Eddy current losses in core of a electromagnetic motor

[markmal]

Hi here,

I am trying to make an efficient ribbon microphone motor.
My target is to get a highest possible output voltage so it can be plugged into a regular mic pre-amp.
It should have possibly flat output on frequences 20Hz to 20KHz.
I know that electric output depends on magnetic field B and velocity of a wire in it. (and velocity is proportional to frequency and amplitude.)
So target is to achieve highest possible B in air gap where ribbon oscillates.

I've made a model of motor using Gmsh and GetDP. It shows more than 1 Tesla in the gap, and as high as 2.5 T in core tip. I plan to use cobalt-iron core because of its high saturation capabilities. But I also read that it is not very good for high frequences like 20KHz because of Eddie currents.

My question is, should I consider to suppress Eddie currents that should appear in the core as a reaction on movind a wire in B field?
Is my understanding correct that Eddie currents should appear in this case?
Will be Eddie currents significant enough at high frequences (5KHz-20KHz) that I should take precausitions to suppress them?

I have this concern because all microphone/speaker motors I saw were made of just solid soft steel, not of electrical steel laminates like transformers are.

Is my concern about Eddie curents valid?
I am not sure how to calculate Eddie curents/losses in core for my case where a wire moves in mag field.
What portion of it will produce a voltage in the wire and what portion will impact mag field B so it will create Eddie currents in core? If such proportion exists, what it depends on? Does it depend on frequency?
Could you please advise?

Thanks,
Mark

 

[Carl Pugh]

This is the first time I ever heard of a ribbon microphone motor.
However after some Googling, it is my opinion that;
As long as there is no current in the ribbon, there will be no eddie current in the steel.
As long as there is no eddie current, solid steel will work as well as laminated steel.

 

[markmal]

This is the first time I ever heard of a ribbon microphone motor.
However after some Googling, it is my opinion that;
As long as there is no current in the ribbon, there will be no eddie current in the steel.
As long as there is no eddie current, solid steel will work as well as laminated steel.

My understanding is that current in ribbon should appear as a result of moving it in magnetic field and electrons should go back and forth in ribbon/wire accordingly. Main part of these electron movement should go to preamp. But some part of this energy, I assume, should create eddy currents in core.

I've googled this article http://www.tymphany.com/files/resources/papers/AES122nd-Impedance.pdf
that I not 100% understood, but it looks it confirms my assumption.

though the article says that eddy currents in speaker core actually helpful, it helps to extend diapasone higher.
Some speakers even have copper rings in core to help eddy currents.
I completely puzzled now. Should I fight with these currents or should I help them?

I found also another, more simplistic article http://www.diy-audio.narod.ru/litr/FaradayRingsVoiceCoilImpedance.pdf
that I am reading now...

 

[markmal]

well, from articles I read I understood that B will not alter if flux is saturated.
Then Eddy currents should not appear? Is it correct?
Typically all ribbon mikes have flux saturated in core and especially in poles where core is narrowing to concentrate the flux to a maximum before it goes to air gap.
Does it mean that movements of ribbon will not modulate flux behind saturated areas?

Here on this picture B is saturated in inside tips of pole pieces, but usually saturation disappears in wider outer volume of pole pieces.
Will that not saturated field that is located in outside volume of pole be modulated by ribbon movements while the field in insider narrow pole tips is saturated?
Other words can modulations of B be transmitted via saturated field areas to farther not saturated areas?

Another thing, according to my GetDP model, B decreases below saturation point in very internal volumes of pole tips, that touch air gap, on surface and a little bit deeper. That means that very tips of poles are not saturated and flux can be modulated in them. Volume of these areas is not very deep, and area is not wide, but still it may impact system on high frequencies, I assume.
So, even despite of saturation in major part of pole, Eddy currents may appear on pole tips internal surfaces and a little bit deeper.

Question is what should I do about this?
a. suppress them with using thin isolated laminates that poles should be made of?
b. place copper ring(s) around poles to create short-circuit secondary for transformer-like system - ribbon-pole-ring? (similarly to what they do in loudspeakers)
c. both

 

[Carl Pugh]

This is not my area of expertise, however these are my beliefs:
The magnetic field produced by the permanent magnets is maybe 1000 to 1,000,000 times larger than the magnetic field produced by the ribbon.
The magnetic field produced by the ribbon is so small that it may be ignored.
Therefore the magnetic field is does not change when the ribbon is vibrated by an audible signal.

