Resonant panel or slot absorber??

When would I want to use a resonant panel, and when would I want to use a slot absorber? To my little mind, seems like there's a lot of overlap there in controlling low mids. Thanks, -lee-

Panel absorbers are traditionally used for Low frequency absorption whereas slots are more Low-mid orientated. It's not feasible to get slots below around 150Hz (i.e. without taking up a lot of depth and space) For mine panel absorbers are competing with hangers for low end reduction where they take up less space than Hangers. Unfortunately you usually want low end absorption in a rear of a control room where the reflective nature of the panels becomes a problem. cheers john

I was just reading parts in Everest's Master Handbook the other day, and he mentioned that you could mount 703 in front of a panel absorber without noticeably affecting the panel absorber - this sounds like maybe you could use the 703 trick to get away from the reflections, and still keep things relatively compact for low mid absorption. Comments??? Steve

Yeah - I've done that steve - we had a wall of panel absorbers but it was too bright so I mounted 703 in front. - fine.:) cheers John

John Sayers wrote:

Yeah - I've done that steve - we had a wall of panel absorbers but it was too bright so I mounted 703 in front. - fine.:) cheers John

John, when you say "in front of it", do you mean stick it to the panel or in front but not touching?? Thanks

Sen - i actually built a false wall infront that mounted the insulation but didn't touch the panels. But I agree with Steve in that it doesn't matter if it touches. cheers john

Sen, what I got (sort of between the lines) from the Everest info was that the FRAMES could touch, but the 703 itself was "stood off" by enough so as not to affect the resonance of the panel absorber itself. As with almost everything in acoustics, there are several ways to skin this cat, and as always, YMMV... Steve

Yeah, as I've mentioned elsewhere, placing material close to the slot openings raises and broadens the resonance peak. Just a general note on slot absorbers versus panel absorbers: Slot absorbers are much more efficient (absorption per unit area). Panel absorbers are very inefficient because the inertia of the panel must be overcome before any absorption can take place. The only inertial impedance in a slot resonator is the small mass of air that resides in the slots. The fact that slot resonators must be very large in order reach low frequencies is evidence of their higher efficiency. In acoustics, efficiency is almost always gained at the expense of increased volume. Similarly, the greater the ratio of slot area to slat area in a resonator, the greater the efficiency. The tradeoff is, of course, extra volume. Resonators with high slot to slat ratios must be deeper in order to reach a given target frequency. Thomas

In acoustics, efficiency is almost always gained at the expense of increased volume.

when I started out in recording in the mid 60's all the studios had what I call original BBC acoustics usually made up with panels of fibreglass (The old stuff) and slots/perforated holes and panel absorbers. Near the late 60's and into the 70's the bands started wanting to record with their stage rigs!! you usually only had one recording space so suddenly we were confronted with a live band producing 130 spl !! so much for isolation between instruments. That's when the new acoustics started in the US, led by Tom Hidley, where whole walls and ceiling spaces were taken up with deep traps and up to 1/3 of the floor space became acoustic traps. We've moved back from that position now as the studios were toooo dead and booths are now built to achieve the isolation but Thomas is right - to achieve the isolation/absorption in those days you needed space. cheers john

John, Following your reasoning, and now with the heavy proliferation of pods, DI, virtual instruments, sub bass synths, 5.1, etc., it seems that the emphasis of studio acoustics has shifted away from the live rooms and much more towards the control room. Would you say this is a correct assessment? Thomas

Barefoot, I think that has been a fair assumption for a while, but I find that the new trend is going back to recording as a full band again in the same space. Eric

studio acoustics has shifted away from the live rooms and much more towards the control room

well the control room has always had attention in pro studios - it's the proliforation of smaller semi-pro studios everywhere. Intitially the emphasis was on gear but that's settling down now and people are turning their attention to the acoustics of their spaces especially the control room, and as you say, most of the recording actually happens there. To exemplify what I was referring to -I watched Paradise studios going up in Sydney in 1979 - designed by Hidley. Whilst the compression ceiling in the control room came down to around 7'6" the cavity above went up to 12' and was totally filled with hangers, the rear wall was 4' deep and full of hangers. Music Farm was similar - (we were building at the same time).

but I find that the new trend is going back to recording as a full band again in the same space.

