29th October 2009
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#1 | | Gear nut
Joined: Oct 2002 Location: Vancouver, BC
Posts: 87
Thread Starter  | Bass Traps - Diaphragmatic resonators & Limp Mass
There seems to be much confusing about bass traps so I thought I would start a new thread that discusses the different types and explains the diaphragmatic resonator we use in some of our products.
Some facts to start:
(a) To contain bass, you need mass. Plastic bags do not stop bass.
(b) To stop bass - decoupling is effective. You still need mass.
(c) Decoupling works by allowing one boundary to freely vibrate without affecting the next. Floating floors and offset-stud walls are examples.
(d) A limp mass, when set to vibration does not make sound.
(e) Sand granules and loaded vinyl barrier are forms of limp mass.
Using a limp mass (diaphragmatic resonator) in a bass trap
Over the past year, we have been doing a lot of research into bass traps and how they work. So I thought I would take a minute to share some thoughts regarding the various types of bass traps, their limitations and so on. But before, I get too far, let me first chime in on the science. To understand how a resonator or membrane works, one simply needs to look at a microphone. Sound waves cause the microphone diaphragm to vibrate which in turn causes an electric impulse. If a given frequency is louder, for instance the low bass from a kick drum, the microphone will resonate at that frequency and deliver bass to the audio system. The microphone is not selective. It is reactive. It simply picks up whatever is in the room and renders it to the best of its ability.
A diaphragmatic resonator works very much the same way. The one inside the Primacoustic Max Trap and Full Trap is basically a huge microphone diaphragm that is suspended so that it can float or vibrate. But unlike a microphone that is made from a very light material so that it is sensitive to all frequencies, the heavy barium-loaded vinyl inside is so heavy, that it will only vibrate at low frequencies where there is sufficient energy in the wave to cause it to move. Herein lays the magic: The diaphragm will sympathetically vibrate at the loudest frequencies in the room and remove them by thermo-dynamic transfer… or in layman’s terms, by converting sound energy into heat as the membrane vibrates.
How efficient is it? To put it bluntly, it completely blew away the techs at Riverbank Labs. It was literally off the chart. They had never encountered such an amazing device. In fact it shows test results with greater than 300% efficiency. Here are the differences:
Acoustic panels
As described earlier, acoustic panels will absorb bass but their performance is tied directly to the thickness and density of the absorption material that is used to build the panel. If the density it too high, then high frequencies will reflect. If the density is too low (like foam) then bass absorption will be minimal. Because the performance is tied to the thickness, they are limited in how low they will work. For the most part, even with an air cavity behind, they will typically only absorb 50% of the energy down to around 75Hz. The advantage here is that they are more cost effective. Bottom line: with foam, fiberglass and air space, size is the issue along with density for bottom end control
Hard membranes
These are typically wood panels that are suspended with some form of retaining spring which is connected to an absorbent material. Because the panel is rigid, it will have a very specific resonant frequency based on the size, thickness and density. Hard membranes are narrow band by nature. This means that they will vibrate at some frequencies and effectively absorb energy in this narrow band but do nothing at other frequencies. So unless your room has a problem at that specific frequency, they will not provide an effective solution. In our view, these are better than simple absorbent material as they do not require as much space in the room. But they only really work well at predetermined frequencies.
Helmholtz resonator
A Helmholtz resonator is best described as a big bottle. And just as you can create a sound by blowing across the Coke bottle like a flute, a Helmholtz resonator can be designed to suck out a certain frequency by building one at a very specific frequency. Helmholtz resonators have same problem as hard panel resonators: they are narrow band and only work so long as you identify the problem and then create a very specific solution. But since every room has slightly different dimensions, a ‘one size fits all’ solution here is impossible. Building a Helmholtz resonator is complex as it must be tuned to work.
Diaphragmatic Resonator or Limp Mass
The membrane or diaphragmatic resonator used inside the MaxTrap and FullTrap is different. It is a limp mass. In other words, it does not have a specific resonant frequency - but will vibrate based on the loudest or most powerful energy in the room. So no matter what the resonant frequency, it will self adjust to the problem and suck it out. To build one, you need to produce a frame, find some heavy loaded vinyl, hang it, then cover the front of the device so that it looks nice. We incorporate a large 3” thick acoustic panel on the front of both the MaxTrap and FullTrap that absorb sound above 100Hz. The rear cavity and diaphragmatic resonator does the magic on the low end. These sell for around $399 each… more expensive than some of the above, but the beauty is that the device will actually work in your room to solve the primary modal problems.
