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Post by Tbone81 on Sept 30, 2022 10:30:07 GMT -6
Reducing the bandwidth of absorption is the goal when putting a thin plastic barrier on the face of a trap. Depending on the design, some people glue craft paper to the front face of the traps, or use wood slats. The barrier reflects mostly high frequencies while still allowing low frequency waves to be absorptive. You’re right though that at a certain point, if the barrier is too good the trap won’t work at all. But we’re only taking about a thin barrier covering only face of a trap so plenty of sound energy is still passing through the trap. I still don't get that logic. If the trap requires air movement thru it to work, hence the use of breathable fabric, even a thin film of plastic would completely cut off that air movement. As such, a thin layer of plastic would render the trap useless, based on my understanding of the necessity of air flow. Can anyone point me to where it is definitively explained that plastic film (or some other thin non-porous material) will still allow a trap to work? It just seems to me like a lot of hard work to build traps is getting undone by a little piece of plastic. well I’m not an expert by any means but think of it this way. It’s not just about air flow, it’s about air pressure. If a thin piece of plastic were able to completely reflect all bandwidths of acoustic energy then you’d be able to build studio walls out of nothing but plastic. But it takes large amounts of mass to stop bass frequencies. Here’s a little thought experiment…suppose you leave the door to your tracking room open, but cover the door jam completely with a thin 2mil sheet of plastic. Let’s assume you achieve an air right seal. Now with that thin sheet of plastic would you still be able to hear a drummer banging away in the tracking room? Of course you’d still hear the drummer, which means that acoustic energy is still being transmitted through the plastic. The way I understand it is that it is the sound pressure, not just the air flow, that is being transmitted. Make sense? |
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Post by svart on Sept 30, 2022 10:55:24 GMT -6
Reducing the bandwidth of absorption is the goal when putting a thin plastic barrier on the face of a trap. Depending on the design, some people glue craft paper to the front face of the traps, or use wood slats. The barrier reflects mostly high frequencies while still allowing low frequency waves to be absorptive. You’re right though that at a certain point, if the barrier is too good the trap won’t work at all. But we’re only taking about a thin barrier covering only face of a trap so plenty of sound energy is still passing through the trap. I still don't get that logic. If the trap requires air movement thru it to work, hence the use of breathable fabric, even a thin film of plastic would completely cut off that air movement. As such, a thin layer of plastic would render the trap useless, based on my understanding of the necessity of air flow. Can anyone point me to where it is definitively explained that plastic film (or some other thin non-porous material) will still allow a trap to work? It just seems to me like a lot of hard work to build traps is getting undone by a little piece of plastic. This is the truth. It's a basic misunderstanding of HOW the absorptive trap works. Using a membrane to bandwidth limit creates a sympathetic membrane resonator with the damping material being just about anything with a non-resonating mass. The mineral wool simply works to add limp mass which changes the frequency knee of the membrane. Limp plastic will NOT resonate with low frequencies nor will it let the sound pressure wave pass with any efficiency. Using soft plastic or paper is rendering the absorption system mostly useless. |
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Post by svart on Sept 30, 2022 10:58:19 GMT -6
I still don't get that logic. If the trap requires air movement thru it to work, hence the use of breathable fabric, even a thin film of plastic would completely cut off that air movement. As such, a thin layer of plastic would render the trap useless, based on my understanding of the necessity of air flow. Can anyone point me to where it is definitively explained that plastic film (or some other thin non-porous material) will still allow a trap to work? It just seems to me like a lot of hard work to build traps is getting undone by a little piece of plastic. Here’s a little thought experiment…suppose you leave the door to your tracking room open, but cover the door jam completely with a thin 2mil sheet of plastic. Let’s assume you achieve an air right seal. Now with that thin sheet of plastic would you still be able to hear a drummer banging away in the tracking room? Of course you’d still hear the drummer, which means that acoustic energy is still being transmitted through the plastic. That's not absorption. That's blocking. The sound in the original room is just as loud as it was before. There is ZERO trapping happening in this scenario. |
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Post by Quint on Sept 30, 2022 11:04:11 GMT -6
I still don't get that logic. If the trap requires air movement thru it to work, hence the use of breathable fabric, even a thin film of plastic would completely cut off that air movement. As such, a thin layer of plastic would render the trap useless, based on my understanding of the necessity of air flow. Can anyone point me to where it is definitively explained that plastic film (or some other thin non-porous material) will still allow a trap to work? It just seems to me like a lot of hard work to build traps is getting undone by a little piece of plastic. well I’m not an expert by any means but think of it this way. It’s not just about air flow, it’s about air pressure. If a thin piece of plastic were able to completely reflect all bandwidths of acoustic energy then you’d be able to build studio walls out of nothing but plastic. But it takes large amounts of mass to stop bass frequencies. Here’s a little thought experiment…suppose you leave the door to your tracking room open, but cover the door jam completely with a thin 2mil sheet of plastic. Let’s assume you achieve an air right seal. Now with that thin sheet of plastic would you still be able to hear a drummer banging away in the tracking room? Of course you’d still hear the drummer, which means that acoustic energy is still being transmitted through the plastic. The way I understand it is that it is the sound pressure, not just the air flow, that is being transmitted. Make sense? I completely understand what you're saying and why you might say it. My consternation is based on the idea that breathable fabric is what is supposed to be used. So maybe using the term "useless" is too strong of a pronouncement, but to what degree is the efficiency of a trap being diminished when using a thin layer of plastic? If the fundamental means (air flow) by which the trap functions, is cut off because of the plastic film, I have to imagine that it is likely preventing the trap from functioning in an undesirable manner. Once upon a time, I had read up on this stuff, but I've since forgotten a lot of it. One of the few things that did stick in my mind though, was the principal of using an acoustically transparent covering. I'm open to being convinced otherwise, but I'd like to see where this concept of using plastic film is discussed in scientific terms. |
