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Old 14th December 2011, 08:43 PM   #11
Neko88 is offline Neko88  United Kingdom
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is this calculator any good? the circuit looks simpler, and the site tells you the component values.

Loudspeaker Diffraction Loss and Baffle Step Compensation Circuits
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Old 14th December 2011, 11:44 PM   #12
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Now i'm running out of suggestions... nasal, cupped is usually a reflection coming back thru the cone...

What happens if you use a diferent amp? What amp are you using?

dave
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Old 15th December 2011, 11:38 AM   #13
Neko88 is offline Neko88  United Kingdom
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it's an NAD C320 BEE, but the drivers sounded fine in an open baffle made from mdf, so I would have thought that it is some design flaw with the cabinet. I haven't really got another amp to test it with. It's frustrating, because after EQing, the speaker sounds really good to me.

I would have thought the central brace would disrupt any standing waves going through the middle of the cabinet, but maybe those parallel rings on either side (as in the cross section) are sustaining some sort of standing wave between them? Though if that's the case, shouldn't the stuffing have dealt with it?

Going back to my first post, is designing a notch filter the wrong way to deal with the problem?
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Old 1st January 2012, 08:59 AM   #14
divad is offline divad  Australia
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First things first; calculate the wavelength of the frequency (keep in mind that your altitude above sea level will affect this dimension, if it is of any magnitude).
Once you know the wavelength (in cm or inches) it is a simple matter to locate where the resonance originates (keep in mind there may be more than 1 locus).
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Old 2nd January 2012, 03:32 PM   #15
r6578 is offline r6578  Australia
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I am a total novice in speakers but since this topic resembles my quest to suppress vibration in the plastic case, I took a liberty to do some calculations. They maybe irrelevant, not applicable or just plain wrong, but the end result is quite striking.

These speakers are made of wood laminate which looks pretty solid. I presumed that it is the solid wood with some kind of surface wave propagating across the grains (just looking at the pictures). As the properties of actual material are unknown to me, I took an average speed of sound for different sorts of wood. (See my compilation attached as a picture).

The sphere made of this hypothetical Ďaverageí wood with the external diameter of .28m (from the authorís post) will have the fundamental frequency of standing surface acoustic wave between 1230Hz (along rings) and 1706Hz (across rings). In presumption that ratio is 50-50, the average (once again!) gives us the frequency of 1468Hz.

Pretty damn close to the troublesome resonance frequency!
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Old 2nd January 2012, 04:40 PM   #16
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Quote:
Originally Posted by planet10 View Post
Now i'm running out of suggestions... nasal, cupped is usually a reflection coming back thru the cone...
dave
That would be my guess too. I doubt the steps on the interior surface are large enough to effectively spread the primary resonance. Any central obstruction to spread the resonance will have to be comparable in size to the wavelength of the problem frequency, around 9", so that's going to be impractical. Any smaller and the wave will just diffract round it, just like what happens on the outside.

Killing the resonance defined by the sphere is tricky and we tend to overlook that the driver cone is practically transparent at all but the highest frequencies.

Try removing the driver, then stick your head close to the driver cutout and use your vocal chords to "sweep" the enclosure, you'll hear the enclosure "come back" at you, and if that cuppiness you hear with the driver in place is still there: you have your culprit. You'll look a complete idiot but it will give you an idea of what contribution that one big resonance is making.

B&W intersect a tube with the sphere to suppress it:

http://www.bowers-wilkins.co.uk/Sphere-Tube.html

French Manufacturer Elipson use an internal Helmholtz resonator to tackle the problem:

http://www.elipson.com 4260BN.pdf

I've heard the Elipson's, and the idea works well IMO.
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Last edited by simon dart; 2nd January 2012 at 04:49 PM.
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Old 6th January 2012, 06:16 PM   #17
Neko88 is offline Neko88  United Kingdom
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Divad - well, if the spike is around 1500Hz, that would give a wavelength of 23 cm, not far off the internal diameter of the sphere. This certainly seems a likely explanation for the problem, but many other people online have made Ikea bowl speakers (of almost exactly the same dimensions as mine) and I haven't noticed this problem reported anywhere. Also, I'm not sure what to do about it if this is the cause - although the picture I've attached below shows a piece of mdf that the author of this thread ( ikea bowl speaker + CHR-70 gen.3 ) placed in the centre of his sphere to deal with standing waves. Does it look suitable?



r6578 - that's an interesting idea, are you suggesting that the wood itself naturally resonates at this frequency? Or that a surface wave is being created around the insides of the sphere because I left the solid wood unfinished? Elsewhere on this site (including two pictures in this thread: ikea bowl speaker + CHR-70 gen.3 ) people have built solid wood spherical speakers of a similar size. The fact that many other people have managed to successfully build spherical designs in solid wood means that I don't want to give up on this idea - also, they look beautiful

I can't find much on surface acoustic waves online that seems to be relevant to audio. If SAWs are the problem, have you any suggestions for reducing this wave?




Simon - looking a complete idiot is really not a problem with me The enclosure seems to resonate when I sing an F sharp 3 (i.e. the one below middle C) into the driver hole. An online table says this is 185 Hz, quite a way off the problem I'm having in the upper mids. Interestingly, the table I looked at (Frequencies of Musical Notes) does suggest that the problem frequency is another F Sharp - only one three octaves higher - and this note (F sharp 6) has a wavelength similar to the sphere's internal diameter. Is this a coincidence? it's only around the F sharp 3 that the enclosure resonated to my voice, not any higher, as far as I could tell.

B&W's solution is interesting, but not really possible for me to build, these were difficult enough.

I'm not sure replicating the Helmholtz resonator from Elipson's £19,000 speakers is that practical either, unless you have any ideas? Is it complicated, or could I just put a smaller wooden sphere with a couple of holes in it inside the main enclosure? I'm new to speaker design so this all quite complicated - isn't a helmholtz resonator sort of like a bass port? Don't they amplify the chosen frequency?

It's interesting you've heard them - how good do £19,000 speakers sound? Don't think I've ever heard anything over £600
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Old 6th January 2012, 06:24 PM   #18
Neko88 is offline Neko88  United Kingdom
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oh, here's the picture I was talking about:
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File Type: jpg Picture 1.jpg (35.0 KB, 199 views)
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Old 9th January 2012, 05:51 PM   #19
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Interesting concept, the use of a helmholtz resonator. Would you tune the port to the resonant frequency of the internal standing wave_?
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Old 9th January 2012, 06:53 PM   #20
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I have a teardrop shaped loudspeaker, however the tube section is a bit shorter than B and Ws. With the tube the cabinet still resonates, would like to try the helmholtz resonator to see if things get better.
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