ISO: Back Wave Attenuation

[Ed LaFontaine]

Before I start cuttting material for my next speakers, I'd like to learn more about reducing the reflection of driver backwaves inside a cabinet.
I can imagine the use of internal baffles angled to capture and direct those reflections away from the back of the driver cone.
I can imagine the benefit of felt on those baffles and adjoining surfaces.
I can imagine the benefit of constrained layer dampening of internal surfaces.

I solicit your approach to these ends. I would like to gain a sense of values so a balanced use of materials results in a most effective effort.

[Svante]

The key is absorbtion. Tilted walls, internal reflectors an whatever will only change the frequencies of the standing waves inside the box to something that is harder to predict. It will not remove the resonances.

So, fiberglass or some other type of absorbant is what you need.

[Ed LaFontaine]

Svante, Thanks for your reply.
I didn't mention the project I have in mind will be a TL. I'll be using stuffing in the range of 1 pound / cubic foot.

In addition to angled baffles to minimize higher frequency standing waves, some other things I'm considering include:

F-11 or F-13 felt, probably 3/8" or 1/2" thick.
Foam/Vinyl sound absorbing sheets
Vinyl composition tile (VCT) in irregular shaped pieces glued with mastic or caulk to the case sides.

It is balancing or improving on these options that I'm interested in.

[hughmon]

Might be old news around here, but check out Vandersteen's cabinet construction techniques. It makes for some really rigid walls, and good absorption within the cabinet.

http://www.soundstagelive.com/factorytours/vandersteen/



[quote]"In this close-up shot (right) you can see how each MDF lamination in the Model 5 head has a different shape inside. You are looking at the midrange (upper) and woofer (lower) transmission model5_transmission_lines.jpg (15114 bytes)lines. The different-shaped holes in the layers of MDF that makes up the head create a labyrinth transmission line which causes the back wave to reflect many more times that it would if the transmission line were smooth. Each reflection sends the back wave through the damping material again; the more times the back-wave energy is reflected though the damping material, the more energy it loses. The objective is to get as close to zero back-wave energy reflecting back onto the speaker cone as possible. Any energy which does reach the back of the speaker cone results in 100% distortion, something that should be avoided at all costs. The transmission lines intentionally do not follow straight lines. This improves their effectiveness for damping the back wave. The midrange transmission line curves upwards at the rear using otherwise unused space behind the tweeter's shallow transmission line. This further lengthens the transmission line -- longer is better for damping the back wave. The woofer transmission line goes up and down and changes cross-section area a couple of times before it too rises at the back."

"In this close-up shot (right) you can see how each MDF lamination in the Model 5 head has a different shape inside. You are looking at the midrange (upper) and woofer (lower) transmission model5_transmission_lines.jpg (15114 bytes)lines. The different-shaped holes in the layers of MDF that makes up the head create a labyrinth transmission line which causes the back wave to reflect many more times that it would if the transmission line were smooth. Each reflection sends the back wave through the damping material again; the more times the back-wave energy is reflected though the damping material, the more energy it loses. The objective is to get as close to zero back-wave energy reflecting back onto the speaker cone as possible. Any energy which does reach the back of the speaker cone results in 100% distortion, something that should be avoided at all costs. The transmission lines intentionally do not follow straight lines. This improves their effectiveness for damping the back wave. The midrange transmission line curves upwards at the rear using otherwise unused space behind the tweeter's shallow transmission line. This further lengthens the transmission line -- longer is better for damping the back wave. The woofer transmission line goes up and down and changes cross-section area a couple of times before it too rises at the back."

[Ed LaFontaine]

Any energy which does reach the back of the speaker cone results in 100% distortion, something that should be avoided at all costs.

THAT'S just what I'm trying to work out!

Thanks, hughmon

[ShinOBIWAN]

B&W has a novel way of tackling the problem.

They effectively use horns with the driver mounted at the mouth. Kinda like an inverse horn.

Along the horn section there's a whole lot of divders with holes drilled into them. I'm sure there's some very complicated math required to get these working optimally. But the aim of these is to strip phase information and further attenuate the rear wave before it exits the port to the rear.

I've heard this in action with the N802 and its quite remarkable just how little sound actually exits the inverse horn. Its virtually nothing at decent SPL's.