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Post by Tbone81 on Sept 30, 2022 11:38:54 GMT -6
well I’m not an expert by any means but think of it this way. It’s not just about air flow, it’s about air pressure. If a thin piece of plastic were able to completely reflect all bandwidths of acoustic energy then you’d be able to build studio walls out of nothing but plastic. But it takes large amounts of mass to stop bass frequencies. Here’s a little thought experiment…suppose you leave the door to your tracking room open, but cover the door jam completely with a thin 2mil sheet of plastic. Let’s assume you achieve an air right seal. Now with that thin sheet of plastic would you still be able to hear a drummer banging away in the tracking room? Of course you’d still hear the drummer, which means that acoustic energy is still being transmitted through the plastic. The way I understand it is that it is the sound pressure, not just the air flow, that is being transmitted. Make sense? I completely understand what you're saying and why you might say it. My consternation is based on the idea that breathable fabric is what is supposed to be used. So maybe using the term "useless" is too strong of a pronouncement, but to what degree is the efficiency of a trap being diminished when using a thin layer of plastic? If the fundamental means (air flow) by which the trap functions, is cut off because of the plastic film, I have to imagine that it is likely preventing the trap from functioning in an undesirable manner. Once upon a time, I had read up on this stuff, but I've since forgotten a lot of it. One of the few things that did stick in my mind though, was the principal of using an acoustically transparent covering. I'm open to being convinced otherwise, but I'd like to see where this concept of using plastic film is discussed in scientific terms. So to be clear, I’m not advocating for a thin plastic sheet on your bass traps. Or for any specific design one way or the other. Im simply pointing out that a thin plastic sheet won’t, by default, “ruin” a broad band trap. There are many factors at play and all of them matter. I think you’re right that it will limit the air flow to a degree, and that may be a bad thing. However it can also be a good thing, it depends on the specific context. When I build my bass traps they killed all the high end in my room. The bass was more controlled but high end was gone and the room sounded way too dull and unnatural. These were 11” thick bass traps, 7’ high and about 3’ wide. After some time on the John sayers forum and reading other sources, I glued craft paper to the front of about 50% of the bass trap faces. The craft paper helped reflect high end energy and really improved the sound of the room. I took REW measurements at every stage and confirmed this. The bass trapping wasn’t effected in a negative way but the higher frequency trapping was diminished (a good thing in my case). |
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Post by kcatthedog on Sept 30, 2022 11:49:49 GMT -6
Isn’t the absorbtion material actually absorbing energy and creating heat, we think of it as sound/freq waves, but ?
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Post by Tbone81 on Sept 30, 2022 11:51:33 GMT -6
Here’s a little thought experiment…suppose you leave the door to your tracking room open, but cover the door jam completely with a thin 2mil sheet of plastic. Let’s assume you achieve an air right seal. Now with that thin sheet of plastic would you still be able to hear a drummer banging away in the tracking room? Of course you’d still hear the drummer, which means that acoustic energy is still being transmitted through the plastic. That's not absorption. That's blocking. The sound in the original room is just as loud as it was before. There is ZERO trapping happening in this scenario. Thats my point. The plastic sheet doesn’t block all acoustic energy, only a certain amount of it at certain frequencies. Therefore, if you have a plastic sheet on the face of a trap, it only blocks a certain bandwidth/amount of acoustic energy from entering the trap. My point is that this single factor when taken in isolation (a plastic film on the face of a bass trap) doesn’t tell you whether it’s good or bad, effective or ineffective. It’s all contextual. It many cases it’s probably a horrible idea, in other situations it probably is great. It’s way too hard to generalize. You need to take into account the thickness of the trap, the density of the mineral wool, the placement of the trap in the room, the desired effect you’re shooting for, the mass of the plastic, the tension (or lack there of) of the plastic and how it’s mounted. Etc etc. I should point out that I’m really not advocating for plastic facing on bass traps. I haven’t used that technique myself so I can’t vouch for it specifically, but I have used craft paper and it worked brilliantly. What I’m talking about is more the IDEA of a thin barrier on the face of a trap and whether it can help or hurt. In my experience it can do both. You have look at the system as a whole, and measure the response, to really know. I may be misstating some of the physics around it all…it’s been a decade since I took physics and don’t remember most of it lol, but this has been my experience and I’ve heard the same from people much more knowledgeable then myself. YMMV |
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Post by Tbone81 on Sept 30, 2022 11:52:30 GMT -6
Isn’t the absorbtion material actually absorbing energy and creating heat, we think of it as sound/freq waves, but ? Yes I believe you’re right. The law of conservation of energy. I believe the acoustic energy becomes heat. |
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Post by OtisGreying on Sept 30, 2022 15:35:35 GMT -6
We’re talking about a very thin sheet of plastic, every single thing I’ve read points to a thin sheet of plastic only reflecting HF energy - which is the point of using it on this tube trap - this isn’t a regular bass trap I was referring to, the tube trap is a cylinder and therefore the side that has plastic acts as a poly diffusor for HF energy.
