have to sell out to corporate sponsorship which would mean the wall would reflect any sound as the Intel jingle.
hippo, Feb 21 2007
  

       NOOOOOOOOOOO!!!!
theleopard, Feb 21 2007
  

       Sound propagation speed is dependent on the bulk modulus elasticity for the medium in question. Instead of air, use Carbon Dioxide (speed of sound = 259 m/s) or vulcanized rubber (speed of sound = 54 m/s)   

       Strike a steel endplate with a hammer, have the sound travel through the medium to appropriate panels, then return to the endplate.   

       Great idea. Could it say, "Where am I going to get 85 tons of rubber?"
shapu, Feb 21 2007
  

       Oh, and you should probably have James Earl Jones record your message for you. Future generations won't know the difference, and you'll avoid what must be one of the worst feelings in the world: having just spent several billions of dollars on a massive echo wall, only to hear it for the first time and think, "Eww, does my voice really sound like that?"
lankybits, Feb 21 2007
  

       When you clap/bang into the wall the sound comes back as speech. If you shout at the wall the correct words would it come back as a clap?   

       If so, you could engineer the wall to only respond to a particular voice command to which it would reply an otherwise undiscernable secret message. An unknown password to unlock the message within, for the post-apocalyptic civilisation that worship the giant mystical relic of yesteryear...
theleopard, Feb 21 2007
  

       Very clever. It's kind of like a sound-based hologram in a sense. Pulse in, information out. I like [theleopard]'s secret message concept. Bun for [lankybits].
Agamemnon, Feb 21 2007
  

       Thanks for the buns, folks. [theleopard], to answer your question, designing the wall to produce intelligible sounds in response to a sound longer than a clap becomes very complex. The output sound you get is the convolution of the input sound with the segment areas, where convolution is an operation that involves shifting, multiplying, and adding. You would have to do some kind of inverse convolution to figure out your wall shape, which might be tough. At the very least, I think longer input sounds would restrict you to lower peak frequencies in your output sound.
lankybits, Feb 21 2007
  

       Good job! if you want to experience this you don't have to go to Chichen Itza of course. I have noticed this dribbling a basketball at an angle to a corrugated steel building but I never made that step to think about engineering the sound. I wouldn't know how to prove or disprove the ability to record any sound wave. I am certain you can get the frequency but there is also more form to the wave that I have a hard time imagining. You get my bun because I can't think of a reason for sure that it would not work. [+]   

       You could curve the wall to make the last part fade less, or you could think about using the floor and concentric circles I am pretty sure that would have to work like you discribed.   

       The hearing things at science centers use an elipse shape with two focci.   

       If you use confocal elipsi (probably making up words as I go here) and trace them on the floor you could have a two person experience and remove yourself from the original clap by a distance.   

       My imagination says you can do a lot with this but you need to limit yourself to one large initial wave, larger and shorter the better. The human ear can probably sort out a lot and voices can probably be understood.   

       The image I have in my mind is a groove like a record player spiralling outward and wider bits where you would expect a wave peak and narrower bits where you would expect a wave valley. Hopefully they don't resonate with the spiral or you will find parts going down hill steeply and parts going uphill. The sound I am discribing will probably be either dirrectionless or very confusing. You would probably do better with one ear plugged. You can probably overcome the sloping by wasting some reverberation by deflecting it.   

       You really got me thinking.
MercuryNotMars, Feb 21 2007
  

       I like it!   

       This brings to mind two cases of hearing echoes that should have inspired this, and make me sure that it will work.   

       I once was setting off firecrackers about a block from a building with foot-deep square corrugations/crenellations, and kept hearing a ripping noise for the echo.   

       I also remember standing at attention while a naval gun salute was fired. The echoes came back from each house across the bay, separately.   

       Cool.
baconbrain, Feb 21 2007
  

       While this does sound cool, I don't understand why, when you clapped, you would get anything but applause in return.
ldischler, Feb 21 2007
  

       Same reason that a prism gives you more than white light - if the sound can be refracted, separated, and recombined in the right order, it can come back as pulses that simulate sound.   

       I'm not sure you can ever actually make it recreate a specific voice (since that would depend on the input sound as much as anything), but it can definitely recreate a phrase of sounds, or a song.
shapu, Feb 21 2007
  

       //...would depend on the input sound...//
If the input sound is as short as possible (clap = good, smacking two rocks together = better), it approaches the theoretical 'impulse' input of zero duration, which effectively contains all frequencies (Fourier transform of a delta function if you're that interested).
Since all frequencies are available, it becomes a matter of using diffraction and absorption to remove those you don't want, then reflecting those you DO want back to the source/target.
As the input is very short, so the sound reflected is too; so by careful arrangement of reflectors etc in space, you could get an essentially continuous response.
Then it's just a matter of deciding how smooth you want your sound, and building enough correctly spaced relectors to accomplish this, eg: CD quality - 44.1 kHz, needs 44,100 reflectors for 1 second of sound (although that's probably far too complicated).
As has been mentioned earlier, later reflectors will need to be bigger, to offset the effect of the soundwave spreading out.
Anyone feel free to carve holes in my arguments - constructive criticism is always welcome!
neutrinos_shadow, Feb 21 2007
  

       I don't have any arguement with that, it sounds good to me. I was thinking maybe an elephant could be your friend with their long beyond hearing waves.   

