Acoustic damping material behind wall-mounted surround speakers?

[bjornb17]

I have noticed in some movies and concert DVDs that there was this annoying high-bass tone that was audible from certain spots in my room. I'd say it sounded like it was in the 150-200hz region. I thought my subwoofer was giving off some weird reflections from the room.

It turns out this noise is coming from my wall-mounted Polk Monitor 30s. They are rear-ported, and the ports are each about an ince or so off of the wall. I discovered that this noise is in fact some sort of "boom" coming off of the close proximity to the wall. Taking the satellites off of the wall eliminated this completely.

I was wondering if i should buy a couple small squares of acoustic damping material to put on the wall right behind the ports of the speakers to get rid of this annoying high-bass spike.

Other than that, things are sounding good :D

Thanks for any help.

[kexodusc]

Do you have them set to "small"? What's the crossover at?
Sounds like port noise to me. Damping material isn't going to elimanate it, and you don't really want to absorb sound around your surround speakers.

[bjornb17]

Do you have them set to "small"? What's the crossover at?
Sounds like port noise to me. Damping material isn't going to elimanate it, and you don't really want to absorb sound around your surround speakers.

Yes they are set to small with crossover at 80. Its not the typical port chuff, but it seems to related to how you can get weird bass spikes and dips by having speakers too close to the walls. I think the sound coming out of the port is having some weird intereactions with the wall.

[kexodusc]

Damping material might not be out of the question then...you could try taping some polyfil behind your speakers for a quick, inexpensive experiment. A few inces deep.

[Peter Duminy]

Although a little pricey, you might want to give this material a try from GR Research:

http://www.gr-research.com/components/blackhole_5.htm

Quite impressive.

[bacchanal]

Well the first thing to try would probably be the old "sock in the port" trick.

[kexodusc]

Uhh, sock in the port isn't always advisable...some speakers need the protection at lower frequencies that a port gives as the driver is unloaded.

[hermanv]

An aside. The best accoustic damping material seems to be wool felt (undyed is best). 1" or thicker wool felt costs more than some hifi components. I found a carpet underlayment made of 65% wool at the "natural fibers" store it was (I think) $9/square yard and solved many reflection problems in my home. Not all that pretty, basically a dark brownish/red color but for hidden places worked great.

Natural felt fibers have ever so tiny scales, these catch and release when vibrated to convert motion into heat and thereby absord sound rather than reflecting it.

:D :D :D Just think, if you use felt and play your system loud enough, you can cut down on your heating bills :D :D :D

[kexodusc]

Best damping materials I've seen are rigid (compressed) fiberglass or mineral wools. Problem is they're quite impractical in a living room, especially if you're married.

[dmb_fan]

Uhh, sock in the port isn't always advisable...some speakers need the protection at lower frequencies that a port gives as the driver is unloaded.

I can't imagine what protection a port can offer a driver. Please explain? I always thought the opposite was true: in a sealed speaker, the cushion of air inside the speaker stops the driver from overexcursion. In a ported design, the driver is more at risk of overexcursion. But I'm wrong all the time... just ask my wife. :)

Anyhow, I'm just trying to understand what you mean.

Thanks!
-Adam

[kexodusc]

I can't imagine what protection a port can offer a driver. Please explain? I always thought the opposite was true: in a sealed speaker, the cushion of air inside the speaker stops the driver from overexcursion. In a ported design, the driver is more at risk of overexcursion. But I'm wrong all the time... just ask my wife. :)

Anyhow, I'm just trying to understand what you mean.

Thanks!
-Adam

It is true that the cushion of air in a sealed speaker acts as a resisting force to the BACKWARD movement of the driver. This helps improve the transient resonse of the speaker. But excursion is really dealing with the forward movement of the driver.

The short story is that in a ported speaker, as you approach the tuning frequency of the enclosure, the mass of air inside the cabinet actually steals the energy from the driver as it resonates...in a perfect system, the woofer doesn't actually move at all, the air is doing 100% of the work at the tuning frequency. In practice, it's not that extreme, but the woofer's movement (excursion) is at a minimum, and most of the work is done by the air inside the enclosure. So we say the woofer is "protected"..the air is providing opposing forces to the drivers movement, while creating the bulk of the sound you hear. This wears off the further away from resonance you go, so the woofer will go back to excursion maximums below the tuning frequency.

If you followed the excursion curve on a graph, where excursion in mm was the Y axis, and frequency in Hz the X axis, you'd see a large dip in excursion at the tuning frequency of the ported speaker, followed by a fast rise again...it's sort of "U" shaped. In the sealed enclosure, there's no dip, but instead a constant increase in excursion as the frequency gets lower.

[Peter Duminy]

This same theory is shared with Passive Radiator designs also, as Celestion did with their Ditton series using their ABR (Auxilliary Bass Radiator) as well as some KEF & B&W designs to name just a few. One could watch the main woofer with a strobe light almost become stationary as the Passive Radiator tuned to lower frequency took over the workload. Starting to make comeback again especially in subwoofers. :)

[bjornb17]

It's neat how that type of stuff works