To answer your question:
a. Will not make any difference.
b. Will not make any difference.

It would appear that the secret to making the ribbon microphone motor is:
Type of material used for the ribbon,
Number and shape of bends in ribbon,
Thickness of ribbon,
How ribbon is supported,
The design of the magnetic field is not important as long as it's uniform and strong.

 

[markmal]

Carl, thanks for response.

Yes, magnetic field produced by moving ribbon is extremely low, but the EMF produced by the ribbon is also very low.
Yes, ribbon's magnetic field is incomparable by level with field from magnets. But magnet's field is constant. ribbon's field is changing. This modulation still can create Eddy currents. Yes, these currents are extremely low too, but they may be same magnitude with "good" current inside ribbon. Let say 25%. I do not know. And they also tend to grow with frequency.

Other words, in mike acoustic energy is converted from mechanical to electrical. Some portion of electrical energy creates current in ribbon, it is "good" energy that is picked up and sent to preamp, but some portion creates Eddy currents in core poles, I assume it is as "bad", as losses.
I am trying to avoid these losses.

What is puzzling me is that loudspeaker's cores are not designed to suppress Eddy currents in pole and core. These are not laminated - solid steel - some even have conductive ring/sleeve/cup. So these intentionally maintain currents in that.
My understanding was it would be better to suppress Eddy currents in core, so more electric energy will go to mechanical. But they made opposite way. I do not understand this for all 100%.
But, by analogy, I am looking, what if to introduce this idea to mic, to add rings around poles there? Will it make sense?

 

[Carl Pugh]

Sometimes it's easier to build something and try it. This may be one of these times.

However from a design viewpoint, the current produced by the ribbon is insignificant. There will be no eddy current in the magnets or pole pieces.
There may be some eddy currents in the ribbon caused by the magnetic field. This eddy current will tend to dampen the ribbon and prevent it from vibrating.

 

[markmal]

Sometimes it's easier to build something and try it. This may be one of these times.

May be. But how can I find from experiment are there Eddy currents or there are no. How could I measure them?

However from a design viewpoint, the current produced by the ribbon is insignificant. There will be no eddy current in the magnets or pole pieces.

What makes you be so sure? Could you prove your point?

There may be some eddy currents in the ribbon caused by the magnetic field. This eddy current will tend to dampen the ribbon and prevent it from vibrating.

These are not Eddy currents. This is good EMF.

 

[Carl Pugh]

Eddy currents are caused by changing magnetic fields.
Insulated wire wound around the poles will detect this changing magnetic field (this voltage can be displayed on an oscilloscope). This changing magnetic field is so low that most likely the only thing detected will be 60 hertz from the power lines.

The output voltage from the ribbon is so low and the load impedance is so high that there will be practically no current. Since there is no current there will not be any magnetic field. Since there is no magnetic field there will not be any eddy current in the magnets or pole pieces.

The ribbon will have a voltage induced in it. Most or all of the voltage will appear on the ends of the ribbon. It is possible for a small portion of the voltage to be shorted internally by the ribbon. This internal voltage that is shorted is usually called eddy current. This shorted voltage will have very little or no effect and may usually be ignored.

 

[markmal]

Eddy currents are caused by changing magnetic fields.
Insulated wire wound around the poles will detect this changing magnetic field (this voltage can be displayed on an oscilloscope). This changing magnetic field is so low that most likely the only thing detected will be 60 hertz from the power lines.

This method is possible, but it is indirect. Though nature of current in this wire will be same with nature of eddy currents, value will be different, and it will be a portion of value of eddy currents.
Some portion of energy will create current in the wire and some still create eddy currents in core.

Another way would be to measure rise of temperature in core.
But both methods require extremely precise instrumentation that I do not have.
For start I'd prefer theoretical, not empirical, analysis.
Something like - current in ribbon Ir will cause field change delta B, this delta B will cause Eddy current Ie in core of width W at distance L for frequency F ...
something like that.

