Rational Acoustics



Arthur Skudra
March 19th, 2014, 09:53 PM
Well over a year ago I started experimenting with accelerometers in a very special project where I needed to sense certain floor vibrations for an alarm. Found an inexpensive accelerometer, designed a inexpensive phantom powered preamp, developed a water tight box, and created a DSP program that worked rather well for the application. Client was really happy with the end result. That left me with a bunch of leftovers from my prototyping stages, and so spurned some thoughts on what to do with it all. In particular, I had one of these:

Measurement Specialties ACH-01-03/10

http://www.digikey.com/catalog/en/partgroup/ach-01/21992

You can buy them for around $40 each. Capable of a frequency response of 2 Hz to 20,000 Hz, with a dynamic range of up to 150 G's of force! Yikes! :)

Although my phantom powered preamp worked ok for the previous project, I found that phantom power did not provide the amount of power that these accelerometers really needed to operate to their full potential. I was going to build a DIY preamp that I spotted in the June 2011 issue of Audio Xpress magazine, but being short on time, maybe I might have time to build one the second Tuesday next week! :p

Then I discovered this gem of a preamp from LinearX:
http://www.linearx.com/products/accessories/LP201/LP201_01.htm

It was built specifically to work with the spare accelerometer I had! And the price was rather reasonable considering the amount of time it would have taken to design and build one for myself that I couldn't justify without billing a client for my time!!

So all I had to do was built a custom power/audio adapter using a spare 9 VDC supply I had kicking around, an F-XLR, and a 1/4" TS connector. Went to a doll house store and bought some quick stick adhesive wax, I was all set to measure vibrations!

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So I have a church client that I specified Guitammer's Buttkicker bass shaker for, one for their bass guitarist, another for their drummer. I purposely specified a separate power amplifier and DSP (QSC CMX500V and a QSC DSP-3) that would be dedicated for buttkicker use. My last task was to program the DSP for the buttkickers, which I did today.

So here are some pics of the bass guitar's shaker platform. First an overview from the top:

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And here's the underside, I spec'd Kinetics Noise Control RDA-125 isolators to decouple this platform from the stage (they worked rather well):

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Here's a closeup, showing where I attached the accelerometer to the platform using the doll house adhesive wax:

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And here's the buttkicker attached directly to the seat on the drum throne (note there is no isolation from the floor of the drum booth):

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I attached the accelerometer directly to the top of the drum throne seat. To be continued.....

Arthur Skudra
March 19th, 2014, 09:54 PM
So now comes the fun stuff, measuring the bass shaker!!

So first comes the bass guitar. The client has an Allen & Heath GLD-80 with a complete ME-1 personal monitor system that I spec'd, works absolutely fabulous for them.

So I take the mono out from the ME-1 mixer, feed that into the DSP-3, which in turn is fed into the power amp, and finally comes to buttkicker.

I fed a pink noise signal into the mixing console, and turned it up on the personal monitor mixer for the bass guitarist, and started to measure the result in Smaart.

So I first measured the bass platform with and without someone standing on it, picture follows. Light blue is without a person standing on the platform, dark blue is with a person standing on the platform. Without the weight of a person, note that the resonance was sharper and lower in frequency. With the weight of a person on the platform, it seemed to smooth things out a bit. There is a resonance peak around 370 Hz that I could not correct at all, I didn't have time enough to investigate further, but I suspect it is a result of the size of this platform.

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Just for fun, I wanted to see how well the Kinetics isolators worked, so here's a pic of the platform without the weight of someone standing on it, with the accelerometer mounted on the isolated platform (light blue), and with the accelerometer attached to the stage floor about a foot in front of the platform (pink). Note that I zoomed out the vertical dB scale different that the other plots shown here, so you can see the bigger picture. Roughly 20 dB isolation between the two! Yep, they work very well. Also helps that the entire stage is covered in a gym floor mat material. This stage is absolutely dead when it comes to foot traffic, just the way I like them designed!

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Finally, here is a pic with the accelerator attached to the isolation platform, with a person standing on it, without any eq (blue trace), and with eq and LPF (red trace):

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I think in total I did 3 eq filters and a LPF at 90 Hz. The difference was certainly noticeable when I listened to a favourite Bela Fleck track!

Arthur Skudra
March 19th, 2014, 09:55 PM
And now the drums!!

Here's a pic of the shaker uneqd, with the accelerometer attached to the top of the seat cushion, without someone sitting on the seat (light blue), and with someone sitting on the seat (black). Wow! What a major difference!!!

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Here's a pic of the shaker uneqd, with the accelerometer attached to the top of the seat cushion (light blue), and one foot away from the throne on the floor of the drum booth (burgundy), without someone sitting on the seat. Note that I zoomed out the vertical scale to the same range as the previous isolation pic that I posted further up this thread showing the isolation of the bass guitar platform. I certainly am going to recommend they isolated the drum throne from the drum booth floor, especially if they want to crank the kick drum mic up!!

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And here's a before and after pic of the bass shaker uneq'd (light blue) and eq'd (purple). I used two eq filters here, one to smooth out the "peak" at 27 Hz, and another at 85 Hz, along with a LPF set at 90 Hz.

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Just for fun, a person of heavier weight sat on the drum throne, here's a pic with the lighter person (purple), and the heavier person (light green). Interesting to see the slight difference in response. Hence why a minimalistic approach to eqing is probably a good idea if there are several different drummers using this drum set (common in churches).

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Was this a useful tool in the tool box? Yep! In the past I tuned by ear, er butt, but was never satisfied with the results. This time I could pinpoint exactly what I needed to do! The results "felt" good! :)

martindale
March 20th, 2014, 01:29 PM
Nice stuff Arthur! I've had to rent accelerometer rigs in some of my consulting work, but your setup is tight! Gonna use those product references for next time it comes up....I've also designed and built transducer platforms used by a music therapy clinic, which turned out very cool (and definitely fun to sit on and groove)....Power to the Vibrations!

merlijnv
March 21st, 2014, 11:05 AM
Excellent work Arthur!

PhillipIvanPietruschka
April 4th, 2014, 07:39 AM
Interesting and unusual work Arthur.

Could you discuss the perceptual differences a little more w/r/t the eq'd/filtered end result vs raw? Did the musicians compare? Did it improve their experience/ability to perform? Was the mid range resonance on the bass platform quite strange and distracting when left untreated? I have a harder time imagining these things when the vibrations are to felt more than heard... Perhaps because I havent used similar devices all that much.

I've just spent the last few days reworking a Planetarium sound system, which presents a number of quite unique challenges of its own.

Best,

Phillip

Arthur Skudra
April 4th, 2014, 12:50 PM
Interesting and unusual work Arthur.

Could you discuss the perceptual differences a little more w/r/t the eq'd/filtered end result vs raw? Did the musicians compare? Did it improve their experience/ability to perform? Was the mid range resonance on the bass platform quite strange and distracting when left untreated? I have a harder time imagining these things when the vibrations are to felt more than heard... Perhaps because I havent used similar devices all that much.

I've just spent the last few days reworking a Planetarium sound system, which presents a number of quite unique challenges of its own.

Best,

Phillip
Kinda hard to describe the perceptual differences before an after, I'd say it "felt" better and cleaner without the extraneous resonances, the musicians certainly noticed it as well. I will say that using the accelerometer was a very quick and efficient way to find the problem frequencies and do the appropriate corrective action, eliminating any guesswork in the process. It would have been very difficult to guess where to put these filters, even if I were to "sweep" a few parametrics in the eq.