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Old 13th December 2012, 03:41 PM   #11
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Originally Posted by mondogenerator View Post
REDUCE standing waves???? reduce to one frequency in ALL directions maybe but not reduce them in amplitude--IMHO BAD BAD idea
This would be true if the driver were in the center of the sphere, but would it still be true with the driver on the perimeter?
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Old 13th December 2012, 04:14 PM   #12
DeonC is offline DeonC  South Africa
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Originally Posted by Keriwena View Post
This would be true if the driver were in the center of the sphere, but would it still be true with the driver on the perimeter?
No, standing waves are set up between two parallel sides. A square box has 6 flat parallel sides, therefore 3 pairs of parallel sides, all of them the same distance from each other, so all standing waves will be at the same frequency. A rectangular box also has 6 parallel sides, but they vary with respect to distance from each other, hence the standing waves will be at different frequencies. A ball has infinite parallel sides, but they are all through the central point of the ball. The point where the frequencies are generated is not of importance.

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Old 13th December 2012, 04:38 PM   #13
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the *volume* of the enclosure sets up a standing wave condition alone.

If it was as simple as making a sphere and putting something in the middle, anechoic chambers would be made this way. And, the dome or sphere would be a more popular shape for buildings.

In fact domed large auditoriums and arenas have proven to be very problematic acoustically...

if there are parallel sides it is true that at high frequencies (where the wavelength is shorter than the distance between walls, that standing waves that are a function of the wavelength can be set up and reinforced.

But you can take a large bowl (half sphere) and perturb the liquid and observe that standing waves still occur - compare to the square or rectangular container... water can be used for fill, you can put a speaker against a side and sweep the freqs... even putting the back end of the speaker against the side probably will be sufficient, etc...

I'll bet that if you put something in the middle of the water the effect will be approximately the same for the bowl as the other shapes...

what will vary is the amplitude at any given frequency depending on the distances... as in cube vs. rectangular solid shape vs. sphere vs. egg vs. pyramid vs. truncated pyramid, etc...

so, at LF it makes no difference really, and at higher freqs you still need absorption to limit the internal reflections and so the resonances... that's where I'd put the focus - on absorbing unwanted energy.

Proper and effective internal absorption will outperform any shape variation by orders of magnitude.

External shape however is a major factor in how the sound is radiated and if there is diffraction or not - flatness of frequency response...

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Old 13th December 2012, 04:51 PM   #14
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Theoretically an egg shape should be better than a sphere.
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Old 13th December 2012, 06:03 PM   #15
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No, standing waves are set up between two parallel sides.
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Just a few thoughts to add:

Parallel standing waves between parallel surfaces is just one very simple and common kind of phase reinforcement and cancellation...just one of many. But it is hardly the only kind of problematic stationary or moving reinforcement or cancellation. For instance in cabinets or rooms there are often significant more complex modes where sound bounces at an angle off the floor, a wall, the ceiling, the opposite wall, back to the floor; that will also reinforce or cancel at some frequency (sometimes called a 'circular' standing mode though obviously not really round). Imagine the ways a superball in weightlessness could bounce inside and reach its original position repeatedly. And of course there are more complex bounces that can reinforce or cancel, like floor / ceiling / wall / floor / other wall / floor. The incident angles may make it travel thru more absorptive material and the additional surfaces encountered may further dampen these modes, but they can still be significant. Imagine a laserpointer inside a reflective ball; as you change the incident angle you get different problematic reinforcing geometries...you get direct return, a triangle, a square, a pentagon, a hexagon, etc. etc.

Ideally the volume and stuffing inside the box is supposed to absorb all the sound. In reality, when stuffed, what calcuates to be a reinforcement might actually be a very powerful absorptive trap tuned a bit lower, due to the stuffing.
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Old 13th December 2012, 06:17 PM   #16
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Quote:
Originally Posted by Keriwena View Post
This would be true if the driver were in the center of the sphere, but would it still be true with the driver on the perimeter?
I dont even remember posting that which you quoted...offsetting the driver may mitigate the standing wave influence on cone movement which would be worst if the cone intersects the antinode of a particular standing wave. Harmonics however present a further issue, where the driver cant physically be placed to avoid antinodes over the harmonic waves and the fundamental. Not to mention that offsetting within a sphere would also be problematic and compromised at best. I suppose a sphere of say 4 times the driver diameter could be placed on the floor, then an offset applied so the driver points up to the listener, but ive not seen a single example of such a design. If the sphere is less than twice the driver diameter, it would be nigh on impossible. Offsetting would merely offset the excittation of the membrane by the standing wave fundamental and harms. Like what occurs in a tube, changing driver position along the tubes length.
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Last edited by mondogenerator; 13th December 2012 at 06:35 PM. Reason: antinode or node? I forget everything lately
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Old 13th December 2012, 06:40 PM   #17
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how about a hemisphere?
id imagine a hemisphere wouldnt be great either. Id guess it would act as a parabolic mic does, and create a focal point. However sound isnt quite as simple as ray tracing, but quite where it departs from ray tracing to pressure driven i am unsure. That is beyond my humble experience.

A hemi with a heavily damped tline placed at the apex 'sinking' the rear wave could work very well. I.e. Nautilus '
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Last edited by mondogenerator; 13th December 2012 at 06:44 PM.
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Old 13th December 2012, 07:47 PM   #18
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You need volume, but you don't want standing waves. Two approaches are
- No walls at all, open baffle.
- No open volume at all = this one could be seen in anechotic rooms. Take it to extreme, and enjoy. Whoa? Pyramids on all the walls, larger than lowest frequency you'll play in there. Another way to say is to sectionize the volume in such way so the sections will be smaller than the wave itself.
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Old 13th December 2012, 09:16 PM   #19
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If a sphere makes all the standing waves the same frequency, wouldn't that make it easier to damp them?
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Old 13th December 2012, 11:37 PM   #20
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you cant damp a standing wave because its not moving. you cant hear standing waves. i thought the curved edges of a sphere would 'focus' sound in some way.
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