The output voltage from the ribbon is so low and the load impedance is so high that there will be practically no current. Since there is no current there will not be any magnetic field. Since there is no magnetic field there will not be any eddy current in the magnets or pole pieces.

what load impedance do you refer to? My understanding is that impedance of ribbon will be just portion of Ohm. However it will be multiplied by transformer (1:37) to increase voltage that will go to preamp. If the load is preamp, then yes, it is going to have at least 5 times higher impedance. 5 times does not mean that current in ribbon will disappear. It should still exist. Am I right?
Also, if there is no current in ribbon, there will be no current in primary of mic transformer, then how a signal will be passed via transformer? Transformer requires some current to create flux in its core. And this current will go through ribbon too. And will/should impact flux in mic core.
Value of this impact I'd like to find.

The ribbon will have a voltage induced in it. Most or all of the voltage will appear on the ends of the ribbon. It is possible for a small portion of the voltage to be shorted internally by the ribbon. This internal voltage that is shorted is usually called eddy current. This shorted voltage will have very little or no effect and may usually be ignored.

I agree this should be ignored. Because these currents will be situated perpendicularly to ribbon surface (coaxial with flux in air gap). So diameter of these eddy circles will be equal or less than thickness of the ribbon, which is ~2 micron. Such small circles may have some impact only on very high frequencies.

 

[Carl Pugh]

"For start I'd prefer theoretical, not empirical, analysis.
Something like - current in ribbon Ir will cause field change delta B, this delta B will cause Eddy current Ie in core of width W at distance L for frequency F ... "

Sounds like a good approach.
Assume that the sound moves the ribbon a certain distance at a certain frequency. You have calculated the flux density previously, so voltage = change of flux/change of time
Next calculate or measure the impedance of the transformer with the operational amplifier connected.
Now calculate the current through the ribbon.
Now calculate the change in magnetic flux. Most of the opposition to the flux will be in the air gap between the ribbon and the pole pieces, so the pole pieces and the magnets can be ignored.

This is getting so complicated that I wouldn't trust the calculations and would do one of the following;
1.Get the proper equipment and measure the change in flux density using a coil having insulated wire.
2.Build two electromagnetic motors, one with laminations and one with solid steel.
3:Just ignore the eddy currents.

 

[markmal]

...
Most of the opposition to the flux will be in the air gap between the ribbon and the pole pieces, so the pole pieces and the magnets can be ignored.

Hmm. That would be very nice.

This is getting so complicated that I wouldn't trust the calculations and would do one of the following;
1.Get the proper equipment and measure the change in flux density using a coil having insulated wire.
2.Build two electromagnetic motors, one with laminations and one with solid steel.
3:Just ignore the eddy currents.

#1. What would be equipment that can measure so weak fluctuations of flux in cores?
I do not believe that can be available for me from budget perspective.
#2. It is possible, but also requires couple of hundreds for machining solid cores and thousands to make custom stamp for stamping laminates.
#3. This is practical! :) However I'd like to know _what_ I've ignored. :)

What I am thinking is to analyze all three cases (solid, laminated, laminated+ring) with SPICE method to get FRCs.
From them I hope to see the impact.

 

[sophiecentaur]

I can't find anywhere in this thread that explains the use of the term "motor", here. A ribbon microphone is a transducer that produces an emf from air motion. A motor produces motion, using supplied electrical power. The two terms seems to be self contradictory. Is this just a language thing and are we really just talking about ribbon mike design?

 

[markmal]

I can't find anywhere in this thread that explains the use of the term "motor", here. A ribbon microphone is a transducer that produces an emf from air motion. A motor produces motion, using supplied electrical power. The two terms seems to be self contradictory. Is this just a language thing and are we really just talking about ribbon mike design?

Yes, it is a language thing. it is about ribbon mike design. "ribbon mic motor" is wide used term, though, I agree, it may be not correct from terminology point of view.

Any dynamic/ribbon mic can be used as a loudspeaker and vice versa, dynamic/ribbon loudspeaker can be used as a mic. I believe that physical principles of one are applicable to another. So if some solution decreases impedance for a speaker it can decrease impedance for mic. If "flux stabilizing Faraday ring" helps in loudspeaker it can help in mic. Am I right?
Or is impact opposite?

 

[jim hardy]

Consider relative currents in the ribbon for motor vs microphone.

Current that doesn't flow can't cause eddy currents elsewhere.

Hence the advice to present a ribbon mike with high impedance preamp.

