The visco elastic thickness is 1mm. If you have an existing box and not want to ruin lot of space you can use 1mm of visco elastic glue and a steelplate.
Bracing is of course extremely effective also.
center panel to opposite center panel brazing is very very good and does not take much air volume.
Bracing is of course extremely effective also.
center panel to opposite center panel brazing is very very good and does not take much air volume.
Granted it is not intuitive, but velocities may be high and pressures low along the side walls where waves are travelling more along them, rather than interacting with them on their way to the back wall. Damping material along this wall would therefore interact normally with some modes and not others.
Absorption of sound whichin other words is slowing down the speed of the soundwave and convert the speed loss to friction heat. That is aschieved most efficient where the speed velocity is maximum. At speed minimum=pressure max, there will be no friction between air molecules and the fibers. And by that no energy loss.
This means that in the proximity to the wall you will have zero damping of the sound...
for 2kHz you find the speed maximum at 1/4 wavelength from the wall = 42mm 1kHz is 84 mm from the wall 500 Hz 17 cm 80Hz 1,1meter 160Hz 53cm from the wall, 320Hz 27cm from the closest wall. We can see a pattern here. If we want to dampen resonances in a box with wool of any kind and we target for a subwoofer working in the sum 100Hz region. there is really not much use for damping material at all. That said we know that close to the woofer itself there is a lot of movement! SO close to the woofer there is good use for damping material. Sometimes you see that they wrap the damping material around the magnet to create an "air velocity damper" It was done on the famous Thyrland transmission line speaker which was commercialized by Jan H Stridbeck in the TL6 speaker. Using KEF B139 woofer and a lot of wrapped wool around the magnet.
This means that in the proximity to the wall you will have zero damping of the sound...
for 2kHz you find the speed maximum at 1/4 wavelength from the wall = 42mm 1kHz is 84 mm from the wall 500 Hz 17 cm 80Hz 1,1meter 160Hz 53cm from the wall, 320Hz 27cm from the closest wall. We can see a pattern here. If we want to dampen resonances in a box with wool of any kind and we target for a subwoofer working in the sum 100Hz region. there is really not much use for damping material at all. That said we know that close to the woofer itself there is a lot of movement! SO close to the woofer there is good use for damping material. Sometimes you see that they wrap the damping material around the magnet to create an "air velocity damper" It was done on the famous Thyrland transmission line speaker which was commercialized by Jan H Stridbeck in the TL6 speaker. Using KEF B139 woofer and a lot of wrapped wool around the magnet.
The Thesis Tyralnd wrote is famous and many speaker manufacturer reffer to this amazing job done by Sven Thyrland back in 74. LIBRIS - Konstruktion av en monitorhog...
What about for a transmission line.. in that case would you have a long burrito within it or should the stuffing reach out to the walls? I'd be going out to the walls..
OK. Do you concur when I put it like this: the concept of reflection of sound isn't useful in spaces relatively small compared to the wavelength of the sound propagated? In other words: the movement of discrete air particles might be influenced by a nearby boundary, but regarding the boundary as a mirror is moot.
Of course the existence of boundaries result in pressure variations on a cone, a reflex port or another boundary. As long as the max size of the internal enclosure < 1/2 wavelength, any discussion about singular reflections is useless and pressure chamber modeling does the job just fine (apart from when resonators come into play). When sizes are 1/2 wavelength or bigger, the dominant nonlinearity effecting cones, reflex ports and (flexible) boundaries is the occurrence of standing waves. Reflections (ray theory) only comes into effect at far bigger internal sizes than the wavelength in effect. All this is basic acoustic theory of enclosed spaces.
This is preposterous... if velocity actually reached 0 just by encountering boundaries there would be no such thing as reflections....its a boundary not a stop light....energy cannot be created nor destroyed thus if the velocity reached 0 (which results in no reflection) that energy went somewhere else (heat, light, chemical reaction etc etc)....We all know reflections are real
Reading this thread is helping me but I feel like I have a pretty good understanding already and have made some post that support that. Damping is all about distance traveled through the damping material vs the effectiveness of the damping material...period.
