limp mass membrane

[audiothings]

how, if at all, can a limp mass membrane assist in bass trapping? i already know its very efficient as a sound barrier...

jai shankar.

[Glenn Kuras]

Originally Posted by audiothings

how, if at all, can a limp mass membrane assist in bass trapping? i already know its very efficient as a sound barrier...

jai shankar.

Think of it as a drum head. The vibrating takes energy which will decrease the sound wave.

Glenn

[audiothings]

thank you sir. can you please elaborate on how it can be used for bass trapping? i have already been informed that they would be counter productive in "superchunk" corner traps.

speaking of which, can anybody point me in the direction of a commercially available limp mass membrane? are lead sheets (which are used for adding mass to studio doors?) considered limp mass membranes?

thanks.

[bpape]

A LIMP membrane does nothing but block mid and high frequency absorbtion.

A mambrane that's stretched tight and is not touching the absorbtion acts like the drum head and can work much deeper IF it's done over a sealed cavity. The advantage here is that you can go much deeper without a lot of physical depth. This is the type Avare was referring to. These are much narrower in usable range

Bonding something like FSK to absorbtion will reduce HF absorbtion but also form a hump in low frequency absorbtion. The center is determined by the material and thickness of what's behind. These are NOT sealed absorbers and still work on velocity.

Bryan

[Eric Desart]

Originally Posted by bpape

A LIMP membrane does nothing but block mid and high frequency absorbtion.

A mambrane that's stretched tight and is not touching the absorbtion acts like the drum head and can work much deeper IF it's done over a sealed cavity. The advantage here is that you can go much deeper without a lot of physical depth. This is the type Avare was referring to. These are much narrower in usable range

Bonding something like FSK to absorbtion will reduce HF absorbtion but also form a hump in low frequency absorbtion. The center is determined by the material and thickness of what's behind. These are NOT sealed absorbers and still work on velocity.

Bryan,

You can NOT distinguish them that way.

1) By NOT sealing a membrane trap it goes deeper in frequency since you weaken the spring of the mass-spring system. You only get a lower Q and somewhat extended bandwidth.
A membrane damper does NOT assume a sealed cavity, it's only easier to calculate.

2) An FSK or whatever still causes it to be a membrane absorber.

Both will also absorp via velocity as well which is defined by the mass and TL of the membrane versus frequency.
When you glue a membrane as e.g. FSK to the mineral wool the mass becomes a complicated thing since it's somehow connected to the wool.
Here we then speak about an apparent mass.

Hence you can't distinguish this saying sealed = membrane, and not sealed something else.

[bpape]

Eric.

To clarify, I never said if it wasn't sealed it wasn't a membrane. What I said was that if you had a membrane stretched tight and not touching anything else, it had a narrower range and that was what Avare was referring to in the linked response.

Yes, you can use a membrane in other ways that are not sealed - which I identified in my previous post. I mentioned that the bonded FSK/FRK did in fact produce a hump in the absorbtion curve but not all through the bass range. The center of that peak is determined by what it is bonded to.

If you have a true limp membrane, not stretched tight, not over a sealed cabinet, not bonded to anything, all it's going to do is block upper mids and highs from being over absorbed.

If you stretch a membrane tight over a non-sealed cavity, then it will react as you described - I didn't mention that option because I didn't want to confuse the OP as that wasn't anything close to what he was trying to do.

Bryan

[jayfrigo]

Originally Posted by Eric Desart

Hence you can't distinguish this saying sealed = membrane, and not sealed something else.

I may have misunderstood, but I don't see him saying it's not a membrane in his post. I see it stated that in the example of attaching it to the frictional absorber that it is not sealed. "Not sealed" does not equal "not membrane" the way I read it, though perhaps there was some subtext that is easily mistaken. Either way, your clarification is a welcome one in case anybody misunderstood the distinction.

[audiothings]

A LIMP membrane does nothing but block mid and high frequency absorbtion.

is this really true? it it the only thing it can do? are we agreed upon what is a limp mass membrane? something tells me that a limp mass membrane can be a great asset in bass trapping but it must be that i am wrong because i don't know how and why. forgive me for my superficial knowledge, but intuitively, i feel that something which is dense and heavy but still "limp" can assist greatly in bass trapping. i cannot justify this and i see no proof in my online scan, so i must be wrong.

If you have a true limp membrane, not stretched tight, not over a sealed cabinet, not bonded to anything, all it's going to do is block upper mids and highs from being over absorbed.

what is a "true" limp membrane please? if i took the auralex sheetblock and stretched it over (or, perhaps, in the middle of) a relatively deep frame stuffed with OC703 or similar, with or without a sealed cavity, would it cease to be a "true" limp mass membrane? can anybody predict what would happen if i put the auralex sheetblok say, over the exposed face of, in the middle of, or at the back of a 6" frame stuffed with OC703?

[bpape]

It's one of those terms that is interpretted differently by many people. To me, a limp membrane is by definition limp meaning not streteched tightly. I also don't consider a membrane that is bonded directly to the absorbing medium to be a limp membrane.

If you took a piece of sheetblock and put it (not tight) over a frame with absorbtion behind it (not touching), then it would reflect highs and let bass pass somewhat. The mass of the membrane will determine somewhat where that point is between reflecting and passing to the absorbtion.

If you take a membrane and stretch it over a frame tightly but again without sealing it and without it contacting anything else, it will act like a drum head - but without the help in damping of a 'spring' (the sealed air). Again, it will pass some frequencies, reflect others, but also will resonate at a band of frequencies which the absorbtion behind will still help to absorb.

If you do just the membrane and the frame as I think you were originally suggesting, it can only really act to reflect some frequencies and physically move absorbing a little of the bass energy. This is how it will function as part of a wall to a certain extent and also depending on exactly how the wall is built and how the membrane is implemented.

Bryan