A typical 250-ohm ribbon microphone may have an impedance modulus as shown in Figure 8.12A. As long as the microphone looks into a high impedance load, its response will be fairly flat. When used with a modern console having 1500 or 3000 ohm input impedances, the frequency response will be altered as shown at B. Moving coil dynamic microphones can also experience similar issues. This is a response problem routinely encountered today and frequently goes unnoticed.

courtesy this free book preview:
http://www.scribd.com/doc/100502271/Microphone


sound logical?

 

[markmal]

preamp is going to be tube preamp with very high input impedance, dozens of K Ohms, according to http://www.aikenamps.com/CommonCathode.htm

Does it mean there will be practically no current in ribbon, thus no currents in core?

 

[jim hardy]

preamp is going to be tube preamp with very high input impedance, dozens of K Ohms, according to http://www.aikenamps.com/CommonCathode.htm

Does it mean there will be practically no current in ribbon, thus no currents in core?

That is what I believe.
Disclaimer - I am not a microphone designer.

if I treat your ribbon microphone as an electric machine ,

without armature current there's no "armature reaction" to perturb the flux.

Since per your sketch the ribbon moves in a uniform field, there's noplace for current to flow except out the ends (QVcrossB) and you're connecting the ends to a high impedance tube amp.. So,, Circular eddy currents in the ribbon look to me impossible.
That's unlike the magnetic braking of a disk by a small magnet where current can circulate around the magnet pole.

Corrections welcome...

Here's an appnote for your audio circuit collection:

http://www.triodeel.com/7199.htm

 

[sophiecentaur]

I find it hard to believe that a ribbon mic (without a matching transformer - which they were always fitted with afaik) could have as high an impedance as that graph suggests. It only consists of a single turn of wire, effectively. Where could 250Ohms come from, let alone nearly 2k at low frequency. The magnets they used were old fashioned low flux type. Were they really like that in the early days?

 

[jim hardy]

Hmmm looks like you're right on as usual - the old timers used transformers..

http://www.coutant.org/ribbons.html

The active element is of course the ribbon, consisting of a very thin corrugated aluminum ribbon clamped under light tension and mounted between the poles of a strong magnet. The extremely low impedance of the ribbon (typically on the order of 0.2 ohm) is fed directly into a step-up transformer to match low-impedance lines (50, 250, or 600 ohms).

So - there's a transformer in the picture too ? the plot thickens...

It's still an electrical machine driving almost an open circuit.

 

[markmal]

That is what I believe.
Disclaimer - I am not a microphone designer.
if I treat your ribbon microphone as an electric machine ,
without armature current there's no "armature reaction" to perturb the flux.
Since per your sketch the ribbon moves in a uniform field, there's noplace for current to flow except out the ends (QVcrossB) and you're connecting the ends to a high impedance tube amp.. So,, Circular eddy currents in the ribbon look to me impossible.
That's unlike the magnetic braking of a disk by a small magnet where current can circulate around the magnet pole.
Corrections welcome...

It is still unclear to me how mic transformer would work if there is no current? How it passess and increases voltage?

Here's an appnote for your audio circuit collection:
http://www.triodeel.com/7199.htm

Thaks for the link. I am thinking which amp to make. I'll take a look. Currently I am thinking about RCA OP-6 because of its huge 90dB gain.

 

[markmal]

I find it hard to believe that a ribbon mic (without a matching transformer - which they were always fitted with afaik) could have as high an impedance as that graph suggests. It only consists of a single turn of wire, effectively. Where could 250Ohms come from, let alone nearly 2k at low frequency. The magnets they used were old fashioned low flux type. Were they really like that in the early days?

250Ohms is after transformer. 2-3K is input impedance of preamp, I believe.
Old magnets were huge, so they may produce flux equivalent to today's small neodims.

 

[markmal]

Hmmm looks like you're right on as usual - the old timers used transformers..

http://www.coutant.org/ribbons.html

So - there's a transformer in the picture too ? the plot thickens...

It's still an electrical machine driving almost an open circuit.

Today's ribbons also use transformers. It is the only way to increase such low voltage without adding noise.

I have another question. There is one transformer in mic ~ 1:35, and another one in preamp.

Is it really required to have two? Can they be replaced by one with "summarized" ratio?
In this case it should be less distortions, right?

 

[sophiecentaur]

Today's ribbons also use transformers. It is the only way to increase such low voltage without adding noise.

