Damping material quantity

[pski]

I've built a volume-scaled-up cabinet of really dense 1" MDF. It's internal bracing is larger and has better contact and the speaker is biscuited entirely rather than being screwed together compared to the 3/4" lower density plan provided in the kit.

The cabinet is essentially a single part. The only screws hold drivers and wire cups.

The damping plans provided include single open cell foam on the sides and double thickness on the top and back of the cabinet, and adds acoustistuf behind the woofers and in the bottom. Done that way the build sounds terrible. The designer is (by his choice) unavailable.

I've spent more time diddling with damping than making the things.

Question: if the cabinets are massively stable (knocking the exterior anywhere produces the same apparent sound,) would a substantial reduction in damping material be justified? There's a definite change with small changes to the acoustisuf and removing the double layer of foam on the back and top was also an improvement.

https://www.facebook.com/media/set/?...1&l=6334890e9d


Thx for your help

P

[rvsixer]

It's my understanding damping affects standing waves, mass/bracing affects resonance. Separate issues. You stated you scaled up the volume of the cabinet, which could lead to something that sounds entirely different from the original design (assuming no other changes were made).

[AllenB]

Cabinet vibrations add to the sound in a way that many get used to. Listening without them for the first time can make a speaker sound dry and dark by comparison. Maybe you are seeing the internal reflections as a reasonable substitute. If it was me, I'd use enough material to banish the reflections, although I notice you're using a reflex enclosure.

Mass market systems, while sounding bad, have something about them that makes all recordings sound OK and somewhat consistent. We could look at box resonances, diffraction and other various distortions to find out why.

Once these are stripped away we are left with only our rooms, and the raw recordings. You'll probably never listen the same again

[rabbitz]

There's a couple of threads here on the ZRT and sound / damping issues.

[pski]

Originally Posted by rvsixer

It's my understanding damping affects standing waves, mass/bracing affects resonance. Separate issues. You stated you scaled up the volume of the cabinet, which could lead to something that sounds entirely different from the original design (assuming no other changes were made).

By scaled-up, I mean making the parts 1/2" larger to account for the volume difference of 3/4" vs 1" material. The internal dimensions are identical.

The increase in brace size reduced effective internal volume about 2 liters ( or 3%.)

I'm really saying that the "plans" provided sound so bad they can't be the result of a real build. I don't think I've changed the design enough . . At this point the damping has dramatic effect. As shown in the plan, two layers of open-cell foam on the top and back make them sound like they are playing through a mud puddle. Removing the double layers and adjusting the acoustistuf on the bottom (and maybe the top) seems the best tweak.

Thanks for your help and comments.

P

[pski]

Originally Posted by rabbitz

There's a couple of threads here on the ZRT and sound / damping issues.

I would really appreciate the search terms you used (or links if you're feeling happy.)

P

[AllenB]

pski, it's possible that your internal reflections happen to be compensating for some room based cancellations.

[jimangie1973]

Couple of thoughts:

How much acoustistuff are you using? For this design, it's being used to damp the top to bottom standing wave. For it to be effective, the acoustistuff must be packed very very dense. I just finished a reflex enclosure that required almost 2 lbs of dense fill at the bottom to fully damp the standing wave. I monitored the impedance while adding fill, until the impedance glitch at the standing wave frequency was gone.

Also, if the internal dimensions are identical, your baffle is now about half and inch wider than the original plan. That will alter the response slightly between 200Hz or so to 1kHz. I doubt this is the issue though.

Maybe there is something wrong with one of the drivers, or possibly the crossover implementation. I would double check the wiring and crossover assembly. If all looks good, try putting white noise through each speaker to verify they both sound the same.

Jim

[rabbitz]

Originally Posted by pski

I would really appreciate the search terms you used (or links if you're feeling happy.)

P

Search for "ZRT bass" which should give only 2 pages of threads.

The ones that spring to mind are:
Zaph ZRT - Bracing question (re: 'holes' in braces)
no bass from vented + floorstander 18w-8531- port problems?

I use a scheme that's been successful in my BR projects and is recommended by a lot of BR designers. The main damping hangs down like a curtain mid box and has an air gap to the back panel. The bottom of the box is filled to help with the pressure node (see Zaph's design comment on the ZRT). Loose fill is not to be used on vented designs and needs to be a blanket such as fibreglass batts or equivalent. Importantly, a clear path needs to be maintained from the rear of the woofer to the port inlet. Most times I use nothing on the walls but sometimes use a thin layer (5-10mm) of felt on smaller boxes.
http://www.diyaudio.com/forums/attac...o-3-way-p3.jpg

Damping may not be the main culprit, so check your crossover and other aspects of your build.

[pski]

Originally Posted by AllenB

pski, it's possible that your internal reflections happen to be compensating for some room based cancellations.

Make no mistake, the speakers sound good. It's just that I've spent more time diddling with the damping than it took to do the rest of the job.

P