       I imagine the sound will always be missing a deminsion of some sort, and that people can see what it represents just like we can make out what a 2 dimensional picture looks like even if it is only good from one perspective. I'm guessing we are made to filter out that stuff. The clap would be stretched out and still be there and the sound likewise will be stretched out. You would probably be able to recognize both. The clap might be harder but you have the original clap to compare it with and you will know what to listen for. You are probably right about the ball bearing route.   

       One other thing you could do is simply think along the lines of cool things to do like know some material's resonance frequency. Hit one rock or something and break another on down the line with the noise, like when martial artists are given a stack of bricks and ask which one you want them to break.   

       I wonder if it is possible to hit the same material and from the sound break the same material without breaking the first. You might have to hit it several times at the right time. That would be a nice experiment. This is probably some proof I have thought of like the one for there being no such thing as truely one way mirrors.
MercuryNotMars, Feb 21 2007
  

       This is very slick. I cannot recall anything similar. This could be done at an art installation. The sound would be best audible at a given spot on the floor.   

       I am trying to think of a way to reverse engineer such a thing or use a computer to predict to topology and position of the various reflectors. Once you have them, it might be possible to alter them in position so different messages come out.
bungston, Feb 21 2007
  

       Ah, but could you design a wall that would return the sound of a duck's quack?
Custardguts, Feb 22 2007
  

       Ah, that's right a duck's quack. I have spent a lot of time pondering this idea but that is the real question.   

       I was way off with the elephant idea. I see my mistake now. //Fourier transform of a delta// Elephants don't heard in delta formations. That's why ducks quack while flying. I am glad I have you guys to straighten me out. I probably would have brought up dolphins next and embarassed myself further.
MercuryNotMars, Feb 22 2007
  

       Several regressive buns.
elhigh, Feb 22 2007
  

       <CLAP> --------------------->I
croissant <-------------------I
  

       Quite right.
theleopard, Feb 22 2007
  

       I can understand how you'd be able to 'stretch' out a clap into a series of pulses - but not how you get a change in pitch.
I've got two possible methods
a) if parts of the echo-wall were moving towards or away from the sound source you might get a kind of doppler shift effect.
b) you might be able to filter out portions of the generated sound, letting ranges of higher or lower frequency through somehow - but I can't think of any structural arrangements that would do this. Would be an interesting problem to figure out.
zen_tom, Feb 22 2007
  

       [zen_tom], the wall doesn't stretch or contract the clap to change its frequency... it echos back many tiny "claplets" (each a quieter but otherwise unchanged copy of the original clap) at a high frequency. You won't be able to discern the individual claps, but you'll perceive the frequency at which they're arriving as a tone.   

       Say you have a bunch of equally sized panels (accounting for perspective), each one 0.17m farther away from you than the last. Each successive claplet will travel 0.34m farther than the last (0.17m outgoing, 0.17m incoming). At 340 m/s, successive claplets will arrive 0.001 s apart, or 1000 times per second, so my belief is that you will perceive this wall as producing a 1kHz tone.   

       I am taking some serious liberties here: 1.) I'm assuming that through some physical means (like banging two metal objects together) you can produce something "close enough" to the ideal impulse [neutrinos_shadow] referred to; and 2.) The system has no final low-pass filter, which is typically present in digital audio systems. I am thinking that your ear will naturally do some low pass filtering, and the imperfectness of the clap (see 1.) will reduce the need for it anyway.   

       After running some more numbers, I found that using a 44.1kHz sampling rate would require that successive panels be less than 4mm apart! This means that the observer's position would be critical, and that changes in distance over the surface of one panel alone would be significant... the panels shouldn't be flat, but segments of concentric shells, with the listener standing at the center. A lower sampling rate (maybe 8kHz) would be more forgiving while preserving most of the tones in human speech.
lankybits, Feb 22 2007
  

       Ahh I see - and an array of x receding vertical surfaces (as found perhaps in a big flight of steps such as you might find in front of a temple/pyramid) will echo your single clap, an x number of times, only bunched up together closely enough to give you the sensation of a single sound with a certain pitch.   

