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Secondary Mix Room Treatment Project
Old 6th August 2020
  #32
Gear Nut
 

Quote:
Originally Posted by Kyle P. Gushue View Post
Quite a few participants go on to reference the Harmon white papers as I have. I’ve had the pleasure to experience the four sub method which lead me decide to implement it in my control room. The only ‘issue’ I have with the initial Harmon discussions recommend two subs......and if all you were concerned with was 100-125hz at the LP i’d agree.........but for 40-60hz it’s utter chaos.......three or four subs cure the entire sub range IME and the work by Earl Geddes I quoted earlier confirms this.

Thanks for the link....it was an excellent read.
Old 10th August 2020
  #33
Quote:
Originally Posted by thethrillfactor View Post
Read this paper:
http://downloads.bbc.co.uk/rd/pubs/reports/1992-10.pdf

Its one of the issues one encounters of adding helmholtz or membrane absorbers after to a structure instead of building it as part of the structure. If its not sealed correctly or if part of the box vibrates it can create peaks and resonances of their own. Also if the walls aren't stiff enough it becomes an issue as well.

Also the more and deeper you build/seal the traps, what the wavefront sees changes as the dimension of the room has changed.
Thank you. Yeah, that seems to be the best explanation of what I’m seeing. I probably wouldn’t have believed this phenomenon would be so measurable, had I not seen it in the data, but if you measure the distance between the membrane (front) surface of the absorbers on the front wall to the same on the rear ceiling, there indeed is the 56 Hz problem.

After calculating that the membrane surface area now makes up 20% of the surface of the front and rear walls they now cover, it’s easier to understand (and believe).

When I started this endeavor, I also thought the membrane absorbers were less sensitive to specific frequencies only - thinking they would act less like notches; more like smooth band filters. But this experience shows that at least the ones I built are very specific to the frequency they were built for. So the new frequency problem at only 1/3 octave up, is not affected very well by the same absorbers.

Great post, and thank you for the information. Mystery solved!

As for the multiple subs arrangement, I appreciate your input, as well. This does seem to me, to be a workaround, rather than an answer to the modal problems; albeit a practical solution. If it fits the budget, we may play around with multiple subs, down the road. We have some inquiries from the surrounding area (including a Jr, High School) that wish to treat some very small rooms for social media teaching. They need some low RT60 times in their space, and the sound sources (instruments) are highly varied and mobile. No live monitoring. Even though such rooms are very different in purpose than our mix room, I’m presently motivated to focus on room problems more than the workarounds.

But all the material presented is very good and helpful, thank you guys. I think, at this point, we’re going to move forward to the next primary modal problem, between floor & ceiling, at 68 Hz. The 58 Hz is not a problem big enough for me to focus on, and it may even out, down the road, as some broadband is likely to be employed.
Old 10th August 2020
  #34
Here for the gear
 

Seeking clarification on Tim's work.

Hi

This thread has been a good read. I appreciate the work. Thank you. It's a real benefit reading your results and following the thinking here. Best wishes for your room.

Seeing you're working off Tim's limp mass bass trap design I'm wondering if you could clear this one up for me please.

Tim gave a couple of sample trap layouts for a room - 6m x 3.5 x 2.5. Those dimensions generate standing waves at 28.6, 49 and 68.6 Hz.

However, Tim said there are 'dominant modes at 50Hz and 85Hz'. So he recommended tuning traps to 50, 85 and 120Hz - the latter as a 'generalist' added measure.

Why choose 85Hz and not address 29 or 69?

I can see Tim's 50 Hz fits for the 3.5m (49Hz). And the 28.6 might be worth ignoring because standard monitoring/bass instruments (without subs) won't resonate that low. And that 85Hz is the 2nd octave of 28.6.

But why not target the 68.6Hz or the 57.2 (octave of 28.6) etc. ?

Did you notice this in his recommendations and what do you think?

Cheers
Old 10th August 2020
  #35
Quote:
Originally Posted by antpain View Post
Why choose 85Hz and not address 29 or 69?

I can see Tim's 50 Hz fits for the 3.5m (49Hz). And the 28.6 might be worth ignoring because standard monitoring/bass instruments (without subs) won't resonate that low. And that 85Hz is the 2nd octave of 28.6.

But why not target the 68.6Hz or the 57.2 (octave of 28.6) etc. ?

Did you notice this in his recommendations and what do you think?

Cheers
Hello, antpain.
I see your point and follow you calculations. Maybe someone else can answer this, but I can’t. I agree that a lot of people won’t likely feel the need to treat the extra low frequencies. But I calculated the same axial modes as you, and if it were me, they’d probably be on my list.... unless measurements proved otherwise.
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