Usher 701 resonance issues ?

[DBMandrake]

Originally Posted by DBMandrake

Speaker dave posted a good reference for this issue in another thread some time ago (I could probably find it if I dug through my subscribed threads enough) which debunks the whole adding damping makes resonances more audible wives tale.

Had a quick look back and found the original post by speaker dave that I was thinking of regarding resonance audibility:

What are the characteristics of a better material for enclosure?

Funnily enough I had forgotten it was planet10 he was responding to back then.

[sreten]

Originally Posted by system7

Doctor Joseph D'appolito clearly knows very little
about designing speakers. Who is he anyway?

Hi,

I assume there is some facetiousness involved in that comment

Usher seem to own the rights to using the good Doctors name
and of course in that context will use as much as they abuse it.

The actual design lacks baffle step compensation. So it needs
to be near walls. Fine for most AV fans. But near wall screws
up the lower midrange and always will and most don't care.

In my book it goes into a good AV speaker, not a good hifi speaker.

rgds, sreten.

[percy]

The cab does have a vertical brace in it - http://www.parts-express.com/pdf/302-732s.pdf

and this is the foam -Acoustic Foam 1-1/2" x 24" x 18" UL 94 260-516

One thing I had noticed earlier was that the kit comes supplied with speaker caulk to be put around the drivers to isolate them from the baffle. But the pre-built version did not have it. No isolation. So I just grabbed some length of weather stripping I had lying around (not sure if that was a good choice or not) and put it around the drivers and mounted them again.

[system7]

Can't agree with you there, sreten. There's HUGE bafflestep built into that crossover.



The Usher 8945A Woofer has Le of only 0.46mH. That's a standmounter.

I think it's broken and unfixable really. You are getting the usual 1kHz peak from the cabinet and driver. You can't really damp more with reflex. You can't put it near a wall because of the rear reflex. You can't notch without knowing the driver frequency response. Just gotta live with it.

[jimangie1973]

Originally Posted by system7

You can't really damp more with reflex. You can't put it near a wall because of the rear reflex. You can't notch without knowing the driver frequency response. Just gotta live with it.

I disagree with that. That PE 1.5" egg crate is thin enough that you can definitely double it, even triple it. I would triple it on top (longest dimension) and double it in the back.

You could do some iterative listening. First just double top and back, then if still boxy sounding, triple top, finally triple back.

I've done this and the port output is not affected much. Just keep a clear path between port and woofer.

Another thought is you may have a strong port resonance, meaning there's a midrange peak at the port output. This can screw up the midrange as well. It's easy enough to place your ear near the port output to check for this. I've noticed I can reduce this resonance behavior by placing a brace between the woofer and port, so most of the port is not in a direct path from the woofer output. I've also placed a foam block around the port itself which seemed to reduce it. I'm not sure of the physics behind it though.

If none of that works to your satisfaction, then you may need to address panel resonances with bracing.

[planet10]

Originally Posted by DBMandrake

Less likely ? How so ?

THe big one is that the energy available to excite the resonance is inversly proportional to the square of the frequency althou it can be argued that this could approach the 4th order above some point.

A high Q resonance will also be less likely to be excited by music, since to get it going you have to pump energy into it continuously within its bandwidth.

A brace should never be placed at the centre of a panel to avoid the situation you describe.





dave

[AllenB]

Originally Posted by jimangie1973

That PE 1.5" egg crate is thin enough that you can definitely double it, even triple it.

I gave up using egg carton foam twenty years ago. There may have been advances in foam since then but I don't much care. Still I agree with you that more damping material is worth trying.

Addressing Planet10, a good wad of fibreglass may somewhat reduce the transmission of those higher frequencies to the panels making your suggestion workable, but on the issue of bracing I'll fall back on the recent discussions as Simon has highlighted.

Originally Posted by system7

You can't notch without knowing the driver frequency response. Just gotta live with it.

If we did the crossover from scratch, aiming to understand why the peaks in the middle are necessary or not, I think we'd find 2 simple limitations in this design. The limits of the system configuration, and those of the budget. I suspect it wouldn't take a week to max out and possibly reach the same conclusions as Dr D'Appolito. His supporting comments lead me to believe that he has done his best within the constraints, but has surely achieved more with other designs... but it may be mainly a matter of time and money to produce a better crossover.

