Bandpass Enclosure Question

[dreadnoughtstrength]

Hey guys, built a quick bandpass Enclosure to see how these Peerless subs go. Modelled it up in BassBox and went with recommended enclosure to the letter.

When playing anything with rolling or constant bass it sounds perfect - very good response down to around 35hz and rolling off around 100. The issue is with anything with a bass 'hit' (ie. dat beat dropping). Even with the amplifier turned right down, there's an initial noise which sounds like the sub going way past excursion limits, but it goes away after a tiny period of time if that note continues. The best description is a 'pop'.

It doesn't sound like any sort of port noise I've heard before, more like distortion. With the top panel removed, the noise is gone.

Thoughts? The surrounds on these subs look a bit dry, so I'm going to get some of that treatment stuff.

 

[reloader650]

Hi Drew,

You may have to use a HP filter. Can you post more info about the Bandpass, like Vf, Vr, FB, S and Qbp. And the driver model, and t/s parameters that you used. Thanks.

Regards,
Matt

 

[TBTL]

It might be the top panel resonating against the box, with an eigenfrequency that is only in the 'hit' part of the bass.

 

[GM]

'Sounds' like either a leak allowing the driver to 'bottom out' and/or the port 'choking up' from being too small, causing the driver to 'hiccup', so to speak.

GM

 

[dreadnoughtstrength]

Hi Drew,

You may have to use a HP filter. Can you post more info about the Bandpass, like Vf, Vr, FB, S and Qbp. And the driver model, and t/s parameters that you used. Thanks.

Regards,
Matt

--Box Parameters--
Chamber 1 - lower-frequency
Vb = 33.46 liters
V(total) = 38.01 liters
Fb = 64.74 Hz
QL = 6.974
F3 = 38.81 Hz
Fill = none
Chamber 2 - upper-frequency
Vb = 23.72 liters
V(total) = 27.86 liters
Fb = 64.74 Hz
QL = 7
F3 = 110.5 Hz
Fill = none
--Vents--
No. of Vents = 1
Vent shape = round
Vent ends = two flared
Dv = 100 mm
Lv = 182.2 mm

--Driver Parameters--
Fs = 38 Hz
Qms = 8.52
Vas = 63.65 liters
Xmax = 12.5 mm
Sd = 353 sq.cm
Qes = 0.497
Re = 2.7 ohms
Le = 0.844 mH
Z = 4 ohms
Pe = 250 watts

On modeling, cone excursion does seem to be going past Xmas at 38Hz (?) so I might have to build an active HPF and have a play. If it's just a tone, the sub easily pays down to 35 without anything funky going on though so I'm not sure what's up.

It might be the top panel resonating against the box, with an eigenfrequency that is only in the 'hit' part of the bass.

I thought that might be the case, but I have 20 2" wood screws holding the panel on. That being said, I went through the non-removable panels and sealed everything up with industrial glue, so nothing can possibly vibrate now (once I've used a rubber seal underneath the removable panel).

'Sounds' like either a leak allowing the driver to 'bottom out' and/or the port 'choking up' from being too small, causing the driver to 'hiccup', so to speak.

GM

Shouldn't be any leaks, although as mentioned above I've doubled-down on all the sealing to make sure. The port is 4" PVC - checking velocity it seems to hit a lot higher than I thought - 26m/s @ 40Hz. Going off the port velocity/noise chart, with that size port I should be right up to 32m/s at 30Hz (I've got [email protected])

Before I left last night, I swapped out the port (4" PVC, ~7.5") for a PVC 90* degree bend. Taking the loss of diameter when the PVC is bent, the area's are roughly the same. A guy I was speaking to suggested that although there might not be port noise, there may be some sort of turbulence going on as the sub is perpendicular to the port.

With the 90* bend in and both ends flared, we have the same problem so I'm guessing that's not the case.

 

[Brian Steele]

Hey guys, built a quick bandpass Enclosure to see how these Peerless subs go. Modelled it up in BassBox and went with recommended enclosure to the letter.

When playing anything with rolling or constant bass it sounds perfect - very good response down to around 35hz and rolling off around 100. The issue is with anything with a bass 'hit' (ie. dat beat dropping). Even with the amplifier turned right down, there's an initial noise which sounds like the sub going way past excursion limits, but it goes away after a tiny period of time if that note continues. The best description is a 'pop'.

Hook it up to another amp to see if it exhibits the same problem when driven by a different amplifier.

 

[dreadnoughtstrength]

Hook it up to another amp to see if it exhibits the same problem when driven by a different amplifier.

That current amp is playing into my ghetto workshop sub and hasn't had any issues - I've got a heap of the same model sitting (and one ready to be installed for this box), so I might try it though.

Now another question - if the port diameter is somehow to small, what can I do to reduce turbulence? One end has a big flare that fits into the box and the other is rounded over (big radius router on the inside, smaller on the outside). I can either go with the 90* port facing the sub, or the straight port being perpendicular. I probably can't go up any size in port as there is physically not enough room (and it would reduce the volume in the ported chamber significantly).

I've seen some information on building port donuts/'velocity stacks' to go on the inside; has anybody had any success with these? If not, do you think a massive flare on the inside (approaching 5-6') would make any difference?

 

[reloader650]

Hi Drew,

I think there is a problem with the parameters that you used. Did you measure the parameters yourself? Is the driver the XLS or XXLS?

