Usher 701 resonance issues ? - diyAudio
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Old 17th June 2012, 02:20 AM   #1
percy is offline percy  United States
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Default Usher 701 resonance issues ?

A few months ago I grabbed a pair of Usher 701s pre-built (not the kit) from parts express as they were on an unbelieably deep hard to pass discount.
Dayton Audio UA701CK Usher Speaker Kit Cherry 302-952
Parts Express DIY Project

These are place about 6" from the wall. I have always felt that the low-mid voice/speech range sounded a little bit boxy/honky, like some kind of a resonance artefact. First of all, I'd like to see if anyone who has heard these speakers share the same opinion or not ?
Secondly, I never really bothered to measure or dig into it but now I have, and I would like to get your opinions if these measurements reflect the sound impression I am getting. I have indicated the sections of the frequency range in the response and csd charts that I think are those resonances. Do you concur or disagree ?
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File Type: png Usher_FR.png (35.1 KB, 195 views)
File Type: png Usher_CSD.png (79.5 KB, 190 views)
File Type: png Usher_CSD2.png (39.7 KB, 189 views)
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Old 17th June 2012, 04:04 AM   #2
AllenB is offline AllenB  Australia
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It's plausible. How's the panel damping and stuffing?

I would take a look at room placement. I'd be reluctant to modify the crossover. This peak is noticed by others it may be a necessary one. It's possible that firstly, your close wall placement is causing some issues in the lower midrange which might be helped by pulling them out a bit or moving them across, and secondly that your off-axis sound is causing colourations that may be helped by placement and toeing, or treatment.
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Old 17th June 2012, 04:39 AM   #3
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Those the Dayton rectangular boxes. They really need sme addition bracing, I have heard the same issue in other speakers using this box.

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Old 17th June 2012, 07:07 AM   #4
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Not a comment on the drivers specifically, but the measurement technique.

If you're looking for resonances, don't use 1/3rd octave smoothing.

High Q resonances will be smoothed out and made ambiguous (or even invisible) by such coarse smoothing. Use the minimum amount of smoothing possible, in ARTA that means 1/24th octave for frequency response, and (from memory) 1/12th octave for CSD.

Also it's critical for CSD that the measurement is gated (a red finish marker is enabled) and that any room reflections are excluded from the gated period using this marker, otherwise the CSD will be invalid and will probably be showing room resonances and reflections as well as driver resonances. The long ridge just below 500Hz is an example of an invalid result caused by reflections entering the measurement. (For looking at resonances in a CSD you'll also want a wider amplitude scale - 25 or 30dB as 15dB will cut off too much information)

The same thing applies to the frequency response as well, if it's not a reflection free gated period you could be seeing room reflections in the response, it's not uncommon to see bumps in the midrange that are actually room reflections.

Trying to measure at 1Khz with enough frequency resolution to see resonances is quite challenging though, as it's difficult to get a sufficiently long reflection free gate time in a typical room.

I would put the speaker right near the middle of the room on a stand with the driver height half way between floor and ceiling and the microphone only about half a metre in front of the driver, then see how long you can get to the first reflection from the floor/ceiling, and take a gated but unsmoothed measurement.

Use the following calculator to help estimate the delay to first reflection if it's not obvious in the measurement impulse itself:

http://mehlau.net/audio/floorbounce/

(For example for a 2.4 metre high ceiling with the driver 1.2 metres off the floor and a 0.5 metre microphone distance the best you can do is 5.6ms)

