Speaker Back And Forth Motion Damping

[bhobba]

Hi Guys

I usually post in the Quantum Physics section because that in my interest these days.

I have recently been in a rather heated discussion about an interesting device a speaker designer I know (Mike Lenehan) recently bought to market:
http://www.eti-research.com.au/index.php/our-products/resonance-control-devices/eti-amg-toppers

What it is designed to do is as the cones of a speaker go back and forth and the cabinet also goes back and forth in response and to damp it - it stretched some rubber the weight sits on that exerts a countering force and dissipates some of the energy in doing so. That's the simplistic explanation the designer told me and it sounded OK.

What I can say is it works on most speakers I, and others, have tried it on (not all but most - only 2 out of the 20-30 failed) - you get a better stereo localisation of sound exactly as you would expect from a reduction in the blurring caused by driver motion - the sound stage seems to just snap into place. And at least one of those was in a blind test.

However one guy on a Hi Fi forum I frequent thinks its all hooey. He claims its like a Helmholtz resonator and could not possibly work. I have a bit of an understanding of Helmholtz resonators and it didn't seem to be like that - it wasn't absorbing a well defined frequency but rather damping a broad range. I explained in all sorts of different ways what was explained to me but he is adamant - it can't work - and was quite uncomplimentary to what he thought was a total lack of understanding of the physics. It is a resonance device and can't do anything outside resonance. I always thought there was something called Q that if small meant it was more broadly based than a particular frequency but he thinks no - it can't work like that. I chatted to the designer and mentioned about the Helmholtz resonator and his eyes basically bulged - it is not a Helmholtz resonator. But the guy says he is a resonance engineer and gave me the following link he claims proves it can't work - it must be individually tuned to each speaker:
http://eprints.soton.ac.uk/45668/1/Pub9121.pdf

I have a background in applied math but that paper was beyond me in a casual reading.

The other issue is for some reason he seems to think if it actually did work as claimed there is no way it could possibly affect the distortion of a speakers tweeter. Now that really left me scratching my head - there is no claim about it affecting distortion - simply a blurring of the sound due to a fuzziness introduced by the back and forth motion of the speaker.

This has me beat - what do you guys think - does the resonance expert have a valid point - is it all hooey?

Thanks
Bill

 

[davidm1732]

My initial thought after seeing the print add for it was that it was a tuned mass damper type of device. Then I read the link to the ISVR paper, and that also provides more detail on that type of device. I don't think it's a helmholtz resonator... that's an acoustic device consisting of a neck and acoustic volume to provide acoustic attenuatian at a specific frequency. I do agree with the audio guy that a tuned mass damper should ideally be designed for a specific cabinet. You'd have to know the structural resonance frequency of the cabinet top to match the tuned frequency of this device if it really works that way. The print ad talks about using concrete pavers to mass load the cabinet, which would drop the structural resonant frequency of the cabinet top to a much lower value. If the cabinet has structural resonant modes in the audio frequency range, then I suppose it could affect the speaker performance... any wall compliance will affect the performance of the air spring that the cabinet creates behind the speaker cone. Adding mass and damping to the cabinet will theoretically change that response, so it's possible you can hear the results on any cabinet...or alternatively, if you know most cabinets are of a similar size, and use similar construction material, perhaps you can get the tuning of this thing close enough so it does work pretty well on most of the stuff out there?

Look for more papers on tuned mass dampers, or multiple vs single degree of freedom spring mass systems.

 

[AlephZero]

Reducing speaker cabinet vibration will reduce distortion. That much is sort of obvious.

So the question is whether this device does reduce cabinet vibration in a meaningful way. For good quality speakers, the answer should be "no, because there is no significant vibration to reduce". On the other hand, people buy good quality speakers and then mount then on flimsy speaker stands, or stand them on flexible wood floors, etc, so it might help it situations like that.

I would have thought it the design would only help damping out a "global" resonance of the entire speaker cabinet (caused by the way it is mounted, as with the above problems). In a typical "box" cabinet, each face of the box (expecially the front, which is the most flexible because it has some big holes cut in it!) will have its own set of resonances, and putting a damper on the top would only affect one set out of the six.

 

[Studiot]

bhobba
What I can say is it works on most speakers I, and others, have tried it on (not all but most - only 2 out of the 20-30 failed) - you get a better stereo localisation of sound exactly as you would expect from a reduction in the blurring caused by driver motion - the sound stage seems to just snap into place. And at least one of those was in a blind test.

It is interesting to hear you say this because all the spatial information is in the mid and upper registers of the sound. Human perception is pretty insensitive to any locational information provided by true bass frequencies.

 

[bhobba]

Hi Guys

Thanks for your replys.

Yea I don't think its a Helmholtz resonator either. I personally think its a tuned mass damper but with a Q chosen by ear to work the best subjectively.

Regarding cabinet resonance the guy that designed them builds his speakers lined with steel or even 1/4 inch thick copper plate to combat resonances and having heard them I can say going to such lengths is clearly audible.

I don't think we are getting any localisation information at such low frequencies - what I think its doing is the back and forth motion of the speaker as a whole blurs the localisation information at the upper and mid frequencies.

I had a friend over yesterday and we did some experimenting with it - it really is strange - subjectively it just seems to lock the sound stage.

Thanks
Bill