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Speaker enclosure vibrations - a few measurements with accelerometer

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Thomas_A

Thomas_A

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The main goal is to have a cabinet that do not add distorsion. A uniform made cabinet should not add distorsion since it should be linear, i.e. only vibrate at the excitation frequency. I have a bit hard tine to understand why distorsion of cabinet vibrations can reach up to 100%. Is it so that the distorsion from the driver cone can be amplified that much? Or is it something else going on such as a non-uniform driver to cabinet transition? The March speaker thread indicates distorsion induced from the driver motor and frame and may be dependent on bolt torque.

I have a plan to make some modifications and measure again.
 

Tangband

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I think that i-or on faktiskt.se on the ongoing thread there, has a big clue to your question, and its the same one that Linkwitz already has written about, - its about the way the driveunit is clamped to the baffle . The best way seems to clamp the driver from behind at the magnet, this way there is no need for screws directly into the front baffle .

Linkwitz wrote :

A) Drivers with a stamped metal baskets are prone to exhibit a high Q resonance when tightly clamped to the baffle. The magnet moves relative to the voice coil at the resonance frequency. Energy is stored and also readily transmitted from the moving mass of the cone into the cabinet.

B) Soft mounting the driver basket to the baffle using rubber grommets reduces the resonance frequency. A 2nd order lowpass filter is formed that reduces the transmission of vibration energy from the moving cone to the baffle and cabinet. The resonance must occur below the operating range of the driver.

C) If the driver is mounted from the magnet and the basket rim touches the baffle only softly, then the magnet-basket resonance cannot occur and the transmission of vibration energy into the baffle is minimized.

072EE70A-48BF-48AA-A459-99253D6BE857.gif


My comment: I would continue measuring without any screws on the driver basket , using glue or mounting the driver from behind at the magnet.

FB50D8C1-31EC-4B7B-9E27-A7457E7030F0.gif
 
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Thomas_A

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With respect to stamped or molded frame, the 19 mm MDF enclousure with signifcant distorsion in the 500-700 Hz range use a molded frame driver. Not stamped. The cabinet with constrained layer damping holds a stamped frame.

Glue would be too much to go for me now. The two things to be made experimentally is 1) a brace between woofer and tweeter 2) Attach woofer with 8 instead of 4 screws to get better and more even coupling.

Ideally I also want the same torque of each screw but these manual tools seem quite expensive. Still looking.
 

Tangband

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You can find an article I wrote on measuring cabinet vibrational behavior like this at https://www.stereophile.com/features/806/index.html

John Atkinson
Technical Editor, Stereophile
The main goal is to have a cabinet that do not add distorsion. A uniform made cabinet should not add distorsion since it should be linear, i.e. only vibrate at the excitation frequency. I have a bit hard tine to understand why distorsion of cabinet vibrations can reach up to 100%. Is it so that the distorsion from the driver cone can be amplified that much? Or is it something else going on such as a non-uniform driver to cabinet transition? The March speaker thread indicates distorsion induced from the driver motor and frame and may be dependent on bolt torque.

I have a plan to make some modifications and measure again
Those cabinet resonances seems to be clearly audible in some cases, and it seems to be more audible If the resonances are at lower frequencies, because of the way music tones work.

John Atkinson wrote:

”The question is begged as to how important this will be subjectively with normal musical material, not test tones or pink noise. This high-Q resonance will need to be hit with sustained tones lasting for a good few milliseconds before it is fully excited. 360Hz is midway between the notes F and F# above Middle C in the second space of the treble staff (frequencies 349.23Hz and 370.0Hz, respectively); it is possible, therefore, that this resonant mode might add a degree of clouding on just these notes, which a very sensitive listener would pick up. If it were lower in frequency, however, where the notes are closer together, it would probably be excited more often, which is why resonances in the tenor region—approximately 120-240Hz—often add a "wooden" coloration to a speaker's sound.”
 
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Thomas_A

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Those cabinet resonances seems to be clearly audible in some cases, and it seems to be more audible If the resonances are at lower frequencies, because of the way music tones work.