Or maybe I’m wrong and thin plastic is capable of reflecting 100hz energy? Is that somehow the case? As far as I know lower frequency content needs density/depth or else it just passes right through.
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Post by OtisGreying on Sept 30, 2022 15:45:09 GMT -6
I still don't get that logic. If the trap requires air movement thru it to work, hence the use of breathable fabric, even a thin film of plastic would completely cut off that air movement. As such, a thin layer of plastic would render the trap useless, based on my understanding of the necessity of air flow. Can anyone point me to where it is definitively explained that plastic film (or some other thin non-porous material) will still allow a trap to work? It just seems to me like a lot of hard work to build traps is getting undone by a little piece of plastic. This is the truth. It's a basic misunderstanding of HOW the absorptive trap works. Using a membrane to bandwidth limit creates a sympathetic membrane resonator with the damping material being just about anything with a non-resonating mass. The mineral wool simply works to add limp mass which changes the frequency knee of the membrane. Limp plastic will NOT resonate with low frequencies nor will it let the sound pressure wave pass with any efficiency. Using soft plastic or paper is rendering the absorption system mostly useless.
How do you define "mostly useless"?
I don't understand how thin plastic could possible reflect any energy below 500hz let alone 1-200 or below.
I've read you have to have very thick walls for any energy below 50hz to even stay in a room and not just pass through the walls to the outside world.
Single pane glass - useful on a wall of a studio in the right circumstance because LF energy will just pass through and not reflect, etc. etc.
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ericn
Temp
Balance Engineer
Posts: 16,107 
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Post by ericn on Sept 30, 2022 15:49:18 GMT -6
Reducing the bandwidth of absorption is the goal when putting a thin plastic barrier on the face of a trap. Depending on the design, some people glue craft paper to the front face of the traps, or use wood slats. The barrier reflects mostly high frequencies while still allowing low frequency waves to be absorptive. You’re right though that at a certain point, if the barrier is too good the trap won’t work at all. But we’re only taking about a thin barrier covering only face of a trap so plenty of sound energy is still passing through the trap. I still don't get that logic. If the trap requires air movement thru it to work, hence the use of breathable fabric, even a thin film of plastic would completely cut off that air movement. As such, a thin layer of plastic would render the trap useless, based on my understanding of the necessity of air flow. Can anyone point me to where it is definitively explained that plastic film (or some other thin non-porous material) will still allow a trap to work? It just seems to me like a lot of hard work to build traps is getting undone by a little piece of plastic. If your lucky enough to have a room with natural decent high mid absorption, it’s a cheap hard to screw up method of a semi tuned trap. It’s getting it right that’s the hard part. |
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ericn
Temp
Balance Engineer
Posts: 16,107
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Post by ericn on Sept 30, 2022 15:51:59 GMT -6
Here’s a little thought experiment…suppose you leave the door to your tracking room open, but cover the door jam completely with a thin 2mil sheet of plastic. Let’s assume you achieve an air right seal. Now with that thin sheet of plastic would you still be able to hear a drummer banging away in the tracking room? Of course you’d still hear the drummer, which means that acoustic energy is still being transmitted through the plastic. That's not absorption. That's blocking. The sound in the original room is just as loud as it was before. There is ZERO trapping happening in this scenario. I’m not sure if it was dumb luck but I, as much as it made no sense had to admit it worked. Hunch has always been it was dumb luck and in this case crappy workmanship being the key. Of course Dumb luck was probably the biggest factor. |
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Post by chessparov on Sept 30, 2022 18:15:47 GMT -6
Much of this discussion is way above my head.
All I know is...
Whenever Chuck Norris sings,
he doesn't have to worry about sound pressure waves.
They salute him.
Chris
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ericn
Temp
Balance Engineer
Posts: 16,107
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Post by ericn on Sept 30, 2022 20:09:27 GMT -6
Much of this discussion is way above my head. All I know is... Whenever Chuck Norris sings, he doesn't have to worry about sound pressure waves. They salute him. Chris Chris you got it wrong they are saluting the AE who hits the mute button. |
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Post by chessparov on Sept 30, 2022 20:19:28 GMT -6
I can't even imagine the punch ins...
Chris
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