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Multi-Way Conventional loudspeakers with crossovers

Furious Wolf Tone
Furious Wolf Tone
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Old 14th March 2014, 03:23 PM   #1
Bamalama is offline Bamalama  United States
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Default Furious Wolf Tone

Hello. I hope that you are well and enjoying life in general.

Over the decades, I have only had two loudspeaker systems with poly ~6.5" woofers, and both of them had furious wolf tones in the low midrange-upper bass area. The resonance was so loud in one of them that I had to strain to tell what song was playing, and in the other (which is present), it is merely torturous.

With the one that I have now (an old Infinity model, if you must know), I was able to abate the resonance a bit by adding fiberglass stuffing in the ends of the cabinet. Still, when a singer hits those notes, the speaker goes "WOOOP!". Since one of the things that I want to do with these speakers is to evaluate recordings, this won't do.

The lowest calculated standing wave in this box is 435 Hz, and I don't think that what I'm hearing is that high. I haven't measured it yet.

Does anyone have any actual experience to share regarding curing this problem?

I lean toward replacing the woofers with something better behaved, and dealing with the peripheral issues separately. I'm not interested in anyone's insights on those right now. I need to get this howling fixed.

And by the way, why in the name of God do people evidently put up with this?

Thank you kindly.

Last edited by Bamalama; 14th March 2014 at 03:26 PM.
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Old 14th March 2014, 04:09 PM   #2
dumptruck is offline dumptruck  United States
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Quote:
Originally Posted by Bamalama View Post
I haven't measured it yet.
Get to it. Also, while I'm being curt, keeping your speaker model a secret is not a good way to get input.
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Old 15th March 2014, 08:46 AM   #3
Lynn Olson is offline Lynn Olson  United States
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If you want to cure a design defect, first you have to figure out where the problem is. This cannot be done just by listening, unfortunately, but requires measuring equipment and a general sense of how to operate it. There are threads elsewhere in the forum about measuring equipment, generally using a computer soundcard and a good-quality test microphone (not a microphone used for musical applications).

Once you start measuring, you can determine if the driver or cabinet are at fault. If the driver is at fault, the problem might be equalized out, or if not possible, replaced with a better driver.

If the "wolf note" is in the cabinet, adding stuffing material will provide a small improvement, but not a complete solution. If the cabinet needs to replaced, then you're really building a new speaker, and there's no reason to keep the old design. If that's the case, there are some really good DIY systems out there, ready to be built, just by following the plans.

Hope this helps.
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Old 15th March 2014, 03:38 PM   #4
Bamalama is offline Bamalama  United States
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Thank you for your kind response!

I have an appropriate set of measuring devices, ranging from a vtvm to the ersatz Parts Express measuring mic. None of it is very modern but it gives me adequate results. And, I have enough listening experience to get an initial focus.

I'm also thinking that if the problem is more ring than rise in the frequency response, I could send an impulse to the speaker and hopefully get enough ripples in the response that I could measure the period. I have primitive gadgets that will do that. In that case, though, eq wouldn't be exactly the right remedy. I would have to approach something physical.

I don't need the results to be perfect, I just need them to be not horrible.
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Old 15th March 2014, 04:58 PM   #5
LineArray is offline LineArray  Germany
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Default Loudspeaker cabinet vibration and unwanted radiation of sound from the cabinet

Hi Bamalama,

resonances in conventionally built cabinets may often occur in the upper bass range. Usually that lowest modes of vibration are not caused by resonances of the enclosed air volume itself (standing waves, organ pipe like resonances and higher order modes) but by bending waves on the enclosure's walls.

Those may be excited by the driving force on the membrane, which acts in opposite sign on the driver's motor and basket. Also if no standing waves (of the air enclosed in the cabinet) contribute, the alternating pressure in the cabinet plays a role too.

The zones around the baffle cutouts of most boxes are rather weak and have high compliance due to bending wave propagation. Typically the mass of the woofer bolted to that zone is much higher than the cabinet material in the cutout would be. Rather low frequent modes ("eigenfrequencies") are the natural consequence.

