Built some DIY Bass traps. Measurements are now worse. - Page 2

Should I not be pushing my GIK tri traps all the way into the corner? Would they be more effective a little ways out from the corners? thanks

The link doesn't work for me.

Carp, from the tests we've performed and had performed by Riverbanks Acoustic Laboratory, directly in the corner would be best.

I've seen that mentioned before, but have never been able to confirm it myself, and I question if it's true. I see no reason for the velocity of the wave to be any different at the 1/4 wavelength distance than any other point. The first waveform peak is at the 1/4 wavelength point, but that doesn't affect the velocity of the wave. As for the slowing of the speed of sound in porous mediums, that's been speculated on since Bradbury and Bailey and their TL experiments, and it didn't prove to have more than a cursory effect even with tightly stuffed pipes. The effect that would be most obvious at the 1/4 wavelength distance from a boundary is the response null, which might lead one to believe that a trap at that point is more effective than one on the wall. I'd want to see middle of the room measured results with traps in both positions.
Agreed, as you have the combined effect of two reflective surfaces. It's not unlike the difference between 1/2 and 1/4 space loading.

Ok, thanks guys. Glad to hear it I don't want to lose any more space for subwoofers in the corners.

Bill, that's a good question. I'm sure you're aware of much of this, but for clarity I'm going to be include all of it.

Particle velocity is not the speed of sound. Sound intensity = Sound pressure × Particle velocity


Sound, as we all discuss it here, is a pressure fluctuation in the air. It's characteristic is that of wave motion, the motion is triggered when an air particle triggers the next one, so on and so on. The entire wave propagates at the speed of sound in the medium. However the velocity, that you inquire about, is the particle velocity of the vibrating air particles as they oscillate around their equilibrium position. The frequency is determined by the number of pressure variation cycles per second. The key distinction here is the difference between the speed of sound, and the particle velocity.


Our hearing works via detecting sound pressure changes. Since our ears don't work by sensing air particle velocity, this is not an element of loudness perception.


Thanks

No so. What you describe is a broad band absorber with good bass extension.

A true bass trap does not have broad band absorption, allowing bass issues to be addressed without affecting overall room acoustics. An example of such a device is found at the end of this BBC report, Figure A1. It peaks at 65Hz, still has 0.7 absorption coefficient at the minimum frequency shown, and more to the point, has less than 0.5 absorption above ~125Hz, and below 0.25 above 1KHz where a 6" resistive absorber coefficient is at maximum.
http://downloads.bbc.co.uk/rd/pubs/reports/1992-11.pdf

No good or bad tools, just some better suited to the job than others.

HAve fun,
Frank

A true bass trap? You'll get no argument out of me, but I believe I'd still call those bass traps. Why? Place the 6" of 703 spaced 12"-18" off the back wall, it's an effective bass trap. The remaining spectral content, as I noted in my previous post, can be handled however ... one is free to face the material in any manner that the room needs most.

I've read the BBC work.


All the best

Here is a thread from gearslutz titled "why do we insist max particle velocity is always at 1/4wavelength from boundary?", suggest read that
http://www.gearslutz.com/board/bass-traps-acoustic-panels-foam-etc/663796-why-do-we-insist-max-particle-velocity-always-1-4wavelength-boundary.html

I'm aware of that, and haven't been able to confirm that there's anything out of the ordinary occurring at the 1/4 wavelength distance other than the cancellation mode. Not that I'm concerned about it, as I wouldn't be putting traps anywhere between three and six feet from my walls anyway. For that matter I don't use any traps, my room just doesn't have the need for them.

Mike, I remember that thread.

The room boundaries establish where the pressure maximum(velocity min), and pressure min (velocity max) occur.
Theoretically; a room's modal characteristics are determined by the boundaries, and their physical relationship to one another. Insert a subwoofer into the room. Depending on placement, one can either maximally excite the modal characteristics, or minimally excite those characteristics. It all depends on if the location is If it's a position of minimal pressure, or maximal pressure (best coupling).

Thus, manipulating sub placement affect modal characteristics.

This pertains

Superb read, as is the entire massive site of links

Wouldn't 1/4 and 3/4 wavelengths have 0 velocity, IF velocity was changing? Intuition has me agreeing with Bill, how is velocity changing. The wave moves at 340 m/s and I don't see how that changes over the course of a wavelength.

tux, check out the link that was mentioned by foh: http://www.walter-fendt.de/ph14e/stwaverefl.htm

That's where viewing a picture of a sine wave confuses the issue. You see a peak at 1/4 wavelength and from that might assume that there's some sort of shift at that peak. The problem is that a picture of a sine wave is only a graphic representation of phase at any point in the wave period. By no means does velocity reach a minimum at the 1/4 wavelength like a rollercoaster car reaching the top of a hill. That is a good graphic representation of the effect of a wave hitting a boundary, but again it's a graphic representation in a format that makes it easy to comprehend.

Ya, I don't actually think velocity changes, just if it were to change I'd think at the peaks. Or at least acceleration would be 0 at those points. I'd tend to think velocity doesn't change at all. It's like a pulse in the air (or any medium). In my field we measure the shear wave velocity of soil and rock. We initiate a pulse that travels through the medium and that velocity is measurable. It doesn't change either. The same thing happens in air. The speaker piston creates an out/in pulse (over and over to create frequency) which travels through the air medium at a constant 340m/s (depending on air temperature and atmospheric pressure, but pretty darn close to 340m/s). When it reflects off a wall and strikes a bass trap, still constant velocity.

Now, it is my understanding that certain trap materials can change the speed of sound, but I don't know much about it.

Those are the MonsterTraps, available at Best Buy.

I'm hearing confusion of particle velocity with wave packet (or pulse) velocity.

Particles engulfed by a wave packet vibrate with real particle velocites, but they go nowhere, just as your rock's don't move. The wave packet, or pulse, does move, engulfing new particles as it does so.

Velocity of a wave packet is dependent on the medium; speed of sound changes in water, solids, etc., but independent of the packet's amplitude.
Particle velocities in the region engulfed by the wave packet will depend on the amplitude of the wave, as well as the medium.

Resistive sonic absorbers use the air particle velocity, which depends on amplitude, to absorb sound. Amplitude of reflected waves varies with location, so resistive absorbers are most sensitive where particle velocity is maximized...

BUT

Energy can be removed at a given wavelength without locating the absorber at maximum particle velocity, the 1/4 wave point. At 1/3 this distance (1/12-wave), you still have 50% of max. particle velocity; at 1/10, 30%. This is why wall-mounted resistive absorbers go as low as they do.

Put another way, for the fundamental room mode, an absorber placed at room center (1/4 wave point) gives maximum absorbance
- 1/3 of the way to the center = 50% of the maximum
- 1/5 of the way to the center = 30%

Note that "1/5 of the way to the center" is 10% of the room dimension. In a 10' room, only 12" from the wall air particle velocity is 30% of maximum for the longest possible wavelength resonance. It's how the sine function operates...

But you still get a broad band absorber, and need to consider it's affect over the full freqeucny range.

HAve fun,
Frank

The driver has no excursion at 0 and 180 degrees, so those are the points where you'd expect zero velocity. But the forward and backward motion of the cone doesn't translate to a forward and backward motion of the wave that it creates.
Not to the extent that it makes any real difference.

Hey, I suppose that would be the norm in Canada. But I don't work in the patch. Wish I did $$ I'm a geotechnical engineer on Vancouver Island. We use shear wave velocity to do liquefaction assessments for when we get the big quake.