John Atkinson wrote:

”The question is begged as to how important this will be subjectively with normal musical material, not test tones or pink noise. This high-Q resonance will need to be hit with sustained tones lasting for a good few milliseconds before it is fully excited. 360Hz is midway between the notes F and F# above Middle C in the second space of the treble staff (frequencies 349.23Hz and 370.0Hz, respectively); it is possible, therefore, that this resonant mode might add a degree of clouding on just these notes, which a very sensitive listener would pick up. If it were lower in frequency, however, where the notes are closer together, it would probably be excited more often, which is why resonances in the tenor region—approximately 120-240Hz—often add a "wooden" coloration to a speaker's sound.”
Well I dont agree. We are not as much sensitive in lower frequencies as in higher within the 100-1000 Hz region where these distortions occur. Drivers have more distortion in bass normally so lowering frequency will cause masking from the driver distortion. As I said the distortion around 500-700 Hz was audible but those around 250 Hz was not.
 

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I was curious about that glue. Spoke to them now. Sold in 10 liter packages, cost $135. That in itself is a perfectly fine price.:)

Although they mostly sell to companies (building companies that need to soundproof for example) they also sell to private individuals. I live nearby and can pick it up when I pass Gothenburg. I think you can only pick up the glue on site from them. IF I now do that. I don't really need 10 liters. I might buy a tube of green glue instead.:)
 
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Thomas_A

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I was curious about that glue. Spoke to them now. Sold in 10 liter packages, cost $135. That in itself is a perfectly fine price.:)

Although they mostly sell to companies (building companies that need to soundproof for example) they also sell to private individuals. I live nearby and can pick it up when I pass Gothenburg. I think you can only pick up the glue on site from them. IF I now do that. I don't really need 10 liters. I might buy a tube of green glue instead.:)
They used to have 1 litre package which I bought 2 of at the time, and I discarded the remaining a few years ago. I don't have any use for more glue, but perhaps there is one DIYer around you that you could split with?
 

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They used to have 1 litre package which I bought 2 of at the time, and I discarded the remaining a few years ago. I don't have any use for more glue, but perhaps there is one DIYer around you that you could split with?
You're right about that.Speaking of split, sharing. New meeting now in mid-November (I think). It shouldn't be hard to find some there who are interested in some glue like that.If you are passing by Gothenburg, come along to the fair::)

 
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Thomas_A

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You're right about that.Speaking of split, sharing. New meeting now in mid-November (I think). It shouldn't be hard to find some there who are interested in some glue like that.If you are passing by Gothenburg, come along to the fair::)

Sometime I would like to go, but I am quite booked up in the weekends. (Also I will soon have a heart surgery so I am prioritising family. There is always a risk of sad complications...:()
 

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Sometime I would like to go, but I am quite booked up in the weekends. (Also I will soon have a heart surgery so I am prioritising family. There is always a risk of sad complications...:()
Oh, I'm keeping my fingers crossed that everything will be fine! Good luck with the heart surgery and hope you recover well (no complications)after the surgery.
 
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Thomas_A

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Oh, I'm keeping my fingers crossed that everything will be fine! Good luck with the heart surgery and hope you recover well (no complications)after the surgery.
Thanks, the risk is relatively small so most certainly I'll be back. :)
 

fineMen

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Disclaimer: don't let it be a matter of luck. But if so, all the best for Your health!

On topic: Please do not fall for the wisdom of the past. Even Big Science has to admit that it wasn't quite right once in a while. And they are proud of it! How could that be different with speaker enclosure design?!

So, excitation of "resonances" by what? Is it the reaction force, namely Newton's conservation of momentum of the cone that drives the motion? Or is it the enclosed air column which may or may not get into resonance?

It could be, and I found that out independently from others, that the air column drives the vast majority of the peaks. With constraint layer You counteract another resonance than that of the panel itself ;-) You do so quite unspecific, hence overly costly. Better to dampen the internal air a bit more; just try.

