Bass traps and diffusers

[Spencerleehorton]

Just wanted to document my journey with making bass traps and diffusers for my studio.
I have had 11 panels up with various sizes some filled with foam some filled with rockwool.
I have done an online course which details what works and how to build bass trap which will make a difference to my mixing room and tracking room.
Basically all of what I did have on the walls and corners fits into one bass trap!!
I am build 12 bass traps to start with four of them will be diffusers.
Below is studio before, with the old hessian panels, which did pretty much nothing!

 

[Spencerleehorton]

Materials
2 x 8’x4’ 18mm thick plywood sheets for the frame.
These would be cut in 100cm x 60cm x 20cm, once drilled and screwed the panel would be 104 x 60 x 20.
Light sanding and varnished.
I would need to make a little internal frame 52 x 96 for the material front cover which I got a 11m x11m dust sheet (cotton) which I cut up into 12 sections.
Stapled on the material to the internal frame and secured within the main frame.
Rockwool was then cut and wrapped in cling film/ clear packing film to stop it smelling and then put within the frame.
I have the left over hessian to cover the back and then two struts to hold it all in on the back.

 

[Spencerleehorton]

I will do some more pictures once I’ve built some more

 

[Spencerleehorton]

First 4 made and room is starting to sound better, needs more though.

 

[Silas]

Very nice!
Am I right in thinking these are open back?

Btw, which course did you do?
I’ve not done any courses on acoustic absorption but have found the BBC docs very helpful - in my own case building some of their membrane absorbers, which helped loads with low end absorption in my studio

 

[Spencerleehorton]

This is the course

https://www.acousticsinsider.com/products/build-a-better-bass-trap
Jesco is the guy, very informative information, really has helped so far.

I’ve put on a sign wave and walked around the room, I can here where I need more panels on the back wall behind the mix position and either side of the mix position, I’ll also put a couple above the mix position and probably some on the front wall in between the monitors

I’ve tested from 42hz up to 130hz everything seems fine after 130hz.

 

[Spencerleehorton]

And yes sorry they are open back so there is a nice air gap also behind in corners which helps

 

[Spencerleehorton]

Just reading up on the BBC modular absorber

 

[Whoops]

Wish you a good work mate,
I'm also doing the same in my mixing/mastering room, but had some much work after the summer that couldn't finish it and now it's too cold (and sometimes Wet) to work outside on the panels and traps, so my work will have to resume in spring

 

[Silas]

This is the course

https://www.acousticsinsider.com/products/build-a-better-bass-trap
Jesco is the guy, very informative information, really has helped so far.

I’ve put on a sign wave and walked around the room, I can here where I need more panels on the back wall behind the mix position and either side of the mix position, I’ll also put a couple above the mix position and probably some on the front wall in between the monitors

I’ve tested from 42hz up to 130hz everything seems fine after 130hz.

Ahh, good to hear. He did an interview on a YouTube video I saw a while back, seemed helpful on it.

For those lower sub 100hz frequencies the membrane absorber can tackle them quite effectively I’ve found

Interestingly, the biggest thing I could do when I moved studios recently, for some very large modes, was speaker placement (it was quite subtle movements in the end) as they just excite so much energy that I was struggling to absorb it well enough.

 

[Silas]

Just reading up on the BBC modular absorber

There’s a few iterations, this is the one I built The design of a modular sound absorber for very low frequencies

I’ve always wondered how easy it would be to build one tuned slightly higher, as that would be a helpful addition…

Good luck with the build!

 

[kags]

This is the course

https://www.acousticsinsider.com/products/build-a-better-bass-trap
Jesco is the guy, very informative information, really has helped so far.

I’ve put on a sign wave and walked around the room, I can here where I need more panels on the back wall behind the mix position and either side of the mix position, I’ll also put a couple above the mix position and probably some on the front wall in between the monitors

I’ve tested from 42hz up to 130hz everything seems fine after 130hz.

Nice work! Remember that you can only make one location sound the best in a room. There will always be low-freq pressure areas at boundaries (walls/floor/ceiling). You are using velocity absorbers and I would suspect more than 10 will be needed to create a nice response at the listening position.

You should definitly use the structured listening test (as described by Jesco) to find the best listening position - even a few inches forward or back will be noticeable. I use Online Tone Generator with sweep tones from 30 Hz up to 200 Hz and watch the freq readout. Do this sweep at few locations forward and behind the ideal location (starting around 38% from the front wall). I make a chart and mark high and low SPL freqs at four or five locations and then you can visualize the most flat location from your notes.

6" panels straddling the corners like you have made should help down to at least 80Hz and sometimes lower.

You probably have a notch from SBIR (front wall to speaker baffle distance) around 190 Hz that could be fixed with some 6" panels on your front wall between your corner traps. Sidewall treatment will help with first reflection and L-R axial mode. Ceiling cloud with first reflection and floor-ceiling axial mode. I would recommend at least 8" for the ceiling cloud as your floor-ceiling mode will be around 70 Hz. Space couplers above the mix position can help with that, too.

If you plan membrane/pressure absorbers, I recommend a "shotgun" approach. Say you want to target 70 Hz. You should build three sets of absorbers, targeting 60, 70, and 80 Hz. Building a resonator that hits your target frequency is very tricky and spreading your frequency coverage this way all but ensures you will hit your mark. I recommend panel traps with 1/4" (6mm) plywood facing. Cut the plywood into half-sheets and make the panels 2 feet by 4 feet or 8 feet. Make 2 absorbers for each frequency and you'll have six absorbers.