Then you'll have the information you need to assess whether there is actually a driver resonance there.
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Last edited by DBMandrake; 17th June 2012 at 07:22 AM.
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Old 17th June 2012, 07:14 AM   #5
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Get modelling clay/Playdoh and cover inside walls, then put denim insulation or batting, etc. You may also want to play with stuffing a handfull of polyfil or equivalent. The acoustic foam provided in the kit never did much for me. I have a stellar 2-way with similar drivers (8948A and 9950) in the taller .75cu rectangular cabs and they disappear with the clay and denim insulation. They are also heavy as hell. Few hours work, but totally worth it.
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Old 17th June 2012, 07:37 AM   #6
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Before going through the trouble of adding bracing or wall deadening material, I would simply add more layers of foam to the top and/or bottom, and the back wall. If that's the thinner PE egg crate, than a single layer is not enough for the back wall especially. I would do 3 layers there. If you don't have enough, I've used the high density 2" thick green foam blocks at Joann's and that stuff is very good. Every time I've ever had a boxy sound, adding more damping fixes the problem.
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Old 17th June 2012, 08:06 AM   #7
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It is unlikely any amount of damping will fix the issue since the energy exciting the resonance is much more likely to be transmitted mechanically thru the box and not thru the airspace.

Wushuliu's technique will lower the panel frequency and make it lower Q -- both which in general make the resonance more likely to get excited and to be more likely to be heard. That he has practical experience saying it works means in this case it is workable.

Additional bracing will push panel resonances up so they are less likely to get excited.

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Old 17th June 2012, 10:41 AM   #8
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Quote:
Wushuliu's technique will lower the panel frequency and make it lower Q
Why would it lower Q rather than just reduce amplitude?

Quote:
Additional bracing will push panel resonances up so they are less likely to get excited.
Push resonance up to what frequency?
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Old 17th June 2012, 11:49 AM   #9
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Quote:
Originally Posted by planet10 View Post
It is unlikely any amount of damping will fix the issue since the energy exciting the resonance is much more likely to be transmitted mechanically thru the box and not thru the airspace.
I'd agree with that. Panel damping and air space damping are two different and largely independent things.
Quote:
Wushuliu's technique will lower the panel frequency and make it lower Q -- both which in general make the resonance more likely to get excited and to be more likely to be heard.
This is so often stated but is actually not true, and based on a misunderstanding of research into the audibility of resonances. Adding more damping to a mechanical resonance never makes it more audible, it makes it less audible.

The studies into the audibility of resonances that found lower Q resonances to be more audible were specifically comparing electronically created resonances where the different Q resonances were manipulated into having the same peak amplitude. (using a PEQ)

For example a 3dB resonant peak with a Q of 1 will be more audible than a 3dB peak with a Q of 2 because a wider bandwidth is affected. (A greater area under the curve...)

Mechanical resonances don't work this way though because adding damping simultaneously widens the response and lowers the amplitude of the peak.

You can't get a wider lower Q resonance with the same peak amplitude by just adding damping in a mechanical system. Different Q resonances always have different peak amplitudes.

It works out that as you add damping the peak amplitude drops more rapidly than the bandwidth increases and they become less audible.

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.

Quote:
Additional bracing will push panel resonances up so they are less likely to get excited.
Less likely ? How so ?

Subdividing a panel in half with a brace is not the same as a panel half as wide either.

If you put a solid brace half way across a panel you are not doubling the panel resonance frequency, you are actually suppressing the fundamental mode resonance of the panel, leaving the 2nd order mode panel resonance as the lowest panel resonance of significant amplitude.

It might seem that the panel resonance has been doubled in frequency but it hasn't, it's still a 2nd order mode resonance, where the panel flex (looked at from the edge) looks like a full sine-wave instead of half a sine-wave.
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Old 17th June 2012, 01:36 PM   #10
system7 is offline system7  United Kingdom
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See the usual ugly spat developing at diyaudio here. Cabinet bracing...didn't we finish that topic?

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

Click the image to open in full size.

This design is clearly your typical boomy rear-reflex standmounter that is best put up on stands with rubber feet and kept away from walls. Our Troels did something similar with his "Studio 101" Proac 100 clone. A nearfield monitor for sure.

Studio 101 Monitorcopyright 2009

Troels put a little notch at 800Hz on the scanspeak cut paper driver though. Higher crossover too. That's it really, isn't it?
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