Simple test: knock it and measure frequency of peak, compare what excitation by driver says. Difference? Most likely. Explanation? Measure the pathlengths involved ... dampen air and measure again. Further: what about break-up modes? I mean it. Are there break-up modes, as expected if the panel would resonate in itself?! No. Spectral analysis would show an odd, specifically non-harmonic spectrum with panel resonances, but with air column resonances it is even 1 2 3 ... .

Reaction forces drive a rocking mode of the enclosure in total.

Sound radiation is a consequence of de- and acceleration (constant speed motion will not make any sound!). But sound interferes with itself, partly destructively. Phase panel (a) versus panel (b)? Why? Again, break-up modes, phase versus position on panel?

And so forth.

Thank You!
 
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Thomas_A

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Disclaimer: don't let it be a matter of luck. But if so, all the best for Your health!

On topic: Please do not fall for the wisdom of the past. Even Big Science has to admit that it wasn't quite right once in a while. And they are proud of it! How could that be different with speaker enclosure design?!

So, excitation of "resonances" by what? Is it the reaction force, namely Newton's conservation of momentum of the cone that drives the motion? Or is it the enclosed air column which may or may not get into resonance?

It could be, and I found that out independently from others, that the air column drives the vast majority of the peaks. With constraint layer You counteract another resonance than that of the panel itself ;-) You do so quite unspecific, hence overly costly. Better to dampen the internal air a bit more; just try.

Simple test: knock it and measure frequency of peak, compare what excitation by driver says. Difference? Most likely. Explanation? Measure the pathlengths involved ... dampen air and measure again. Further: what about break-up modes? I mean it. Are there break-up modes, as expected if the panel would resonate in itself?! No. Spectral analysis would show an odd, specifically non-harmonic spectrum with panel resonances, but with air column resonances it is even 1 2 3 ... .

Reaction forces drive a rocking mode of the enclosure in total.

Sound radiation is a consequence of de- and acceleration (constant speed motion will not make any sound!). But sound interferes with itself, partly destructively. Phase panel (a) versus panel (b)? Why? Again, break-up modes, phase versus position on panel?

And so forth.

Thank You!
Thanks,

Just a note about the damping material within the cabinet, yes it is a factor but... I did try with various amounts in the past but no luck reducing the main panel vibrations. The interesting question (to me) is if there are non-linearities due to the transition of the driver to cabinet and if fastening techniques (glueing, number of bolts, torque) has anything to do with this (there is another thread about this regarding Purify drivers).
 

fineMen

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Thanks,

Just a note about the damping material within the cabinet, yes it is a factor but... I did try with various amounts in the past but no luck reducing the main panel vibrations. The interesting question (to me) is if there are non-linearities due to the transition of the driver to cabinet and if fastening techniques (glueing, number of bolts, torque) has anything to do with this (there is another thread about this regarding Purify drivers).
Ja, rattling is a prominent source of distortion. It makes itself visible by raising all HD components at the same spot in the frequency range. See the thread You mention. Otherwise I would not expect relevant contributions from the fastening.

Today I build my enclosures from 12mm MDF. Even bigger ones with some strategic bracing as to make it as stiff as--reasonable. No issues, really. Feels and looks flimsy, but it works perfectly. Damping with more or less material by the way is not the thing. It is the effectiveness ;-) Once internal air resonances are gone, panel movement is gone likewise. There must be a connection ;-)
 
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Ja, rattling is a prominent source of distortion. It makes itself visible by raising all HD components at the same spot in the frequency range. See the thread You mention. Otherwise I would not expect relevant contributions from the fastening.

Today I build my enclosures from 12mm MDF. Even bigger ones with some strategic bracing as to make it as stiff as--reasonable. No issues, really. Feels and looks flimsy, but it works perfectly. Damping with more or less material by the way is not the thing. It is the effectiveness ;-) Once internal air resonances are gone, panel movement is gone likewise. There must be a connection ;-)
Just a note about distortion; quite many speakers show a peculiar distortion pattern where distortion naturally is highest in the bass, and drops fast to 100-200 Hz and then increases again in a second peak, commonly 400-800 Hz.

E.g.


While a few ones does not show this, e.g.


