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skidmorebay

Tin Man

Posts: 25 Member Since: 07/08/2015

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Nov 8 16 12:45 PM

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Hello,
I am using FuzzMeasure to check the response of my new room. Is it best to take measurements from one speaker at a time, or both at once? And, is it a good idea to measure with one speaker in a low corner, and the mic in a high corner in order to identify the modes in the room, as I have read about?

And once this is done, do you tend to tune low-frequency trapping (DIY perforated panel traps, in my case) for the observed peaks in the LF response at the listening position, or do you target the peaks in the modal response that is observed from measuring in the corners, probably in combination with the theoretical modal response based on room dimensions? 

I assume that the peaks in the LF response I observe at the listening position are partially from constructive interference of sound waves of different frequencies than the resultant peaks.
thanks for your replies,
JS
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morespaceecho

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Posts: 2,304 Member Since:29/01/2011

#2 [url]

Nov 16 16 2:53 PM

try posting this in the acoustics forum at GS, you will probably have better luck there. there's a small handful of people there who know what they're talking about, you will quickly figure out who they are.

when i've done any measuring, it was both speakers at once with the mic at the listening position. but i'm an acoustics beginner, so my advice isn't worth much.

but i'm very curious to hear more about your perforated panel bass traps, if you care to share!

www.oldcolonymastering.com

morespaceecho.bandcamp.com

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hank alrich

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Nov 16 16 3:35 PM

How much time have you spent with F. Alton Everest's Master Handbook of Acoustics? There's a lot to this, and studying close to the roots of the tree of functioning knowledge can save a lot of time.

hank alrich
http://hankandshaidrimusic.com/
http://www.youtube.com/walkinaymusic

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skidmorebay

Tin Man

Posts: 25 Member Since:07/08/2015

#5 [url]

Nov 19 16 10:31 AM

Thanks for the replies. 
I've spent a lot of time reading and re-reading The Master Handbook of Acoustics over the years. It's fantastic, but because it covers the entire field so broady it sometimes lacks more specific "tricks of the trade" that a designer might use in a situation like mine (control room design). It has definitely informed everything I've done to date. 

Morespaceecho, the panel traps I have started installing are based on the design in the book, and by this spreadsheet that I found online:
https://www.dropbox.com/s/dqv8g35vx6mj5jq/Porous%20Absorber%20Calculator%20V1.57.xlsm?dl=0

Here's my room, measured with a speaker in a low corner and a mic in a high corner, per Everest in order to reveal existing modal resonances:

image

And here's the room, measured from the listening position. Yellow is L, purple is R. This is after treating the reflection points with absorption, but without any LF control:



image
The room is 12.5 feet high, 14.88 feet wide, and 18 feet long (3,347 cu. ft., ratios chosen from Trevor Cox's paper on optimum room ratios). This makes it 1 x 1.19 x 1.44. It's actually a wood frame room built inside of a large commerical shop. A lot of LF leaves the room easily. 

I built the cavities for three large panel absorbers on the back wall, 11.25" deep. I haven't designed the panels yet. 
I'm not sure if I should "tune" the panel absorbers to broadly address the bump between 100hz-700hz seen from the listening position, or the 40hz-120hz seen in the corners. Or if I should start somewhere else. What would a pro do?

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morespaceecho

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#6 [url]

Nov 19 16 11:53 AM

heh, we are in similar boats here. i have that whealy calc too. i asked one of the guys on GS for some advice re: panel traps, i'll let you know if i hear anything.

again, i'm a beginner, so lots of grains of salt, but i would use the panel traps for the 40-120 stuff. 100-700 is easily dealt with using normal 703/rockwool traps. this is basically my plan for my new room. along with thin walls that the bass just goes right through.
 

www.oldcolonymastering.com

morespaceecho.bandcamp.com

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podgorny

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Posts: 2,330 Member Since:27/01/2011

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Nov 19 16 1:15 PM

If you are locked into a specific location for a pressure-based absorber, and want to make it most effective, put your measurement mic in that location and sweep. No point trying to absorb energy which isn't there. Based on the dimensions you gave, at the back of the room you are most likely to have consistent issues at 62Hz and octaves thereof.

