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# Acoustical Impedance Tube

12/15/2009 3:56 AM

Hi everyone, I am working on a project for a school in Germany. An acoustical impedance tube is actually quite simple. You have a quality loudspeaker on one end of a rigid tube (ours is 100mm dia. 1.5 meters long of glass) and a movable mostly sealed piston on the other end. Normally, to measure acoustical impedance you put the sample roughly in the middle and put microphones through the glass on either side. You can do a single frequency but then you have to take into account distance and phase. Or you can do white noise and just take several dB measurements and average them. You could even do a FFT and compare them to see how the material responds to different frequencies. To measure reflectance is more difficult but it is not bad.

But this is not how they want me to do it (even though it's simple). I keep telling the PhD student and the professor that it won't work their way. I have a background in audio so I already know quite a bit about this stuff. They want me to use the speaker and no microphones to measure acoustical absorptivity. How you might ask? Well their first idea was to use the speaker somehow to find the resonance of the tube (which I showed you can't do). If you compare the resonance of the tube with and without a damping material inside you could relate that to damping, because in their theoretical world, IN THEORY damping changes the natural frequency. This is not the case, or at least significantly. I guessed that MAYBE there would be an impedance peak in the speaker when the tube is at resonance (it is not the case). It doesn't effect the speaker because air is such a poor coupler.

I also measured the mechanical and electrical properties of the speaker (known as Quality: Qts, Qms and Qes.) Qms is the important one in this case because it represents mechanical damping of the speaker (and the tube which it is coupled with). But comparing a tube stuffed with crap and empty, there was no difference (aside from allowable error). The resonant frequency of the speaker was only modified by the length of the tube (but even then only by a few Hz). This is the way of the audio world. The properties of the speaker are only hugely modified by the shape, construction and design of the enclosure. The damping material inside a well designed enclosure makes very little difference in the electrical properties (of which I am trying to find differences in of course).

So we actually have all kinds of neat equipment. I have a \$75,000 vibration analysis tool and software suite. It's very precise and accurate. I wasn't able to find any significant difference in any electrical property of the speaker. But to be fair, the only way I know to measure the electrical properties of the speaker is by basically measuring the voltage across a known resistor (which is in series with the speaker) or measuring the voltage across the speaker itself. With these, you can calculate the impedance of the speaker and the current through the system because the input voltage is constant and the resistor is constant. I keep telling them that this won't work, because everyone who has taken a circuits or mechanical vibrations class knows that with an underdamped system (which this should always be), the amount of damping doesn't significantly change the resonant frequency of the system. The resonant peaks of the tube do not seem to show up in the speaker in any form that I know how to measure.

So I'm sure there is someone out there that has some amazing idea. What do you think? Do you agree with me?

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#1

### Re: Acoustic Impedance Tube Help

12/15/2009 4:25 AM

It sounds like the usual story of the Prof and phd student know SFA about the subject they are working with. I've met prof's with egos bigger than their common sense

I think you need a clear definition of who is driving this project.
I think you have 3 choices.

1 Get on and do what you know works. (assuming you are driving it).
2 Play along build whatever they ask you to and let them dig themselves a big hole.
3 Walk away from the whole sorry mess

I dare say it may be possible to measure the electrical impedance of the loudspeaker as it's driving the different loads, same as it may be possible to drive your car whilst sitting in the back seat, using a couple of sticks, buy why would you want to?
It seems nonsensical to exclude the use of micophones from audio measurements.
You have my sympathy, as it sounds like you know your stuff.

Del

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#2

### Re: Acoustical Impedance Tube

12/15/2009 11:26 AM

...'tis not wise to argue with the person(s) who "sign-off" on your thesis or dissertation!

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#3

### Re: Acoustical Impedance Tube

12/15/2009 11:29 AM

Your professor and TA are trying to utilize an old technique that some disfavor because it does not always accurately model what actual sonic qualities that happens in your room. First, I assume that your speaker quality numbers were measured using this technique for Measuring Loudspeaker Parameters from Elliot Sound Products. These Thiel/Small parameters (Small was also an acoustic engineer's name not a speaker size) for calculating a first approach to cabinet design. The speaker's natural resonant frequency get presented by the peak in the impedance curve diagram. The effect you professor is looking for is the difference in the measured impedance of speaker plus cabinet and room. This is the only paper that shows this effect. You will see a tiny difference in the misspelled "speaker and zoom curve". To see this phenomena more readily one must have a more efficient speaker in the frequencies that the cabinet and room effects will be happening. For this measurement technique utilizes the speaker as both a speaker and microphone. The key to this is buried in the first link's following paperwork. This exact circuit simulation of a speaker can be found at several web sites so I'm not sure if Elliot Sound Products is the original source for this, but it is where I copied it.