Do you think this is due to internal air resonance only?
 

fineMen

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Just a note about distortion; quite many speakers show a peculiar distortion pattern ....

Do you think this is due to internal air resonance only?
Nope, the internal air resonances are quite linear, no non-linear distortion from that. With my 6" and even 12" I identified the surround as the culprit. JBL at times doesn't get it right. Anyway, generalizations are not readily at hand. There is *nothing* an amateur could do, regardless of the effort, to target the issues without actually taking relevant measurements.
 

Tangband

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Well I dont agree. We are not as much sensitive in lower frequencies as in higher within the 100-1000 Hz region where these distortions occur. Drivers have more distortion in bass normally so lowering frequency will cause masking from the driver distortion. As I said the distortion around 500-700 Hz was audible but those around 250 Hz was not.
You are unfortunately wrong. John Atkinson just explained to you ( in the article ) how music tones work . He is right about that. The conlusion of this fact is : its much better to have the resonanses at a higher frequency with a higher Q, than to have them at lower frequencies where the disturbance of real music tones are much higher.

There is much confusion in the audio world about how music tones work, even engineers get it wrong sometimes.

John Atkinson wrote:

”The question is begged as to how important this will be subjectively with normal musical material, not test tones or pink noise. This high-Q resonance will need to be hit with sustained tones lasting for a good few milliseconds before it is fully excited. 360Hz is midway between the notes F and F# above Middle C in the second space of the treble staff (frequencies 349.23Hz and 370.0Hz, respectively); it is possible, therefore, that this resonant mode might add a degree of clouding on just these notes, which a very sensitive listener would pick up. If it were lower in frequency, however, where the notes are closer together, it would probably be excited more often, which is why resonances in the tenor region—approximately 120-240Hz—often add a "wooden" coloration to a speaker's sound.”
 
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Thomas_A

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Nope, the internal air resonances are quite linear, no non-linear distortion from that. With my 6" and even 12" I identified the surround as the culprit. JBL at times doesn't get it right. Anyway, generalizations are not readily at hand. There is *nothing* an amateur could do, regardless of the effort, to target the issues without actually taking relevant measurements.
So non-linear distortion could be due to other reasons including


?
 
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You are unfortunately wrong. John Atkinson just explained to you ( in the article ) how music tones work . He is right about that. The conlusion of this fact is : its much better to have the resonanses at a higher frequency with a higher Q, than to have them at lower frequencies where the disturbance of real music tones are much higher.

There is much confusion in the audio world about how music tones work, even engineers get it wrong sometimes.

John Atkinson wrote:

”The question is begged as to how important this will be subjectively with normal musical material, not test tones or pink noise. This high-Q resonance will need to be hit with sustained tones lasting for a good few milliseconds before it is fully excited. 360Hz is midway between the notes F and F# above Middle C in the second space of the treble staff (frequencies 349.23Hz and 370.0Hz, respectively); it is possible, therefore, that this resonant mode might add a degree of clouding on just these notes, which a very sensitive listener would pick up. If it were lower in frequency, however, where the notes are closer together, it would probably be excited more often, which is why resonances in the tenor region—approximately 120-240Hz—often add a "wooden" coloration to a speaker's sound.”
Well it is related to JNDs, order of overtone and masking, as always. And where in the frequency spectrum our ears are most sensitive. It is not in the 200 Hz region.
 

Tangband

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Well it is related to JNDs, order of overtone and masking, as always. And where in the frequency spectrum our ears are most sensitive. It is not in the 200 Hz region.
This link are the important frequencies in the tones that music are made of. This has nothing to do with pink noise or sine sweeps. The human ear has no problem at all to hear a distinct G3 at 196 Hz.


If the resonances in the cabinet hits one of this specific tone frequencies, it gonna be delayed and the pitch will be slightly altered/ changed in the total sound from the speaker. The result will be that the perceived pitch tonality that you hear will be slightly compromised, and the distinct pitch of the tone will be slightly blurred. Here we can see that the problems with disturbed pitch because of resonances are potentially worse in the lower frequencies than in higher , because of how the tones work.
 
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