Acoustic damping inside enclosures

jcaut wrote: »

I agree with doro and think that Polk probably would have put more material in there if they thought it would be of benefit. Don't go crazy as any improvement you make will surely be small. That said, here are some of my observations from speaker building:

I haven't looked inside my SRS 2's to see what kind of material is in there, but if it's a realtively thin "pad" of material, chances are its intended purpose is less about increasing enclosure volume and more about absorbing the back wave off the midwoofers. In that respect, corrogated foam would seem, to me, to be more effective, particularly at higher frequencies. Basically all speaker enclosures can benefit from a little acoustic-absorbing material inside the cabinet, to keep the back wave off the drivers from reflecting back into the cone. A thick layer of fiberglass, or "stuffing" with Dacron would tend to increase the apparent enclosure volume somewhat, both through adiabatic process and from the slowing down of the sound waves inside the cabinet. Denser, multi-layer materials (the "Black Hole" or "Sonic Barrier" types, and vinyl or asphalt sheets) are better for damping cabinet panel resonance, but they're more expensive and a little more difficult to experiment with.

I doubt that a layer (<2") of foam on the back wall, even in addition to what's in there now, would have much effect on apparent enclosure volume, but probably would absorb standing waves over a larger (or just different) frequency range. Whether or not that would be beneficial, I don't know. I do think it would be harder to wind up "over damped" with the PR enclosure than if it was a vented system.

This is one of the areas where you can experiment pretty freely to find out what sounds better to you. You can always pull it back out. Just be careful removing and reinstalling the drivers so you don't tear anything up.

Good advice!
Over the years, I have done some experiments with different damping materials and amounts of same.

I am pretty fasmiliar with the sound of the drivers backwave.

Polk has done a great job in the SRS 2's except in the lower range of some male voices.
Particulary Bob Seger.

It MAY be a cabinet resonance I hear, but it is there.

It is not there on the 801's ,VMPS RM 40's, or the Von Schweikerts.

My instintcts tell me a LITTLE more damping is needed.

I MIGHT simply just add an inner sheet of fiberglass to the dacron damping roll/pad in there already, see what that does ?

I also have a bunch of Kapok, an excellent damping material.
http://www.freepatentsonline.com/6338395.html

Might try some of that inside the stock Dacron Pad ?

dorokusai wrote: »

You should leave what's inside, inside. The tweaking of cabinets with additional this or that changes more than the bass response, so keep that in mind.

I would suggest spikes or some manner of cabinet coupling to the actual floor. It isn't an "insulation" issue.

Good advice in this case.
The RTA 12B has a passive radiator instead of a port.
IF it had a port, it could be shortened a little to loosen up the bass for a fuller sound, although at the expense of some deep bass.

To raise the tuning of a passive radiator, you need to reduce it;s mass.
Not likely!
so spikes, better boundary reinforcement, perhaps even weights on top of the cabinets can help slightly.

Now, let me turn you on to something Bob Carver taught me years ago ?

go to Radio Shack, and get 4 - 1 ohm white sand cast power resistors 10 watt.

Start out with two resistors in parallel, and add them to ONE amplifier terminal ONLY per amp side.
Connect the speaker wire with the resistor inline on one side only per channel.
It does not matter what terminal you use for the resistor, but use the same terminal on both sides of the amp.

This will decrease the damping factor of your amp, resulting in a "fuller" sound with looser bass.
You can TRY both inductive resistors, and NON Inductive both.

This is a "quick and dirty" was to achieve a quasi tubey sound with a solid state amp.

The inductive resistor simulates the inductance of a tube output transformer, and gives a slightly greater effect then a non inductive resistor does.

If 1/2 ohm does not give you the sound you want, simply unparallel the resistors, and use a single one ohm per side instead.

Of course, the T Mod Bob did to his amps is a lot more sophisticated then this, but I have seen this trick he taught me work well.
It is cheap, safe, and easy to try.
You will only be out less then 5 bucks, and if you do not like it, remove it.

It can have amazing effects on some amps.

It works real well on Phase Linear 400's, transforming them from a harsh, powerful sounding amp into a semi tubey sounding 3 dimensional imaging machine.

I used one ohm inductive resistors on my old Phase 400.

Back them I owned Spica TC 50's that imaged very well.

The Phase Linear 400 was flat sounding until Bob suggested the resistor mod.

Worked for me.

Speaker cable diameter can be important too, especially with longish runs into a 4 ohm load.

Generally, the shorter the run and or the larger the cable, the more control the amp has over the speakers.

Sometimes, this may not be the sound you want, and the bass may sound over damped.

Solution ?

Perhaps go to a longer run of cable of smaller diameter.

This also will reduce the amps control of the speaker, sometimes resulting in looser, fuller sounding bass.

avguytx wrote: »

I've heard of doing that with the resistors. Matter of fact, Bob and I were talking about that at CEDIA last month. Well, in between the Sunfire/Elan booth being swamped. That was one of the mods that made the T amps what they were (he said). It's always interesting to hear it come from him. Even though most of it was over my head!

Bob Carver is an old friend of mine from back in 1984 when I lived in Seattle.
He is an great person, as well as a genius.
I was at his home up in Snohomish, Wa north of Seattle the day he came back from Albuquerque for the Stereophile challenge.
Remember that ?
When he claimed he could make his amp sound like a tube amp ?
Well, he did it for Stereophile.
The night he flew into SeaTac airport, he called me.
My friend Bob Pennington, a music teacher and audiophile and I drove up to Bob and Diane Carvers home that night.
Bob had the amp hooked up to his Dahlquist DQ 10's.
Jon Dahlquist and he were good friends.
Bob had a pair of old B&W 801 F's sitting in the corner.
I asked him WHY he had the Dahlquists and not the 801's hooked up ?
he gave me a look that could kill, LOL

He also has a Conrad Johnson Tube amp, the same amp that Stereophile challenged him to duplicate.
We first listened to 3 cuts on the Dahlquist/Conrad combo, then he put his amp in.
NO difference!
It sounded identical.
I heard it, Stereophile heard it, and so did several Seattle area audiophiles.
This was the start of the Carver T Mod.
What Bob does is measure the transfer function of an amp on a given speaker/wire combo, and make HIS amp null identical.

The PROBLEM is he can't do it on EVERY speaker and wire combo!

That is the problem with the T Mods, and why they will not sound like whatever amp they are supposed to sound like on EVERY speaker.

Transfer functions are quite complex.

BUT, on a given speaker, Bob Carver CAN make hs amp sound identical to whatever amp he chooses to.

That does NOT mean Carver amps are "no good" because they are.

Bob designs for "guidry dip", so they are as a rule more on the relaxed, laid back side.

A good thing, IME.

Seattle in the 1980's was a "small town" in many ways, and most audiophiles knew one another back then.

Bob Carver was quite active in the Pacific NW Audio Society, and frequently brought demo stuff to our meetings.

I would like to hear the new carver mini ribbons ?