# Newton's Third Law of Motion...?

#### secretagentman

Joined Aug 14, 2012
12
Greetings to ALL.

I post a hypothetical question of sorts. I have no physics background other than what I have been able to "pick Up" on my own. This question may be a product of the sinlpeton that I am, but here goes. If one were to place identical speakers " face to face", sealed them together in such a way as to keep them from making physical contact at any time and then applied a driving signal to one of them, would the non-powered speakers cones movement be a mirrored reaction to the powered speaker. I understand there may be some losses involved( perhaps friction, air loss through the paper cones, etc.). Any opinions will be happily accepted.
This is concerning an experiment in the transfer of a range of frequencies from one speaker(active), to another speaker(basically used as a non-powered microphone.It's just a curiosity "thing'. Thanks in advance for any and all replies. secretagentman.

#### WBahn

Joined Mar 31, 2012
30,301
The movement of the non-powered cone should be highly correlated to the movement of the powered one, but I think saying that it would be a (possibly attentuated) mirror image is a bit strong. Aside from reflections off things, the waves coming off the powered speaker are diverging and there is nothing to make them converge back onto the unpowered speaker. I would imagine that you could rig up a pair of parabolic reflectors that would result in a much higher correlation (i.e., closer to being a mirror image).

#### Austin Clark

Joined Dec 28, 2011
412
There would be a time delay (phase-shift), loss in amplitude (by what degree at different frequencies I do not know.) and some weird "distortion" as reflected signals will arrive at different times.
I had the idea to send data via sound once upon a time, but eventually gave up because I didn't know how I could convert different frequencies into signals I could use. I think maybe you could use a band-pass filter and rectify it's output, and then put that through a low-value capacitor and have that connected to a comparator or something, to produce a digital value of 1 when the right frequency is hit... idk, just random ranting idea.

#### DerStrom8

Joined Feb 20, 2011
2,390
Theoretically, the answer is yes, though there will be the differences discussed earlier in the phase and amplitude. In an ideal world there would be no distortion or loss in amplitude, but of course our world is far less than ideal

A speaker can work as a microphone, if used in the right way. Your idea is simply an example of this. Have you ever taken a microphone and plugged it into a speaker output jack? Or have you ever plugged a speaker into a mic input? In most cases it works, though the differences in the sound are often very noticeable. Speakers are made to vibrate when an electrical signal is driven through their coils. Their cones are not designed to vibrate with ambient noise, but in certain conditions it will act as you describe.

A simple test you can do yourself, if you have an oscilloscope:

Connect one speaker to the audio source, and also connect the output from the source to one channel of the oscilloscope. Take another speaker (un-powered) and connect its wires to the channel 2 input of the scope. Set up the speakers in the way you describe, face-to-face, and power up the audio source. You should see similar waveforms on both channels, though the waveform for the non-driven speaker will have shorter peaks and lots of spikes. This will demonstrate the lower amplitude and distortion of the sound received by the second speaker.

On a side note, computer speakers will not work in this case, since they have extra electronics inside to drive it. I'm talking about the bare-bones speakers you can pull out o fa boombox or something similar.

#### Wendy

Joined Mar 24, 2008
23,477
Anyone remember acoustic modems? I used to use them.

#### DerStrom8

Joined Feb 20, 2011
2,390

#### secretagentman

Joined Aug 14, 2012
12
Per Bill_Marsden's advice I started a new thread(I hope.). Being new to theory, forums, and computers I may be "mucking up" everything that I try to do here. If I am still causing any problems, any advice is appreciated. Do not worry about "talking down" to me, as you are probably not able to get "low" enough. I appreciate the advice of DerStrom8 concerning the use of a scope in my "experiment'. That is one problem that I have at present. I haven't yet been able to purchase one, and I realize that I will need one to complete my project. I actually had one in the past, but have been out of electronics for some time, and let much of my equipment get away from me(DUMB). I am interested in the signal of the "non-powered" speakers "output", at it's normal input. Yes, I fully understand that a speaker can be a crude microphone, and vice-versa. A woofer,at times is used as a microphone for the bass drum in a set. I have already determined that with my crude prototype, I measured(True R.M.S. DVMM)
that the attenuation of signal voltage was over 90%. This is in line with the inefficiency of loudspeakers. Perhaps, the right scope for this project is the only way to get much more information. I was just curious if theoretically, identical speakers(used as I previously mentioned) should produce, at the "un-powered" speakers terminals, very nearly identical frequency characteristics. I guess, to put it bluntly, will they work identically(or nearly so) in "reverse" ? Also, If I may ask further,
I have not yet tried speakers identical speakers, except for rated impedance, such as 1 at 4 Ohms and 1 at 8 Ohms. I found that moving coil phono cartridges appear to have a higher output when they have a higher D.C. resistance(I realize not the same as impedance). I wonder if this would generally hold true for speakers. The goal(my curiosity held back by limited funds) is to attenuate amplitude(which I have done), and most closely reproduce frequency between 50hz and 5000hz. Again,I realize that an oscilloscope is the true test for frequencies here. I am even concerned to reproduce the 2nd and the 3rd harmonics of the mentioned range. Not knowing any theory except Ohm's Law well, I had hoped to gather some opinions concerning this project, which I have the parts on hand to at least have gotten this far. I heard a quote some time back which seems to somewhat apply here, pardon the paraphrasing : All science is physics, everything else is in story-books. I don't intend to waste anyones time. If I'm asking
too much, or am improper in any way, I apologize. Any and all facts, advice, or opinions will be accepted with gratitude. I thank you in advance
for your patience and tolerance. secretagentman.

