Its not something that's obvious or easy to measure. But its a slight woody cabinet ambience to the notes. I listen mostly to metal and it comes out in the drums especially. The thing is if you are going to have resonance you want a nice sound.
Supposedly Wilson speakers researched the use of special composites in the cabinet to remove any trace of sound.
And what you will find is pressure maximum in the corner and close to the walls. Pressure minimum in the center. And in the center you will have velocity maximum.
Thats why the stuffing is best IN THE CENTER of the box and NOT close to the side walls.
For practical reason you just stuff the whole box....
Adding special foam to the side walls is a waste of money and effort.
(Long hair sheep wool is ok, and smells nice.)
I've heard this before. But no one seems to be able to explain it. So why is the soundwave moving slower as it reaches a surface. I supposed it would stop for a moment at the surface. And why would a velocity maximum make absorption more effective. Obviously my ears tell me to stuff the middle because the cabinet sounds more "solid". There are a lot of unknowns with cabinets.
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Hi,
don't forget to use Google. speakers obey physics
The problem is that soundwaves are often visualized with lines that look like a vibrating string when in reality soundwaves propagate longitudally.
Here is one link, I'll paste some more in a minute that will visualize the standing waves so they are more understandable and easy to see where particles move and where they stand still (but "compress"). Near wall they cannot move but to hit the wall = pressure node But there are pressure nodes on the "center" as well.
Standing Sound Waves (Longitudinal Standing Waves)
Also a note: only the odd harmonics have velocity node in the center of the enclosure. The lowest harmonic being often the most troublesome so filling material in the middle of the enclosure would be the best place to tame it. Even better would be to put the driver in the middle to prevent the lowest harmonic happening. As a bottomline, sides are the least effective place for filling material (particles not moving much) but often the most convenient and apparently it works well enough as most people line the sides.
I suggest everyone to make a prototype box, if there is a problem then fix it. End result is a much better speaker than the first try by luck version would habe been
Have fun!
Edit:
These Khan academy videos seem nicely produced: part onr here and search for more Standing waves in open tubes (video) | Khan Academy Spending half an hour watching some videos will help to understand some problems associated with Helmholtz resonators, aka vented speakers, as well
don't forget to use Google. speakers obey physics
The problem is that soundwaves are often visualized with lines that look like a vibrating string when in reality soundwaves propagate longitudally.
Here is one link, I'll paste some more in a minute that will visualize the standing waves so they are more understandable and easy to see where particles move and where they stand still (but "compress"). Near wall they cannot move but to hit the wall = pressure node
But there are pressure nodes on the "center" as well.Standing Sound Waves (Longitudinal Standing Waves)
Also a note: only the odd harmonics have velocity node in the center of the enclosure. The lowest harmonic being often the most troublesome so filling material in the middle of the enclosure would be the best place to tame it. Even better would be to put the driver in the middle to prevent the lowest harmonic happening. As a bottomline, sides are the least effective place for filling material (particles not moving much) but often the most convenient and apparently it works well enough as most people line the sides.
I suggest everyone to make a prototype box, if there is a problem then fix it. End result is a much better speaker than the first try by luck version would habe been
Have fun!
Edit:
These Khan academy videos seem nicely produced: part onr here and search for more Standing waves in open tubes (video) | Khan Academy Spending half an hour watching some videos will help to understand some problems associated with Helmholtz resonators, aka vented speakers, as well
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The velocity is zero at the boundaries because air cannot flow through a wall, the wall
forces a velocity zero.
There are several analogies that help to explain what is going on. Air has mass and compliance
per unit length just as a guitar string and a guitar string also has a
zero velocity at the ends where it is tightly fixed. It resonates at a fundamental and
harmonic frequencies just as a tube of air does. If you look at the string the maximum
excursion is at the middle for the fundamental resonance. The frequency of fundamental
resonance is the one that fits the length and the forced zero velocity points of the string.
It is a 2 dimensional problem with air but the fundamental that you are trying to damp
works the same way.
After thinking about it, I think one should line the walls, doesn't have to be very thick.
There are essentially 2 mechanisms at play here.
1) the reflection of the sidewalls which leads to an echo inside the box, and escapes through the port or speaker cones. These happen at all frequencies. Think: echo in a concrete wall room.
2) the standing waves caused by the waves in the box resulting in resonance of the enclosure. Think room resonance. Happens only at low frequencies.
The burrito addresses 2) but not so effective in 1). To address 1) lining the wall should help, very much like an anechoic chamber but not so effective in 2).
