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PostPosted: Sat Aug 30, 2003 3:30 am 
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John,

Is anyone building studios with active bass absorbers rather than passive traps? It's not a new concept. I recall reading an article in Stereophile magazine when I was in college, about 15 years ago, describing an active servo feedback bass trap product. With the advancements in sub woofer designs these days it seems like active systems would be a cost effective alternative to passive traps... especially when you consider the real estate and building costs associated with some of these high-end "20Hz" studios.

Any thoughts?

Thomas

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PostPosted: Sat Aug 30, 2003 7:50 am 
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Never heard of them mate - could you please elaborate?

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PostPosted: Sat Aug 30, 2003 9:28 am 
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The basic design principle is very straightforward - as represented in its simplest form in the schematic below.

Even for those of you who know nothing about electronics this circuit is very easy to understand. The Op Amp is basically a device with 3 connections - a positive input, a negative input, and an output. The Op Amp's only mission in life is to keep it's positive and negative inputs the same. It will try to produce whatever output is necessary in order to achieve this goal.

Our circuit schematic has the positive input tied to ground. In other words, the positive input sees no signal at all. So, where does the Op Amp want the negative input to be? That's right, at ground. We also see that the negative input is connected to a microphone, and the output is connected to a speaker. The microphone is positioned close to the speaker cone.

Now what happens if a sound wave hits the microphone? Well, it starts to produce a signal at the negative input - i.e. it starts to move the negative input away from ground. But like we said before, the Op Amp hates this. It absolutely wants to keep those inputs the same. And since the microphone is connected to the negative input, the Op Amp will start to swing its output in the opposite direction of the signal until, hopefully, something happens to bring the negative input back to ground.

So the output sends a signal to the speaker opposite to the microphone input signal. And its only satisfied when the speaker output equally and oppositely matches any external sound waves such that the microphone signal always rests at zero.

Since there is a finite time required for the signal to travel from the microphone, through circuit, out of the speaker, then back to the microphone, this type of "active feedback system" is restricted to low frequencies where the wave cycle time is large compared to the circuit cycle time. Buy this is fine. We're interested in bass traps, and this is just the frequency range where this type of circuit works.

As we see, this microphone/speaker system acts like a black hole for sound. Whenever a sound wave comes by, the system produces an opposing sound wave to cancel it out. If we strategically place these devices in our studio, it's very much like knocking down the walls as far as bass frequencies are concerned. The cancellation doesn't necessarily have to be 100% and we can even add equalization to the feedback circuit so the cancellations occur more or less at various frequencies, allowing us to tailor the bass response of the room.

In practice, these systems would look just like subwoofers with small microphones mounted in front of the speaker cones. A big control room might require 4 or 8 of them and, depending on the quality of the sub, they could seriously be effective down to 20Hz. Say, you built 8 monster versions of these for $2000 a piece. That's $16,000. My guess is, high end "20Hz" studios spend far more than $16,000 on those giant concrete outer shells and massive trapping systems. Am I right?

Thomas


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PostPosted: Sat Aug 30, 2003 9:42 am 
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.... And of course, much more modest and affordable systems could be built for small studios with no space to put deep bass traps.

Thomas

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PostPosted: Sat Aug 30, 2003 11:28 am 
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Sounds like the system they are trying to develop to quieten airplane noise - hasn't worked so far but it's a possibility I suppose. ;)

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john


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PostPosted: Sun Aug 31, 2003 12:59 am 
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Thomas,

> Whenever a sound wave comes by, the system produces an opposing sound wave to cancel it out. <

As best I can tell this type of design is fundamentally flawed. I believe the concept can be made to work, but only with substantial modification.

Last year my partner Doug and I pondered this at length. Our idea was to mount a piston driver within a panel trap, to make the entire panel work like a huge loudspeaker cone. First we thought to use negative feedback, as proposed above. The problem with negative feedback is it makes the wall infinitely stiff! So there's no difference between an active trap with negative feedback and a massive cement wall. Then we considered positive feedback, but that too won't work because when a low level sound wave strikes the front the panel will bottom out immediately.

The key is understanding that a panel trap works like a shock absorber in a car. It offers just enough resistance to wave pressure to absorb the energy. Too much resistance and the waves are reflected. Too little and the panel flexes too easily as if it wasn't even present.

So it seems to me the only way to create an active trap is to have a pressure sensor in the loop that influences the amount of negative feedback, so the panel offers the optimum resistance. What would be fabulous about a design like this is it could be made efficient over a very wide range of frequencies.

--Ethan


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PostPosted: Sun Aug 31, 2003 3:45 am 
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This whole thing really isn't abstract theory. It works. This design is already widely used in noise protection headsets http://www.sennheiserusa.com/pages/products/index.htm Velodyne Acoustics also uses a variation of this idea to linearize their subwoofers. Instead of connecting the positive op amp input to ground they connect it to the amplifier output. The signal from an accelerometer attached to the speaker cone is fed to the negative input. The op amp then outputs a compensating signal to cancel out any difference between the amplifier output and the cone motion. In other words, it cancels any distortion created by the speaker. I think Meyers Sound makes a studio monitor with a similar bass linearization system that actually uses a microphone mounted in front of the woofer.

