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Tracking down those persistent modal peaks
Old 10th August 2017
  #1
Gear Head
 

Thread Starter
Tracking down those persistent modal peaks

Thought I might share my recent experience with trying to track down and treat some modal peaks in my small room.

After having read a busload of advice on this site and, as a result, managing to tame my room somewhat, I still have pesky modal peaks at about 50, 90 and 125Hz.

In an effort to find where the best spots were for more bass traps (to date my Bass traps have all been DIY corner chunks) I have been using REW in the Real time mode using frequency limited pink noise and wandering around the room with a mic trying to find bass 'build up'. I might not be doing this right, but to date I've had limited success.

Anyway...
Whilst messing around with ETC and trying to find some HF refection problem points, I put a camping mattress (single bed size about 3" thick and filled with light polyester - I have no idea what the GFR might be) spanning one of the room's back vertical corners. I'd always heard that mattress type material was not a good choice for bass trapping but I was using this for HF not Bass so that wasn't an issue.

After taking some measurements (yes it revealed the HF reflection issue I was looking for) I also noticed that one of my problem low frequencies (90hz) had dropped by 5db in the frequency response graph! In addition the waterfall also revealed a reduction in decay time for the same frequency.

Armed with this knowledge (that my mattress was a potentially an effective -and mobile - bass trap) I continued to change it's position to various other corners of my room including the front vertical corners and the front wall ceiling/wall dihedral. (suspended between the front wall's curtain rail and an existing cloud) To my delight I found this tracked down and reduced each of my problematic peaks in turn by between 5-10db!!

I couldn't use this size of mattress for permanent trapping as it is far too intrusive into the room's real estate, but as a mechanism to find where to apply bass traps it proved very effective!
Attached is a photo of what's now been dubbed my Sherlock mattress. Not a great photo but its the green thing above the window.

Now I'm off to build more bass traps!
Attached Thumbnails
Tracking down those persistent modal peaks-canva-photo-editor.jpg  
Old 10th August 2017
  #2
Gear Guru
 
DanDan's Avatar
Interesting

Nice to hear of such big improvements. Not sure about a HF problem behind you, flutter? Polyester seems to be a very good absorber. Caruso Isobond show very good data.
DD
Old 10th August 2017
  #3
Gear Head
 

Thread Starter
Thanks DD
Wasn't so much chasing a relevant reflection behind me, rather saw a spike on the ETC at 14ms (so, from my calculations) about 15ft further than direct speaker to mic and was calculating the distance as to where it might have come from. Back untreated corner of my very small room was the culprit (it seems) as putting some HF absorbtion in that corner killed the spike
It was more a mathematical exercise than anything relevant.

I was amazed how significant a 3 inch thickness of polyester can be in bass absorbtion. admittedly it was wide and spanned the corner, so had plenty of air behind it to assist.
Old 10th August 2017
  #4
Lives for gear
 

Decay plot

Quote:
Originally Posted by flicka View Post
...After having read a busload of advice on this site and, as a result, managing to tame my room somewhat, I still have pesky modal peaks at about 50, 90 and 125Hz.
Yes modal, and you need to trap them in order to fill the valleys between them.
It's also evident that these modes decay much slower than the higher frequencies.

Modal frequencies will show as coloured bars along the frequency axis if you insert room dimensions in the EQ section of REW.
BTW Are you familiar with mode numbers?

Quote:
Now I'm off to build more bass traps!
Attached Thumbnails
Tracking down those persistent modal peaks-decay-plot-r-.jpg  
Old 11th August 2017
  #5
Gear Head
 

Thread Starter
Quote:
Originally Posted by akebrake View Post
BTW Are you familiar with mode numbers?
Thanks akebrake
I have read a little about the pqr numbers but frankly my eyes glazed over
Don't they assist in determining where there are 'clusters' of modes and where there are 'holes'?
Old 11th August 2017
  #6
Lives for gear
 

Modes

Quote:
Originally Posted by flicka View Post
Thanks akebrake
You're welcome

Quote:
I have read a little about the pqr numbers but frankly my eyes glazed over
Don't they assist in determining where there are 'clusters' of modes and where there are 'holes'?
"Assist"... OK. pqr or xyz does not matter. Modes got identification names, (like: 2.0.0 ) which is fairly practical. More modes (close in frequency/"clustering") often means more SPL. Lack of modal support e.g no modes in a wide frequency span can create 'a hole' . Hi or low SPL along a mode is position dependent.
But a mode cannot exist in isolation. Everything interacts in a complex mix...

More in this link:
https://www.gearslutz.com/board/stud...l#post12749424

RE the mode equation:
It looks more complicated than it is. This is a simplified equation AFAIK (I am no scientist so if my terminology is not exact pls excuse)
The listing of modes (in various mode calculators) is the result of plugging numbers into the equation below (figure from sengpiel audio, Berlin).

Room modes calculator calculate 3 modes rectangular room control room apps eigenmodes eigenfrequencies formula frequency standing waves room acoustic node equation - sengpielaudio Sengpiel Berlin
Attached Thumbnails
Tracking down those persistent modal peaks-sengpiel-equation.jpg  
Old 13th August 2017
  #7
Gear Head
 

Thread Starter
Quote:
Originally Posted by akebrake View Post
You're welcome
"Assist"... OK. pqr or xyz does not matter. Modes got identification names, (like: 2.0.0 ) which is fairly practical. More modes (close in frequency/"clustering") often means more SPL. Lack of modal support e.g no modes in a wide frequency span can create 'a hole' . Hi or low SPL along a mode is position dependent.
But a mode cannot exist in isolation. Everything interacts in a complex mix...
Thanks for all the info
Looks like I have some reading to do
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