How about these for Bass traps?

  • Thread starter Thread starter Gorty
  • Start date Start date
Rick,

> frequencies below 125 hz are prone to errors. <

There's a related discussion going on now in one of the Home Theater type forums I visit. One of the experts there pointed out an interesting section in the ASTM C-423 standards for measuring absorption I had forgotten about. That section reports the result of a "round robin" test that was done among a large number of certified labs. Each lab was sent in turn the exact same physical sample to measure, and all of the results were tabulated. Even at 125 Hz, which is within the certified range, the labs varied by 50 percent! So forget below 125 Hz. Even at 125 Hz these certified measurements can vary wildly.

--Ethan
 
I've managed to have the product which I posted pictures of at the beginning of this thread (pipe insulation, rockwool) cut without the centre taken out for the pipe!

In other words the product will be cut from a solid cylindrical piece of rockwool into 4 pieces of pie so they will fit straight into my corners. The diameter will be approx 450mm, so from the corner of my room they will protrude 225mm in radius (approx 9").

Once again I am seriously limited in room size so I think these will be a good effective product for my Bass traps!

Any thoughts!

Thanx in advance! :)
 
RICK FITZPATRICK said:
What frequency has a 1/4 wavelength 9" long.
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Forgive my lack of acoustic engineering wave length frequency knowledge....

Will I be wasting my time with this product?...............hopefully I'm not wasting your time Rick! :confused:
 
Will I be wasting my time with this product?...............hopefully I'm not wasting your time Rick!
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Hardly. This was just my old fart way of getting you to WONDER HOW, resistance absorbers work. I ment no sarcasm Gorty. :)

Forgive my lack of acoustic engineering wave length frequency knowledge....
Click to expand...
Gorty, I must confess :D I am NO expert and didn't mean to make it sound as if I were. However, I do understand SOME of the principles.
Think about this. Here is a fellow member by the name of Knightflys explanation of this phenomena. It really is quite simple. And also, products like Rockwool, absorb at different COEFFICIENTS at different BANDWIDTHS at different THICKNESS's. What I infered by 9", is THEORETICALLY.......

First, the way bass traps work is that they present an "acoustic resistance" to sound waves.
This is the function of the rigid fiberglas inulation board. Moving air (sound wave) can't get thru the insulation board as easily as it can move in free air, so the energy it uses
to pass thru is partially converted to heat. Less sound energy, less sound.

Every sound has its own wavelength, which is expressed in (normally) either feet or meters; in feet, the formula is Wavelength=1130/f, with 1130 being the speed of sound in feet per second (approximate) at sea level, and f being the frequency in Hertz.

A wall, floor or ceiling is referred to as a "boundary" in acoustics - if we consider these boundaries to be impenetrable for sake of discussion, then at any boundary the air velocity from a sound wave striking it perpendicularly, will be ZERO, while its sound
PRESSURE will be maximum.

If you move away from the wall a distance of 1/4 wavelength of the sound (just one frequency for this example) at that point, the sound VELOCITY will be maximum, and the sound PRESSURE will be zero. This is important, becauseThe only place it does any good to place an acoustic resistance in order to trap sound, is where there is air movement, or velocity. The
more velocity, the more sound energy is converted to heat.

This is why absorbent traps need air space behind them - the more distance included in the trap cavity, with all other things equal, the
lower frequency the trap will attenuate. The closer to 1/4 wavelength the absorbent is placed from the boundary, the better that frequency is absorbed.

Using the wavelength formula, if we plug in 100 hZ we find that the wavelength is 11.3 FEET ?!?!! sooo, 1/4 wavelength would be 2.825 feet, or nearly 34 inches. That would be the optimum depth of a trap that would work at 100 hZ - just one example, so you can see
some of the problems of making low bass traps.

One of the benefits of a corner trap is that the depth is varied, so the trap works at a broader range of frequencies.

Keeping in mind that in order for the trap to work, the sound wave must pass THROUGH it, not AROUND it, then it makes sense that one important criteria is that the absorbent material is SEALED around the edges so the sound must pass THROUGH it. The frame is mainly there to support the absorbent, and give you a place to tack a cloth cover, and for looks. The important part is the type and thickness of absorbent, the depth of the cavity behind the absorbent, the thoroughness of seal,and somewhat the angle of
incidence that the sound strikes the absorbent. Perpendicular gives more absorbency than a glancing blow, for example.
Click to expand...

Ok, I hope that helped. See, I may be an old fart, but that doesn't necessarily mean I'm ALWAYS an asshole. :D
:) fitZ
 
Thanx Rick for your time, I do appreciate it!

I have done a lot of reading and I get confused when I read some arcticles referring to the so called "Chunks" in the corners for Bass absorption. I just had another read through some more arcticles and the gap is referred to as being the important decisive factor in relation to absorption and trapping! As you have quoted and mentioned earlier!

My main problem is that I have a lot of resources at my disposal and I have a very small room to work with, so I was trying and are still trying to find a happy medium between constructing effective Bass traps without eliminating too much of my so badly needed space!

But thanks to your help and time I'm getting there, slowly but surely! :)

So thanx again Rick, and like you said you may be an old fart, but that doesn't necessarily mean you are always an asshole! :)
 
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