Originally Posted by system7

I think it's broken and unfixable really. You are getting the usual 1kHz peak from the cabinet and driver. You can't really damp more with reflex.

Usual? I don't think damping material in a reflex box should be such an issue. Texts refer to box losses reducing the reflex action, maybe so, but this is a lower priority here I would think and I'd damp it as much as practical, as necessary, and/or as workable.

[DBMandrake]

Originally Posted by planet10

THe big one is that the energy available to excite the resonance is inversly proportional to the square of the frequency althou it can be argued that this could approach the 4th order above some point.

How do you figure that ? Whilst displacement drops with increasing frequency for the same SPL, peak acceleration stays the same, and it's the reaction force from the drivers acceleration directly feeding the panel that is the main source of panel vibration at least for the front panel, which is the one radiating towards the listener.

All irrelevant though, because it's the ratio between the direct signal from the driver and the re-radiated signal from the resonating panel that determines its audibility. The panel might vibrate far less at high frequencies in terms of excursion, but so does the drivers cone so the ratio isn't changing just by pushing the frequency higher.

So the "less energy to excite resonances" is bogus. The excursion of both cone and panel vibrations are just less at higher frequencies, thats all.

A high Q resonance will also be less likely to be excited by music, since to get it going you have to pump energy into it continuously within its bandwidth.

Most music has a wide enough spectra to excite any resonances at least periodically if not continuously. A high Q resonance is audible as harshness, particularly at high frequencies, and results in a fatiguing listening experience. A low Q resonance is perceived as a simple tonal imbalance.

A mechanical high Q resonance usually can't be corrected electronically because it's too unstable to be accurately targeted, while a low Q resonance is often easy to fix in the network or with EQ.

There is no time when a high Q mechanical resonance sounds preferable to a low Q resonance. It may not be excited in all music all the time, but when it is excited it is usually very obnoxious sounding.

A brace should never be placed at the centre of a panel to avoid the situation you describe.

It doesn't matter where you place the brace, you are suppressing different resonant modes to varying degrees, not simply pushing the resonant frequency up like stiffening the panel would.

As I said, bracing a panel is not equivalent to stiffening the panel nor is it equivalent to multiple separate panels of the subdivided sizes, because the different parts of the panel are connected together. Your diagram above actually demonstrates this well as none of the above are fundamental modes, they're all 2nd order and higher modes as the brace has suppressed the fundamental.

Much like placing multiple subwoofers around a room can suppress certain modes without actually changing what the fundamental modes of the room are.

dave

The reference you show there doesn't make a lot of sense as it only lists a single resonant frequency for each configuration, but any rectangular panel braced or unbraced will have different resonances in each axis.

More likely they mean lowest measurable resonance not fundamental resonance, as when the box is subdivided in half the lowest measured resonance is not the fundamental resonance but the 2nd harmonic resonance.

[AllenB]

Originally Posted by planet10

THe big one is that the energy available to excite the resonance is inversly proportional to the square of the frequency

So less energy leaves through the box walls at higher frequencies but this will still amount to the same level.

I think everyone knows that when a resonance is pushed it can cause extraneous noise or harmonics, be it creaking in the walls or simply a level dependent non-linearity in them as a medium, which could lead to the common misconception (IMO) that a certain amount of energy is needed to start a cabinet resonance, or that more energy makes them more loud/noticeable. Speaker Dave was refuting this as well if I remember correctly.

Originally Posted by planet10

A high Q resonance will also be less likely to be excited by music, since to get it going you have to pump energy into it continuously within its bandwidth.

A high Q resonance may be harder to 'find', if this is what you are saying but a good speaker really doesn't leave many places for something like that to hide.

[system7]

Quite interesting to look at the basic cabinet dimensions internally.

400 H x 216 W x 270 D

Not your classic golden ratio cabinet (5x8x13) by any means. Doing the sums for speed of sound at 340 m/s, you get 850 Hz with a wavelength of 400 mm. Exactly right for a strong resonance on the height and width. You need some mass of wadding in the middle of the cabinet to fix that. Can't do it with reflex really.

Broken and unfixable. Eat that, Dr. D'Appolito!