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Redoing an old build
Redoing an old build
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Old 7th February 2018, 03:10 AM   #21
Brian Steele is offline Brian Steele  Grenada
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After repainting and sealed around the vents a bit more, I'm getting the attached FR. Looks like the dip at the upper end of the passband has almost vanished. I added a little stuffing in the vented section, and that seems to have appreciably reduced the resonance at 400 Hz

The impedance curve still looks unusual however.
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File Type: png 20180206-INF10-BP (stuffing)-FR.png (60.6 KB, 43 views)
File Type: png 20180206-INF10-BP (stuffing)-IMP.png (58.8 KB, 43 views)
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Old 7th February 2018, 11:02 AM   #22
David McBean is offline David McBean  Australia
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Quote:
Originally Posted by Brian Steele View Post
I would really like to know however where the up to 6dB of theoretical output in the passband has disappeared to.
Me too :-).
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Old 7th February 2018, 11:35 PM   #23
Brian Steele is offline Brian Steele  Grenada
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#%(@Q#@$()&*!!

Apparently for the last test there was some leakage in the vented section through some of the screw holes in the access panel. Now that everything's buttoned up properly,. the hump in the response is back.

Grr...

I'm going to call this one "Enigma", because it's still a mystery to me why there's such a big difference between the sim and the actual result. And you'd think that a 6dB difference would make it rather insensitive, but it seems to be the other way around - it's like it's gained a few dB at low frequencies rather than losing a few at higher frequencies.

Oh well, at least the cutoff at the upper end is pretty smooth, and out of band noise is 20dB down from the passband. This 4th order BP box can actually be incorporated into a full range system without having to use a HP HP filter. And now that I know the "secret" about how to eliminate the lowest out of band resonance in a 4th order BP system, I'm tempted to try it out on another build..
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Old 8th February 2018, 01:13 AM   #24
anatech is online now anatech  Canada
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Redoing an old build
Hi Brian,
Since the sub is normally rolled off well below 400 Hz, it might not be such a big deal anyway. That's probably what they were thinking when they used that woofer. It would be nice to be able to find out where it's coming from.

Do you have another different woofer you could place in there temporarily? It would just prove whether the 400 Hz hump is in the box or the driver.

-Chris
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Old 8th February 2018, 01:37 AM   #25
Brian Steele is offline Brian Steele  Grenada
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Originally Posted by anatech View Post
Hi Brian,
Since the sub is normally rolled off well below 400 Hz, it might not be such a big deal anyway. That's probably what they were thinking when they used that woofer. It would be nice to be able to find out where it's coming from.

Do you have another different woofer you could place in there temporarily? It would just prove whether the 400 Hz hump is in the box or the driver.

-Chris
That 400 Hz is definitely a by-product of the 4th order BP alignment. Its caused by a combination of vent "organ-pipe" resonance and the location of the vent's entrance in the vented chamber of the build.

Where this build is going "off the reservation" is the rolloff at the upper end of the passband, which is not predicted by the sim. The sim suggests that the passband will slope upward, and it's sloping downward.
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Old 8th February 2018, 03:27 AM   #26
anatech is online now anatech  Canada
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Hi Brian,
Increasing inductance in the woofer VC would tend to roll the higher frequencies off. I don't know what the magnitude of the inductance and air load that looks like inductance to the amplifier is. But it seems that the 400 Hz peak is stronger than the mild effect I just mentioned. It really does look like some backwave is being reflected out the port. I think that was what you were saying here:
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Its caused by a combination of vent "organ-pipe" resonance and the location of the vent's entrance in the vented chamber of the build.
The resonance of the port looks like a reactive mass to the system, it wouldn't resonate at 400 Hz because the box would damp that behavior. I was just seeing if there was a way to prove what it was for certain, and maybe some method to eliminate it. Sometimes aiming the inside entrance of the port in a different direction is all it takes. Or a simple deflector to break up the direct path to the port either from the back wall or the rear of the woofer.

I hate having unexplained deviations from the predicted response. That 400 Hz peak is crying to be solved as it is pointing to a problem.

-Chris
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Old 8th February 2018, 03:37 AM   #27
Brian Steele is offline Brian Steele  Grenada
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The resonance of the port looks like a reactive mass to the system, it wouldn't resonate at 400 Hz because the box would damp that behavior.
The vent itself will have resonant modes (pipe resonances) based on its effective length.

In this case, I think the 400 Hz peak is related to the dimensions of the box. It used to be huge when the vent was longer, but decreased significantly when the vent was shortened. I suspect that if I shorten the vent so its entrance in the box is halfway between the front and rear panels, that 400 Hz peak would likely disappear . At the moment it's 20dB below the passband and basically inaudible when I use the subwoofer as part of a three-way system with no x-over.

The woofer's inductance is taken into consideration in the Hornresp sim. However the upper part of the passband, and impedance around that area of the response is not a good match for the sim, and I haven't figured out why yet.
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