The No.1 Website for Pro Audio
 Search This Thread  Search This Forum  Search Reviews  Search Gear Database  Search Gear for sale  Search Gearslutz Go Advanced
BBC Type D2 - Safe VLF treatment?
Old 29th October 2010
  #1
Lives for gear
 
audiothings's Avatar
 

Thread Starter
BBC Type D2 - Safe VLF treatment?

Been looking for a relatively safe solution for VLF treatment. Tuned absorbers seem hit or miss, and relatively narrow band. What about this BBC spec trap? It has a sound absorption coefficient of .8 @ 125 Hz and is steadily climbing till the edge of the graph @ 63 Hz.

This looks almost too good to be true. Any comments?

BTW, can anyone explain the principle... how does it work?
Attached Thumbnails
BBC Type D2 - Safe VLF treatment?-bbc_d2_1.jpg   BBC Type D2 - Safe VLF treatment?-bbc_d2_2.jpg  
Old 29th October 2010
  #2
Gear maniac
 
mikahanau's Avatar
 

which year was this paper?
i can`t find it at the moment

cheers
Mika
Old 29th October 2010
  #3
Lives for gear
 
audiothings's Avatar
 

Thread Starter
It is this link from an ongoing thread. I'm guessing it was written early to mid 80s.
Old 29th October 2010
  #4
Lives for gear
 
jhbrandt's Avatar
That unit is a resonant absorber. The Masonite or hardboard resonates, the fiberglass damps the panel, and the vibrations are turned into heat.

The document was written in 1992. "The Design of a modular sound absorber for very low frequency - Available here: http://www.bbc.co.uk/rd/pubs/reports/1992-10.pdf

Cheers,
John
Old 30th October 2010
  #5
Lives for gear
 
audiothings's Avatar
 

Thread Starter
Quote:
Originally Posted by jhbrandt View Post
That unit is a resonant absorber. The Masonite or hardboard resonates, the fiberglass damps the panel, and the vibrations are turned into heat.

The document was written in 1992. "The Design of a modular sound absorber for very low frequency - Available here: http://www.bbc.co.uk/rd/pubs/reports/1992-10.pdf

Cheers,
John
John,

that document makes only a passing reference to the D2 type absorber... no details... Its mostly about the A series. Also in type D2, the panel is perforated, maybe there is some Helmholtz principle being used.

I'm wondering about the effect of the interlocking hardboard grid with 10cmx10cm cells.

Sadly (confusingly), there is a graph in there (attached herewith) which shows a different sound absorption coefficient from the one shown earlier.
Attached Thumbnails
BBC Type D2 - Safe VLF treatment?-d2.jpg  
Old 30th October 2010
  #6
SAC
Registered User
 

The D2 absorber is described in some detail in pp. 96-97 and in figure 40 and p 116 - 117.

The D2 is described via the description of the A2 behavior described in the previously posted document. Only the tuning is lower as they indicate. You might also want to note the limitations described on pp. 92, 95,96...
Old 30th October 2010
  #7
Lives for gear
 
audiothings's Avatar
 

Thread Starter
The first two images I posted are from the very pages you refer to. I was unable to find the limitations on those pages.
Old 30th October 2010
  #8
Gear Guru
 
DanDan's Avatar
True

Audio, the BBC design was used a lot in BBC local stations.
I would have no doubt that it works as described. G.E. has a thread testing sort of similar designs. I believe his are open back, but some of his results are very good. Any of these sealed or even partially sealed absorber/resonators stop a lot of the incoming energy, reflecting it. Of course at resonance or within the pass band they are fully (or more) absorbing but I always wonder what is the cost/benefit here.
A 30cm depth of fibre should be fully absorbing from 50Hz or so up.

I believe there is a thin line between absorbing and re-radiating. The damping needs to be right. Those cardboard things help by breaking up internal box resonances. I am sure a light fill of fibre would do the same.
There seem to be conflicting views as to whether the resonating panel should be touched by fibre or not. Discussed by G.E. as far as I can remember.

The 'blocking' suggests to me that you will need a large number of these to get a good result. I believe the BBC would typically cover a ceiling with a pattern of them alternating with one or two of the other modules.
In any case, like pretty much any other LF design, more seems to be the key.

DD
Old 30th October 2010
  #9
Lives for gear
 
G. E.'s Avatar
 

Lightbulb Clarifications

Quote:
Originally Posted by DanDan View Post
... G.E. has a thread testing sort of similar designs. I believe his are open back, but some of his results are very good. ...
For clarification: all the membrane and perforated panel Helmholtz absorbers that I've shown in various threads are closed at the back (well, the need to be closed). I've opened a perforated panel absorber once at the front to check the absorbtion characteristic but it didn't make a big difference. Note: the absorbent in a perforated panel Helmholtz device absorbs even without the Helmholtz principle through the holes of the panel just as any porous absorber...

