Pressure based motional feedback. - diyAudio
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Old 12th August 2006, 01:40 PM   #1
tade is offline tade  United States
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Default Pressure based motional feedback.

I wasn't able to turn up anything in searching, however if this info exists already please point me to it.

could I mount a microphone to the front of my subwoofer and use that for feedback? The voltage input should be proportional to the acoustic output. Also, Is it true that most microphones are fairly accurate in the low range but it takes a decent one to measure high frequencies? So perhaps I could use an inexpensive eletret mic, amplify it, then use an opamp to compare the input and acoustical output, and then adjust accordingly. Maybe also put a low pass filter on this effect to place the feedback below any room modes.
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Old 12th August 2006, 01:58 PM   #2
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Meyer Sound used to have a studio monitor that used a mic for feedback. I believe it (mic) was mounted in front of the woofer. I don't see it advertised on their website now(????). This aproach might not work so well on shorter wavelengths (tweeter). I think your largest distortion contribution will be from the woofers



I found the link: http://www.meyersound.com/products/studioseries/x-10/
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Old 12th August 2006, 02:07 PM   #3
Ron E is online now Ron E  United States
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I've never done this, but I have thought about it in the past as a way to avoid purchasing an accelerometer. Accelerometers are starting to get cheaper, but they are usually SMD, so more difficult to work with.

If you try this, it would probably be best to use a sealed box and a microphone inside the enclosure. It would then work at low frequencies where the enclosure is non-resonant. Make sure it is an omni mic.
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Old 12th August 2006, 02:37 PM   #4
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I like the accelerometer idea too!! Velodyne does that on their upper end subs. I think a thing to watch out for is cone modes. If the accelerometer is placed in the wrong spot, the FB info will be out of phase and the thing may bust into oscillation. You could probably test drive the accelerometer in a couple of location and make a phase plot.
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Old 12th August 2006, 02:41 PM   #5
poobah is offline poobah  United States
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Any distance between the mic and the cone will create a delay. This delay should not be confused with a pole... 2 different beasts entirely. Is it a showstopper? No, but it sure complicates things. This is why accelerometers mounted close to the voice coil are used.

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Old 12th August 2006, 06:55 PM   #6
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I am planning to try this out, I already glued a microphone on my subwoofer and the measurements look very good, it's about the same as winisd calculated, including the phase.

If you want to look at it on the scope use something like a 5 volts power supply and a 4k7 resistor in series with the mic.
Be sure not to wire the mic reversed or you won't get any measurements

Also, you will need a lowpass, use a first order after the differential stage, not on the microphone signal or you might get lots of noise and a rising top end, something you don't want on a subwoofer.
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Old 12th August 2006, 10:15 PM   #7
forr is offline forr  France
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In his book, in french, available free on the net at
http://www.brouchier.com/livre/index.html
or
http://www.brouchier.com/Le_son/LE_LIVRE.pdf

author Francis Brouchier has proposed to use a tweeter or a closed back (or closed front...) little loudspeaker as a pressure sensing low cost device and to place it inside the enclosure. An experimental easy setup.

Such a technique has been criticised by John Watkinson in one of his Electronics World articles however I think his arguments are valid only at high sound levels.
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Old 13th August 2006, 06:41 PM   #8
Svante is offline Svante  Sweden
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Quote:
Originally posted by Ron E
I've never done this, but I have thought about it in the past as a way to avoid purchasing an accelerometer. Accelerometers are starting to get cheaper, but they are usually SMD, so more difficult to work with.

If you try this, it would probably be best to use a sealed box and a microphone inside the enclosure. It would then work at low frequencies where the enclosure is non-resonant. Make sure it is an omni mic.
Ah, but if the mic is placed inside the box, one would have to differentiate the signal twice to use it in a feedback loop. There is a difference of 12 dB/oct between the pressures inside and outside of a box at low frequencies.

The accelerometer is IMHO better, since it needs no differentiation, but still is as immune to "other" sounds on the outside as the in-the-box microphone placement.

...but the mic-in-the box version would work for BR enclosures too.
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Old 13th August 2006, 07:02 PM   #9
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Feedback for BR enclosures is absolutely useless, first the sound from the bass reflex pipe is 180 degrees out of phase which makes it very very hard to get good feedback, second a bass reflex is a resonator, which means the sound builds up slow when signal is applied and decays slow when the signal is turned off.

This means the feedback circuit will applie huge amounts of power at the beginning to get the resonance started and it will apply huge amounts of power at the end to stop the resonance.

You might as well just plug up the pipe and use it as a sealed enclosure.
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Old 14th August 2006, 07:32 AM   #10
Calvin is offline Calvin  Germany
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Hi,

Mics could be used for bass applications up to app. 500Hz
Commercially the German company Backes&Müller (quite renown for their active and fed back speakers) used this methode.

The DIY-magazine Elektor published this circuit
Click the image to open in full size.

jauu
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