So think it won't affect the woodiness sound but should help with midrange escaping through the port.
My 2cents.
Oon
why soundwaves are slower when they reach the surface
I'll, try. The waves in sound waves are pressure waves. At the peak of the waves there is less pressure, less molecules = more speed of molecular movement. When a sound wave hits a wall (straight on) it stops then starts again (at that point it is high pressure. Then at 1/4 wavelength from the wall is the first peak of low pressure. That is the best place to remove energy from the sound - as the fast moving molecules bounces through insulation some heat energy comes off. But inside the cabinet (or a room) is like a pool table, and the sound is coming at all angles to the walls, so some bass sound and all frequencies are skimming along the sides (not coming to a stop either!) and coming at different angles, so even on the sides insulation has some effect on all frequencies. Lower bass sound also builds up in corners and edges. Oh and the low bass is going to go into and some through the walls of the cabine (molecular vibration).
I know about studio design, just starting to build my first speakers from plans. But my idea is to also defract that sound, cutting wedge shapes of wood and sticking them on especially the back internal wall (that reduces the volume of the cabinet size so I have to add a little size to the cabinet to compensate), then insulation to that. The sound is still getting through the insulation and will bounce off according to the angle it hits. I figure it is going to be a mess of sound in there but if I can smash it in all directions it will at least reduce echo and modal issues (which if I understand it correctly - the pressure of standing waves will affect the back of the driver cone negatively).
I'll, try. The waves in sound waves are pressure waves. At the peak of the waves there is less pressure, less molecules = more speed of molecular movement. When a sound wave hits a wall (straight on) it stops then starts again (at that point it is high pressure. Then at 1/4 wavelength from the wall is the first peak of low pressure. That is the best place to remove energy from the sound - as the fast moving molecules bounces through insulation some heat energy comes off. But inside the cabinet (or a room) is like a pool table, and the sound is coming at all angles to the walls, so some bass sound and all frequencies are skimming along the sides (not coming to a stop either!) and coming at different angles, so even on the sides insulation has some effect on all frequencies. Lower bass sound also builds up in corners and edges. Oh and the low bass is going to go into and some through the walls of the cabine (molecular vibration).
I know about studio design, just starting to build my first speakers from plans. But my idea is to also defract that sound, cutting wedge shapes of wood and sticking them on especially the back internal wall (that reduces the volume of the cabinet size so I have to add a little size to the cabinet to compensate), then insulation to that. The sound is still getting through the insulation and will bounce off according to the angle it hits. I figure it is going to be a mess of sound in there but if I can smash it in all directions it will at least reduce echo and modal issues (which if I understand it correctly - the pressure of standing waves will affect the back of the driver cone negatively).
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Don't bother I'd say. You'll only diffract some high frequencies which can be managed with absorbing material already. An obstacle must be rather large compared to wavelenght to have any effect = doesn't fit inside the enclosure just build a rectangular box or something else that fullfills some of your other requirments for a speaker box, measure it, if there is a problem try to handle it without sacrificing the more important stuff, like size, weight, cost, looks
All this is relative to the bandwidth a speaker is going to produce. If it is a smallish sub (smaller than a fridge), there is no problem. If it is small bookshelf speaker, there might be a problem but should be ok with absorbing material. If it is a large two or three way box where a longest dimension goes much over 50cm I'd say only the lowest, or few lowest modes can be problematic, for example "leak" through a port and be audible if the port is on the front. These lowish standing waves seem to be really hard to tame by absorbtion. Just stick the driver in the middle and the lowest mode doesn't develop or use closed box. No reason to over think, concentrate on more important issues like diffraction and radiation pattern, dampening the enclosure, having fun and getting results It is all compromises after all. Have fun!
just build a rectangular box or something else that fullfills some of your other requirments for a speaker box, measure it, if there is a problem try to handle it without sacrificing the more important stuff, like size, weight, cost, looks All this is relative to the bandwidth a speaker is going to produce. If it is a smallish sub (smaller than a fridge), there is no problem. If it is small bookshelf speaker, there might be a problem but should be ok with absorbing material. If it is a large two or three way box where a longest dimension goes much over 50cm I'd say only the lowest, or few lowest modes can be problematic, for example "leak" through a port and be audible if the port is on the front. These lowish standing waves seem to be really hard to tame by absorbtion. Just stick the driver in the middle and the lowest mode doesn't develop or use closed box. No reason to over think, concentrate on more important issues like diffraction and radiation pattern, dampening the enclosure, having fun and getting results
It is all compromises after all. Have fun!
