How-to create DIY Tri-Traps

[Millo 3.1]

Originally Posted by DanDan

Milo. The FRK supplied on 705 etc. is quite heavy. I have read elsewhere that a 4 inch FRK batt resonates at very approximately 250Hz. Certainly I can hear something like this when it tap a hanging panel. I have also read that a 0.5 mil or so sheet of plastic will work just fine. Some have even used clingfilm around homemade tube traps. Even paper should work. Many prefer the brightness which FRK bounces back into the room. I don't, so I recommend a layer of foam or fibre to cut the HF. I suggest 20-30 mm is appropriate. This is not an exact science. In fact sometimes a little science can really get in the way of what is ultimately an ear/taste issue.
DD

Very, very cool... Have you built anything like this yourself?

[TRA]

Originally Posted by Ethan Winer

Youngster!

Haha...reminds me of my grandpa yelling at "the kid" at Firestone because his quote for tires was too high. That "kid" was a 60-something year old man probably with grandchildren of his own.

[DanDan]

mil is indeed millimetre. Clingfilm is kitchen wrap. I haven't built traps with membranes. Laziness perhaps. I am also very comfortable with the full range predictable and linear behaviour of simple large absorbers. A sheet in there has to bend the frequency response.
DD

[Bonemeister]

Originally Posted by DanDan

Milo. The FRK supplied on 705 etc. is quite heavy. I have read elsewhere that a 4 inch FRK batt resonates at very approximately 250Hz. Certainly I can hear something like this when it tap a hanging panel. I have also read that a 0.5 mil or so sheet of plastic will work just fine. Some have even used clingfilm around homemade tube traps. Even paper should work. Many prefer the brightness which FRK bounces back into the room. I don't, so I recommend a layer of foam or fibre to cut the HF. I suggest 20-30 mm is appropriate. This is not an exact science. In fact sometimes a little science can really get in the way of what is ultimately an ear/taste issue.
DD

===========================

I noticed a few posts here where folks discuss the resonance of a fiberglass panel plus questions regarding the use of as limp mass or diaphragm to control bass. I posted the following in another area, but thought you may want to give this some some consideration.

Note: The vinyl we use in the Primacoustic MaxTrap and FullTrap weighs in at 1 pound per cubic foot. Point beeing, to control bass, you need mass. Try standing outside a night club and the only thing you hear coming through the concrete walls is bass.

Further, fiberglass or foam work by converting sound energy into heat by causing minute fibers or the cellular sturcture to vibrate. These in fact do not actually resonante as a complete sturcture like a piece of wood (marimba). One could say that for it to resonate, it must be able to generate a sound. But this is also not completely accurate... a limp mass in the form of a diapragm will resonate, quietly.

Thus my posting.... enjoy.

==================

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

[DanDan]

Peter, interesting post. 1 pound per cubic foot? Hardly massive. Is that a typo. 705 is six pcf.
DD

[Bonemeister]

Originally Posted by DanDan

Peter, interesting post. 1 pound per cubic foot? Hardly massive. Is that a typo. 705 is six pcf.
DD

=============
Sorry, 1 lb per square foot... must have been thinking acoustic panels.
PJ

[PaulP]

Originally Posted by DanDan

mil is indeed millimetre. Clingfilm is kitchen wrap. I haven't built traps with membranes. Laziness perhaps. I am also very comfortable with the full range predictable and linear behaviour of simple large absorbers. A sheet in there has to bend the frequency response.
DD

In North America, and for pre-metric oldtimers, a mil is .001 inch.
0.5 mil is roughly the thickness of cling/film wrap.

I've never heard the term used for anything else (but then I'm getting on
in the years and am not European).

Paul P

[nucelar]

Peter thanks for your post, very informative. I believe that the diaphragmatic resonator is a very good idea, but I have a question: How is the dynamic absobtion behaviour? Isn't it too heavy to react quickly to bass impulses? for example a kick drum?
Absoption has always been measured as a static property, but in this case (a moving mass) I think it makes sense to look at what happens in the time domain.
Thank you!

[Ethan Winer]

Originally Posted by nucelar

Isn't it too heavy to react quickly to bass impulses? for example a kick drum?

A tuned resonator absorbs most at a center frequency, and less at higher and lower frequencies. The idea of "fast bass" is an oxymoron. A frequency is what it is. Now, a kick drum has a mix of low and mid frequencies, so a trap tuned to the kick's fundamental won't do much two or three octaves higher. But it's not trying to. This is why most professional acousticians prefer broadband absorption for "home sized" rooms.

--Ethan

[Bonemeister]

Originally Posted by Ethan Winer

A tuned resonator absorbs most at a center frequency, and less at higher and lower frequencies. The idea of "fast bass" is an oxymoron. A frequency is what it is. Now, a kick drum has a mix of low and mid frequencies, so a trap tuned to the kick's fundamental won't do much two or three octaves higher. But it's not trying to. This is why most professional acousticians prefer broadband absorption for "home sized" rooms.

--Ethan

==============

Ethan is correct. the low frequency content of the kick drum will have longer, more powerful sound waves that will be absorbed by the diaphragm while the upper harmonics that create the 'click' or transient will be absorbed by the acoustic panels.

A heavy diaphragm will only work on powerful low frequencies. Thus the reason we combine it with a 3" thick panel, with air cavity.... Broadband absorption will deliver better, balanced results.

Peter