The major technological hurdle these days have to do with developing active noise cancellation systems that are effective at high frequencies and which don't require the cancellation system to be in close proximity to the area one wants to protect. So yes, developing an airplane noise cancellation system that works on even the higher frequencies and doesn't require a headset is indeed an extremely difficult challenge at this point in time. But low frequency cancellation where the system is in close proximity to the affected area (the wall reflection in this case) is far from rocket science.


Ethan, I'm not sure I understand your points. Either the system you were envisioning is different than what I've described, or you may have some misunderstandings about the nature of active feedback. The circuit I drew just described the basic principle, but there are simple refinements that can account for the issues I think you're trying to raise. The "just enough" aspect of the system is simply controlled by the gain of the feedback loop. The idea of an active panel, whether it be an absorber or a loudspeaker, has been around for a long time. The only reasons such panels don’t work are because 1. They aren't stiff enough relative to their dimensions. They flex and do not radiate like pistons. 2. They are too heavy and, therefore, extremely inefficient. (This is the very same reason why passive panel absorbers are very inefficient for their size).

As far as car suspensions go, there are active systems that work on the very same basic principles I've outlined. http://www.carlist.com/autoglossary/aut ... ry_27.html Some of these sytems use "complex algorithms", but this is only because the optimal alignment of a car's suspension under various conditions is far more complicated than the one dimensional problem of positioning of a loudspeaker to cancel low frequencies.

These sytems can and do work. My question mainly centers on the economics of the solution. Does it make sense relative to the cost of building large traps?
Thomas

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PostPosted: Sun Aug 31, 2003 10:13 pm 
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Thomas,

> This whole thing really isn't abstract theory. <

I understand, but it seems to me that active noise cancellation and bass trapping have entirely different and opposite goals. In the first case you are trying to counter a sound source with an inverted version to cancel it. In the second case you are trying to avoid the cancelling reflections. So using active noise cancellation in a control room will remove the primary content too, no? That is, instead of unwanted dips in the response at only some frequencies, you'd reduce the level at all frequencies. Unless I missed something in your drawing. Do you mean to place the system near a wall and have it cancel only the reflected sound from the wall but not the direct sound from the speakers? How would you get the microphone to distinguish the source direction?

> Velodyne Acoustics also uses a variation of this idea to linearize their subwoofers. <

That is yet another variation on the same theme. Mackie does this too in their HR series loudspeakers, and (I was told) they do so without the added complexity of a cone-mounted transducer. They simply compare the woofer current that should be drawn at the moment with what is actually being drawn, and compensate by sending more or less voltage. Or something close to that.

> Either the system you were envisioning is different than what I've described <

I thought you were describing exactly the same thing I was describing. :D That is, a panel or other surface that reacts to wave pressure in a manner that absorbs the pressure instead of reflecting it. Again, such a panel would have to be mounted against a room boundary, and it would have to provide a controlled resistance to the wave pressure.

> This is the very same reason why passive panel absorbers are very inefficient for their size. <

Jeez, not this again. :roll: Read my fingers: The panel traps I build have an absorption coefficient near 1.0 at their center frequency, as measured at a real acoustics lab. That is very efficient, not inefficient! I don't know where you got the idea that panel traps are inefficient, but it's very wrong.

Anyway, back to active trapping, if I'm missing something in how you envision these panels working and where you would mount them, please explain. I understand all about op-amps and feedback and servos. That's not the problem.

> My question mainly centers on the economics of the solution. Does it make sense relative to the cost of building large traps? <

When we were selling panel traps we found it difficult to get people to pay $400 each for eight of them. I think it will be even harder to get people to pay $1000 or more each!

--Ethan


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PostPosted: Tue Sep 02, 2003 5:21 am 
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Ethan Winer wrote:
I understand, but it seems to me that active noise cancellation and bass trapping have entirely different and opposite goals. In the first case you are trying to counter a sound source with an inverted version to cancel it. In the second case you are trying to avoid the cancelling reflections. So using active noise cancellation in a control room will remove the primary content too, no? That is, instead of unwanted dips in the response at only some frequencies, you'd reduce the level at all frequencies. Unless I missed something in your drawing. Do you mean to place the system near a wall and have it cancel only the reflected sound from the wall but not the direct sound from the speakers? How would you get the microphone to distinguish the source direction?


Whether it be a perfect panel absorber, a window open to an abyss, or an active feedback mechanism, absorption is absorption. The only way an absorber can completely eliminate the primary wave at its source is if it is also located at the source. Otherwise it is only eliminating a potential reflection.

