Bass trap query

[madmuso]

Hey guys,

Ive been researching bass traps and it seems that density/mass and particular coefficients seem to be reoccurring themes!
I have a few questions.

1) Most of the designs I have seen on the net are 100mm (4inches) thick. Apart from the space factor, is it not a good idea to build traps that are maybe 150mm (6inch) or thicker even?

2) It seems owens corning 703 keeps popping up which is fine for our American friends but what about in Australia (Melbourne), can anyone recommend and equivalent or something better?

3) Has anyone tried making something like this? http://www.primacoustic.com/fulltrap-specs.htm
Would the loaded vinyl really make a difference? In theory it sounds logical. And if it really works im gonna try to utilize this construction technique when I build mine, I manged to source some 4kg and 8kg loaded vinyl at a fair price.

4) Does hanging the traps slightly off the wall really help lower its trapping range?

Ive read so much on the net that im not sure what to believe anymore! Im going looney!

thanks, hope you are all well and making plenty of music!

[Soundman2020]

1) Most of the designs I have seen on the net are 100mm (4inches) thick. Apart from the space factor, is it not a good idea to build traps that are maybe 150mm (6inch) or thicker even?

Absolutely! The thicker the better. That's why Superchunks are the most recommended bass traps here: At the thickest point, they are about 24" deep. You make them by cutting panels of 703 into large triangles, and stacking up the triangles in the room corners, floor to ceiling. They are extremely effective down to very low frequencies.

2) It seems owens corning 703 keeps popping up which is fine for our American friends but what about in Australia (Melbourne), can anyone recommend and equivalent or something better?

Pretty much any good mineral wool product with a density of around 50 kg/m3 will work well, or any good fiberglass product with a density of around 30 kg/m3. The neat thing about 703 is that it comes in semi-rigid panels that are easier to work with and easier to cut than typical "pink fluffy" fiberglass, but pink fluffy works well too.

3) Has anyone tried making something like this? http://www.primacoustic.com/fulltrap-specs.htm
Would the loaded vinyl really make a difference? In theory it sounds logical. And if it really works im gonna try to utilize this construction technique when I build mine, I manged to source some 4kg and 8kg loaded vinyl at a fair price.

That's a membrane trap, not just a simple broad-band bass absorber. It is tuned to a specific center frequency, which is given by the surface density of the membrane (in this case, the MLV) and the depth of the air cavity behind. Those work too, but they are tuned to that specific range of frequencies, which might or might not be right for YOUR room.

4) Does hanging the traps slightly off the wall really help lower its trapping range?

Yes. For a simple panel absorber, it extends the range down lower. A 4" trap with 4" of air behind it is very effective, but an 8" trap would be even more effective: Technically, the distance of the front surface of the trap from the wall surface is what affects the lowest frequency, and the amount of absorption behind that surface is what affects the effectiveness of the trap. (It's a bit more complicated than that, but that's the basic rule).

- Stuart -

[madmuso]

>Absolutely! The thicker the better. That's why Superchunks are the most recommended bass traps here: At the thickest point, they are about 24" deep. You make them by cutting panels of 703 into large triangles, and stacking up the triangles in the room corners, floor to ceiling. They are extremely effective down to very low frequencies.

cool, thanks.

>Pretty much any good mineral wool product with a density of around 50 kg/m3 will work well, or any good fiberglass product with a density of around 30 kg/m3. The neat thing about 703 is that it comes in semi-rigid panels that are easier to work with and easier to cut than typical "pink fluffy" fiberglass, but pink fluffy works well too.

