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MA CHBW70 sims - vent air speed
MA CHBW70 sims - vent air speed
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Old 26th March 2020, 08:00 AM   #1
jimk04 is offline jimk04  United Kingdom
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Default MA CHBW70 sims - vent air speed

I got a couple pairs of drivers from Scott recently, including a pair of CHBW70.
I put the data into WinISD to the best my knowledge allows. I got the red warning about dodgy parameters.

The model seems to work. The curve looks as one would expect soi believe the parameters are ok. Surprising LF extension in small enclosures the only 'alarm' that rang for me. But hopefully this is correct as Scott did mention they would go down into the 50s.

The biggest issue I see is the vent air speed. Even with a 1w signal, the curve just shoots up off the graph intimidating huge air speeds. Maybe this is a function of the parameter input being bad?

Best regards
Jim

Last edited by jimk04; 26th March 2020 at 08:03 AM.
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Old 26th March 2020, 08:49 AM   #2
Scottmoose is offline Scottmoose  United Kingdom
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Without knowing what you've entered it's impossible to say. Typically WinISD does a mathematical calculation of a number of parameters from the data entered, so while I don't generally use it, a solid approach would be to enter: Qe, Qm, Sd, B*L, Xmax and let it do the rest. There will always be some variation and it also depends on how the brand decides to present data. MA are generally quite consistent & the published data agrees with itself, although there are usually some minor roundings off or slight variation as there are different mathematical methods for back-calculating based on what data is entered.

Air velocity in ducts can be reduced by a larger vent CSA, it's a balancing act between that and length, and whether it fits into a small box. There is no 'alarm' however in small boxes being tuned low as a broad principle though: if Fs, [effective] Qt and Vas are balanced to allow that, that's what happens. This is largely what Small [following Thiele, and Novak] achieved: by applying electrical filter theory to drive units & enclosures, you can get a low tuning out of the smallest possible box. The price is acoustic efficiency, as you'd expect. Similar situation to what happened when the acoustic suspension came along: the drivers suited to it could go low in a relatively small box volume, but compared to the older, larger sealed boxes & the drivers typically used in them, efficiency was usually significantly lower.

Note that a duct position on the rear panel can often reduce the effects of high air velocity in the vent itself; this is a matter of degree and depends on the design.
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Old 26th March 2020, 09:22 AM   #3
jimk04 is offline jimk04  United Kingdom
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Thanks Scott. I just re did the parameters as you suggested. I just got a flat line for transfer function. So I added MMs and VAs and Fs.

Looking good. 'Green light ' for the parameter integrity which is a start, and now my port speeds are sensible. I just have messed up before.

Models F10 39Hz in 8litres tune 52Hz.
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Old 26th March 2020, 10:03 AM   #4
Scottmoose is offline Scottmoose  United Kingdom
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Sorry, you need Fs, Re as well. Let it do Vas, Mms, Cms, Rms et al.
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Last edited by Scottmoose; 26th March 2020 at 10:07 AM.
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