Quote:
Originally Posted by DanDan View Post
... There seem to be conflicting views as to whether the resonating panel should be touched by fibre or not. Discussed by G.E. as far as I can remember. ...
This is what theory says (which I found to be true):
  1. the membrane should be able to move freely to reach the highest possible velocity maximmum at the resonance
  2. the absorbent should be as near as possible to this area of highest velocity (again, without touching the membrane)
Depending on the mounting conditions (ceiling, wall) a few millimeters [~3/16"] distance between absorbent and membrane will do it...
Old 1st November 2010
  #10
Lives for gear
 
avare's Avatar
 

Sory for the delay. Why the D2? The A10 has significant advantages. (For light readers of the thread, the A10 is detailed in the document John linked)

Andre
Old 1st November 2010
  #11
SAC
Registered User
 

Old 3rd November 2010
  #12
Lives for gear
 
audiothings's Avatar
 

Thread Starter
Quote:
Originally Posted by avare View Post
Sory for the delay. Why the D2? The A10 has significant advantages. (For light readers of the thread, the A10 is detailed in the document John linked)

Andre
Thank you Andre and SAC.

The A10 is very interesting, for its relatively shallow profile and absorption at low frequencies.



But:

* it seems to rely on Supawrap 600 insulation, and its equivalent needs to be found...

* the A10 has a coefficient of around .25 @ 125 Hz, while the D2 reads at around .75

* The better looking of the two graphs I first posted shows the absorption coefficient of the D2 steadily rising until 63 Hz and IT NEVER FALLS! That graph shows it performing way better than the A10. This is what I find too good to be true.

The question remains as to which of the two graphs are to be referenced when building small rooms.

The graph in one in post #1



Or the graph in post #5


As for the absorption coefficients, the plot that is less optimistic says 'measured in an ISO-standard reverberation room' and the other plot with the ever rising absorption coefficient simply says 'typical absorption coefficient.....' Is the key in there somewhere?

If the graph in post #5 is the correct one for our purposes, the A10 begins to look much more interesting.
Attached Thumbnails
BBC Type D2 - Safe VLF treatment?-a10jpeg.jpg  
Old 3rd November 2010
  #13
Lives for gear
 
avare's Avatar
 

I don't think anyone has written it yet: congratulations on researching first, and researching with one of the best libraries of acoustic data, the BBC RD publications.

Quote:
* it seems to rely on Supawrap 600 insulation, and its equivalent needs to be found...
Yes. And the point is?

Quote:
* the A10 has a coefficient of around .25 @ 125 Hz, while the D2 reads at around .75
Are you looking for VLF absorption as per the title? high absoprion at 125 Hz is easy to achieve.

Quote:
* The better looking of the two graphs I first posted shows the absorption coefficient of the D2 steadily rising until 63 Hz and IT NEVER FALLS! That graph shows it performing way better than the A10. This is what I find too good to be true.
Again what are you looking for? Have you calculated the desired absorption vs frequency?

Quote:
The question remains as to which of the two graphs or the one in post #5) are to be referenced when building small rooms.
I would look at fig 21 in 1992-10 for the A10 absorption.

You are doing it right studying published reports. Have you studied RD 1994-12? It provides some furthere insight to low frequency absorption and has some references to the D2.

Andre
Old 3rd November 2010
  #14
Lives for gear
 
audiothings's Avatar
 

Thread Starter
Quote:
Quote:
* it seems to rely on Supawrap 600 insulation, and its equivalent needs to be found...
Yes. And the point is?
The point is that I do not know how to find the equivalent. I have a feeling that the gas flow resistivity figures may help... it is just a complication I am trying to avoid... but I will deal with it if you advice me to, or I figure out that I have to, anyway.
Quote:
Are you looking for VLF absorption as per the title? high absoprion at 125 Hz is easy to achieve.
A broader band absorber is more useful IMHO. Glad to be corrected.
Quote:

Quote:
* The better looking of the two graphs I first posted shows the absorption coefficient of the D2 steadily rising until 63 Hz and IT NEVER FALLS! That graph shows it performing way better than the A10. This is what I find too good to be true.
Again what are you looking for? Have you calculated the desired absorption vs frequency?
This is not about a particular situation. I am trying to understand the concepts.
Quote:
Quote:
The question remains as to which of the two graphs is to be referenced when building small rooms.
I would look at fig 21 in 1992-10 for the A10 absorption.
The confusion is about the D2, not the A10.
Old 4th November 2010
  #15
Gear Guru
 
DanDan's Avatar
Points

audiothings, I did a spot of searching earlier. RW2 was mentioned. This was around 30KG I believe. Supawrap was later mentioned as being lighter.
I expect we are talking about attic insulation here. 10-20KG.
You have noted the spectacular curve rising all the way to 63Hz.
That would be the limit of the testing, it doesn't mean it keeps rising beyond the extent of the graph. Testing even at 63Hz is dodgy enough, and only the brave or foolish go below that.

DD
Old 4th November 2010
  #16
Lives for gear
 
audiothings's Avatar
 

Thread Starter
Quote:
You have noted the spectacular curve rising all the way to 63Hz.
That would be the limit of the testing, it doesn't mean it keeps rising beyond the extent of the graph.
Thanks Dan... I am aware of this. If it falls as gradually as it rises, there will still be substantial absorption absorption below 40 Hz.