Absorption and cancellation of a potential reflection are physically equivalent. The source doesn’t sense the outgoing wave. It is only affected by reflections from nearby barriers. Those reflections are what cause interference either at the source location or at the listening position. As long as those reflections are eliminated we don't care what happens to the outgoing wave. By the time the wave reaches a barrier its intensity has dropped off according to the 1/r^2 law. This reduced intensity is what the absorber senses. In the case of a passive absorber this wavefront is completely transmitted, never to return. In the case of an active absorber the wavefront is reflected, but another wavefront of equal magnitude and opposite phase to the refection is also produced. This opposing wave completely cancels out the reflection. Since the resulting magnitude of the reflection and its opposing wave are zero, the source or the listener experience it as if there were no reflection at all. [In fact, if you setup the problem in terms of boundary conditions, you find that in essence there is no reflection from the active absorber.]

The cool thing about an active absorber is that the amount of absorption can easily be adjusted by setting the feedback loop gain. Infinite gain (effectively infinite) yields 100% absorption. Zero gain yields 100% reflection. And any setting in between can easily be adjusted with the turn of a potentiometer, allowing one to tailor the low frequency "reverberation" field of the room. (reverberation isn't technically the correct term when discussing wavelengths of similar or larger scale than the room dimensions). Furthermore, the feedback mechanism automatically compensates for intensity variations at any frequency within the active absorber's bandwidth. Standing waves are attenuated greatly while other frequencies are attenuated less, leveling out the room, and without any need to know the modal frequencies ahead of time.

Of course, the main advantage is size. Trading space for active power consumption an active absorber can provide 100% absorption across a broad frequency range (20-200Hz) all in the size of a subwoofer box. When placed in the corners (major antinodes) active absorbers can significantly attenuate both complex room modes as well as bass lift in even small rooms.

Thomas

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PostPosted: Tue Sep 02, 2003 6:46 am 
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Have you tried this thomas -i.e. built one and measured it or is it theory?

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John


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PostPosted: Tue Sep 02, 2003 7:03 am 
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John,

I've had plans to build a scale model of a room to test out a few other ideas. I'll put this on the list as well. Not sure when I'll get to it, but it should be fun to see these things in practice. :)

Thomas

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PostPosted: Wed Sep 03, 2003 12:20 am 
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Thomas,

> In the case of an active absorber the wavefront is reflected, but another wavefront of equal magnitude and opposite phase to the refection is also produced. This opposing wave completely cancels out the reflection. <

Okay, let's try this from a different angle:

You say you plan to put the proposed active panel in a corner. Forgetting feedback and op-amps and all that, how will the panel respond physically as a sound wave strikes its front surface? Will it respond to wave pressure by pushing forward in opposition, or will it move inward? Either way, how much will it move and with how much force?

--Ethan


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PostPosted: Wed Sep 03, 2003 2:49 am 
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An acoustic pressure rise will cause the panel to move outward (out of the room) with an appropriate velocity such that it creates an equivalent pressure drop, maintaining a net pressure equal to atmospheric near its surface. An acoustic pressure drop will cause the panel to move inward (into the room) with an appropriate velocity such that it creates an equivalent pressure rise, once again maintaining a net pressure equal to atmospheric near its surface.

This is that same thing that would happen if the panel were made of a near perfect absorbent material. The wavefront would penetrate into the material and almost instantaneously its energy would be dissipated into heat - net pressure equal to ambient. The more perfect the material, the shallower the penetration depth.

Thomas

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PostPosted: Wed Sep 03, 2003 4:05 am 
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Thomas,

> An acoustic pressure rise will cause the panel to move outward <

Okay, then how is that different from a passive panel that also moves in and out in sync with the wavefront? What role would the active components play? Do you see what I'm getting at?

Or put another way, what you seem to be describing is positive feedback, in that a positive pressure on the panel will cause it to move into the enclosure (out of the room) by even more than it would were it passive. Yes?

--Ethan


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PostPosted: Wed Sep 03, 2003 5:16 am 
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There is a very big difference.

A passive resonator is limited by its mass and resonance characteristics. The only force on the panel is the sound wave itself. It must accelerate the panel into motion. Off resonance the mass and reactance of the system allow very little energy transmission into the absorber. Even on resonance the panel inertia causes a phase lag between the incident wave and the opposing wave generated by the resonator (lighter panels with larger cavity volumes yield higher efficiencies). And, of course, maximum efficiency requires a sharp resonance. Broadening the bandwith (lowering the Q) necessarily reduces efficiency.

With an active system, however, the only input inertia to overcome is the mass of the microphone diaphragm. At low frequencies, therefore, the input signal is effectively 100% in phase with the incident wave. The inertia of the output (loudspeaker cone) is actively overcome by the amplifier. The system output can stay effectively 100% out of phase with the input (negative feedback) within its pass band. The high frequency bandwidth is primarily limited by the time delay caused by the necessary physical separation between the microphone and the speaker cone.

Of course, the universe never gives a free lunch. The price you pay for the vastly superior characteristics of the active system is the power input. If you want a small, broadband, high absorption (cancellation) efficiency device, you must provide the power.

Thomas

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