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How far to go? And treating low frequency ringing
Old 16th May 2019
  #1
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How far to go? And treating low frequency ringing

Hi everyone,

I've been reading a lot of info here and doing various treatments to my room, moving monitors and listening position and have got it to this point...

The room is 3.5m wide x 5m long x 2.4m high.
Mainly production and mixing - but also tracking vocals and other instruments.

See my attached files. All floor to ceiling corners are straddled, 2 (short) wall to ceiling are straddled... First reflections treated and a massive cloud. Front, back and side walls have more panels on them too...

I'm quite happy with the results so far - if anything it's too dead in here. I'm planning to build some LeanFusers to put around the room.
What do you think of the response?

I have a lot of low ringing going on from 30 to 40hz - what's the best way to attack this?
I was thinking of building some tuned bass traps - helmholtz resonator style boxes.

Any and all assistance appreciated
Attached Thumbnails
How far to go? And treating low frequency ringing-lwsub.jpg   How far to go? And treating low frequency ringing-lwsubwf.jpg   How far to go? And treating low frequency ringing-rwsub.jpg   How far to go? And treating low frequency ringing-rwsubwf.jpg  
Attached Files
File Type: mdat L wsub.mdat (2.89 MB, 13 views) File Type: mdat R wsub.mdat (2.89 MB, 9 views)
Old 16th May 2019
  #2
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boggy's Avatar
Quote:
Originally Posted by Dryza View Post
.....
I have a lot of low ringing going on from 30 to 40hz - what's the best way to attack this?
I was thinking of building some tuned bass traps - helmholtz resonator style boxes.

Any and all assistance appreciated
I see "ringing" from 30-40Hz but not "a lot", that may be acceptable.

Bigger problem is dip in response at 80Hz.
- Are you sure that you cannot solve it with loudspeaker/listener positioning.
- Are you sure that phase of subwoofer is adjusted right. Did you try to reverse it, and measure?
- Are you sure that your subwoofer position is best?

Attached Thumbnails
How far to go? And treating low frequency ringing-gs1.jpg  
Old 16th May 2019
  #3
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Hi boggy

Yes, I have experimented with moving monitors closer/further from both the front wall and side walls, and their height. This is the best response I have got - the null is at its highest frequency and least dip in this response.

I did test with the phase of the sub, this is in phase and has only a slight difference from the out of phase position.

Subwoofer position has also been tested and measured - currently slightly off centre to the left in the room and facing me - this gave the best results.

FYI the sub is the new part of the studio - which I got to try to fill in that null. While it did help with it by about 4db, there was another big null at 111hz before and the sub has somehow cleared that up. This was most evident when experiencing with the HPF on the sub to the tops. (Sub is a PreSonus T10 and mid/tops are dynaudio acoustics BM6A mk1)

I figured the 70-80hz null is the octave of 30-40hz ringing - so treating the 30-40 would help the 70-80? Or am I reading this wrong?
Old 16th May 2019
  #4
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boggy's Avatar
Quote:
Originally Posted by Dryza View Post
....I figured the 70-80hz null is the octave of 30-40hz ringing - so treating the 30-40 would help the 70-80? Or am I reading this wrong?
Dip at 70-80Hz is non resonant interference (people call it SBIR), it’s just cancelation because out of phase reflection from boundary, so your “ringing” is not correlated with it.
Old 16th May 2019
  #5
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I see, thank you boggy.

I did a lot of testing with SBIR in mind - the monitors are almost touching the front wall, which when first put there did yield the best result.

So, considering I have moved the monitors to their best possible position physically in the room - what would you suggest is the most effective way to treat the 70-80hz null?

When playing a sine in these frequencies, from the listening position they are much lower, but can be heard in different parts of the room - which confirms the SBIR.
Old 16th May 2019
  #6
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You should have posted in your first thread here:

New room opinions - waterfall/FR graphs

...so we have the full story and all infos needed to help you.

Seeing the first waterfall from your very first post (thread linked) and your last measurement here. We can see there a very small improvement regarding freq. under 100hz.
How far to go? And treating low frequency ringing-screenshotfr.png


Any new pics to share?
Attached Thumbnails
How far to go? And treating low frequency ringing-screenshotfr.png  
Old 16th May 2019
  #7
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Hi JayPee,

No new pics other than the new graphs already shared in first post of this thread.
Do you have a suggestion on how to treat the 70-80hz null?
Old 16th May 2019
  #8
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Pics of your room I meant.
Old 16th May 2019
  #9
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Hi JayPee

No, no new pics of the room. Nothing has changed other than the sub being introduced, which is discussed earlier in this thread.