       By shortening the horizontal distance between each step, you ought to be able to raise the pitch, and by spacing them further apart, lower it.   

       A broad flight of steps of with varying step-widths along its length, (say flat at one end, and opening out to say 1metre or more per step at the other) should, if you stood closer to the flat end, return a sound descending in pitch - while if you stood at the other end, would return a sound that increased in pitch.   

       Since a flat (i.e. unstepped) wall would provide you with the highest pitched sound possible, and step widths allowing for lower tones (down to some point at which the manufactured tone breaks up into individual claps) it would make sense to start with as high-frequency and short a sound as possible - since this ought to give you the greatest range. But I think you said that already - metal balls would be great.
zen_tom, Feb 22 2007
  

       I'm picturing a sort of stadium with scalloped walls. Each little shelf would be tuned, and each face tilted slightly more downward than the one under it, so the surface is always normal to the desired input/output direction. this would also decrease the overall size of the (contraption?) by allowing the distance to be vertical as well.
Hunter79764, Feb 22 2007
  

       Has anyone else noticed that this idea is related to "talking rumble strips", sorta?   

       As far as pitch goes, I'm thinking that it may be possible to use the width of each step as a pitch selector. Something about the wavelength and the size of the reflector certainly holds true in radar reflectors, satellite dishes and such--I dunno what happens with sound waves.   

       So perhaps if the reflective step was only an inch high, only high-pitched sounds with a less-than-one-inch wavelength would be reflected. That's assuming that the original sound contains all frequencies.   

       ----   

       Hmmm, that's a false analogy, there, as sound waves are pressure waves, while lightwaves are transverse waves. Well, maybe someone will be inspired to think of something more appropriate.   

       I know that a didgeridoo uses length to reflect the internal pressure wave. And so do tuned exhausts, so there may be something similar useful here . . . maybe.
baconbrain, Feb 23 2007
  

       It would seem possible to simulate this quite accurately before going to the huge bother and expense of building it. As [lankybits] says, convolution is the key. But inverse convolution doesn't have to be more complex than forward.   

       A windowed FFT could be taken at a suitable frame rate, and appropriate delays and amplitudes derived from the coefficients. These could be modelled by a bunch of parallel delay lines.   

       Since it need not be in real time, any modern computer system should be capable of doing the calculations.
csea, Feb 23 2007
  

       //I came up with this idea while visiting Chichen Itza on my honeymoon.//   

       That's funny, I thought honeymoons were used for purposes other than coming up with halfbaked ideas.
SledDog, Feb 24 2007
  

       Sorry to hear about your honeymoon, [Sleddog].
bungston, Feb 25 2007
  

       Maybe the size could be limited by reflecting the sound to a distant point. Imagine someone clapping and the reflectors are at some angle to a distant obsever (following an elipse: 2 foci). The furthest reflector is nearest the 2nd observer. This would mean that it needn't be any larger, so all the reflectors are much the same size. All sound would travel roughly the same distance.   

       I think that if the input is a pulse, then the reflectors need only be shaped according to the time domain variation of the required sound, and not the frequency domain. That means the reflectors pretty much look like the actual wave shape, but with a stepped approximation, which will give higher harmonics, according to the pitch of the steps. This could be possibly tuned out using a clever passive low pass filter. I will try to think how to do that.   

       edit: link found. Use a duct with an expansion chamber.   

       A fantastic idea - I would like to see this implemented. ++
Ling, Feb 25 2007
  

       You might use this to convey a message at that Yucca Mountain nuclear waste dump. Not sure what the sound would be though. Maybe "TURN BACK!" in a number of languages, or maybe just someone screaming? Hmmm, maybe this wouldn't keep people away.
discontinuuity, Feb 25 2007
  

       This really deserves additional consideration. I seem to recall having read an article on a similar subject within the last few years. Here are some [link]s to similar ideas, except with the goal of reversing the acoustic signal.   

       I grew up near the Echo Ampitheatre area in NM, and recently visited there, and found it just as awesome as I remembered.   

       The recording in clay link came about because I remembered reading a Daedalus article some years ago.
csea, Feb 26 2007
  

       how about something similar except transmitting sounds rather than reflecting them. This could be useful for sound barriers next to roads. input>cars zooming past, output>birds chirping.
xaviergisz, Feb 26 2007
  

       [xaviergisz], This could be accomplished by conventional means, involving loudspeaker horns reproducing a prerecorded message, and triggered by standard motion detectors.   

       I guess I'd prefer real birds chirping, maybe enticed by cultivating the local flora.
csea, Mar 05 2007
  

       Wait, how big does this chamber have to be fore the echo to continue echoing into the next millenium?
spiraliii, Mar 06 2007
  
      
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