I agree Eric, spill is coming back into fashion :):) I was talking with a bass player the other day who reckoned his bass used to resonate with the kick drum when he stood next to the kit and his bass sounded fuller with his amp spilling into the drums. cheers john

Yeah, my voice sounds fuller after a beer spills into my mouth... :=)

With me it usually takes about 7 -8 to make my voice sound better.

OK, so I'm a cheap drunk - on the other hand, even a half-case has been known not to accomplish said task from my wife's point of view... :=)

Thomas, > Panel absorbers are very inefficient because the inertia of the panel must be overcome before any absorption can take place. < I'm not sure what you're basing that on, but it is not true. The panel traps I've built and tested have very high absorption coefficients. Yes, panel traps have a center frequency with peak absorption that falls off at either side, but over a range of an octave or so they are very efficient. --Ethan

Ethan, I'll crack open my texts and get back to you with some solid analysis. However, the point about needing to overcome panel inertia should seem intuitively clear. If the mass of the panel gets large, it simply acts as a reflective wall (zero absorption efficiency). As it's mass goes to zero it acts more like an open window to the cavity behind it (absorption efficiency = 1, assuming the cavity volume scales accordingly). But, if you're realizing high absorption coefficients, maybe there's some basic mechanism I'm missing? Thomas

Thomas, > needing to overcome panel inertia should seem intuitively clear. < Not at all. Saying "overcome inertia" implies a non-linearity, like a noise gate whose threshold must be exceeded before anything happens. This is clearly not the case - if it were, adding a panel bass trap to a room would literally create distortion! > As it's mass goes to zero it acts more like an open window < A panel trap is essentially a shock absorber for audio waves. With the proper combination of spring and mass, it can absorb very well. --Ethan

Ethan, A mass limited response doesn't imply any non-linearity. It simply implies an impedance mismatch which reduces energy transfer efficiency. Acoustic impedance is the missing factor in your system model. These sorts of mismatches are everywhere. Probably one of the most ubiquitous examples is sound transmission between water and air. Water is an excellent medium for sound transmission and the animals in the sea can create quite a ruckus. So, why do we only hear the waves when standing at the shore? The answer is: because there is a huge acoustic impedance mismatch between the air and water. Most of the energy is reflected at the interface. Yes, a panel resonator is simply a damped mass on a spring which, when excited, likes to dissipate energy around its resonance frequency. The question is, however, how well can the air actually excite the panel? The answer is primarily a function of wavelength, size of the panel, and its mass per unit area - relative to the density and compliance of the open air. I'm trying to dig up an analytical expression that relates absorption coefficient to these parameters for a simple idealized system. My guess is this expression will show that the typical air/panel resonator interface is analogous to trying to bounce a pickup truck up and down on its suspension by mounting a trampoline to the bed then asking a 5 year old child to jump on the trampoline - an inefficient coupling at best. Thomas

Thomas, > A mass limited response doesn't imply any non-linearity. < Well, you said "overcome intertia" which to me implies a gate effect. > These sorts of mismatches are everywhere. < Yes, I understand impedance matching fully. I knew all about standing waves long before I became interested in bass traps, when I was a ham radio operator as a teenager. > how well can the air actually excite the panel? < Apparently pretty well, since the tests performed on the traps I sell have a very high absorption coefficient. Have you seen the specs on my www.realtraps.com site? I have obviously never measured the traps in my DIY plans, since they are built directly onto a wall and can't be brought to an acoustics lab. :) But I have no reason to think they aren't at least similar in performance. --Ethan

