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roger strummer said:

 

VERY interesting stuff, thanks, I'm seeing what I can find about previous measurements of vibration modes of speakers. I'm doing the approach you suggested, just instead of using a mic I'm using accelerometers right now since it is what I have immediately available, but shortly thereafter I'll get mics, and maybe I can do something for measurements of the coils (its sweet having friends with good equipment or access to it).
 
I think I'll make a thread in the sound science forum to post the (rough and preliminary) first results I get and expose my thoughts of whatever can be read of the results, and also see what Tyll  might say about all this (although I bet he's very busy right now with the measurements at CJ).

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Microphones wouldn't give you the mechanical response of the coils.  It's the mechanical response (measured by accelerometers) that pushes the air and generates a sound wave that we hear (and that a microphone "hears" by converting back to mechanical --> electrical energy). 
 
FYI, doing the analysis for the acoustic modes wouldn't be too tough, particularly for closed headphones.  You could treat it as an acoustic volume.  Assuming it's cylinder shape, the solution may even be closed form (it is for rectangular rooms, for example).  The modes wouldn't necessarily line up with the structural modes of the headphone, and actually you wouldn't want them to.  You don't want that coupling as you'd get a significant bump in response.

 
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garyjw said:

I've seen this done on satellites (a little larger than headphones I agree).  Take a satellite and place it on the world's largest subwoofer and do a frequency sweep to check that the vibration modes are where they are expected.  (I say subwoofer deliberately, when I asked, I was told that the vibration table (I guess 4m x 4m in size (from memory)) was actually driven by a large set of coils. I am not 100% sure how the response was measured but may have been as simple as a microphone (since one could hear the resonances) or maybe through reactance measurements through the coils themselves.

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Response measured via accelerometers.  You'd also have accels on the table for control.  The more exciting spacecraft tests are random vibe (the typical test is 20-2000 Hz with all frequencies simultaneously active rather than a sweep) and of course acoustic (air horns generating 140-160 dB levels in either air or nitrogen (for it's lower density)). 

 
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tigermilk said:

 
If "tapping" on the phone like an impulse, you are exciting all frequencies at once.  Unless you have "magic ears" and a mind that can pick out the frequencies, this is meaningless.  Measurements (e.g., microphone for acoustic resonances or accelerometers for structural responses) are required to come to a hypothesis.  Otherwise you are just hearing things...

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You're right, it doesn't prove anything, hence I've rescinded the excited tone of my earlier post.  I know it excites all the frequencies and that is why I wasn't even able to post an approximation of the frequencies.  However tapping on different phones that are on your head does produce a different tone or pitch of the resulting "thud",  I think these are called "overtones" (not sure).

 
Quote:

roger strummer said:

 

VERY interesting stuff, thanks, I'm seeing what I can find about previous measurements of vibration modes of speakers. I'm doing the approach you suggested, just instead of using a mic I'm using accelerometers right now since it is what I have immediately available, but shortly thereafter I'll get mics, and maybe I can do something for measurements of the coils (its sweet having friends with good equipment or access to it).
 
I think I'll make a thread in the sound science forum to post the (rough and preliminary) first results I get and expose my thoughts of whatever can be read of the results, and also see what Tyll  might say about all this (although I bet he's very busy right now with the measurements at CJ).

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I'm curious Roger, what is your experience in these fields?  I can't comprehend entirely what it is you are doing, is there any links you can provide me, as I am keen to understand your endeavour.  Tyll should be back soon and I hope he will be able to lend his expertise to your experiments.
 
I did have a quick thought.  If one was to connect the headphone to a microphone input and bring up a spectrum analyser, one can tap at the housing and check the frequency extremes - using the driver itself as a microphone.  Of course this is also flawed as each phone has a different driver with different response.  Ideally the same driver should be in all the headphones - are you going to attach a microphone to the headphones to eliminate this variable?  Using the human ear as the universal microphone eliminates this variable, but we cannot record our ear responses. 
 
 
 

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sp wild said:

 
I did have a quick thought.  If one was to connect the headphone to a microphone input and bring up a spectrum analyser, one can tap at the housing and check the frequency extremes - using the driver itself as a microphone.  Of course this is also flawed as each phone has a different driver with different response.  Ideally the same driver should be in all the headphones - are you going to attach a microphone to the headphones to eliminate this variable?  Using the human ear as the universal microphone eliminates this variable, but we cannot record our ear responses. 

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Doing this you will get a read of the acoustic modes of the cavity but I'm not sure what that will really tell you.  Microphone placement would be critical - even in a room moving a few inches in either direction will affect the perceived sound.  And here you don't have that supercomputer (i.e., your brain connected via the ear canal) in the loop.  In other words, there's more to sound perception than just raw measurements.  And unfortunately what I perceive as "good sound" is not necessarily what you or others perceive as the same. 
 