Cheers
G
Old 17th May 2019
  #10
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Does anyone have any suggestions on treating the LF dip?
Old 18th May 2019
  #11
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johannburkard's Avatar
My uneducated guess: You need to find out what wall the interference is happenning on and then trap these frequencies there. I think if you move the microphone left and right and the frequency doesn't change, it's the ceiling, the side walls otherwise.
Old 18th May 2019
  #12
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boggy's Avatar
Quote:
Originally Posted by johannburkard View Post
My uneducated guess: You need to find out what wall the interference is happenning on and then trap these frequencies there. I think if you move the microphone left and right and the frequency doesn't change, it's the ceiling, the side walls otherwise.
Yes, thank you!


But, as you can see, his main problem is that he already has treatment in the room, which didn’t address anything below 100-150Hz, obviously. Absorbing 70-80Hz is not possible to improvise now, when everything is finished.
To cover ghost speaker interference from the boundary you need at least bass trap with size (WxL) of minimum half wavelength of the sound wave frequency of this dip, which can absorb frequencies down to 60Hz at least. And that will be THICK(!) even with best rockwool (GFR and density)!

This is why I use combination of slotted diffusers as Helmholtz absorbers (MyRoom Acoustic Design) and thick LF wide-band absorption in small rooms, so I address this very problem from the beginning, covering whole walls and ceiling with combination of adequate designed low frequency absorbers. Its task is to hide and isolate all main ghost speaker images from room boundaries (reflections which cancelling direct waves at the listener position). Later, if there are still some dips... I do couple of tweaking using parametric equalizer, usually, I got flat response from+/-2 to +/-3dB below 300Hz. Here is 8-9dB dip... that cannot be corrected with equalizer.

As you can see... addressing just room modes in small rooms is easy, but we still won't have (without luck) a mixing room with decent frequency response, after only room modes are damped.

This situation is a reason why I newer assyme Room Modes as real problem in Control Room acoustics design. Also, I never acoustically measure Control Room before design, because there will be nothing relevant to measure.



Last edited by boggy; 18th May 2019 at 09:37 PM..
Old 20th May 2019
  #14
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Thank you johann burkard, boggy and JayPee.

The room is not finished however - hence why I am researching the best and most feasible method for treating the LF dip.

It is looking like tuned helmholz absorbers could be the best way - so am most likely looking at building something covering most of the back wall - approx 1.8m wide x 2.4m high x 20cm deep... or maybe build 2 for the side walls - approx 1m wide x 2.4 high x 20cm deep...
Old 20th May 2019
  #15
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Quote:
I am researching the best and most feasible method for treating the LF dip. ... It is looking like tuned helmholz absorbers could be the best way
Maybe you didn't see what Boggy wrote:
Quote:
Dip at 70-80Hz is non resonant interference (people call it SBIR), it’s just cancelation because out of phase reflection from boundary,
If it is NON-RESONANT, caused by a phase cancellation, then it is impossible to fix it with a RESONANT device! Think about it: For a resonant device to work, you would have o place it at the location of the resonant peak in the room. A non-resonant issue has, by definition, no resonance at all, and thus no peak: Therefore, there is no possible place in the room where you could place such treatment... You cannot fix a phase cancellation with resonant devices, just as you cannot fix it with EQ. You can only fix it by, in some way or other, changing the condition that is causing it in. One option is to re-arrange the geometry of the mix position and speakers so that they are no longer in that phase-cancellation relationship, another option is to add one or more sub-woofers to the room, to create a different interference pattern that "cancels the cancellation", and yet another is to modify the room itself in some way that breaks the current phase cancellation problem.

- Stuart -
Old 20th May 2019
  #16
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boggy's Avatar
Quote:
Originally Posted by Soundman2020 View Post
............
If it is NON-RESONANT, caused by a phase cancellation, then it is impossible to fix it with a RESONANT device!
Yes, it is, because not everything is black and white. Same as it is possible to reduce traffic noise on highways with Helmholtz based absorbing barriers. That is well known practice.
Quote:
Originally Posted by Soundman2020 View Post
Think about it: For a resonant device to work, you would have o place it at the location of the resonant peak in the room.
Yes, it is most effective place for resonant absorber to reduce resonant problems in the room. But we do not have resonant problems here... so we need to place it on different location.
Quote:
Originally Posted by Soundman2020 View Post
A non-resonant issue has, by definition, no resonance at all, and thus no peak: Therefore, there is no possible place in the room where you could place such treatment...
Fortunately, Helmholtz based absorber doesn't care too much about way of generating sound wave if it is from room resonance or it is a just test signal tone from speakers... It will absorb as best as it can in the place where it is used. That is what Helmholtz absorber do.
Quote:
Originally Posted by Soundman2020 View Post
You cannot fix a phase cancellation with resonant devices, just as you cannot fix it with EQ.
Phase cancellation can be fixed with carefully designed absorber over whole wall/ceiling... To cover a ghost image of speaker which introduce phase cancellation. Same as people cover traffic noise with barriers on highways.

This Helmholtz type need wide spread slats or holes... not just couple of holes to target single frequency... It MUST absorb over all surface... or at least 60% of it.

Usually, my Helmholtz absorbers-barriers have 3000-5000 liters of volume...with 2-5m2 of absorptive surface.