I found a relatively cheap product today which I ordered, its around 35kg/m3 but I got in touch with the manufacturer and got hold of the acoustic coefficient charts and the results were good for a product of this price. The cost doubled (sometimes trippled) for a product that performed better only slightly, I cant afford it. From memory the product I ordered showed at 125hz it was 0.46 and at 250hz it was 1.15. I figure I'll just add more traps, its still cheaper than buying the overpriced stuff. I started making the frames for them today like a production line! hehe

>That's a membrane trap, not just a simple broad-band bass absorber. It is tuned to a specific center frequency, which is given by the surface density of the membrane (in this case, the MLV) and the depth of the air cavity behind. Those work too, but they are tuned to that specific range of frequencies, which might or might not be right for YOUR room.

Isnt it supposed to take care of the resonant freq's in whichever room they are placed in? AT least thats what I understand from the fine print under the performance chart.

>Yes. For a simple panel absorber, it extends the range down lower. A 4" trap with 4" of air behind it is very effective, but an 8" trap would be even more effective: Technically, the distance of the front surface of the trap from the wall surface is what affects the lowest frequency, and the amount of absorption behind that surface is what affects the effectiveness of the trap. (It's a bit more complicated than that, but that's the basic rule).

Excellent, due to having such a small space to work with my traps wont be hanging to far off the wall, the traps rear timber frame itself will sit hard up against the wall but the actual mineral wool panel will be off the wall by 40mm, which is the width of the backing frames I made holding it all together.

thanks for your help Stuart,

[madmuso]

Forgot to ask, due to budget restraints the fabric I purchased to cover and dress the bass traps is very thin but has quite dense weaving/stitching. Its a cotton/polyester blend. Is this ok? You need to blow fairly hard to get air to pass through it. I dont mind if the mids and highs are reflected back somewhat into the room and not killed by the traps entirely. I dont like working in an overly dead room.

[Soundman2020]

You normally want "breathable" fabric for acoustic panels, meaning that you hold it over your nose and mouth and breathe through it freely, without much resistance, can since the air has to be able to move through it to get to the treatment behind. But for bass traps, that isn't so important. In fact, a fabric that is hard to breath trough will likely reflect some of the high mids and even highs back into the room, and that's a good thing! So what you have is probably fine for bass traps, but not for your ceiling cloud or absorption panels at your first reflection points. Those really do need very breathable fabric.

- Stuart -

[madmuso]

Hey guys,

the last few days I have been putting up some bass traps and general "reflection zone" traps and experimented with a small radio to hear the difference with placement and adding the acoustic treatment. Before I explain my strange experience i'll describe the room.

My control room is small parallel square, 3 meters long X 3.5 meters wide X 2.2 meters high. The walls are all plastered. FLoor is floating timber floor. The ceiling is "inside out" so the plaster is on top of the ceiling joists and the entire ceiling cavity is filled with 90mm polyester insulation. The rear wall of the control room has bass traps covering almost all of the wall area. The side walls have absorption panels at the reflection points. Front wall at the moment has nothing apart from the window and a door. The bass traps I made using a glass insulation with very similar coefficients and density to OC703 and they are covered in a more reflective "hard to breath through" fabric to keep some mids and highs in the room. The side wall panels have a very open weave fabric covering the polyester insulation.

Here's the experience I had today:

I placed my old panasonic boom box on top of a tool box against the front wall in the control room, I played a song through it at a decent level. I stood with my back right up against the rear wall. So at this point the radio is pointing directly at my shins. The bass gtr in the track was very boomy, after a few minutes I squatted down and the difference was huge, no boominess at all, in actual fact just lowering myself down by half a meter from a fall standing position the boominess wasnt present. I grabbed my 4 ft ladder and place the boom box on that to get it at around neck height and stood back against the rear wall again, it was nowhere near as boomy as when it was on the tool box closer to the floor.

Is this a classic example of sever peaks and nulls? Off the top of my head id say the boomines was around 115 to 250hz.

[madmuso]

Another experience regarding bass!