Now if all you want to do is compare peaks (signifying acoustic modes) what you suggest wouldn't be that bad.  It's the air volume that is important here, not the commonality of drivers.  Remember, the driver is merely changing electrical energy to mechanical to acoustic.  Knowing the acoustic modes you'd be able to start to backtrack and see how the driver should respond to give you a desired response.  The tap test gives you the frequency content.  Unfortunately to truly get all the data you'd need more rigor (a known input so you could generate a true frequency response function).
 

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iponderous said:

With respect SP Wild, I think that you are tilting at windmills here. The sound being produced by the headphone driver would overwhelm the negligible sonic influence of the headphone's "resonant frequency" as you describe it.
 
Closed headphone shells have dampening material, earpads, bits and pieces screwed and glued on to them. All of these additions would dampen any innate resonance that the material being used to form the shell might have. Your theory looks even shakier when applied to an open headphone.  
 
Tapping the outside of a headphone and hearing a tone is not analogous to tapping a head on a drum and listening for a note. Drums are empty vessels full of air. Tapping on the batter head moves the air between it and the resonant head, which causes the drum shell to vibrate or resonate in order to produce sound. It is free of internal dampening material to maximise resonance. Drummers usually find themselves adding dampening material to reduce the drum's resonance, and the unwanted overtones that it produces.
 
Whereas headphones are designed to minimise the unwanted resonances that could colour the sound between it leaving the driver and reaching the listener's ears, which is a very short journey indeed. I suspect that the wood used in closed headphones such as the D7000 and the W5000 is there to serve a cosmetic purpose rather than an acoustic one, regardless of what the companies marketing would have us believe. Those are the flagship headphones for Denon and Audio Technica so they should at least look the part.
 
A brick has a tone when tapped as does a piece of iron. So what? Wrap some cloth around them and their unique "resonant frequencies" have been silenced. I'm afraid that it's back to the drawing board for you.
  
 
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how can you say that the enclosure of a headphone doesn't affect the sound?  Are you then saying that an RS1 driver in an SR60 shell would sound like an RS1?  Or that an HP1000 driver in an SR60 shell would sound like an HP1000 (fyi these phones are housed in metal)?  There are actually examples of this, the SR200's with HP1000 drivers, and they are reported to have a similar sound signature, but not as good.  What about the Symphones Magnums, that place grado drivers in all metal housings?  What about speaker cabnets?  Is a speaker housed in a solid metal box going to sound the same as a speaker housed in the normal wooden enclosure?  It's not just for looks, my mackies are painted all black, and it would make no difference to the look if it was a plastic/metal housing.  The front is all plastic.

^ I did not state that the enclosure of a headphone does not affect the sound; you inferred it. I was specifically referring to closed phones with dampening material and doing so within the context of this thread topic. 
 
However, I do suspect that the composition of the material that an open headphone enclosure is fashioned from, and the affect that it has on the sound produced by the driver, is probably overrated. I tend to think that other variables such as the dimensions of the driver enclosure, distance and positioning of the driver from the ear, ventilation and the type of earpads used, would have a greater affect on the sound produced.  
 
For example, isn't the depth of the RS1 enclosure more than the SR60? Haven't we read numerous posts regarding changes in the sound signature of Grado headphones as a result of swapping earpads? And don't the Grado drivers contained in the Symphones Magnums undergo a proprietary treatment process prior to being installed in the modified metal enclosures? Are these enclosures the same dimensions as the original Grado enclosures or have they been altered as well?
 
I think that there's more than a drop of snake oil added to the marketing of headphones by the manufacturers. But those Japanese ebony wood cups on my W5000's look deluxe, even if they don't add anything to the sound. 
 
 
  

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iponderous said:

^ I did not state that the enclosure of a headphone does not affect the sound; you inferred it. I was specifically referring to closed phones with dampening material and doing so within the context of this thread topic. 
 
However, I do suspect that the composition of the material that an open headphone enclosure is fashioned from, and the affect that it has on the sound produced by the driver, is probably overrated. I tend to think that other variables such as the dimensions of the driver enclosure, distance and positioning of the driver from the ear, ventilation and the type of earpads used, would have a greater affect on the sound produced. 
 
 

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you're contradicting yourself here a bit   Either it does or it doesn't.   I didn't infer anything, you specifically said the affect was minimal.
 
how about the wood used on the HF2 which is inside the pads, and cannnot be seen?  little/no affect?  Or the SR200/HP1000 example?  The HP1000 doesn't have big cups, the difference is primarily metal.  Or the speaker example?  Why are my Mackies placed in a wooden housing, and then painted black with a plastic front panel, if it makes no difference?
 
I agree that in many cases, wood is for aesthetic purposes.  The way it's used on the LCD'2 for example, doesn't seem like it could affect the sound because it is just on the outside.  Or the HE-5, which head-direct admitted didn't affect the sound. 
 
But I don't agree that it has little/no affect in some situations.

^ With respect, if you didn't infer then you've misinterpreted my post. There is no contradiction.