Best way is to integrate absorber with diffuser as described here:

A Novel Approach of Multichannel and Stereo Control Room Acoustic Treatment, Second Edition, 140. AES Convention, Paris, 2016

This way, air mass loading on the neck of Helmholtz absorber will be increased, and resonant frequency will be decreased...

After everything designed carefully... absorber doesn't have pronounced just resonance peak... but become wide band absorber with a little extended low end absorbing characteristic, as it's shown in paper above.


Quote:
Originally Posted by Soundman2020 View Post

You can only fix it by, in some way or other, changing the condition that is causing it in. One option is to re-arrange the geometry of the mix position and speakers so that they are no longer in that phase-cancellation relationship,
There may be a chance to scrap what is done and reorganize everything... but no guarantee... also... after adding absorbing materials... room acoustical conditions changes.
Quote:
Originally Posted by Soundman2020 View Post
another option is to add one or more sub-woofers to the room, to create a different interference pattern that "cancels the cancellation", and yet another is to modify the room itself in some way that breaks the current phase cancellation problem.
...
Room is too small to find another position to "cancel the cancellation"... it is 70-80Hz... it is not enough space to avoid cancellation. Also... adding a subwoofer is similar as trying to equalize, especially because subwoofer have the nearly same conditions as satellite speakers, so it will also suffer from the phase cancellations, because they isn't attenuated.

There is no better way, AFAIK, for small rooms, than to make barrier to cover reflected, "ghost", loudspeaker image, then, avoid source of sound wave cancellation.

For bigger rooms,... only thick wideband absorbers is just enough... Same as Philip Newell do in N-E rooms.



Last edited by boggy; 20th May 2019 at 07:26 PM..
Old 20th May 2019
  #17
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Quote:
Originally Posted by boggy View Post
Yes, it is, because not everything is black and white. Same as it is possible to reduce traffic noise on highways with Helmholtz based absorbing barriers. That is well known practice.

Yes, it is most effective place for resonant absorber to reduce resonant problems in the room. But we do not have resonant problems here... so we need to place it on different location.

Fortunately, Helmholtz based absorber doesn't care too much about way of generating sound wave if it is from room resonance or it is a just test signal tone from speakers... It will absorb as best as it can in the place where it is used. That is what Helmholtz absorber do.

Phase cancellation can be fixed with carefully designed absorber over whole wall/ceiling... To cover a ghost image of speaker which introduce phase cancellation. Same as people cover traffic noise with barriers on highways.

This Helmholtz type need wide spread slats or holes... not just couple of holes to target single frequency... It MUST absorb over all surface... or at least 60% of it.

Usually, my Helmholtz absorbers-barriers have 3000-5000 liters of volume...with 2-5m2 of absorptive surface.

Best way is to integrate absorber with diffuser as described here:

A Novel Approach of Multichannel and Stereo Control Room Acoustic Treatment, Second Edition, 140. AES Convention, Paris, 2016

This way, air mass loading on the neck of Helmholtz absorber will be increased, and resonant frequency will be decreased...

After everything designed carefully... absorber doesn't have pronounced just resonance peak... but become wide band absorber with a little extended low end absorbing characteristic, as it's shown in paper above.



There may be a chance to scrap what is done and reorganize everything... but no guarantee... also... after adding absorbing materials... room acoustical conditions changes.


Room is too small to find another position to "cancel the cancellation"... it is 70-80Hz... it is not enough space to avoid cancellation. Also... adding a subwoofer is similar as trying to equalize, especially because subwoofer have the nearly same conditions as satellite speakers, so it will also suffer from the phase cancellations, because they isn't attenuated.

There is no better way, AFAIK, for small rooms, than to make barrier to cover reflected, "ghost", loudspeaker image, then, avoid source of sound wave cancellation.

For bigger rooms,... only thick wideband absorbers is just enough... Same as Philip Newell do in N-E rooms.


I'd love to see your theory on how to fix SBIR with a Helmholtz resonator...

- Stuart -
Old 20th May 2019
  #18
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Jens Eklund's Avatar
All interference related issue (constructive or destructive) can be fixed by removing he aditional phasors (causing the interference). Any effective absorber, including pressure based such as Hslmholtz or membrane (assuming correctly tuned and on relevant surface area) will get the job done.
Old 4 weeks ago
  #19
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boggy's Avatar
Quote:
Originally Posted by Soundman2020 View Post
I'd love to see your theory on how to fix SBIR with a Helmholtz resonator...

- Stuart -
“My” theory you can find in books and scientific papers about acoustics. Similar principle is used in many other/different products and in many ocasions for a very long time. I also have active experience, doing that for years. So it is far from just a theory.

I will rather like to see one document or one book where you find theory that resonant based absorbers cannot solve (absorb destructive components of) non-resonant interference in the room. I never read that until now, from you.


Old 4 weeks ago
  #20
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an HR is an hard surface and simulate a rapprochement of the wall. So it's can correct sbir.
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