Peter Janis
President- Primacoustic
A division of Radial Engineering Ltd.
__________________
Peter Janis
Radial Engineering
Primacoustic
Tonebone | 
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29th October 2009
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#2 | | Lives for gear
Joined: Jan 2009
Posts: 1,341
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Dude...what the hell are you doing? I've got spam all over my shoes.
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30th October 2009
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#3 | | Gear nut
Joined: Oct 2002 Location: Vancouver, BC
Posts: 87
Thread Starter | Understanding bass traps
Quote:
Originally Posted by Brainchild  Dude...what the hell are you doing? I've got spam all over my shoes. | ==============
Sorry, not meant to offend. i simply CCd work i was going for our web site.
peter
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30th October 2009
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#4 | | Gear Guru
Joined: Oct 2002 Location: New Milford, CT, USA
Posts: 13,066
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Peter, I like you and respect your work. But your posts I've seen here read like Primacoustics product descriptions for a commercial web site. Versus containing technical advice meant to educate 'Slutz. I occasionally write about how my company's products work too, but only when someone asks directly.
--Ethan
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19th January 2010
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#5 | | Gear maniac
Joined: Apr 2004 Location: mpls, mn
Posts: 185
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Peter...
Exactly the information I was looking for.
Thank you.
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22nd January 2010
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#6 | | Gear maniac
Joined: Mar 2009 Location: Athens, Greece
Posts: 172
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I also agree that the sales pitch was a bit prevalent.
But I will focus on the core detail. Limp mass. A sizeable panel made of a material that doesn't have a resonance frequency. Did I get it right?
I could add bitumen and lead sheets as candidate materials. And they have been used extensively in loudspeakers (especially bitumen sheets) to dampen the enclosure vibrations. And they are effective.
But I think that they operate slightly differently. The certainly have a resonant frequency but its very low. Perhaps that helps with absorption in the low frequencies. And their large mass helps absorb a lot of energy.
I might be wrong but think them as resonant panels. Perhaps the difference is that the panel resonators are not free hanging (they are stiffly mounted as faces on a sealed enclosure. And since the panel is thin and doesn't have enough mass to absorb energy, the absorbing material is mounted inside the enclosure to do just that.
But I find the idea intriguing.
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22nd January 2010
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#7 | | Lives for gear
Joined: Sep 2009
Posts: 1,196
| Quote:
Originally Posted by SaSi_SiDi I might be wrong but think them as resonant panels. Perhaps the difference is that the panel resonators are not free hanging (they are stiffly mounted as faces on a sealed enclosure. And since the panel is thin and doesn't have enough mass to absorb energy, the absorbing material is mounted inside the enclosure to do just that. | In a panel resonator the box is sealed so the panel is forced to vibrate.
What's to stop sound simply going around a freely hung limp mass ?
Paul P
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22nd January 2010
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#8 | | Gear Guru
Joined: Oct 2002 Location: New Milford, CT, USA
Posts: 13,066
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^^^ Yes, sound goes around bass traps. This is why you need to cover a reasonable amount of surface area. Maybe we could invent an acoustic funnel that directs all the bass reflections into a single small trap. Yeah, that's the ticket. 
--Ethan
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22nd January 2010
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#9 | | Registered User
Joined: Dec 2009
Posts: 2,622
| Quote:
Originally Posted by PaulP In a panel resonator the box is sealed so the panel is forced to vibrate.
What's to stop sound simply going around a freely hung limp mass ?
Paul P | Any wavelength longer (greater) than an encountered object 'flows' (diffracts) around the object and is not effectively blocked and reflected.
Only wavelengths shorter than the dimensions of an encountered object are effectively blocked and reflected.
This is characteristic of the divide in room behavior between sound wavelengths that are greater than the room dimensions which are modeled as a pressure wave defining the modal region, and the region above the 'critical frequency, fc, where the sound wavelengths are smaller than the room dimensions and are modeled as rays which are effectively reflected like billiard balls and constitute specular reflections.
Hence the dual nature of not only the model of sound, but also of how it applies to how a small acoustic space is measured, modeled and treated.
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22nd January 2010
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#10 | | Lives for gear
Joined: Jun 2002 Location: Santa Monica, CA
Posts: 7,441
| Quote:
Originally Posted by Ethan Winer Peter, I like you and respect your work. But your posts I've seen here read like Primacoustics product descriptions for a commercial web site. Versus containing technical advice meant to educate 'Slutz. I occasionally write about how my company's products work too, but only when someone asks directly.