Now one must remember that this is an effective model of a speaker, so all of the elements are ideal. This means that none of the power storing elements, inductors and capacitors, translate electric power into any other form of power. What is not explained in this model is that the 6.2 ohm resistor translates electric power into heat and the 44 ohm resistor is the effective impedance that moves air. This circuit model is a nominal model for today's high fidelity speaker. Modifications of the effective load that appear as changes in the air moving impedance (44R) from cabinet and room loading do not appear as easily measured variations with most of today's inefficient high fidelity speakers. (The simulated model is a very efficient model.) So to measure your cabinet impedance characteristics using the valid approach your professor is trying to use, one must have the cabinet resonant frequency loading near the free air resonance of the speaker. Often when the cabinet's anticipated resonance is significantly different from the final driver's resonance, a test speaker driver possibly with an unreasonably low Q gets used in place of the final speaker.

Hope this helps.

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#12

### Re: Acoustical Impedance Tube

12/16/2009 8:44 AM

Well put! Great paper too.

I've been out of the field for over 10 years, so can't add anything.

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#24

### Re: Acoustical Impedance Tube

09/05/2012 2:36 PM

Great post, Redfred.

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#4

### Re: Acoustical Impedance Tube

12/16/2009 12:14 AM

Reverse the polarity of the speaker

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#11

### Re: Acoustical Impedance Tube

12/16/2009 8:28 AM

What would this do?

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#13

### Re: Acoustical Impedance Tube

12/16/2009 11:05 AM

converts the speaker to a microphone

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#14

### Re: Acoustical Impedance Tube

12/16/2009 11:07 AM

Ha Ha Ha Ha, Oh you're so funny.

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#15

### Re: Acoustical Impedance Tube

12/16/2009 11:33 AM

But doing that is like switching the polarity on an incandescent light bulb. It doesn't do anything

Anonymous Poster
#23

### Re: Acoustical Impedance Tube

02/03/2010 11:30 PM

Doesn't do anything? Doesn't DO anything?

Reversing the polarity on an incandescent light bulb causes it to absorb dark!

There, I feel much better now.

Thanks!

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#25

### Re: Acoustical Impedance Tube

09/05/2012 2:38 PM

Oh, that. Use a working bulb.

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#5

### Re: Acoustical Impedance Tube

12/16/2009 2:33 AM

"Well their first idea was to use the speaker somehow to find the resonance of the tube (which I showed you can't do)"

Hi,

A.:

if you want to use a speaker as a detector (converting small vibratory velocities into induced voltages) then you need a coil with very many windings.

This is derived from the relation that the ratio of force/current equals the ratio of induced Voltage/velocity.

You get a very small velocity to detect and the speakers have a low force/current ratio as designed for 4 Ohm resistance.

So if you can a second speaker to be modified as a detector by changing the number of turns of the coil to between 1000 to 10,000 then this may work depending on the velocity you want to detect.

This is the sensor type that is usually used in balancing machines. To be operated with a high impedance - low noise amplifier, if high precision is needed at lock-in

B.:

Measuring damping by frequency change is ridiculous. Look to the slope of the phase curve near resonance. This is the one and only to evaluate. Phase-steepness (Phasen-Steilheit) equals the Quality factor of an oscillator (may be a factor of 2 to be introduced, I have to look if important.) We measured very small damping coefficient of clamped high quality flexures by this in the low damping region (Q< 0.001) and damping of gas-bearing gaps (Q> 10) in the high damping region.

C.: (but even then only by a few Hz)

If you resolve to ÂµHz this may be a pretty big signal!

D.: (The damping material inside a well designed enclosure makes very little difference in the electrical properties)

It makes a big difference if you measure damping only! But make sure there is no serious nonlinearity where the damping may occur in excited higher harmonics and not seen in the phase curve at the fundamental resonance.

E.: Recommend to the PHD student a visit to his local library (and the Prof. too) to read more about oscillators. Old books of Grammel and Magnus to start with. If necessary I can give them a tutorial. (Only 400Km from Munich).

RHABE

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#6

### Re: Acoustical Impedance Tube

12/16/2009 3:33 AM

If you want to use a speaker as a detector ... then you need a coil with very many windings.
So if you can a second speaker to be modified as a detector
.

Yeh, but this is the exact bonkersness of the whole thing...

Such a device is called a MICROPHONE.
Del <Arggghhh Yowl yrawlllll mrawwwww FTZZZZZZTTTT>

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#7

### Re: Acoustical Impedance Tube

12/16/2009 5:32 AM

I agree,

some advantage: you can modify the sensitivity - much space available for copper (or Al) windings,

some disadvantage: waste your time and money by gaining experience, bulky, needs suited amplifier ...

RHABE

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#8

### Re: Acoustical Impedance Tube

12/16/2009 6:05 AM

Great pic

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#9

### Re: Acoustical Impedance Tube

12/16/2009 7:52 AM

My cats are faster than yours!

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#10

### Re: Acoustical Impedance Tube

12/16/2009 8:28 AM

Cool thanks everyone for the advice. I'm really glad that Del agrees with me, I remember when you first joined, I know how smart you are lol. I've been on a bit longer but haven't done much posting, just reading.

