Sound transmission loss in Loudspeaker diaphragms - diyAudio
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Old 20th August 2009, 03:06 PM   #1
thadman is offline thadman  United States
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Default Sound transmission loss in Loudspeaker diaphragms

I'm in a bit of a dilemma, I'm not sure whether or not loudspeaker diaphragms are acoustically transparent or the degree to which it effects their motion.

A few ideas,

Below the fundamental mode of the enclosure, the air behaves with a single phase. The acoustic forces (and their effects on diaphragm motion) within the enclosure could be described accurately by the air compliance.

Within the modal region of the enclosure, the air has a multi-dimensional phase property, however Ray acoustics still do not apply. The acoustic forces (and their effects on diaphragm motion) within the enclosure could be described accurately by analyzing the modal surface encompassing the diaphragm.

Above the Schroeder frequency of the enclosure, we'll assume Ray acoustics apply. In this region, acoustic waves should be propagating through the enclosure space (ie reflections will be present due to the impedance mismatch between the air and enclosure walls/diaphragm). It is over this bandwidth that the effects of the acoustic transparency of the diaphragm have me curious.

At the interface between two dissimilar masses, an impedance mismatch is found. This is the reason we observe reflections.

Above the Schroeder frequency, reflections are believed to occur. If we assume a reflection occurs, we must also assume the presence of an impedance mismatch.

Let's assume we have two masses (Mass A = air, Mass B = loudspeaker diaphragm). We assume a propagating wave (Mass A --> Mass B) contacts the interface between the two masses. At the interface we observe a reflection (wave is reflected back to Mass A), however some of the energy is absorbed by Mass B.

Wouldn't this dictate that propagating waves have an influence on diaphragm motion due to the impedance match (ie 100% reflections are not observed) and we must thus consider their influence if we wish to accurately simulate a loudspeaker system?

Over what bandwidths should the impedance match be considered?

If any of my assumptions are inaccurate or incomplete, I would much appreciate some feedback.

Thanks,
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Old 20th August 2009, 03:40 PM   #2
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The only feedback I could give you , is that , your question is too high tech for me .
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Old 20th August 2009, 04:29 PM   #3
SY is offline SY  United States
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This is potentially an important factor. A few observations:

1. Stuffing reduces this issue considerably.
2. So does lining the walls.
3. Having the cone nonparallel to the rear wall can make a big difference.
4. Light thin cones do better in open-back cabinets.

The reductio is the planar diaphragm- anyone who has tried putting an ESL diaphragm in a box knows how severe the problem is. There has even been a product marketed (I haven't tried it myself) claimed to greatly reduce reflection when attached to the cabinet rear wall.
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Old 20th August 2009, 04:32 PM   #4
thadman is offline thadman  United States
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Quote:
Originally Posted by SY View Post
This is potentially an important factor. A few observations:

1. Stuffing reduces this issue considerably.
2. So does lining the walls.
3. Having the cone nonparallel to the rear wall can make a big difference.
4. Light thin cones do better in open-back cabinets.

The reductio is the planar diaphragm- anyone who has tried putting an ESL diaphragm in a box knows how severe the problem is. There has even been a product marketed (I haven't tried it myself) claimed to greatly reduce reflection when attached to the cabinet rear wall.
I've never seen transmission loss ever cited with regards to loudspeaker diaphragms. However, I believe it must be considered for an accurate simulation of the complete system. It would seem to suggest that diaphragm motion (and thus Frequency Response) will be modulated by the rear wave if the driver is placed within an enclosure.

Is this a reasonable conclusion?
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Old 20th August 2009, 04:39 PM   #5
thadman is offline thadman  United States
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I know Mige0 wrote a paper on Back Diaphragm Mirror Distortion, although he did not specifically analyze sound transmission loss through the diaphragm. Other than his work, I have not found much information available with regards to this specific phenomenon.

It would be interesting to hear Dr. Geddes' position on this issue as I believe he may have done significant work in this area working at Ford (minimizing road noise in Vehicles, etc).
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Old 20th August 2009, 09:43 PM   #6
badman is offline badman  United States
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To some extent, I'd say that the CSD is a useful tool for examining transmission loss in diaphragms. The more rapid the decay the more loss there is. The more consistent the decay the more linear the loss is. This is one effect that Enabl seems to have according to the linked chart. The decay is much more consistent and doesn't tend to plateau as much.

http://planet10-hifi.com/johnK-test/
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Old 20th August 2009, 10:00 PM   #7
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Default Cone effects

Seems you're leaving out the issue of cone/membrane break-up, and only considering what happens to the diaphraghm at rest (i.e., not driven pistonically) wrt cabinet effects.

