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Old 8th July 2013, 10:43 AM   #1
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Default Negative bulk modulus: new principle for woofer design

I just read about a new principle that seems counter-intuitive:

Redesigned Window Stops Sound But Not Air, Say Materials Scientists | MIT Technology Review

With that material as a port/window on a small sealed box, you could mount a woofer in a small box that didn't raise the resonance of the driver (because the box isn't really sealed) but no anti-phase sound comes out of the port (because that new stuff absorbs a lot of the sound on its way through the window).

Whether or not that makes any sense even speculatively, a really interesting concept.

You read it here first!

Ben
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Old 8th July 2013, 07:28 PM   #2
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Quote:
Originally Posted by bentoronto View Post
I just read about a new principle that seems counter-intuitive:
Redesigned Window Stops Sound But Not Air, Say Materials Scientists | MIT Technology Review

With that material as a port/window on a small sealed box...

You read it here first!

Ben
Ben,

The "soundproof" window passes air and low frequency sound and attenuates frequencies in the multiple Helmholtz resonators.
I don't see new principles or materials, but plenty of bad translation .

As Sang-Hoon Kim mentions, the attenuated frequency range can be adjusted by changing the size of the "bird hole", AKA port.
"The separation between sound and medium has been successfully demonstrated. The key is the Helmholtz resonators.
It consists of a three-dimensional array of strong diffraction-type resonators with many holes centered at each individual resonator.
With the help of a modified Helmholtz resonator, we designed a soundproof window or wall where air is allowed to flow freely within some specific frequency ranges.
"

The "window" contains a large number of ports.

I fail to see any use of the "soundproof window" in the construction of a speaker enclosure, unless you might be implying that the tuned diffraction-type resonators might be helpful in reducing air turbulence port noise, which has not been tested, as the test level was only 80 dB, and no "bird hole" air velocity was indicated.

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Old 9th July 2013, 04:47 PM   #3
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It shows from the linked pic that a transmission loss of 20~30dB in the frequencies of 600 Hz to 2K3Hz is present, what might be related to the size/dimensions and construction of the "sound absorbing panels".
At those frequencies I don't see any innovation in woofer design but in the design of soundproofing materials for auditoriums, studios, etc.

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Old 9th July 2013, 05:28 PM   #4
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Geez, they're such fools over at MIT. They should have posted here first to find the right way to look at this old invention.

For enthusiasts of sealed boxes, there have been various compromise solutions to the problem of resonance rising when the box gets smaller. Small holes is one. Another approach is to have a lot of the rear panel open but stuffing the box with stuffing (I did this with a Bozak speaker... seems like yesterday).

But I wonder if there is some way to use this concept (new, old, re-invention, or whatever) to address that challenge with a "semi-sealed" box. It would be a great advance.

From the point of view of the interior of such a semi-sealed box, the port would be transparent to sound outside its passband (which is what I think the authors mean by "wind" passing through the window or in our terms, DC or static condition in the speaker box). But would the window turn acoustically "solid" at the system resonance - thereby confusing or counter-acting the very reason for using it?

If you know how active shock absorbers work in cars, you might wonder about creating an active port analogously.

Ben
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Old 10th July 2013, 01:17 AM   #5
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Quote:
Originally Posted by bentoronto View Post
... From the point of view of the interior of such a semi-sealed box, the port would be transparent to sound outside its passband (which is what I think the authors mean by "wind" passing through the window or in our terms, DC or static condition in the speaker box). But would the window turn acoustically "solid" at the system resonance - thereby confusing or counter-acting the very reason for using it? ...
You would need a rather special window:
- "solid" at frequencies below the driver resonance, to prevent over-excursion.
- "transparent" at frequencies around the driver resonance.
- "solid" at higher frequencies, to prevent leakage of upper bass / lower mids.

You would still have the problem that the leaked sound would be out of phase with the sound from the front of the driver, tending to cancel it, right at the frequencies where you need all the sound pressure you can get...
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Old 10th July 2013, 12:08 PM   #6
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Thanks for analysis. Might well be that my dreaming was wrong: if you want an "unsealed" box, you end up with a port. Can't "unseal" unless you are venting!

What you describe is, of course, a filter with a passband. I suppose you could find some application that uses a filter of that sort made by familiar techniques today (although at this hour none comes immediately to my little mind). But the new/old invention might make it all the more feasible (just as microscopic coatings on glass are doing that with light now).

But what if the filter behavior was different (or even the opposite) of what you describe? True, you lose the protection from over-excursion a sealed box provides, but you gain sound output.

Ben
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Old 11th July 2013, 02:47 AM   #7
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... but then you are back to the problem of a too-small box and its effects on frequency response.
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