I'm actually a graduate engineer. I finished last spring. I seem to have a really good practical understanding of the engineering world (for example I designed a full scale continuously variable transmission for a Porsche track car from design to prototype for my senior design project, all in 10 weeks lol).

I'm just here "helping out". My school asked me "hey, you want to study in germany?" So I was like, "Yeah, Sure I mean I'll already be there for my internship at BMW in Munich." I'm just here for fun, I don't get credit for it.

So enough bragging... down to business

So today I continued to argue with the PhD student and the prof but by some miracle I finally convinced them they were wrong!!! lol. It only took 8 weeks (and I've only been here 8 weeks )

Redfred, thanks for your advice. That is very good info. So it definitely is theoretically possible. And I did use the exact same method by Elliot Sound Products. I remember trying to use that method when I first started college with not very good equipment and got really odd numbers

But this time I have extremely good equipment, even besides the \$75k thingamajig. So, to think things through, I need to move the length of the tube to the resonance of the speaker. Unfortunately, since the resonance frequency is ~75 hz (it's a small 4" driver) the tube would have to be 2.28 meters long (since it's effectively a double closed tube, it hast to be half wave instead of 1/4 wave). So the closest I can get is about 115 hz if I capped the end.

And I like your description of the idealized model using just RLC components. I've seen it before but somehow never thought about how the parallel higher value resistor represents the effective resistance of moving the air. It shows the scale between the speaker impedance and the air impedance.

Sorry for taking so long to reply back. I But I will read that other article you sent as well and maybe try something to see what happens.

Also, RHABE, about "A" although I think your idea would work it seems like it would be rather difficult. We have lots of microphones already. And "B" I'm really glad you agree as well. I couldn't believe they wanted to measure damping by frequency shift, and that's just from my basic knowledge from my circuits and mechanical vibrations classes. "C", I suppose that would be correct lol. "D" That also makes me feel good that my intuition about the setup was correct. Now I know I wasn't doing anything too technically wrong for trying to measure any electrical differences because they would be hard to detect. "E" well I think it's unlikely that I could get them to read any books lol. I found several doctoral thesis publications on the subject, read them all yet wasn't able to get them to read them. I even highlighted key sections lol. But I just realized that I haven't updated my location recently. I'm unfortunately not in Munich anymore, I'm in the city of Jena (ThÃ¼ringen). Where are you with respect to Jena?

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#16

### Re: Acoustical Impedance Tube

12/16/2009 11:36 AM

Glad you got a result and ta' for the compliment.
Del

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#19

### Re: Acoustical Impedance Tube

12/16/2009 12:24 PM

Hi,

Jena is a great place too,

don't miss a visit to the Carl Zeiss optical museum!

I had some limited cooperation with Fraunhofer IOF in Jena, Mr.Risse and Guyenot.

I am 70km from the French border near the (biggest worldwide) US/NATO airbase Ramstein.

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#17

### Re: Acoustical Impedance Tube

12/16/2009 11:44 AM

That project sounds simuliar to the Bose Wave system.

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#18

### Re: Acoustical Impedance Tube

12/16/2009 12:08 PM

notice the speakers on the bottom

I don't really know exactly what makes the bose wave system special but I always thought it was essentially a really optimized transmission line enclosure. I built and sold a few t-line enclosures, they are pretty sweet things but are hard to make them small. What the clever person in the image above did was essentially make a very crude t-line enclosure. I've always wanted to do that with big concrete forms but my 500 lbs of home built speakers barely fit in my apartment as it was

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#20

### Re: Acoustical Impedance Tube

12/19/2009 10:22 PM

I have an electro-magnet speaker (post WWII) sitting here, about 10" across, so it may be your speaker-microphone combination ?

Pulse it and then listen, like a sonar or radar ?

I also have a Baldwin speaker (WW I vintage).

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#21

### Re: Acoustical Impedance Tube

12/20/2009 1:25 PM

A bigger speaker would be more sensitive, but then I'd need a tube the same size diameter. I was actually going to try a pulse then listen technique. Basically play white noise for a short time and then suddenly turn it off. Then read the signal from the speaker. But when you use the speaker as a microphone, you might as well use an actual microphone anyways.

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#22

### Re: Acoustical Impedance Tube

02/03/2010 6:21 PM

Hi Nick,

I've found this thread by chance: I was looking for some people trying to do the same measurements I'm planning to do (with a much much cheaper instrumentation).

As far as I know somone else did (quite) the same experiment: Martin J. King and Robert A. Robinson.

Me, I had some idea to extend their work and I was planning an appropriate impedance tube to do the job.

I would like to better understand your difficulties (they could become mine), help you in some way and get some work done for me.

Can we continue this discussion offline ? I could share some calculations and simulations that are difficult to share on the pages of a forum.

Thanks

Teodoro (teodorom@hotmail.com)

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