If I walk from the middle of the room to towards the back while music is playing the bottom end seems to blossom nicely when im about half a meter away from the rear wall. This spot is actually nice and even and well balanced with the mids and highs on the song, walking towards the centre of the room the bass really thins out and the nice balance is lost. Moving closer to the boom box (towards front) brings the bass back a little but doesnt have the nice low bass extension like the rear spot. From a mix point of view, I personaly would instinctively set my kick and bass gtr levels similar to the balance in the rear spot, I really like to feel the kick and hear the bass gtr. So if im to setup in the front half of my room which has less bottom end, my mixes will be too bass heavy, can I boost the bass freq knob on my monitors to simulate the rear spot that I like which would hopefully prevent me from producing bass heavy mixes?

thanks

[Soundman2020]

I would suggest that you provide a detailed diagram of what you have done to the room so far with your bass traps and other treatment (and also mention how you built each of those, in terms of exact materials), then get rid of the boom box, which is not telling you anything useful at all about the room (and in fact is probably just confusing the hell out of you), set up some good, accurate, full range studio monitors in the position they will be when the room is finished, set up a measurement mic at the mix position, and run a full REW analysis on the room. Post the resulting MDAT file here, so we can download it and analyze it, then figure out what is wrong with the room and suggest how you can fix it.


- Stuart -

[madmuso]

Hey Stuart,

Cool, I will have to see if any of my friends has a usb audio card (and measurement mic) I can hook up to my laptop, I cant setup my recording rig in there right now because im in the middle of mixing an ep for a client. I will however unplug my krk V6 monitors and set them up in the room. Thats no issue.

thanks,

[madmuso]

I did some tests today, it seems the results are definitely showing what I was experiencing. Check out the difference at 60hz between the rear and middle positions. I will do the diagrams of the room treatment next.

[madmuso]

100hz is a consistent problem through all the test positions.

[Soundman2020]

Please post the actual MDAT file. It is not possible to draw any conclusions at all from the graphs you are showing, since they 1) Show the entire spectrum, 2) you have applied smoothing, 3) they are only frequency domain, and 4) you didn't say which speaker each graph corresponds to (left, right, or both).

Also, you either didn't calibrate REW correctly, or you didn't run the tests at a high enough level: The graphs show that they were run at a level of a bit under 60 dB, but in reality they should be run at a level of 83 dB per individual speaker, which should be about 86 dB for both speakers.

So please calibrate REW correctly for the SPL levels, using a hand-held sound level meter, then re-run the tests at a level of 83 dB, then post the actual MDAT file here on the forum (or if it is too big, then post it elsewhere on a file sharing site, such as Dropbox, then post the link here).

Check out the difference at 60hz between the rear and middle positions.

The middle of the room is the worst possible position in any room, since all of the first order modal nulls and second order modal peaks are located right there. Also, the "60 Hz peak" is actually very likely your 0,1,0 axial mode, which should be at 57.4 Hz, theoretically, and certainly looks to be a bit under 60 Hz, (but it's impossible to be sure since you are using major 1/12th octave smoothing on those curves.). The "100 Hz" issue is very likely your 1,0,0 axial mode, which should be at 98.4 Hz, but it probably also combined with the 0,1,1, tangential mode at 97.1 Hz, which could be why you are seeing the double-peak on the last graph you show. The sharp dip at around 150 Hz is very likely your 3,0,0 axial mode, at 147.6, possibly combined with the 1,2,1 oblique mode at 147.4. The other sharp dip, between 70 and 80 Hz, is very likely your 0,0,1 axial (78.3 Hz), or possibly your 1,1,0 tangential mode (75.6Hz).

You say that you did one measurement at the listening position, but where exactly is that located in the room? In other words, where precisely did you have the tip of the mic when you did that test? It was on the room center-line for sure, but how far from the front wall and the rear wall, and at what height above the floor? If you don't specify exactly where you took each reading, the it basically doesn't mean anything since there's no way to correlate it properly to the graphs produced by REW and the predicted response of the room.

Also, what speakers did you use for the tests, and what mic? The graphs show that you have the soundcard calibration turned off and don't mention the mic calibration (suggesting that you don't have a mic calibration file), but all of that is critical to producing accurate graphs.