--Ethan | A bit spammish, yes, but it contained some clear explanations and useful information. Plus it tells you how to build your own if you wish. Peter should wade in here more often and become part of the give and take.
In any case, give Primacoustic a break. They make great products and don't take every opportunity here on gearslutz to trumpet their own products in the guise of giving advice.
Try googling Primacoustic. Guess what pops up as the first listing on the first page--RealTraps!! Nice web optimizing, dude. Did you use "primacoustic" as a key word to direct traffic to your site?
-R
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22nd January 2010
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#11 | | Gear Guru
Joined: Oct 2002 Location: New Milford, CT, USA
Posts: 13,066
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Quote:
Originally Posted by RKrizman A bit spammish, yes, but it contained some clear explanations and useful information. Plus it tells you how to build your own if you wish. Peter should wade in here more often and become part of the give and take. | I agree, and I think Peter's great and has a lot to offer. I made that comment only because Peter made the exact same post three or four times in one day in different forum sections.
--Ethan
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22nd January 2010
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#12 | | Lives for gear
Joined: Jun 2002 Location: Santa Monica, CA
Posts: 7,441
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Well I for one hope Peter sticks around and finds a way to work his way into the community. I love what I've tried of his products and think the limp mass thing is interesting and innovative.
It's a big world out there, and frankly, between Real Traps and GIK, these Gearslutz acoustical forums seem a bit parochial.
-R
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23rd January 2010
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#13 | | Gear Guru
Joined: Oct 2002 Location: New Milford, CT, USA
Posts: 13,066
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Quote:
Originally Posted by RKrizman the limp mass thing is interesting and innovative. | As far as I know RealTraps was the first to use that approach, going back at least seven years now. I say this only to clarify the history.
--Ethan
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23rd January 2010
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#14 | | Lives for gear
Joined: Jun 2002 Location: Santa Monica, CA
Posts: 7,441
| Quote:
Originally Posted by Ethan Winer As far as I know RealTraps was the first to use that approach, going back at least seven years now. I say this only to clarify the history.
--Ethan | In what way?
-R
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23rd January 2010
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#15 | | Gear Guru
Joined: Oct 2002 Location: New Milford, CT, USA
Posts: 13,066
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I don't fully understand your question. As far as I know, RealTraps was the first to use a semi-reflecting membrane to increase bass absorption over what you get from plain rigid fiberglass. Back then the main choices were tube traps, corner foam, and RPG's Modex Corner which is a tuned absorber.
--Ethan
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24th January 2010
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#16 | | Lives for gear
Joined: Jun 2002 Location: Santa Monica, CA
Posts: 7,441
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I don't know how your traps are made. The Primacoustic trap, I believe, has the limp mass hanging freely behind the fiberglass. Do you have something attached to the fiberglass? Can you say what it is without revealing a trade secret?
BTW, I apologize for questioning your integrity. I know for years you have been a source of good information on these boards.
Cheers,
-R
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24th January 2010
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#17 | | Gear Guru
Joined: Oct 2002 Location: New Milford, CT, USA
Posts: 13,066
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Quote:
Originally Posted by RKrizman I don't know how your traps are made. | Here you go: Made in Connecticut Quote: |
BTW, I apologize for questioning your integrity. I know for years you have been a source of good information on these boards.
| Accepted. I piped up only because I saw two such posts from you in one day.
Peace. 
--Ethan
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26th January 2010
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#18 | | Lives for gear
Joined: Jun 2002 Location: Santa Monica, CA
Posts: 7,441
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LOL. Great infommercial. For all the hooha I'm still unsure as to exactly what the plastic coating on the OC is, and whether it goes on the front or the back, or both.
-R
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26th January 2010
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#19 | | Lives for gear
Joined: Jun 2002 Location: Santa Monica, CA
Posts: 7,441
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BTW, I did buy one of your portable vocal booths, and am curious--is that just two pieces of 705 or is there a plastic coating as well?
-R
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26th January 2010
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#20 | | Gear Guru
Joined: Oct 2002 Location: New Milford, CT, USA
Posts: 13,066
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^^^ PVBs are made the same as our HF style traps, with the plastic membrane behind the rear fabric instead of behind the front. So go for it! Peel off the fabric and you'll see exactly the thickness and weight of the plastic.
--Ethan
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26th January 2010
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#21 | | Lives for gear
Joined: Jun 2002 Location: Santa Monica, CA
Posts: 7,441
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Hey, I paid a lot of money for that thing!