Ethan, Please don't take my elaborations as any reflection on what I think you know personally. I try as much as I can to expand my comments with examples and such, so those with less background in this area can get a handle on the discussion as well.:) "Overcoming inertia" is just a another way of looking at impedance. It's easiest to look at impedance mismatches in terms of sound velocity differences between two mediums. But some people have difficulty thinking of something like a panel resonator as being a "medium" with its own inherent sound velocity characteristics. So, I thought inertia (which directly determines the system sound velocity) would be a simpler concept for most to grasp. I looked at your data, but like any good scientist, I have to approach it with a great deal of skepticism - especially when my understanding of the physical model seems in contradiction. My first question regarding the ASTM data would be, is this reverberant room method appropriate for a system which could potentially be better characterized as resonant rather than absorptive? For example, the phase of any reflected signal might influence the data depending on where the test microphone is positioned. Also, your control room/sine wave data doesn't seem to jive with your explanation. Why would traps increase the deep bass response? In the very deep bass where the wavelength is large compared to the room dimensions the acoustics are not "standing wave" in nature. In this range you are in the true near field and it's meaningless to speak of increasing or decreasing "cancellation" effects. There are no null positions. All possible phase shifts within the scale of the room are by definition small compared to the wavelength and, therefore, have no significant effect on the amplitude. The room response is fairly uniform and simply characterized by the rate at which the room can bleed or dissipate the acoustic energy. The only way to raise the response at any position is to decrease the energy dissipation. So, your sine wave data seems to indicate that you actually decreased the low frequency absorption in the control room. This could make sense if the bass traps had virtually no effectiveness in this frequency range and simply tightened up the room by adding mass to the walls. The ASTM data is certainly plausible, but I'd need some convincing that the test method works for a resonator. Likewise, your cancellation reduction argument for the sine wave data seems plausible in the 100Hz + range, but not at all in the low frequency range. I think the only thing you can reasonably conclude from this data is that you decreased the absorption in this range. Thomas

Thomas, Good points, and all are worthy of further discussion. First let me say that there is no question that panel traps work, and work very well. This started when you said "Panel traps are very inefficient" which simply is not true. Aside from the ASTM measurements performed on the traps I sell, I have been building panel traps for many years. In every case they changed a room that was impossible to mix in, into a room that sounds great and is a joy to use. I have sold more than a hundred panel traps through my company, and every customer has been extremely pleased. Nobody has ever asked for their money back, and I have received many positive comments. Our last sale was to Jerry Barnes at Avatar Studios in New York City, and he was beside himself with praise for how much they improved his mix room. I'm sorry if this sounds like an ad :) but there is no question that panel traps, when properly built, are extremely effective. How they work and how they might be measured is surely worthy of discussion. > "Overcoming inertia" is just a another way of looking at impedance. < Fair enough, but panel traps are linear in their operation. If you excite them at low volumes they respond with small movement. At higher levels the panels move more. You can no more point to "inertia" as a negative in a panel trap than you can in every cone-based loudspeaker ever made. > I looked at your data, but like any good scientist, I have to approach it with a great deal of skepticism - especially when my understanding of the physical model seems in contradiction. < I have no problem with skepticism. I subscribe to four different skeptic magazines, so I am very aware of the need for skepticism in every field, not just audio! > is this reverberant room method appropriate for a system which could potentially be better characterized as resonant rather than absorptive? < Sure it's appropriate. Why wouldn't it be? Further, how else would you measure the absorption of panel traps at various frequencies? > For example, the phase of any reflected signal might influence the data depending on where the test microphone is positioned. < That is way off. When an ASTM lab measures absorption, the microphone is constantly in motion. Further, hundreds of separate tests are performed and the results are all averaged. At IBM's lab which we use, each test takes about 40 minutes to complete because so many tests are performed and with so many mike positions. > In the very deep bass where the wavelength is large compared to the room dimensions < I agree that the sine wave data I show is not as useful as a legitimate test. The problem is that no lab in the world can test at those low frequencies, so a half-assed test using tones was all we could come up with for very low frequencies. Also, bear in mind that the room dimensions at very low frequencies are much larger than where the walls are physically. Very low frequencies go right through normal sheetrock walls, so the real walls are whatever lies beyond that is more massive. In this case the control room is inside a basement, and the cinder block walls are quite a distance away from the inner sheetrock walls. > This could make sense if the bass traps had virtually no effectiveness in this frequency range and simply tightened up the room by adding mass to the walls. < That could well be the case. I have no idea, nor do I even have a suggestion for a better way to test that. But it's a mistake to dismiss panel traps for not absorbing much below, say 45 Hz., when they work so very well at higher bass frequencies. And for most studios, the important frequencies are 80 Hz. and above. More to the point, how effective is any other type of broadband absorber below 45 Hz.? --Ethan