Measuring blowing up a guitar...help!?!

[lerone]

Right, this is hard to explain, so please bear with me, its not mastering related, but i'm on this forum a lot and you guys are great sooo .....I've been hired by a guitar designer / builder, to help devise an experiment to see how pressure effects the tone of acoustic guitars. Now, the designer wants to basically blow up an air tight acoustic guitar (with air) and measure what happens to the frequency response as the pressure increases and the guitar explodes. I know this sounds like I've been smoking crack, but the designer has done guitars for Status Quo, Tony Iomii + others, so he obviously does have some reasoning behind this. Right, now i'm thinking we need to get pink noise into the guitar (as flat from 20-20) but how the hell could I reproduce this inside a guitar!?! Putting the guitar in the listening position in the mastering room seems a little bit crude, but what else could reproduce flat from 20-20? What other approach could I take? What would I use to take the measurements? I've got to repeat the experiment 3 times with 3 identical guitars and make it as fair as possible for each test. I'm looking for the change in frequency response in relation to pressure over a period of time? I need this to be as accurate as possible, but not so accurate that it become a completely impractical pain in the arse. Stumped.. Please help guys?!?!?

Thanks,

Lerone

[wado1942]

I'd probably look into attaching some kind of voice coil driver to the bridge of the guitar itself. Take a baseline reading with pink noise before you seal the guitar and start ramping up the pressure. A small omni directional mic inside the guitar (or several) recorded at 24-bit 96Khz should do the trick for measurements. Also, I'd highly recommend getting a 16mm camera that can shoot at 400fps to film the moment of transition. Of course, you'll need a LOT of work lights because even with 500ASA film, you're talking at least 80 foot candles of light for a proper exposure.
The slower you can ramp up the pressure, the more accurate the results will be. But nothing will explode if you bring up the pressure slowly. It would require extremely rapid pressurization to cause anything to fly through the air, in which case, the measurement periods would be too short to give you any data.

Wouldn't you rather launch a burning accordian with a trebuchet?

[lerone]

Originally Posted by wado1942

I'd probably look into attaching some kind of voice coil driver to the bridge of the guitar itself. Take a baseline reading with pink noise before you seal the guitar and start ramping up the pressure. A small omni directional mic inside the guitar (or several) recorded at 24-bit 96Khz should do the trick for measurements. Also, I'd highly recommend getting a 16mm camera that can shoot at 400fps to film the moment of transition. Of course, you'll need a LOT of work lights because even with 500ASA film, you're talking at least 80 foot candles of light for a proper exposure.
The slower you can ramp up the pressure, the more accurate the results will be. But nothing will explode if you bring up the pressure slowly. It would require extremely rapid pressurization to cause anything to fly through the air, in which case, the measurement periods would be too short to give you any data.

Wouldn't you rather launch a burning accordian with a trebuchet?

Thanks! I think blowing bits of guitar through the air is a bit much eh But I've got a starting point. This isn't happening til september so I can refine it more and talk to Ian in more depth.

Again thanks for the input mate.

[masteringhouse]

Back in my college years I performed a similar experiment measuring the acoustic properties of good sounding guitars versus lousy ones. We basically had the setup above where a speaker was mounted over the guitar and ran an oscillator through the speaker with a mic or piezo pickup and some sort of graphic plotting device which I forget offhand (it was from the physics lab). I would expect an acoustic to blow up somewhere near its helmholtz resonant frequency if possible.

Then again I'm sure Pete Townshend would probably know off the top of his head.

[Jesse Graffam]

If you're looking for a single frequency tone at super high levels, you can't go wrong with a diaphragm. A guy which I seem to be able to reference far too much here so far, lol... Tom Danley... built some sonic boom simulators for NASA capable of modulating the walls of a house several feet back & forth, at 3 Hz. High pressure, high volume compressed air through diaphragms mounted to horns.

However I've never played with anything like this, or went very in depth to it's research, but I would think the first thing you would want to find out is how frequency agile they are, and how smooth the frequency response would be.

Conversely... i would be more inclined to think that something like a high precision, high speed actuator of some kind would be best to run sweeps with. and use the actual pickups on the guitar to test the quality of the audio. considering strings resonate and provide slightly different pressure to each bridge depending on the note and the string played... taking that into consideration could complicate things greatly.

As far as surface resonance, it depends on the budget, but i would think that reflecting lasers off the guitar would be able to pick that up with more precision than anything else I can think of off hand.

Finally... have you thought about using an E-bow to resonate one of the strings while you slowly pressurize it? Low, middle, and top string... Just a thought.