I have another question. There is one transformer in mic ~ 1:35, and another one in preamp.

Is it really required to have two? Can they be replaced by one with "summarized" ratio?
In this case it should be less distortions, right?

Unless you are intending to use a valve amplifier in the pre-amp, it shouldn't be necessary to have one there. There are many more options for pre-amp designs now and those would not usually involve a transformer. The reason for the transformer in the microphone is to make sure that the impedance it presents to the line is more suitable and reduces the effects of interference and hum. Nowadays, I would imagine that a suitable pre-amp could easily be placed inside the microphone body and so you'd need no transformers at all. But it depends whether you want optimum performance or authentic 'retro' equipment.

 

[jim hardy]

It is still unclear to me how mic transformer would work if there is no current? How it passess and increases voltage?

A good transformer requires only small magnetizing current.
Load current is in addition to that.

So the old advice about connecting the ribbon microphone to a high Z preamp is to reduce the current demanded of it.

Doubtless those old-timers calculated the damping provided by electrical loading... and designed partly 'by ear'.
This could become a VERY interesting project.

Interestingly your turns ratio of 35^2 is 1225 which X ~0.2 ohm Zribbbon is your 245 ohm Zout...

In your pre-amp schematic in last post, that input transformer I think provides noise rejection in addition to stepup. Pins 2-3 are differential input for the long microphone cord..
Here's all I could find on that transformer part number:
http://www.tomshardware.com/forum/48023-6-looking-transformer-parts

With today's opamps I would think you can get by with with one less transformer - but which one to skip?

As I said this is out of my field. My freshman physics is not too bad but you need a genuine audio guy.
To me this is just a small dynamo.

thanks - I'm learning new stuff.

old jim

 

[sophiecentaur]

Providing the optimum amplifier is not simply a matter of considering the volts. It's the Power that counts and how to match the source resistance (0.5Ω) best to the most appropriate amplifying device in the best configuration. The last thing you'd choose to do, I reckon, would be to use a bog standard Common Emitter, voltage amplifier, configuration. Common Base mode would probably be most suitable - or the equivalent thing with an OP amp. You want Volts out for Current Into a low input impedance. But I'm no expert in the details of low noise system design. It's never straightforward - I do know that! I'm pretty sure that you can do well enough without a transformer with the available devices.
But we're still not sure what the OP actually wants to make - optimum or retro.

 

[markmal]

A good transformer requires only small magnetizing current.
Load current is in addition to that.
So the old advice about connecting the ribbon microphone to a high Z preamp is to reduce the current demanded of it.

This means to me that current still appears. Especially on high frequencies. If these current can induce flux in step-up transformer, can it also induce some amount of feedback flux into core?
Yes, I realize that the amount will be very low, but you know, if we take a glass as an analogy, even low impurity in glass will cause visible effect on its transparency and clarity of image that comes trough it. That what I mean when I ask about that backward flux caused by ribbon movements.

 

[markmal]

Providing the optimum amplifier is not simply a matter of considering the volts. It's the Power that counts and how to match the source resistance (0.5Ω) best to the most appropriate amplifying device in the best configuration. The last thing you'd choose to do, I reckon, would be to use a bog standard Common Emitter, voltage amplifier, configuration. Common Base mode would probably be most suitable - or the equivalent thing with an OP amp. You want Volts out for Current Into a low input impedance. But I'm no expert in the details of low noise system design. It's never straightforward - I do know that! I'm pretty sure that you can do well enough without a transformer with the available devices.
But we're still not sure what the OP actually wants to make - optimum or retro.

I want to build, so you call, "retro", but, this is based on contemporary elements. Retro design is often simplistic and because of this it creates less noises, distortions and coloration to sound.
That is why lots of audiophile level devices made on tubes. Yes these are bulky, eat power, but they produce good sound, and, that is most important for me, these are more DIY-able than solid state integrated circuit based. It is matter of personal preferences though. I prefer tubes.
Also, with tubes it is quite simple to add that famous "tube warmness/overdrive/distortion", that sounds lovely for guitars and for some cases for vocal. Though tube distortions is not first goal for me. I'd like to have clear sound first of all.

And my question was: can two transformers - one in mic and one in tube preamp - be replaced by one, and does it make sense?

