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How shape/material of speaker footers impact their effectivness

Don Hills

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... Townshend's idea is a bit different than other people. They contend the speaker sends out bass notes, which come back to a speaker via the floor causing movement. Enough movement to equal the movement of tweeters which can smear or blur sound. So decoupling is to prevent this from occurring. They have this video to show what they have in mind. Also in some cases vibration from other sources can interfere with the speaker providing sound cleanly. My question would be if the amount of movement is enough to really make an audible difference. ...
It's not enough to assume that the baffle (and therefore the tweeter) moves back and forth a similar amount to the movement of the tweeter diaphragm. You also need to take into account the frequency of the movement. In theory, it would impart a doppler effect on the tweeter output. In practice, it's simply inaudible. For proof, look at any fullrange driver, where the problem of bass moving the treble source back and forth is hundreds of times worse. If it was that audible, fullrange drivers would sound awful. Or any coaxial driver, where the cone forms a waveguide for the tweeter.
 

March Audio

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It's not enough to assume that the baffle (and therefore the tweeter) moves back and forth a similar amount to the movement of the tweeter diaphragm. You also need to take into account the frequency of the movement.

In theory, it would impart a doppler effect on the tweeter output.

In practice, it's simply inaudible. For proof, look at any fullrange driver, where the problem of bass moving the treble source back and forth is hundreds of times worse. If it was that audible, fullrange drivers would sound awful. Or any coaxial driver, where the cone forms a waveguide for the tweeter.
Yes, you can see sidebands on tweeter output, which was the next test I was going to do.
 
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March Audio

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Acoustic measurement with 100Hz and 5kHz

20170427_131245.jpg


Noise floor (Fs not ref 20uP)

acoustic noise floor.png


100Hz Bolted Mount
100hz bolted.png



5kHz Bolted
5khz bolted.png



100Hz + 5kHz bolted
dual bolted.png



Zoomed - 100Hz sidebands
dual bolted zoom.png



100Hz Sorbothane feet
100hz sorb.png


5kHz Sorbothane
5khz sorb.png


100Hz+ 5kHz sorbothane
dual sorb.png


zoomed sidebands
dual sorb zoom.png


This should demonstrate (along with the previous post) that the baffle is vibrating pretty much the same regardless of how the speaker is fixed to the stand.

The sidebands on the acoustic measurement demonstrate Dons assertion that a Doppler effect is happening and the tweeter is moving at 100Hz caused by the woofer.
 
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RayDunzl

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Have you considered using different test frequencies so the products don't overlap the noises in the noise floor and stand out on their own?
 

March Audio

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Have you considered using different test frequencies so the products don't overlap the noises in the noise floor and stand out on their own?
I have but I am limited for time, the real purpose was to show the sidebands on the tweeter indicating its vibrating whatever the speaker mount is doing.
 

March Audio

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How much are you playing when testing this? A single period of the frequencies?

I'd probably tested what happens with a sinc if I could.
Its a 65536 point FFT linear averaged - you can see the avg number on the plots, so its measuring for some time.
 
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Blumlein 88

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Perhaps Nightlord is asking for the equivalent of a Dirac pulse. A good hammer blow to the speaker should do it. :) Oh, no please don't do that.

Yes, it does appear doppler distortion is alive and well. Such effects seem likely to overwhelm the jitter sidebands at picosecond levels everyone worries so much about.

I have an old DIY speaker building handbook from the 1960's, and they spent half the book going on about Doppler / FM distortion of the tweeter.
 

March Audio

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Doesn't say what you played for that, so I assumed it to be by hand.
I literally gave it a wack with a soft mallet. The impact is like a stimulus at all frequencies.

Do this a lot in the vibration testing I perform in my job.

The second graph was the speaker playing pink noise, again a wideband stimulus. Thiswas fairly consistent with the bump test


One thing I wondered about, at least some of this modulation must be due to the acoustic pressure on the tweeter diaphragm and not just direct vibration???
 
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Nightlord

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I think that a single (or half) period of a sine - and progressing that down in frequency to find when you pass below resonance of the system would be interesting. That would work continuous as well, but if we want to see the difference in decay time, it's better if it's limited.

The feet I use will put the resonance point at 7Hz or lower... so if I would do it, I'd start at say 20Hz and go down 1Hz at a time and see what happens when you pass it. How you find at what point you should do the same for a spike based setup is (for me) more difficult, I have no idea how much higher that is. For that the dirac or whack might give the answer, and having found the resonance point for the soft feet, there might be a clue in whack/dirac graph for the soft feet once you know what to look for?
 

Sal1950

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Thought I'd pass along a Cheapskate tip. While gathering items to protect my new wood floor from furniture, etc; some time back I came across these at Lowe's that are perfect for going under spikes. I've got them under both my subwoofers. At $4.97 for a package of 8, they're a whole lot cheaper than the fancy-dans being sold by the audiophool retailers. ;)
https://www.lowes.com/pd/Waxman-8-Pack-Brown-Round-Felt-Pads/3027868
 

Blumlein 88

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Thought I'd pass along a Cheapskate tip. While gathering items to protect my new wood floor from furniture, etc; some time back I came across these at Lowe's that are perfect for going under spikes. I've got them under both my subwoofers. At $4.97 for a package of 8, they're a whole lot cheaper than the fancy-dans being sold by the audiophool retailers. ;)
https://www.lowes.com/pd/Waxman-8-Pack-Brown-Round-Felt-Pads/3027868
Dammit Sal! I was planning on selling those rebadged as Blumlein soft points. One way diode action in a soft floor safe finish and constrained mode damping decoupling disc in one. $497 for 8. I know, I know compared to some items a 100x mark up isn't much. Doesn't matter now. You ruined it. There goes my chance to pocket a bundle and move to Florida.

:p

I'll have to go with my hocky puck feet now. Those are such stiff rubber though you need 1000 lbs of speaker for them to be effective. I use them at jacking points on my sports cars.
 

March Audio

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I think that a single (or half) period of a sine - and progressing that down in frequency to find when you pass below resonance of the system would be interesting. That would work continuous as well, but if we want to see the difference in decay time, it's better if it's limited.

The feet I use will put the resonance point at 7Hz or lower... so if I would do it, I'd start at say 20Hz and go down 1Hz at a time and see what happens when you pass it. How you find at what point you should do the same for a spike based setup is (for me) more difficult, I have no idea how much higher that is. For that the dirac or whack might give the answer, and having found the resonance point for the soft feet, there might be a clue in whack/dirac graph for the soft feet once you know what to look for?
As shown the feet arent going to stop the baffle vibrating so we are looking at a different issue here, that of vibration transfercinto the floor.

I will do some further measurements on the stand itself to show the difference between bolting and sorbothane.
 

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