So please try to correct all that, then do another test and we should be able to see what is going on with the room.


- Stuart -

[madmuso]

Hey Stuart,

I will run the tests again but heres some info anyway.

I used one speaker only, my krk V6. I used an mbox2 hooked up to my laptop and a behringer ecm8000 measurement mic, not the best but all I can afford. I thought I had the levels set right, I viewed the tutorial vid a few times to make sure I knew what I was doing but obviously I dont and misunderstood part of it, will check it out. Theres was no mention of using an spl meter to calibrate anything either. Will check again. If so will have to try and get my hands on one.

The speaker was placed dead centre of the room as far up to the front wall as possible at roughly the height it will be at. The mic was 800mm from the front wall and height wise I put it at my ear level when I am sitting in a chair ( I have no measurement cause I physically sat next to it and set up the height like that then locked the mic stand), the mic was pointing up (vertical). For middle position it was 1500mm from front wall and rear position 2800mm from front wall.

I got a wedding tomorrow and a few hectic days after that so as soon as I get a chance will run tests again. Thanks!

[Soundman2020]

I will run the tests again but heres some info anyway.

Cool! Useful stuff.

I used one speaker only, my krk V6.

According to the manual, that's pretty flat from 58 Hz up to 22 KHz, so that's fine.

I thought I had the levels set right, I viewed the tutorial vid a few times to make sure I knew what I was doing but obviously I dont and misunderstood part of it, will check it out. Theres was no mention of using an spl meter to calibrate anything either. Will check again. If so will have to try and get my hands on one.

There's a section in the manual that tells you how to do that, I think. If not then let me know, and I'll PM you the procedure I use. It's fairly simple.

a behringer ecm8000 measurement mic,

It's fine. There's a "standard" calibration file for that some place, which you should load into REW for your measurements. It tells REW about the response of the mic itself, so it can subtract that from the data and give more accurate results.

I used an mbox2

The calibration procedure in the REW manual shows you how to produce the calibration file for that, which you should also load into REW for all your measurements. It's the same concept as for the mic: it compensates for differences in the frequency response of the interface, if any.

The speaker was placed dead centre of the room as far up to the front wall as possible at roughly the height it will be at.

OK, that might be an issue. Try to set it in the location where either the left or right speakers will be, instead of in the middle, and also angle it 30° so it is pointing at the mix position. Set it up so the acoustic axis is about 1.2 m above the floor (47-1/4"). Note! that is "acoustic axis"! Not the top or bottom of the box! If you can't find out where that is in your speaker (check the manual), then you can "guesstimate" it as being on an imaginary line that joins the middle of the woofer to the middle of the tweeter, and very much closer to the tweeter. Probably right on the edge of the tweeter cone, roughly. The mic should also be set up so the tip is where the middle of your head will be when mixing, and at the same height as your ears, which is also 1.2m above the floor, standard, and the mic should be pointing straight upwards (some people say) or forwards at an angle of about 45° (my personal preference). You had it straight up, and that's fine if the ceiling is treated properly.

The mic was 800mm from the front wall ... My control room is ... 3 meters long

Ooops! That's a problem! Your mix position is almost exactly at the second worst point in the room! 800 mm back in a room 3m long puts you at 26% of the room depth. The worst point is 50%, and the second worst is 25%. You are at 26%.... The theoretical best position is 38%, so your head should be 1.14m from the front wall. Or rather, that's where your ears should be, and therefore that's where the tip of the mic should be: 1140 mm from the front wall.

Once you get the mic set up properly, then measure the exact point where the tip of the mic is in the room, with great precision, since that's the place where you have to get it every single time from now on, each time you take a new reading. It needs to be accurate to with about 1/4", in all three dimensions, so that you can validly compare measurements with each other.

as soon as I get a chance will run tests again.

Great! Looking forward to seeing the new data!


- Stuart -