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Exactly.
More than 2 cents. I've been saying this for years. It's frustrating when so many seem to ignore what one says...
Actually i was wrong, the highest velocities is both in the center of the box and in the proximity of the speaker itself, so stuffing with wool close to the woofer is an old trick.
box vibrations is most effectively damened with viscoelastic glue between tha box wall and a second stiff plate Viscoelasticity - Wikipedia
some companies has registered names for it. Some companies is not mentioning it but just keep it as a secret. I can mention at least 5 brands that uses this method..
If you are a DIY person you can glue a relatively thin aluminum or steel plate with visco elastic glue with good result. I used birch plywood on a set of Beveridge 2sw2 speakers (2 meters high, BIG boxes) with SWEDAC DGA2 viscoelastic glue and that reduced the vibrations with >15 dB playing pink noise above 100Hz. But that is another topic.
box vibrations is most effectively damened with viscoelastic glue between tha box wall and a second stiff plate Viscoelasticity - Wikipedia
some companies has registered names for it. Some companies is not mentioning it but just keep it as a secret. I can mention at least 5 brands that uses this method..
If you are a DIY person you can glue a relatively thin aluminum or steel plate with visco elastic glue with good result. I used birch plywood on a set of Beveridge 2sw2 speakers (2 meters high, BIG boxes) with SWEDAC DGA2 viscoelastic glue and that reduced the vibrations with >15 dB playing pink noise above 100Hz. But that is another topic.
"concentrate on more important issues like diffraction and radiation pattern" That is what I was thinking of, so why some designs use curved loudspeaker enclosures (curves back, flat face, some rounding of front edges) addressing internal and external defraction issues. I was thinking how I could otherwise defract inside a reactangualr box. Are internal baffles are a common approach?
I'd be interested in hearing about dampening the surface with viscoelastic glue. How thick does your aluminium and glue have to be ?
Oon
So what do you propose? Broadly speaking what about a curtain wall of absorption, top to bottom and side to side would intercept any wave twice, once travelling out to the back wall etc, in other words a burrito with side wings, or lining the side walls?
Just that the ideal would be graded absorption material, from dense at the sidewalls (where pressures are high, velocities low), to less dense towards the centre of the cavity (pressures low, velocities high). You can approximate this with thick felt on the sidewalls and normal stuffing in the rest of the cavity.
The main point is that this will damp standing waves, and the lined sidewalls will have the added benefit of reducing reflected sound (at all mid and high frequencies). Additional benefit, the stuffing will increase effective volume by about 20% if the "optimum" density is used.
Choice of stuffing material is a tricky one - polyester stuffing is seemingly a lot less effective than fibreglass, but a lot more user-friendly. If the fibreglass can be enclosed in polyester to form a "burrito", then this seems a good compromise to make.
Why do you think this is true and relevant?
Modal behavior of sound fields isn't some kind of wizardry, it can be solved by the right application of the (universal) wave equation. This way one can calculate air pressure and velocity at any point and time in a given enclosed space (or field with boundaries). The standing wave itself is only one aspect of the sound field. Whether there is a signal to be carried or not is not relevant, at least not to me.
Transfer of energy from air to solids and reverse also isn't misunderstood that much. Standing waves in loudspeaker enclosures in short bring issues as nonlinear behavior a cone driver because of non-resistive behavior of the air behind it ('leaking' sound via the cone) and higher and non coherent sound transmission through panels and ports. 'Carry signals'? I think they do.
Curving the front/edges will change diffraction (to better). Curving the back / sides is mostly aesthetics and might stiffen the panels somewhat which internal panels do as well. But they don't do much for standing waves inside the enclosure. Might shift standingwave frequencies for better or for worse depending on your overall system design. Aesthetics is very important thing and joy of woodworking (or crafting some other material) is another
Ease of constrution is very important parameter for succesful hobby project, too hard or too easy doesnt feel as satisfactory than a finished project that one can be proud of. If you are building a commercial product, it is mostly for targetting a current trend and aesthetics and marketing skills that define success. In my opinion of course For me satisfaction comes from prototyping, developing crafting skills, testing stuff like stuffing and whether they make any difference. Latest prototype was just too difficult to build and didn't provide meaningfull advantages so the next one will be more rectangular or CNC machined and not directradiating but multiple entry horn- Status
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