dealing with modal resonances in midrange transmission lines

[thadman]

I feel modal resonances in midrange enclosures are significant enough to warrant thorough examination as to alternatives to a traditional sealed midrange, but am currently not interested in a dipole or cardiod dispersion pattern. For a traditional sealed enclosure, the largest internal dimension would have to be 1.67" to place the first modal resonance one octave past a 2khz crossover point. This is of course highly impractical.

This has got me interested in midrange transmission lines.

As long as the line is stuffed, the resonances along the length of the line should be attenuated significantly due to the high particle velocity at the open end of the tube...but what about the resonances that propagate along the radii?

Is there anyway to deal with the radial resonances other than building appropriately sized diffusors in the interior? Wouldn't the particle velocity be 0 at the walls and the pressure be at maximum, thus negating any absorptive effects of foam for radial modal resonances.

[Jed]

It would be interesting to see a few measurements of this to confirm.

[thadman]

anybody?

[Dennis H]

Build a big mid enclosure and stuff it -- no modes because they get absorbed by the stuffing.

[thadman]

Stuffing only works when particle velocity is high, in a closed box pressure will be maximum and particle velocity will be 0.

[Davey]

Particle velocity is zero in a closed box?

Dave.

[severian]

My understanding is that stuffing helps to dissipate heat caused by the increased pressure in the enclosure thus causing the pressure to decrease. PV=NRT (ideal gas?) This makes the enclosure seem like it is much bigger than it actually is. This should help to dampen any resonances to a certain extent. I hope this makes sense.

JC

[Paul W]

what about the resonances that propagate along the radii?

So I understand the question, do you mean across the diameter of a round pipe?

[thadman]

So I understand the question, do you mean across the diameter of a round pipe?

yes

[thadman]

Particle velocity is zero in a closed box?

Dave.

pressure is maximum and particle velocity is zero, thats why room treatments ie absorbing foam work best in the center of the room as opposed to on the walls. On the walls pressure is maximum and particle velocity is 0 whereas it is the opposite in the center of the room.

[Paul W]

Consider a cylinder of felt suspended in the center of the pipe by a light fiberglass stuffing (or whatever you are using in the T-line). Or, reshape the pipe into a truncated cone or triangular pyramid.

[Dennis H]

Stuffing only works when particle velocity is high, in a closed box pressure will be maximum and particle velocity will be 0.

Nice theory but the measured results of stuffing enclosures show otherwise.

[davey_m]

Nice theory but the measured results of stuffing enclosures show otherwise.

Dennis is correct I think. If you model stuffing as a pure flow resistance then the particle velocity theory holds. In practice stuffing at pressure maxima / velocity minima still dissipates energy due to deformation. It may not be optimal there, but it still works. I think medium density foam is a better energy dissipator at pressure maxima though. I use stuffing at velocity maxima, and foam at pressure maxima, a compound solution if you would like to call it that. But many people use this approach.

To experiment and test this theory you can connect your measurement mic to the enclosed volume. Use the same setup you would use to measure your driver response, but measure the transfer function to the enclosed volume instead of the driver - by exciting the driver as you normally do. You will see the standing waves this way - measure in various positions and compare.

[thadman]

I've considered using quadratic residue diffusors of appropriate size (5" depth) along the length of the line (sides and top, the woofer gets in the way of the bottom) to diffuse all of the modal energy within the tube, but this option makes the enclosure very large (about 20" wide, 8" tube + 5" QRDs + 1" walls). Any opinions on this option?

[thadman]

I guess the real question is where stuffing is used along the standing wave, the node or anti-node. Intuitively, As long as stuffing is used at the position of the node, where velocity is highest, the standing wave should be attenuated, correct?

[Dennis H]

I guess the real question is where stuffing is used along the standing wave, the node or anti-node. Intuitively, As long as stuffing is used at the position of the node, where velocity is highest, the standing wave should be attenuated, correct?

Well, as long as you stuff it full, you should have all that covered, right, no matter where the peaks and nulls at different frequencies happen to occur along the length of the box? You're thinking too much and making this too hard.