#### secretagentman

Joined Aug 14, 2012
12
Per Bill_Marsden's advice, I started a new thread( Ihope). I am new to theory(physics anbd electronics), forums, and even using a computer, so if I "muck" up anything further, please let me know. I thank as well DerStrom8 for the advice to use a scope. I know that I cannot complete my "project" without one. I had one years ago and let it "get away"(DUMB). I have been out of electronics for some time. I have had great luck repairing, modifying, and building guitars, amps, and effects since 1981, but I am essentially an experimentalist. This project I now discuss, is more complex than anything I have tried to do before. I have found that with the dual-speaker configuration that I described in my last post, I was able to attenuate the signal voltage at the "non-powered" speakers terminals by over 90%. I am now turning to the goal of reproducing the frequencies(50hz-5000hz) plus their 2nd and 3rd harmonics. Even though this raises the uppermost frequency range, it's within the limits of most full range speakers of quality.Is it wrong to think that such frequencies could be faithfully reproduced in this fashion ? If anyone for-sees problems, I would like to hear any thoughts about them.
I have found that seperating the speakers, instead of sealing them together increases attenuation, and I wonder if it might also affect frequency response in the "receiving' speaker(perhaps positively). I know there are many ways to achieve what I am trying to do in what must seem a strange methodology, but I have my reasons. I do not wish to waste anyone's time, but I will be greatful to hear any advice, facts, or opinions. Also, you can't possibly "talk down" to me too much. I leave off with a paraphrase of a quote I read some time ago which made me realize it's significance(at least to me) : All science is physics, everything else is in story-books. Thanks in advance for your patience and tolerance.
secretagentman.

#### Wendy

Joined Mar 24, 2008
23,477
I don't remember saying that, except in an email concerning a automotive thread with another user. Doesn't matter either way, we generally do not like multiple threads on the same subject from the same user, so I merged them.

Audio can be a complex subject. You are creating what could be considered a resonant tube, which can cause other problems. A simple pipe cut to a sound wave length will be resonant at that frequency, an effect that is used in many ways. The longer the pipe, the lower the resonant frequency.

#### #12

Joined Nov 30, 2010
18,224
You must consider that air is compressable and that cone type speakers have mass, therefore, inertia. There will be a frequency limit on the high end. There will be an increasing phase shift as frequency increases. If you look up Thiele-Hall it will give you ideas about how to model speakers as a springy mass.

#### WBahn

Joined Mar 31, 2012
30,301
I'm having a hard time grasping what you are trying to accomplish and why you are going about it this way. Why not just use a microphone and, if necessary, equalize it?

#### Wendy

Joined Mar 24, 2008
23,477
Per Bill_Marsden's advice I started a new thread(I hope.). Being new to theory, forums, and computers I may be "mucking up" everything that I try to do here. <snip>
secretagentman.
Please point to where this advice I gave was, I do not recall it.

Second notice, do not start a new thread when you have an existing thread on the same subject in place.

Again, I merge the thread you created with the old one, this is becoming tiresome. Please don't.

There are no stupid questions, but we do have several rules. Multiple threads on the same subject is one of them, as is hijacking.

You can build equalization circuit to compensate for durn near everything, it depends on how much time you want to spend on it. As has been noted, speakers can be used for microphones. True microphones have light weight components to reduce the physical inertia to increase fidelity though. I don't know of any cases, but I am sure there are speakers that make better mics than others, and I bet size is a consideration.

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