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Old 9th September 2012, 11:11 PM   #1
tvrgeek is offline tvrgeek  United States
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Default Build or Research

Coming off three good builds, I am looking to my next project. Instead of another build, maybe a little research project. I did a simple test to see the relative performance of different stuffing on Fs. I guess I could do one on dampening reflections. I was thinking about internal diffusers. Any ideas? Any pressing questions an armature can take on? Any SOP that has not really been verified?
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Old 9th September 2012, 11:57 PM   #2
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Do tell us, what three builds have you concluded........The stuffing question has been bandied about almost endlessly.........perhaps you could collimate(sp?) the lengthy discourse...(Lots of reviewing posts).

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Old 10th September 2012, 12:31 AM   #3
VaNarn is offline VaNarn  Australia
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Suppressing standing waves with the minimum amount of fibrous material could be a worthwhile research project.This is in the context of vented boxes where excess material results in Qb losses.James Moir advocated,many years ago,that a single sheet of material stretched diagonally from the top of a rectangular enclosure,to an opposite diagonal at the bottom,is to be preferred over the usual practice of lining the walls.Basically the standing wave has to pass through the half spiral curtain,twice, and the wave will have a higher velocity at this point than at the cabinet rear surface. Also a layer of material cowled over the rear of a speaker(like a teapot cosy)may also be part of the experiment.
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Old 10th September 2012, 12:40 AM   #4
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Perhaps the enclosure could have a easily detachable front baffle so that material/arrangement variables could rapidly change while all others are fixed.????


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Old 10th September 2012, 12:46 AM   #5
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Quote:
Originally Posted by Richard Ellis View Post
Perhaps the enclosure could have a easily detachable front baffle so that material/arrangement variables could rapidly change while all others are fixed.????


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Or Rear baffle removable which would place less strain on the speaker fixing over time.
Either is reasonable if using T-nuts or similar
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Old 10th September 2012, 03:19 PM   #6
tvrgeek is offline tvrgeek  United States
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No question on a test box. Stuffing and accessing the crossover through a 4 inch hole is not fun. Removable baffle means easy to switch drivers, but as mentioned and I have confirmed, weak and resonance prone. So I have wound up with completely glued boxers. For testing, back access seems most logical.

It is a logical step to do similar tests on a vented box. I much prefer low Q sealed, but space and drivers available keeps making my use vented. Fill, lining, shapes, curtains. A lot of options out there. I'll think about that. I try to combine a few woodworking skills with every one too. I am still in the regular polygon space.

Three builds:
1: Started with Zaph SR-71 parts (good discount) and changed tweeters to the metal dome version of the same motor and slight box differences. Passes my wife's critical ear but something very slight is not right in Join Mitchel's voice. Confirmed the larger radius on all 12 edges of the box had a major improvement in diffraction induced response issues. 3/4 inch beats 1/2 by a lot. Also confirmed I still can't get a decent painted finish on MDF as it soaks in the seams. More work to do.

2: Fountek 85's for my desk. Applied what I learned trying to damp the breakup modes of the Fostex to the 85's pretty well. Good enough for the use for half-decent to listen to classes online. 100 to 12K speakers driven by a little USB DAC/Amp.

3: Rebuild of my old Dayton RS-150/Vifa with Seas tweets and total new crossover. Voicing came out excellent. It is within 1 dB 100 to 18K. I used what I learned on stuffing to reduce the hump before roll-off and reduce the mid range reflections. This is the first time I was successful with combining the BSC into a third order crossover. ( second order on the tweet). I went from hating metal cones on anything to coming to terms with them. No notch on the 150, as the crossover is low and steep enough.
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Old 10th September 2012, 08:15 PM   #7
sreten is offline sreten  United Kingdom
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Hi,

If I was testing damping material at various densities I'd use PVC piping
components with a small test driver about 2x the Vas of the main pipe,
not as easy as it sounds to get a suitable low Fs / high Vas test driver.

You can look at the optimum density for various materials to minimise
the sealed box resonance. By leaving the pipe open at the other end you
look at the shift in the 1/4 wave resonant frequency, i.e. the velocity drop,
and more contentiously (i.e. more difficult to measure) the damping effect
at higher frequencies compared to lower frequencies, and if they diverge
or correlate well with the low frequency result.

Pulse testing might give some idea of attenuation loss.

IMO regarding damping materials there is little useful testing you could
with a vented box in terms of identifying damping material properties.

It depends on the nature of the information you want to ascertain.

rgds, sreten.
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Last edited by sreten; 10th September 2012 at 08:18 PM.
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Old 11th September 2012, 12:22 PM   #8
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Quote:
Originally Posted by tvrgeek View Post
No question on a test box. Stuffing and accessing the crossover through a 4 inch hole is not fun.
Who obliges you to put it inside the box ?
This is DIY and you're not selling the box as it is ( a...box)
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Old 11th September 2012, 12:37 PM   #9
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reflective damping or diffusion of reflection is a good test, there are many you have mentioned. Skyscraper diffusors, holey 'helmholtz' absorbers with damping, or a 'pierced' inner cavity wall with absorbent in the cavity. These are things that i would find interesting to investigate. In particular the last one.
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Old 11th September 2012, 05:36 PM   #10
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Also, if you built a test box with a removable panel, you could try different panels of different materials with different bracing schemes, and perhaps shed light on that age old conundrum, "What's the best thing to make my box out of?".
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