For the record, the kind of limp mass treatment that Primacoustic is using is something that is free-hanging behind the absorbent, not mounted on the panel like yours. So it really is a different approach, and to my mind, innovative, despite the fact that you've been using limp mass plastic in your construction for 7 years.
-R
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26th January 2010
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#22 | | Lives for gear
Joined: Jan 2008 Location: Midwest
Posts: 4,739
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Given my recent track record, perhaps now isn't the time for asking...
I have attached examples of the questions I am about to ask.
Currently I have 2" thick 703 panels in my room. The backs are breathable, and they are spaced an 1" off the wall...as shown in the first attachment.
If I were to apply the limp mass method, as described by the op, would the rendering I have in the second attachment achieve it? And to that point...
Is the spacing required only enough to allow the vinyl to hang freely, or is there more to it?
Also, is it required and or preferred to have the area between the back of the trap and the wall sealed, or is the open design I have pictured acceptable?
In regards to suspending the vinyl, can the top of the sheet be sandwiched between two strips of wood, or would it be required to hang it more freely...say from a series of hooks?
Anything else I may be missing?
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26th January 2010
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#23 | | Lives for gear
Joined: Jan 2008 Location: Midwest
Posts: 4,739
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Here's a link to a supplier, seems one roll would be enough to augment 12 traps (2' x 4'), for a cost of under $12 a trap. Interesting Mass Loaded Vinyl Barrier | |
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26th January 2010
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#24 | | Lives for gear
Joined: Jun 2002 Location: Santa Monica, CA
Posts: 7,441
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What you need is Realtraps in all your corners.
Just kidding.
From what I've seen of the Primacoustic stuff, your plan looks great, at least worth trying. I was thinking of trying the same thing myself, so if you do it report back.
-R
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26th January 2010
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#25 | | Lives for gear
Joined: Jan 2008 Location: Midwest
Posts: 4,739
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After I get a few pointers as to the specifics, I will shoot the room before the vinyl is added and after. Bear in mind, all of this will take some time.
It seems to me, that sealing the sides wouldn't do much, as the front is breathable anyway...
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26th January 2010
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#26 | | Gear maniac
Joined: Aug 2006
Posts: 157
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If you already have 2'x4'x4" or 6" bass traps made, could you build a 2" frame on the back of the bass traps and hang the vinyl there? thanks
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26th January 2010
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#27 | | Lives for gear
Joined: Jan 2008 Location: Midwest
Posts: 4,739
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The primacoustic website's detail of the membrane appears to show it supported vertically along both sides of the cabinet... Primacoustic - Acoustical Solutions | |
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26th January 2010
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#28 | | Lives for gear
Joined: Jun 2002 Location: Santa Monica, CA
Posts: 7,441
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I think the key idea is that it is "draped loosely" so that it vibrates without resonating. I don't see why your plan for just hanging it wouldn't work. Perhaps Primacoustic did it their way to make it possible to ship without falling apart.
-R
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26th January 2010
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#29 | | Lives for gear
Joined: Jan 2008 Location: Midwest
Posts: 4,739
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Sure. Here's some food for thought... and perhaps these numbers are apples and oranges...
Here's the absorption coefficients as posted by primacoustic for the maxtrap: (3" of fiberglass 6lb. density plus the vinyl)
50Hz- .65
80Hz- 1.01
125Hz- 1.20
250Hz- 1.25
500Hz- 1.29
1kHz- 1.22
2kHz- 1.15
4kHz- 1.05
5kHz- 1.00
And the numbers I got from Ethan's website regarding his "mini" trap (3 1/4" thick fiberglass)
50hz- .31
80hz- .93
125hz- 1.66
250hz- 1.10
500hz- 1.19
1khz- 1.10
2khz- .98
4khz- .86
5khz- .80
For the record, I only used Ethan's numbers, because they are vaguely comparable traps in terms of size...not to compare the actual products.
[EDIT] This post originally compared the mondo trap...but in terms of surface area and fiberglass thickness...I found the mini trap to be a more suitable comparison
Last edited by johndykstra; 26th January 2010 at 10:59 PM..
Reason: found a better match for fiber thickness
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27th January 2010
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#30 | | Lives for gear
Joined: Jan 2008 Location: Midwest
Posts: 4,739
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I have also just realized, that judging from the photo on Primacoustic's site, that there isn't any division between the limp mass and the absorption. If this is indeed the case, I could simply staple the vinyl to the back of the frames, and use the spacers that are currently in place.
Thinking out loud here.
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