I am still hoping someone will know exactly why those two graphs look so different.
Old 4th November 2010
  #17
Gear Guru
 
DanDan's Avatar
Slopes

I see. In all of the visible graphs the absorption below resonance falls off very rapidly. I think it wise to assume your D2 will do likewise. I am not sure that I fully understand your question. However, if I do, I see an aborption of 1.2 vs 0.9 for the D2 in different tests. Well within the bounds of test variation at 63Hz I reckon. Is that what you are asking.
DD
Old 4th November 2010
  #18
SAC
Registered User
 

Quote:
Originally Posted by DanDan View Post
the absorption below resonance falls off very rapidly. I think it wise to assume your D2 will do likewise.
ditto.

With perhaps the exception being at 0Hz, where the absorption reaches near ideal infinite levels.

hehhehheh
Old 4th November 2010
  #19
Lives for gear
 
audiothings's Avatar
 

Thread Starter
Quote:
In all of the visible graphs the absorption below resonance falls off very rapidly.
You are correct, thanks. I had failed to notice it.
Quote:
I see an aborption of 1.2 vs 0.9 for the D2 in different tests. Well within the bounds of test variation at 63Hz I reckon.
Are we looking at different graphs? One of them shows a coefficient of over 1 @ 63 Hz, and the other is showing less than .4, from where I'm looking...
Old 4th November 2010
  #20
Lives for gear
 
Jens Eklund's Avatar
When looking at data derived from reverberation chambers, it’s more or less pointless to care about absorption coefficients below approx 100 Hz for two reasons; if not a huge room was used, the limit of reliable data will be around 100 Hz. Even if the data is “accurate” at 80 Hz, the absorption coefficient is valid only for random incident and a normal control room is not a diffuse field below approx 150-300 Hz depending on volume. Room modes, that totally dominate the “modal region” (and that’s the reason for the name), are not random incident. Another issue with reverberation chambers is the edge effect, causing absorption coefficients above 1.0. If “true” absorption coefficients is needed for normal incident, the most reliable test procedure is the impedance tube. Unfortunately, the testing of pressure absorbers is a bit difficult to do in these since the tuning sometimes is related to the dimension. Testing porous material is on the other hand, very simple in an impedance tube but the values will naturally not look as “good” compared to the reverberation room. I wonder why some manufacturers of porous “bass traps” frequently use the latter method even though it’s a lot more work and gives less reliable results …
Old 4th November 2010
  #21
Lives for gear
 
audiothings's Avatar
 

Thread Starter
Thank you Jens.

I think the bigger picture that is confusing me is this - Pro designers, if I understand correctly, have the entire design finalized on paper, guaranteed to translate to the final build. How do they do it if prediction of VLF response is so uncertain? FEM/BEM analysis seems to be the only way? And is that sort of system (EASE?) totally accurate from model to implementation, in the real world?
Old 4th November 2010
  #22
Lives for gear
 
Jens Eklund's Avatar
Not saying that I’m a "pro designer", but I do have some experience and I would never be totally convinced that a design performs exactly as predicted and it’s also a definition regarding what the predictions include. If there’s room in the budget, I often divide a project in steps and make measurements between each to confirm and perhaps change the next step accordingly.

Sincerely Jens Eklund
Old 4th November 2010
  #23
Lives for gear
 
johndykstra's Avatar
 

Not to mention tunability.

G.e.'s design has shown to have that ability. Design a treatment within tolerence, and adjust as needed.
Old 4th November 2010
  #24
Lives for gear
 
Nordenstam's Avatar
 

Quote:
Originally Posted by audiothings View Post
I think the bigger picture that is confusing me is this - Pro designers, if I understand correctly, have the entire design finalized on paper, guaranteed to translate to the final build. How do they do it if prediction of VLF response is so uncertain? FEM/BEM analysis seems to be the only way? And is that sort of system (EASE?) totally accurate from model to implementation, in the real world?
They typically use standard units that have been thoroughly tested. Designing a new sort of bass trap for every installation is a waste of energy!
Old 4th November 2010
  #25
Lives for gear
 
avare's Avatar
 

Quote:
Originally Posted by Lupo View Post
They typically use standard units that have been thoroughly tested.
+1. If it is a new then that is thoroughly tested before being installed.

Andre
Post Reply

Welcome to the Gearslutz Pro Audio Community!

Registration benefits include:
  • The ability to reply to and create new discussions
  • Access to members-only giveaways & competitions
  • Interact with VIP industry experts in our guest Q&As
  • Access to members-only sub forum discussions
  • Access to members-only Chat Room
  • Get INSTANT ACCESS to the world's best private pro audio Classifieds for only USD $20/year
  • Promote your eBay auctions and Reverb.com listings for free
  • Remove this message!
You need an account to post a reply. Create a username and password below and an account will be created and your post entered.


 
 
Slide to join now Processing…
Thread Tools
Search this Thread
Search this Thread:

Advanced Search
Similar Threads
Thread
Thread Starter / Forum
Replies
loopzilla6 / Gearslutz Secondhand Gear Classifieds
0
monkeychow / Rap + Hip Hop engineering and production
18
GravityRobert / So much gear, so little time
0

Forum Jump
Forum Jump