Control Room tuning - Frequency peak (36,3 Hz)

Hello everyone, I hope you're all enjoying the holydays... Here at Mikrokosm in France (Lyon), we are spending holidays in the studios, and we hope you can help us improving our set up. We've been working over the last 10 month on the building of a 2000 sq.ft recording space, with a large live room, control room and iso booth. We built everything ourselves and sort of made a point doing so because we are indeed very interested in understanding and learning always more about sound and well, we have to admit that we financially can't afford much more than that. Ha. We recently brought equipment in and started the acoustic treatment process. We designed before hand a treatment solution with Gik Acoustic's Brian Pape and applied it to our control room. After listening to our productions, we felt happy with the general acoustics but noted low end troubles. A serie of measures (QTC 40 + "fuzzmeasure" soft) allowed us to spot what seems to be the problem, and that's where we kindly ask you, dear acoustic connoisseurs, to help us think about some options. We uploaded graphs, pictures, and quotes of the control room at the URL below, so you can have a precise idea of what we're talking about: http://www.mikrokosm.fr/measures thank you so much in advance for sharing your thoughts with us. So..... We can observe on the frequency response curve that the trouble we heard comes from a 36,3Hz peak redundant "around" upper octaves (69 - 109 - 132 - 172 - 220). Then the response seems to be acceptable -minus the 2k valley due to absorbtion panels, but that's another topic. First, are we reading this correctly? Is our analyze coherent? Then, as we tend to look for solutions that we can build ourselves, we found spreadshits to build Helmholtz filters. But the frequency to kill (36,3Hz) is so low that we'd need to space the slates with 1mm (approx. 0,04 inches). A fairly tough and precise job, but not impossible at all. a/Our first big question here is about where to place this filter in our space. If you look at the pictures and plans we uploaded (see link above), there is still space below the Schroeder filter. Intuitively, that's where we'd place the treatment, but is it a wise position? b/Second doubt, is such a filter the only option for us to get rid of that resonance? c/We understand why a flat response wouldn't be ideal to achieve a coherent work (home environment is never flat), so we don't necessary look for a flat curve. Maybe the actual frequency response is acceptable in a control room? d/We played EQed tracks (-10dB at 36,3Hz) and the difference is quite big. Should we just attenuate those uncomfortable peaks (72 - 109 - 132 - 220) and harmonize the overall response (not bother the 36 Hz original frequency)? e/Also, could the desk resonate? We didn't sound check the room without it, but we covered and filled the empty spaces and the resonance is still there. We tried placing the speakers on stands to avoid physical contact to the desk; no improvement in sound. Can you see on the pictures (link above) any evidence of missplacement? We hope this is an interesting problem for you all to think and solve, and look forward to all your answers and advices. Thank you! Benoit & Johannes.

welcome! very nice looking control room! based on the measurements, it looks like the room is almost square and the ceiling height (just guessing from the SU drawing) appears to be nearly a multiple so you have not only length and width approximately the 32-36hz (based on calculation) but a height of 62hz (based on calculation) and any of these could readily be both the culprit and reinforcement of the problem. not sure what you have available for experimenting (several large absorber panels 4" (100mm) thick would be handy) plus the ability to move things around a bit. does changing the measuring position behind the desk change things? does changing the speaker positions change things? as a general rule, your listening position tests start @ 38% of the room length from the front wall. your monitors are on stands (not on the desk!) about 0.3m from the front wall and forms an equilateral triangle with the listening position. does putting an absorber in front of the desk help? gradually adjust the position of the speakers and measuring mic to determine if your issue is modes or front/rear wall reflections.

Hi Gullfo, happy new year everyone, thanks for the compliment. We're slowly getting there, ha! As for experimentations, we measured the same signal in several positions, from equilateral with speakers, to the back of the room (centered with speakers), as well as in the corners. Response spots the same peaks (and gets more flat in the corners, where we built bass traps). We tried with speakers on stands, from approx. 0,3m from the front wall, no changes. After trying different placements, we felt like we adjusted the position of the speakers to their optimal sounding situation, and we come up with the same measure. Maybe there are still a couple things we can try though. I'd like to insist in a test we did; we EQed the signal according to the measures and the result was quite convincing pretty much all around the room (a couple phasing issues when moving around though). Is there something to deduce from that experience? Also, we generally don't have a lot of room to move things around and our set of absorbtion panels is limited. Again, many thanks for your help. Mikrokosm.

if it's persistent across the room and changing system component positions doesn't alter it, then your looking at modes which need to be treated. this may mean using some panel traps tuned around the 36hz and/or increasing the number of absorbers - in the corners + triangular ones in the tri-corners. you may benefit from larger clouds overhead as well.

ok. If we build a 36Hz tuned trap, first thought is to place that on the back wall, below the Schroeder absorber (link for pictures above). is there a strategic placement to observe in the room for that one? Thanks. Mikrokosm.

Another question, does it make sense to build a 109 Hz trap that would diminish the peaks we see on the tests? This trap would affect 1 octave up and 1 down, which are the main disturbing peaks. Or really, we should tune the trap to 36Hz...? Also, does the size of the panel affect the treatment? Many thanks, Mikrokosm.

for the higher frequencies, a slat resonator is ideal but a flatter Q on a panel trap would work. for the 36hz, you'll need a large panel trap to address that. something like 1200mm x 2400mm on a sealed frame. positioning of it will be dependent on where in the room that 36hz is exerting the most pressure and you can find it using a sine wave played through your system and your ears and a sound level meter. there are likely several places in the room where this will exist and you can trap those positions. damping the panel will increase the bandwidth at the cost of efficiency. you might find several damped panels to be more effective than one or two high Q panels.

Just to offer a (surprising coming from me) different perspective: 36 Hz is so low and musically insignificant that I'd probably use an EQ to reduce the peak a bit. :? --Ethan

cannot disagree with Ethan, but there may be more of a range involved so the damped panel - wider Q - might still be in play.

Thanks Ethan for your thoughts! So, if we understand well, a strategically positioned, wide-Q, 109Hz tuned panel would be a clever option..? Glenn, you're talking about a slat resonator, and we're thinking of building a perforated absorbing panel.. Does it make a difference? Slat resonators seem really tough to self build for such low frequencies... how do you control the Q when you build your panel? We french people with our english, ha! Mikrokosm.

the Q is managed by the damping. in the slat or perforated panel your air gap behind the openings and the overall cavity damping (cloth and insulation) determine how sharp it will be (assuming all slat/slots/space-opening are the same). in a membrane trap, the proximity and contact of the insulation to the panel will widen or lessen the Q. you could go with the perforated panel if that is what you're comfortable with. et nous en anglais avec leur français ! touche !

touchdown, ha! ok, so bigger air gap = wider Q? If so it's cool, so we can experiment a bit with filling or emptying the damping..

bigger gap = narrower Q

Great! Thank you so much, we'll keep you posted on the experiments. Mikrokosm.