Rythmik 4th order Bandpass experiment - diyAudio
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Old 2nd August 2009, 07:48 AM   #1
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Default Rythmik 4th order Bandpass experiment

I have two Rythmik sealed 12" servo subs and I've just tried a new experiment - 4th order bandpass. I'm considering a 3rd sub - a coffee table bandpass, so this is a quick experiment to see what I think of it. I want to find out if it's an improvement, a step back, or if I can get it to sound the same.

Why bandpass?
Firstly, the acoustic filtering allows closer placement without localisation. Secondly, a bandpass filters not only the acoustic response, but distortion as well with potentially cleaner output. Thirdly, the driver is protected from drink spills, etc.

So in front of my existing 70L sealed box I placed a 20L front chamber with a vent I had lying around - 85mm PVC x 500mm long (39 Hz tuning). Tuning is a bit low, but sufficient for a test.

First result wasn't good. I had serious leaks, so last night I went to work sealing up all the joins, then today putting it all together again. Now it seems the leaks are gone.

Now I have a turbulence problem!
I've been testing with an acoustic double bass track which has quite low bass, playing at a moderately loud level so the cone would normally move a few mm at most. When the cone moves the most, I'm getting turbulence!

I estimate from sims that this occurs when vent velocity is less than 3m/s!

Yes, the vent is too small and yes it needs flares on both ends, but under these conditions I don't expect to have a problem!

Have anyone had experience with bandpass subs who can shed some light on this?
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Old 2nd August 2009, 03:57 PM   #2
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Hard to comment without any experience with this driver, particularly since it's a servo controlled one, but I can say that the few BPs I've built that was tuned low with constant area (straight) pipes required some form of vent damping to quell its harmonics and that the vent's area had to be at least 25% of driver Sd.

GM
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Old 2nd August 2009, 04:09 PM   #3
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Not too worried about the servo aspect right now. If I get this working well enough, I'll need to get a custom servo board for it.

That would be a 6" vent!

Actually if I do build a serious version, I have a slot vent in mind with some serious flares. What I just don't get is that it's chuffing when there should be such a low velocity. I'm wondering if a 4th order BP has much stricter requirements with velocity. Hmmmm .... just baffled right now.
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Old 2nd August 2009, 04:49 PM   #4
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Yeah, there's a big difference. Think about it, what happens when smaller vented alignments are required due to a low Vas driver? The vent size increases to get a low vent mach, so the tiny front chamber of a typical BP working against the stiff acoustic 'spring' of the rear chamber requires a decent sized TL for a low vent mach, making PRs your friend in both these apps.

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Old 3rd August 2009, 06:46 AM   #5
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I see your point, but what baffles me is that I'm getting turbulence at such a low velocity ~3m/s! It would be understandable if it were happening at 34m/s, but it isn't anywhere near that.
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Old 3rd August 2009, 03:09 PM   #6
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Measured? Regardless, port 'chuffing' is a high impedance throat distortion that at its extreme shreds driver radiators while port 'noise' that comb filters with the mains's output is port speed harmonic distortion. IOW, it's the high throat impedance that's limiting port speed. In a properly implemented compression horn this equates to one that has less efficiency, but greater acoustic power/watt due to high compression.

Anyway, this is just what passes for my common sense understanding of the situation based on my own results and what I think I know about compression horn theory since I don't know the math or have the smarts to work through it, so best to query Mark K., Dr. Geddes, etc.. Me, I just designed slightly under-damped 4th order BPs and either flared or 'critically damped' the vent as required to 'kill two birds with one stone', i.e. damp both ends of its response since I did all my BP building decades before proper PRs, simming software, etc. became readily available at affordable prices.

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Old 3rd August 2009, 03:17 PM   #7
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Not measured, just a guess from sims. Looked at excursion of the sub without the front chamber - just a few mm. Found out that this corresponds to about 10w input power, then used this input in the sim to find that the vent velocity should be about 3m/s around 40 Hz.

Well, when I get more time, I'll use a 100mm vent and turn up some flares on the lathe - probably 18mm inside and 36mm outside. For now it just has to be good enough to test SQ and localisation. It doesn't need to perform at the limits of the sub.
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Old 3rd August 2009, 06:03 PM   #8
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Are you still using the servo circuit? Perhaps this is control runaway rather than a port issue?
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Old 3rd August 2009, 06:11 PM   #9
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Yes the servo is still operating. It can't be disabled. I'm not sure what you mean.

I've used the sub in a H frame dipole without any problems, although it's not ideal to use it in anything other than sealed unless the servo board is suited to the alignment.
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Old 4th August 2009, 01:36 AM   #10
Ron E is offline Ron E  United States
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The rythmik sub uses an auxiliary coil to sense position.
The port reduces excursion, the ckt supplies boost, you get port noise. Measure voltage at the speaker terminals as you sweep frequency to eliminate this possibility.
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