Kyle Mann :: www.kylemann.com

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skidmorebay

Tin Man

Posts: 25 Member Since:07/08/2015

#8 [url]

Dec 23 16 11:31 AM

Thanks, that was good advice. I adjusted my speaker placement and built some perforated panel absorbers that I "tuned" for the 30-60hz bump I found at the back wall. I got decent results I think.
Here is my room response at the listening position, both speakers firing at the same time, before and after:

image

image
NOTE: I discovered that the huge dip around 1K in the "before" plot was the result of phase cancellation from my DSP-controlled JBL LSR 4328 speakers having a 0.5ms delay in the right channel. This took a long time to figure out. A firmware update fixed the issue. Beware of this if your speakers have DSP control for room corrective eq! (No corrective eq used in the plots.)

Here are before/after waterfalls from the same measurements:

image

image

Here are the two traps I made:

image

I would like to continue with the improvements. I was thinking about building Hemholtz resonators to address the 45hz and 62hz resonances shown in the second waterfall. Does this sound like a good approach? Any advice on this?



 

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thomas northward

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Posts: 139 Member Since:10/02/2011

#9 [url]

Dec 26 16 6:37 AM

skidmorebay wrote:
Thanks, that was good advice. I adjusted my speaker placement and built some perforated panel absorbers that I "tuned" for the 30-60hz bump I found at the back wall. I got decent results I think.
Here is my room response at the listening position, both speakers firing at the same time, before and after:

image

image
NOTE: I discovered that the huge dip around 1K in the "before" plot was the result of phase cancellation from my DSP-controlled JBL LSR 4328 speakers having a 0.5ms delay in the right channel. This took a long time to figure out. A firmware update fixed the issue. Beware of this if your speakers have DSP control for room corrective eq! (No corrective eq used in the plots.)

Here are before/after waterfalls from the same measurements:

image

image

Here are the two traps I made:

image

I would like to continue with the improvements. I was thinking about building Hemholtz resonators to address the 45hz and 62hz resonances shown in the second waterfall. Does this sound like a good approach? Any advice on this?




 

While it's good to somehow focus on the modal response, the ideal target is to get the 20-200Hz to be tightly controlled at once. So you have even & short decay at all frequencies if possible. That's when the rubber meets the road. Frequency response is important, but time response is always more important. It you get a good time response, your frequency response will flatten as a result. The opposite is often not the case. 

What isn't good in focusing too much on a particular set of frequencies is that it tends to shorten their decay, but likely won't shorten the rest of the spectrum around them.

My advice is to go with pressure based treatment rather than helmholtz that are a bit of a "diva" to deal with - with membranes being more efficient and easier to use with a more broadband behaviour should that be needed.

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morespaceecho

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Posts: 2,304 Member Since:29/01/2011

#10 [url]

Dec 26 16 1:00 PM

thomas northward wrote:

My advice is to go with pressure based treatment rather than helmholtz that are a bit of a "diva" to deal with - with membranes being more efficient and easier to use with a more broadband behaviour should that be needed.

hi thomas, 

any chance you can elaborate on that? i.e. practical solutions for bass trapping for the DIYer? thank you!

www.oldcolonymastering.com

morespaceecho.bandcamp.com

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skidmorebay

Tin Man

Posts: 25 Member Since:07/08/2015

#11 [url]

Dec 28 16 12:28 PM

Frequency response is important, but time response is always more important. It you get a good time response, your frequency response will flatten as a result. The opposite is often not the case. 

Thanks for the useful advice. I've studied the concepts extensively, but this is the sort of practical information that you rarely find in the books. 
I'll build some more panel absorbers and keep measuring. 

Last Edited By: skidmorebay Dec 30 16 10:04 AM. Edited 1 time.

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dandan

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Posts: 73 Member Since:02/02/2012

#12 [url]

Fuzzy

Good work, it is rare to see success in the 30Hz region. The BBC publish their R&D freely. You will find LF pressure traps good  which are about a foot deep, constructed using simple fibre board fronts, etc. 
Cancellations are the result of reflected reinforcements, so the boundaries, in particular corners are prime treatment locations. When going for particular identified modes I suggest that you light them up using a Sine Generator. You will readily identify the loudest spots, at corners, wall, or ceiling/floor.
The Back Wall can be very significant in terms of creating deep nulls and many. http://forums.musicplayer.com/ubbthreads.php/topics/1647339/Do_room_modes_even_matter 

FuzzMeasure sometimes comes up with those odd blurry slices at the end of the decay. Sometimes tweaking the gating will clear it up. 

DD

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