I would think that as long as the VC maintains control of the membrane, the electrical input would dominate the response (if it didn't, we wouldn't hear individual notes, just the modal/reflective textures)

Above cone breakup, all bets are off...

John L.
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Old 20th August 2009, 10:45 PM   #8
thadman is offline thadman  United States
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Seems you're leaving out the issue of cone/membrane break-up, and only considering what happens to the diaphraghm at rest (i.e., not driven pistonically) wrt cabinet effects.
Why? If we reduce the system to a single degree of freedom over our desired bandwidth, a 2nd order Partial differential equation should accurately define the motion of a dynamic system. If not, we would simply have to do FEM analysis of the diaphragm. If we could calculate the sound transmission loss energy, we could sum that with the PDE and achieve a more accurate simulation of the loudspeaker response.

Quote:
I would think that as long as the VC maintains control of the membrane, the electrical input would dominate the response (if it didn't, we wouldn't hear individual notes, just the modal/reflective textures)
Sure, the VC provides a much greater force wrt the reflection, however I'm not sure that means we should simply ignore it. You must remember our perception of sound energy is logarithmic, therefore something much lower in level is able to be noticed. I consistently see references to -40dB as a usual distortion threshold, this is 4 orders of magnitude below the original signal!
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Last edited by thadman; 20th August 2009 at 11:12 PM.
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Old 20th August 2009, 11:51 PM   #9
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Default modelling

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Originally Posted by thadman View Post
Why? If we reduce the system to a single degree of freedom over our desired bandwidth, a 2nd order Partial differential equation should accurately define the motion of a dynamic system. If not, we would simply have to do FEM analysis of the diaphragm. If we could calculate the sound transmission loss energy, we could sum that with the PDE and achieve a more accurate simulation of the loudspeaker response.
I guess I'd have to see the eqn. Not sure you can ignore possible boundary conditions as implied. Your FEM analysis would also have to take in to account the non rigid wall absorption, etc. no?

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Originally Posted by thadman View Post
Sure, the VC provides a much greater force wrt the reflection, however I'm not sure that means we should simply ignore it. You must remember our perception of sound energy is logarithmic, therefore something much lower in level is able to be noticed. I consistently see references to -40dB as a usual distortion threshold, this is 4 orders of magnitude below the original signal!
Didn't say to ignore it... only offering some thoughts; take 'em or leave 'em

Not sure I buy into all your assumptions/observations in an attempt to simplify... not all distortions are equally audible (to wit: the Geddes discussions)

John L.
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Old 21st August 2009, 01:19 AM   #10
thadman is offline thadman  United States
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Originally Posted by auplater View Post
I guess I'd have to see the eqn. Not sure you can ignore possible boundary conditions as implied. Your FEM analysis would also have to take in to account the non rigid wall absorption, etc. no?
Couldn't we classify the system as a Driven Harmonic Oscillator?

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Originally Posted by auplater View Post
Didn't say to ignore it... only offering some thoughts; take 'em or leave 'em
This is an intellectual forum, I am very appreciative of your feedback and participation in this thread.

Quote:
Originally Posted by auplater View Post
Not sure I buy into all your assumptions/observations in an attempt to simplify... not all distortions are equally audible (to wit: the Geddes discussions)

John L.
? If a distortion mechanism is present, why not attempt to understand it? Once we understand it, we can evaluate and/or quantify it and determine if it is a problem that relates to our specific application, otherwise I believe our understanding of the system may be incomplete and we may reach a suboptimal solution.

I am in total agreement with your statement that not all distortion components are equally audible. In fact, that is why I'm interested in having the most complete understanding of the system, to determine what effects each part of the system has on the resulting response. If it is found to be undesirable we can attempt to engineer a better solution, otherwise we can ignore it. However, we will never know if we limit ourselves with a primitive understanding of their significance.

FYI I believe this would equate to waveform modulation
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Last edited by thadman; 21st August 2009 at 01:22 AM.
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