Campolo, you are right, the energy goes from velocity to pressure increase.
So the energy is bouncing between the walls.
When you measure sound with a microphone you will see that the low frequencies is amplified close to the wall with 3dB, and in the corner between two walls the sound pressure is increased to 6dB. Going to a corner wall-wall-roof you will se 9dB increase in sound. Thats why you place low frequency absorbers in the corners and you make dem thick, to reach 1/4 of the wavelengt. and typically you see in studios quite bulky arrangements of absorbers up to 1meter deep or more.
I have posted information on my active noise bass traps, in another thread, if you are interested in room acoustics.
So the energy is bouncing between the walls.
When you measure sound with a microphone you will see that the low frequencies is amplified close to the wall with 3dB, and in the corner between two walls the sound pressure is increased to 6dB. Going to a corner wall-wall-roof you will se 9dB increase in sound. Thats why you place low frequency absorbers in the corners and you make dem thick, to reach 1/4 of the wavelengt. and typically you see in studios quite bulky arrangements of absorbers up to 1meter deep or more.
I have posted information on my active noise bass traps, in another thread, if you are interested in room acoustics.
Ok, I see...my perspective tells me to channel the energy and dampen it with my material of choice along the way, borrowing from the epitome of an actual proven technique ie the “aperiodic” TL. It’s how ended up with a design for a half wave line aka closed transmission line. I can’t foresee a reason why any closed design would not be better as some type of closed TL...
Damping material, uniform in density, wall to wall, makes the most sense to me, and the farther you can have that energy travel through the damping material, the more effective it is.
I also believe that there are some type of sonic disadvantage to trying to cheat distance travelled by trying to implement shorter paths with a high density of damping material but I’m speculating. The higher the density the higher he pressure this raising system Q.....a longer path with less density can match the short path with high density damping in effectiveness while not increasing pressure as high, directly behind the woofer thus, not raising Q as much.
Where the line of too much pressure occurs, is the question.
Maybe not a direct answer to this, but I thought about the bandwidth of the driver resonance, and how far up in frequency the box would be working on that resonance, if it weren't for the need to deal with modes. I simmed 0.3@50 and 0.9@150. Could be noticeable up to several '00Hz, as collateral for absorbing low.
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In fact it is, in more ways. E.g. constructing a subwoofer enclosure small enough so that the biggest internal dimension is not near half the wavelength of the highest frequency you want to reproduce. Or the same for enclosures for mid and high speakers.
But please stop referring to this sound field as 'reflections'. Reflections have no frequency. They are a oversimplification of the sound field that in most cases doesn't fit.
The trouble is, they may be easier to absorb, but there's less stuffing in the front/back direction, so less absorption. Also, I'm not sure that depth is any more important than width/length (if you see what I mean). Cavity resonances will exist, whether in X, Y, or Z directions.
Obviously best to avoid similar dimensions in width/depth/length though.
I repeat that one of the important factors in this is the need to suppress reflections from the cabinet walls - and that's done by making the air/wall boundary less acoustically abrupt.
Is slowing down really the case? The mechanism for cabinet effective volume to be increased is down to making a thermodynamically adiabatic effect more like an isothermal one (at low frequencies).
Quarter wave, in the case of front-back resonances. Half-wave for side wall modes.
And multiples thereof.
"Reflections" refers to multiple non-resonant paths within the box which present themselves to the listener mainly through the (mostly) acoustically transparent cone (eloquently referred to as "echoes" by an earlier contributor).
It all about what you can hear at your seat. Easy to speculate about cab vibrations and even to measure wall motion. Another thing entirely to tie that to audibility.
All this endless jaw-jaw and not a bit of evidence in any of these posts.
B.
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