Intern Helmholtz Absorber / bassrefleksport

[Audiodidakt]

Hi

Does anybody know whether the internal Helmholtsz absorber Tony Gee uses in his newest design to prevent / lower the effect of vertical standing waves has significant effect?

Humble Homemade Hifi

Best regards

Gisle

[speaker dave]

Reminds me of Helmholtz resonators that RCA put in console radios in the 30's. It should work if you get the tuning correct and the damping within it correct.

But really, going to these lengths to try and remove all the other damping within a box? To what end? Very misguided.

David S.

[Audiodidakt]

Originally Posted by speaker dave

Reminds me of Helmholtz resonators that RCA put in console radios in the 30's. It should work if you get the tuning correct and the damping within it correct.

But really, going to these lengths to try and remove all the other damping within a box? To what end? Very misguided.

David S.

Well, I am no expert on this but;

a. Some dislike damping material due to "effect" it can have on for instance bass sound ("dull"). I have no experience here.

b. I would believe that damping a third order 83 Hz standing wave with damping material would mean implementing a lot (more than the enclosure can hold) of damping material.

As I said, I have very little knowledge about these things, hence this thread :-)

[simon dart]

I believe the french loudspeaker manufacturer Elipson patented this idea for their groovy grp spherical enclosures. Its a good solution to supress that one killer mode you get in a sphere. Not sure whether it still applies, but their flagship 4260 info claims so.

[Audiodidakt]

Thanx! :-)

There must be an effect out of this, and it is easy to implement in the enclosure. I will use such a device if I go on making enclosure.

Br

Gisle

[jamikl]

I don't think this is far off topic but after reading this I wondered if the sides of the rear radiation part of an H Baffle or U baffle could be built as absorbers
for the resonance caused by the baffle depth? Would they be large enough in area?
jamikl

[speaker dave]

Originally Posted by jamikl

I don't think this is far off topic but after reading this I wondered if the sides of the rear radiation part of an H Baffle or U baffle could be built as absorbers
for the resonance caused by the baffle depth? Would they be large enough in area?
jamikl

Do a google serch for Boston Acoustics Lynnfield tweeter resonance absorber. They put little tube resonators in front of a 1" dome tweeter to knock down one of the dome resonances. (Not sure it worked better than the typical "phase plug" but it was better for the marketing department.)

If it works for a tweeter it should work for a woofer.

Back to the initial concept, my understanding of resonators as absorbers is that they magnify the effectiveness of absorptive material at resonance. In other words you must have the absorption for it to work. Since the designers starting premise is that damping is bad....

Note also that the cabinet as drawn will still have side to side resonances not treated by the back side diffusor (symmetry will kill some of them).

Fiberglass is cheap and it does an awfully nice job of absorbing a wide range of frequencies. There is no technical basis for thinking it impairs any quality of the bass (perhaps a spiritual one?).

David s.

[AndrewJ]

Dave, not in total agreement with you here!
In tall enclosures it is often difficult to absorb the standing waves sufficiently without killing the box Q and hence the effectiveness of the reflex loading.
I try to place semi-rigid absorbing barriers at the nodal points of the standing wave, but it still hampers the box Q.
The best approach is two fold: Dual woofers placed either side of the resonance mode nodal points will effectively nullify the excitation, along with careful positioning of the vent so it doesn't couple to the mode.
In addition, we have found a quarter wave tube with appropriate damping applied works extremely effectively to eliminate the next higher order in the cabinet. As you say, it magnifies the effect of the damping, but ONLY at the reonance frequency of the tube, so it kills the vertical mode in the enclosure without having much effect on the box Q

Also, one has to be careful with the kind of stuffing used in cabinets. Remenber the early experiments on impulse testing vs sine wave testing. The results did not track at all signal levels. The cause was movement of the damping causing added mass effects, but only at the higher sound levels. At low levels there was not sufficient energy to overcome static "stiction" type effects in the material.
Hence, damping can be non-linear and therefore could conceivably impair bass quality if not correctly applied.

Andrew

[speaker dave]

Originally Posted by AndrewJ

Dave, not in total agreement with you here!
In tall enclosures it is often difficult to absorb the standing waves sufficiently without killing the box Q and hence the effectiveness of the reflex loading.
I try to place semi-rigid absorbing barriers at the nodal points of the standing wave, but it still hampers the box Q.

This is a bit as being discussed in the "unexpected resonances" thread. Yes, tall cabinets, especially with a woofer near the end create a difficult to damp primary resonance. I've had success with multiple layers of thin damping with middle of the box material having the most effect. You may have to compromise box Q to kill resonance but a good compromise is usually available.

The best approach is two fold: Dual woofers placed either side of the resonance mode nodal points will effectively nullify the excitation, along with careful positioning of the vent so it doesn't couple to the mode.

Agreed. The same technique some use for room standing waves.

In addition, we have found a quarter wave tube with appropriate damping applied works extremely effectively to eliminate the next higher order in the cabinet. As you say, it magnifies the effect of the damping, but ONLY at the reonance frequency of the tube, so it kills the vertical mode in the enclosure without having much effect on the box Q

This sounds interesting. I'd love to see curves on how well it works. I'm not surprised it works but I haven't tried it.

Also, one has to be careful with the kind of stuffing used in cabinets. Remenber the early experiments on impulse testing vs sine wave testing. The results did not track at all signal levels. The cause was movement of the damping causing added mass effects, but only at the higher sound levels. At low levels there was not sufficient energy to overcome static "stiction" type effects in the material.
Hence, damping can be non-linear and therefore could conceivably impair bass quality if not correctly applied.

I'm aware of this but I'm not sure it isn't more of a technical curiosity than a real problem. It sounds like the solution is damping material that isn allowed to "flap around".

The real underlying issue is: "is damping so undesireable that it must be replaced with complex alternative fixes?" I consider this a typical audiophile approach where a notion is taken to an extreme and conventional wisdom is thrown out the window. The test for all cabinets and internal damping arrangements is to put a microphone in a corner of the inside of the box and take a sweep. Lets see how well a stuffingless box with all the tricks does compared to a simple, properly dimensioned cabinet lined with fiberglass.

David S.