Acoustic treatment can be iterative, so build a few devices and them measure to see the change. Then build more as needed. In small rooms, I rarely use diffusion and also rarely need tuned pressure absorbers.

best wishes!
Adam

 

[Spencerleehorton]

Nice work! Remember that you can only make one location sound the best in a room. There will always be low-freq pressure areas at boundaries (walls/floor/ceiling). You are using velocity absorbers and I would suspect more than 10 will be needed to create a nice response at the listening position.

You should definitly use the structured listening test (as described by Jesco) to find the best listening position - even a few inches forward or back will be noticeable. I use Online Tone Generator with sweep tones from 30 Hz up to 200 Hz and watch the freq readout. Do this sweep at few locations forward and behind the ideal location (starting around 38% from the front wall). I make a chart and mark high and low SPL freqs at four or five locations and then you can visualize the most flat location from your notes.

6" panels straddling the corners like you have made should help down to at least 80Hz and sometimes lower.

You probably have a notch from SBIR (front wall to speaker baffle distance) around 190 Hz that could be fixed with some 6" panels on your front wall between your corner traps. Sidewall treatment will help with first reflection and L-R axial mode. Ceiling cloud with first reflection and floor-ceiling axial mode. I would recommend at least 8" for the ceiling cloud as your floor-ceiling mode will be around 70 Hz. Space couplers above the mix position can help with that, too.

If you plan membrane/pressure absorbers, I recommend a "shotgun" approach. Say you want to target 70 Hz. You should build three sets of absorbers, targeting 60, 70, and 80 Hz. Building a resonator that hits your target frequency is very tricky and spreading your frequency coverage this way all but ensures you will hit your mark. I recommend panel traps with 1/4" (6mm) plywood facing. Cut the plywood into half-sheets and make the panels 2 feet by 4 feet or 8 feet. Make 2 absorbers for each frequency and you'll have six absorbers.

Acoustic treatment can be iterative, so build a few devices and them measure to see the change. Then build more as needed. In small rooms, I rarely use diffusion and also rarely need tuned pressure absorbers.

best wishes!
Adam

I have built all 8” deep panels thanks so much for this info.

 

[kags]

I have built all 8” deep panels thanks so much for this info.

Excellent ! I’m glad to hear that

Jesco has been a fantastic evangelist for practical acoustic treatment.

I’ve been designing studios and teaching acoustics for over 20 years and it’s nice to see people assembling good sounding rooms without resorting to “black magic” and secretive acoustic designs. It’s not that hard to make a typical room sound great.

I wish you great enjoyment of your space!

 

[Spencerleehorton]

So in between my speakers on the front wall you would put two more panels in there but round the other way, rather than vertical they would be put horizontal?

 

[Spencerleehorton]

here are the basic dims to my room using room sim in REW.


these are both with the monitors 0.51m away from the front wall and listening position 0.51m from the monitors in the centre of the room and with no absorbtion.


this one is now with absorbtion front, back, each side, ceiling and floor.
ive moved the monitors and listening position around a fair bit and this seems to be the best postion to get the flattest response so far.
next will be room testing with ECM8000 and REW, also will try the other miniDSP UMIK-1 calibrated USB measurement microphone.

 

[kags]

The REW sims look great, and you can clearly see the predicted effects of the modal issues (especially the axial width bass suckout at the listening position). Since your room is already set up, the real measurements are easy to do and you might find some variations from the prediction. I would guess the height of the center of your monitors is a bit higher than 1m - probably more like 125-130 cm. Also, your listening position is about 27% from the front wall, which may be perfect, but it's worth testing a bit further back - move about 10 cm at a time and see where the sound really locks in.

For quick, but accurate low- and mid-frequency testing, you might want to use pink noise and Charles Sprinkle's moving mic technique with REW. Sweeps are great when you want the ETC, decay time plots, etc. but the pink noise will quickly provide a picture of what the room is doing in the low frequencies around your listening position.

The horizontal vs. vertical placement of the front-wall panels shouldn't matter much, it's really the amount of surface area that you're treating that will affect the efficiency in this case. If you're not actually finding an SBIR problem in your measurements, the front wall may not be an issue. The corner traps may be effectively handling that issue. If you have some panels built, you can measure with and without them (on the front wall) to see how they affect the room.

If you get close to the REW sim in the last image, you will be extremely happy with your room - but be sure to check the decay times, to really hone in on any low-frequency ringing that might not be obvious from the frequency graph.

best!

 

[kags]

next will be room testing with ECM8000 and REW, also will try the other miniDSP UMIK-1 calibrated USB measurement microphone.

In practice, a mic with a calibration file is best, but both of those mics will be extremely accurate up to at least 1 kHz. Above 1 kHz so many things interact that the accuracy of most measurement mics is fine. I've compared several measurement mics and determined that I wouldn't make any different treatment or correction decisions based on the information from different microphones. The same goes for all of the interfaces I've used.

 

[Spencerleehorton]

Ah that’s good to know that the ecm8000 mic will be just the same as the others.
When adjusting the back position for absorption it seems to make the most difference but all positions do get it better and flattern the bottom end.
I’ll hopefully resume finishing these other 8 traps and then test the room with the ecm8000 mic.

 

[Spencerleehorton]

So my question would be this:
My problem with mixing seemed to be I was adding too much bass in general to the mix which would I think mean that there was a low end null within the mix position, does the data that room sim outputs so far support this?