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Old 22nd November 2009, 11:30 PM   #1
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Default Bandpass mystery...

The Subwoofer DIY Page v1.1 - Projects : An INF10 Bandpass Subwoofer

A project that I started several years ago. I found some time to do some more measurements today.

This bandpass sub is showing an extended upper frequency response that's not predicted by the LDC's tables (or my equations - which produce identical results to the LDC). In some cases the response is over 6dB higher. Measured Fb also proved to be significantly lower than predicted. Any ideas what actually might be causing this? I've made a couple of guesses, but I would like to know for sure what's causing it, and if the effect can be accurately predicted.
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Old 22nd November 2009, 11:45 PM   #2
jleaman is offline jleaman  Belgium
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Quote:
Originally Posted by Brian Steele View Post
The Subwoofer DIY Page v1.1 - Projects : An INF10 Bandpass Subwoofer

A project that I started several years ago. I found some time to do some more measurements today.

This bandpass sub is showing an extended upper frequency response that's not predicted by the LDC's tables (or my equations - which produce identical results to the LDC). In some cases the response is over 6dB higher. Measured Fb also proved to be significantly lower than predicted. Any ideas what actually might be causing this? I've made a couple of guesses, but I would like to know for sure what's causing it, and if the effect can be accurately predicted.
I used to drool over all your projects I also remember talking to you long time ago too, MAN time fly's.
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Old 22nd November 2009, 11:52 PM   #3
Collo is offline Collo  Australia
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Hi Brian,

I'll be interested in responses to this too!

I built a 6th-order bandpass sub that had a similar result:

Click the image to open in full size.

Actual measured in blue
WinISD predicted in green
WinISD predicted after adjusting volumes to get frequency peaks to line up (in case my volume calcs were out)

Like you, the HF peak was higher in frequency and value than expected.
Mine had both chambers ported, - the LF chamber had the same issues as the HF chamber.
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Old 23rd November 2009, 12:20 AM   #4
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Originally Posted by jleaman View Post
I used to drool over all your projects I also remember talking to you long time ago too, MAN time fly's.
LOL - I didn't think that my projects were actually drool-worthy...
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Old 23rd November 2009, 12:30 AM   #5
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Quote:
Originally Posted by Collo View Post
Hi Brian,

I'll be interested in responses to this too!

I built a 6th-order bandpass sub that had a similar result:

Click the image to open in full size.

Actual measured in blue
WinISD predicted in green
WinISD predicted after adjusting volumes to get frequency peaks to line up (in case my volume calcs were out)

Like you, the HF peak was higher in frequency and value than expected.
Mine had both chambers ported, - the LF chamber had the same issues as the HF chamber.
Did you measure the T/S parameters of that driver to see if they corresponded with the published specs? Assuming that WinISD's modelling is fairly accurate, those peaks suggest that the driver's Qts might be higher than the published figure.
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Old 23rd November 2009, 12:37 AM   #6
Collo is offline Collo  Australia
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Just used the published figures - could be worth my while to read up on the testing methods...
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Old 23rd November 2009, 12:43 AM   #7
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Originally Posted by Brian Steele View Post
Did you measure the T/S parameters of that driver to see if they corresponded with the published specs? Assuming that WinISD's modelling is fairly accurate, those peaks suggest that the driver's Qts might be higher than the published figure.
Just looked over your design...

The Qts of that driver is a bit higher than what I'd use for a 6th order BP. I tried modelling your alignment and got 5dB peaks at the ends of the passband.

Did you measure the impedance to see if you hit the target resonance frequencies? As you used flared vents, this might have shifted the resonance frequencies upwards a bit unless you took this into consideration in the port calcs.
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Old 23rd November 2009, 12:53 AM   #8
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Originally Posted by Collo View Post
Just used the published figures - could be worth my while to read up on the testing methods...
Eeek, no, don't do that, LOL. Actual specs could sometimes be off published specs by quite a bit for various reasons (including some specmanship on behalf of the manufacturer, LOL). It's particularly important that if you going to tackle one of these higher-order alignments, you use actual t/s parameters. In fact, when I was a lot less lazy, I used to manually measure the parameters at a power level somewhat higher than "small signal" and use those results to generate an alignment. It's not that difficult to do, and would save you a lot of grief later on when an alignment doesn't measure as expected.
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Old 23rd November 2009, 01:13 AM   #9
jleaman is offline jleaman  Belgium
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Quote:
Originally Posted by Brian Steele View Post
LOL - I didn't think that my projects were actually drool-worthy...
They sure were
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Old 23rd November 2009, 01:51 AM   #10
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Quote:
Originally Posted by Brian Steele View Post
The Subwoofer DIY Page v1.1 - Projects : An INF10 Bandpass Subwoofer

A project that I started several years ago. I found some time to do some more measurements today.

This bandpass sub is showing an extended upper frequency response that's not predicted by the LDC's tables (or my equations - which produce identical results to the LDC). In some cases the response is over 6dB higher. Measured Fb also proved to be significantly lower than predicted. Any ideas what actually might be causing this? I've made a couple of guesses, but I would like to know for sure what's causing it, and if the effect can be accurately predicted.
It stopped raining for once, so I've been doing some measurements of a front loaded horn versus a tapped horn. Both enclosures use the same woofer, and the foot print is nearly identical.

I noticed a few things:
  • At some frequencies, the TH has 12db of gain over the FLH. This is waaaaaay beyond what you'd expect. Tapped horns have some crazy gain.
  • The FLH is smoother than the sims predict
  • The sims are "in the ballpark", but the measured response is a lot different than what you'd expect.

Since front loaded horns and single reflex bandpass boxes are basically the same thing, and tapped horns are basically dual reflex bandpass boxes, I thought this might be of interest of you.

One thing that was particularly "fun" was adding pieces of plywood to the tapped horn to change the response. By rearranging segments and angles you can change the response quite dramatically, both for the better and for the worse.

Having a good measurement setup makes this hobby a lot more rewarding

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