Those low frequent bending wave modes can have different shape (dependent on the individual cabinet), but typically a zone around the baffle is vibrating in antiphase to many or almost all of the other cabinet panels, which is often misinterpreted as a "balloon like" motion of the cabinet walls, which it is not. It is more comparable to the modes of a bell.

On my website i show two subwoofer cabinets made from 19mm MDF, which are rather stiff due to small baffle dimensions and overall size, that exhibit a resonance at approx. 210Hz. While being well built as a subwoofer, that particular cabinet would not be prefereable as e.g. housing for a 2-way system.

In this example that lowest mode can already be made visible (!) using the Chladni Ernst Chladni - Wikipedia, the free encyclopedia method, which is over 200 years old. Of course the system has to be driven hard exactly at that resonant frequency to let the grains move and show a node line of the eigenmode, here on the top wall on either of the cabinets. I would not suggest that method as a "standard", as you risk overheating the woofer's voice coil unless controlling the power to be a small fraction of the woofer manufacturer's rated power. Also higher order modes won't usually cause amplitudes of the walls, which could make the grain move.

Accelerometers and laser interferometry are more sensitive methods to be used, but nevertheless old Chladni method gives a picture of the node line in "high resolution":

http://www.dipol-audio.de/projekt-sc...ekgestoppt.jpg

Maybe try throwing this at "google translate":
Schwingungen an Lautsprechergehusen

I am currently working on new methods, how to make cabinets that have really low unwanted sound radiation, nevertheless modifiying an existent cabinet is not that easy: In the anglo-american forums - at least as i observe it - there seems to be a large tendency towards bracing, leading to very stiff enclosures.

But as can be pointed out, this is as a strategy only viable for subwoofers. You cannot make a cabinet having low radiation up to the midrange just by increasing the stiffness: The performance will get worse in the midrange unless mass and dampening components are applied in the right way too. This is because the coincidence frequency of the cabinet's walls will be lowered as the cabinet gets stiffer without mass being added to the walls. That causes unwanted radiation from the cabinet getting more efficient in the midrange.

Some alternative proposals you can try with your given cabinet to "change" or "detune" your nasty eigenmode without guarantee, that the result will be "better" (maybe it will turn out just "different"):

- Magnet mount the woofer using a massive structure inside the cabinet and use the old mounting surface on the baffle just as a gasket.

E.g. use a slab of stone/concrete for mounting the magnet, which may be just be connected with the bottom wall.

- Make a support from the woofers magnet to the rear wall - given this is solid - using a "dead" material that can be put unter pressure a little. Do not use excessive force that could seriously deform the woofers basket.

- Add some mass to the woofer's magnet, do not exagerate especially with weak stamped steel baskets. That may "detune" the resonance, more kind of gambling but could mitigate the effect in some cases.

- Stiffen (brace) the front baffle, but prefereably add some damping pads to the other walls.

- Make a solid/dead/damped brace between side walls directly behind the woofer that is at the same time clamped to the woofer's magnet. But do not disturb any "breathing" of the woofer or port ...

- Make sure (for the higher modes), that sufficent and high quality dampening material fills the volume of the speaker and is right in place at the velocity maxima of expectable standing waves. Net like structures e.g. from cord attached to the walls by hooks can be used to control the position even of loose damping material
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Last edited by LineArray; 15th March 2014 at 05:27 PM.
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Old 15th March 2014, 06:26 PM   #6
LineArray is offline LineArray  Germany
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Quote:
Originally Posted by LineArray View Post
...
Those low frequent bending wave modes can have different shape (dependent on the individual cabinet), but typically a zone around the baffle is vibrating in antiphase to many or almost all of the other cabinet panels,
...
Should better read "often a zone around the woofer's baffle cutout is vibrating in antiphase to large portions of the cabinet's surface area ..."
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Last edited by LineArray; 15th March 2014 at 06:40 PM.
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