 

[sophiecentaur]

I want to build, so you call, "retro", but, this is based on contemporary elements. Retro design is often simplistic and because of this it creates less noises, distortions and coloration to sound.
That is why lots of audiophile level devices made on tubes. Yes these are bulky, eat power, but they produce good sound, and, that is most important for me, these are more DIY-able than solid state integrated circuit based. It is matter of personal preferences though. I prefer tubes.
Also, with tubes it is quite simple to add that famous "tube warmness/overdrive/distortion", that sounds lovely for guitars and for some cases for vocal. Though tube distortions is not first goal for me. I'd like to have clear sound first of all.

That is clearly not the case or valves and ancient transducers would be used in all serious laboratory measuring equipment and in all studios. The fact is that people 'like' the kind of distortion that valves produce (in the same way that they 'like' the shortcomings of vinyl and, even tape!) because they are familiar and identifiable. The problem with old design was lack of open loop device gain, poor noise figures and saggy frequency response.
A modern, solid state amplifier can be made as linear as you like, up to its cracking point and, if levels are appropriately controlled and defined, distortion can be made arbitrarily low.
One should really be wary of the HiFi market and reviewers, who will claim that anything can make the sound better / warmer / livelier / crisper / better controlled etc. etc - even the colour of the connecting leads! Most people listen with their wallets and their opinion of what they hear tends to be coloured by what they read and is seldom supported by any objective measurements.
A (genuinely priced) £nk speaker, system or microphone should be operated in a room which has been treated with at least £nk worth of acoustic modification if that money is to be worth spending.

 

[markmal]

That is clearly not the case or valves and ancient transducers would be used in all serious laboratory measuring equipment and in all studios. The fact is that people 'like' the kind of distortion that valves produce (in the same way that they 'like' the shortcomings of vinyl and, even tape!) because they are familiar and identifiable. The problem with old design was lack of open loop device gain, poor noise figures and saggy frequency response.
A modern, solid state amplifier can be made as linear as you like, up to its cracking point and, if levels are appropriately controlled and defined, distortion can be made arbitrarily low.

I agree. It is subjective and may be psychological. I mentioned DIY-ability as well.
Regarding SS amps, circuits I saw in past were frightening big, with hundreds of elements. May be today's IC schematics are more simple. What schematic would you recommend for a ribbon mic preamp with 80-90dB gain and low noise?

 

[jim hardy]

If these current can induce flux in step-up transformer, can it also induce some amount of feedback flux into core?

Theoretical versus practical - that's where experience comes in. An old hand can tell you what you can ignore and whet you cannot ignore. I don't have microphone experience just motor experience. And this is an unusual form of dynamo.

I'm pretty sure if you consider the amp-turns per inch of airgap in the magnetic circuit of your microphone you'll find the Δflux from ribbon current is infinitesimal compared with the flux from your magnet.
In other words it's calculable; but I think it's so small as to be not measurable and an old hand would likely say 'don't worry about it'..
only way to be sure in measure it.
Carl Pugh addressed this in posts 9 and 11.

The little transformer has many more turns and no air gap. That's where the flux from your ribbon current will be significant - inside the transformer where you want it.
If it's a worry perhaps you could use nonconductive ceramic magnets for pole pieces?
Or place a few turns of tiny wire in the airgap and measure Δflux with an oscilloscope.

Is it moot anyway? Since eddy currents in iron oppose a change in flux it seems to me they're helping you cancel out what in a motor would be called "Armature Reaction".
"Armature reaction" is modulation of field flux by armature current.
Maybe that's why you don't see laminations in the pole pieces of old microphones. Something to ask on a microphone design forum.

Also I point out - a motor has two large mmf's that interact to produce lots of torque.
You have one huge mmf(magnet) and one infinitesimal mmf(current in the ribbon) that you'd prefer don't interact.
How many amp-turns are required to push your 1 tesla through your air gap?
1/1000th as many amps through your 1 turn ribbon will affect your flux by a part in a thousand.
That puts a number on it for you.

I looked at TI's application guide and they recommend this opamp for a microphone transformer replacement, but I think they mean the one at preamp input:

http://www.ti.com/lit/ds/sbos003/sbos003.pdfA

Time for some experimenting.

I haven't heard you mention your acoustic design. This ribbon has inertia and stiffness, and you want it to move with feeble variations in air pressure... F=MA ...
Look at its output when subjected to square wave sound at various frequencies and with a range of resistive loads..