Oh sorry oh well

^ No problem. Just to restate, I wasn't attempting to assert in this thread that the material used to form a headphone enclosure has no bearing whatsoever on the sound that a headphone produces. But your responses to my posts suggest that is how you interpreted what I wrote.
 
My initial post was responding to SP Wild's proposition that a headphone shell has a "resonant frequency" and that this will be revealed simply by tapping on it and listening for the sound or tone, which I thought was an erroneous conclusion to reach.

Quote:

iponderous said:

^ No problem. Just to restate, I wasn't attempting to assert in this thread that the material used to form a headphone enclosure has no bearing whatsoever on the sound that a headphone produces. But your responses to my posts suggest that is how you interpreted what I wrote.
 
My initial post was responding to SP Wild's proposition that a headphone shell has a "resonant frequency" and that this will be revealed simply by tapping on it and listening for the sound or tone, which I thought was an erroneous conclusion to reach.

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The proposition that tapping on the shell to get resonant frequencies is valid.  But mind you that's the STRUCTURAL modes of the shell.  A tap test excites all structural modes at once (do a FFT of a short impulse and you'll see broadband frequency excitation).  The frequencies will ring out structurally and generate sound at the same frequency.  Tap tests are common in vibe testing if you want to get information quickly without the overhead of modal testing or sine sweeps.  What can't be said, however, is if these frequencies affect the sound we hear ultimately due to the frequency response of the acoustic volume itself.  Say the shell has a dominant mode at 100 Hz.  If the acoustic cavity has a dominant mode at 100 Hz as well, you'll get amplification.  If the acoustic cavity has a mode at say 90 Hz, that 100 Hz tone will not be as loud given the same tap intensity (and unless you've got some well calibrated fingers, it's doubtful you could reliably tap with equivalent taps).

^ I think I understand what you're saying and we don't disagree. I don't have your obvious technical background but from a novice's perspective, the approach seemed too simplistic to me. It did not factor in the effect of dampening materials and any other fixtures attached to the shell of the headphone enclosure that could alter the sound/tone/pitch produced when tapped. Given that that OP was coming to his conclusions based on tapping and listening, this approach appeared imprecise at best.   

 
Hey all, sorry for being MIA for the last couple days, quite a few things came out of nowhere and kept me very busy, and I avoided Head-fi due to lack of time to spare .
 
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Originally Posted by tigermilk /img/forum/go_quote.gif

 

Response measured via accelerometers.  You'd also have accels on the table for control.  The more exciting spacecraft tests are random vibe (the typical test is 20-2000 Hz with all frequencies simultaneously active rather than a sweep) and of course acoustic (air horns generating 140-160 dB levels in either air or nitrogen (for it's lower density)). 

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I've been reading about some of the spacecraft tests lately (not extensively, just out of pure curiosity), and they are definitely cool an über interesting. What do you work in or study? Your posts (and GaryJW's) have lead me in the right path to learn a lot about how to test for normal modes, and quite a bit more actually, very educative.

 
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Originally Posted by SP Wild /img/forum/go_quote.gif
 

I'm curious Roger, what is your experience in these fields?  I can't comprehend entirely what it is you are doing, is there any links you can provide me, as I am keen to understand your endeavour.  Tyll should be back soon and I hope he will be able to lend his expertise to your experiments.
 
 
 

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My experience in the particular field of testing for normal modes or vibrations of bodies is not that extensive, just some testing of some materials when I was  in some robotics projects. But the point of the experiment is to test your hypothesis but removing some of the variables, which is to use accelerometers to measure the vibration instead of your ears/fingers to sense the vibrations, and using a vibrating table/subwoofer (aka the thing producing controlled vibrations) instead of finger tapping to know the input that we are giving the system, OTOH  producing a controlled tap (say by dropping an object nearby and repeat is as accurate as possible or something like that, I haven't thought about that yet) might do the trick too, since it would give you the calibrated finger that tigermilk mentions. I have some links and bibliography, I'll post it later (don't have it at hand right now)
 
BTW thanks for the tip about acoustic modes, I'll get into that after I finish a little fight that I'm having right now with Fréchet space stuff that's giving me a headache. .
 
Although I've decided to wait for Tyll's Canjam results before doing anything (I know this seems kind of unrelated, but I've got my reasons), and I am still thinking/investigating some things.
 
 
I hope I haven't said anything incoherent/incorrect, I haven't slept in quite a while .

 
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roger strummer said:

I've been reading about some of the spacecraft tests lately (not extensively, just out of pure curiosity), and they are definitely cool an über interesting. What do you work in or study? Your posts (and GaryJW's) have lead me in the right path to learn a lot about how to test for normal modes, and quite a bit more actually, very educative.
 

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Spacecraft (International Space Station to be specific).  Witnessed acoustic tests in Japan, modal tests in Europe, random vibe and modal tests at my facility, flight testing, you name it.  Also have to make sure the ISS is "quiet" for science payloads - vibration sources are a combination of mechanical vibrations and acoustic impingement (so in many respects, just like the source of loads from launch).