Regards,
Matt

 

[dreadnoughtstrength]

Hi Drew,

I think there is a problem with the parameters that you used. Did you measure the parameters yourself? Is the driver the XLS or XXLS?

Regards,
Matt

XLS, but they aren't off the shelf speakers as they were all custom made for a company (apparently). The specs were measured by somebody at the workshop and massively different to a few data sheets I found around for similar models.

Should I measure them myself across a few drivers and get an average?

 

[reloader650]

Okay that makes sense. I found several independent sites that had measured the XLS, and had similar parameters to Peerless's published data. The standard XLS doesn't seem to work very well in a Bandpass enclosure. I think it is a good idea to measure the parameters if you can. I use the Dats v2, it seems to work pretty good. I couldn't get it to work with Windows 10, but works great on XP. It's also helpful when testing enclosures.

Regards,
Matt

 

[Brian Steele]

That current amp is playing into my ghetto workshop sub and hasn't had any issues - I've got a heap of the same model sitting (and one ready to be installed for this box), so I might try it though.

The problem you described suggests a problem with the driver, the amp or the source IMO. If you believe the amp to be Ok, try a different another driver in the same box. If the problem still exists, try swapping the source for something else.

 

[GM]

Going off the port velocity/noise chart, with that size port I should be right up to 32m/s at 30Hz (I've got [email protected])

Now another question - if the port diameter is somehow to small, what can I do to reduce turbulence? One end has a big flare that fits into the box and the other is rounded over (big radius router on the inside, smaller on the outside). I can either go with the 90* port facing the sub, or the straight port being perpendicular. I probably can't go up any size in port as there is physically not enough room (and it would reduce the volume in the ported chamber significantly).

I've seen some information on building port donuts/'velocity stacks' to go on the inside; has anybody had any success with these? If not, do you think a massive flare on the inside (approaching 5-6') would make any difference?

I don't see any flaring in the pics large enough to do much of anything beyond cosmetic and while you could pack the vent with soda straws or other damping material it will just kill much of its LF output, so opening up the panel and installing the vent on the outside such as adding on a rectangular duct along its length.

Flaring if large enough definitely will lower vent mach, though needs to be on both ends and likely won't lower it enough with just a 3.93" dia. pipe. IIRC the flare has to have at least a radius = to the pipe radius or ~2", so the outside dia. = 8" plus its length will have to be longer based on its mean or ~ how long it would be if it had a ~5.65" dia.; again, all IIRC as it's been a long time since I did any building that needed a flared vent.

Looking at the specs though, it appears to ideally need a TL sized vent to handle any high power, so recommend dual PRs since the box doesn't appear large enough for one big one; otherwise build a 'sonic cannon' like I use to with 8" woofers, i.e. add an 8" concrete former tube to it and cut to length as required and damp to 'taste' with a bit of stuffing. These worked great laid out along a wall or stood up in a corner or put behind the listening position.

GM

edit: just remembered this site: http://www.subwoofer-builder.com/port-flares.htm

 

[GM]

The problem you described suggests a problem with the driver, the amp or the source IMO.

You really believe a ~3.93" dia. vent is big enough for this tiny BP when typically a PR is spec'd?

GM

 

[dreadnoughtstrength]

The problem you described suggests a problem with the driver, the amp or the source IMO. If you believe the amp to be Ok, try a different another driver in the same box. If the problem still exists, try swapping the source for something else.

Will do tomorrow when I get back to the project. As the drivers are fairly old (but unused), do you think a surround that is too stiff and dry could contribute? If so, what should I apply?

I'll build a HPF too and see if anything improves.

I don't see any flaring in the pics large enough to do much of anything beyond cosmetic and while you could pack the vent with soda straws or other damping material it will just kill much of its LF output, so opening up the panel and installing the vent on the outside such as adding on a rectangular duct along its length.

Flaring if large enough definitely will lower vent mach, though needs to be on both ends and likely won't lower it enough with just a 3.93" dia. pipe. IIRC the flare has to have at least a radius = to the pipe radius or ~2", so the outside dia. = 8" plus its length will have to be longer based on its mean or ~ how long it would be if it had a ~5.65" dia.; again, all IIRC as it's been a long time since I did any building that needed a flared vent.

Looking at the specs though, it appears to ideally need a TL sized vent to handle any high power, so recommend dual PRs since the box doesn't appear large enough for one big one; otherwise build a 'sonic cannon' like I use to with 8" woofers, i.e. add an 8" concrete former tube to it and cut to length as required and damp to 'taste' with a bit of stuffing. These worked great laid out along a wall or stood up in a corner or put behind the listening position.

GM

edit: just remembered this site: Port Flares

I used the port speed guide off there which put me in the clear. It did recommend a huge flare so I will attempt that tomorrow. Even though the 90* bend is the type recommended, given that the noise got worse I might go back to experimenting with straight pipes and see if I can get it sounding better.

The exit is flared with the biggest rollover router bit I have access too, if it's not big enough I might go buy a bigger one. The inside is just smoothed over so there are no sharp edges.

 

[dreadnoughtstrength]

Updates: flared the inside of the port to 5.5" but very short (the flared length was only about 3/4" long). No change. Built giant port donut. No change. Took the port out completely. Noise quieter, but sounds terrible. Guessing it is some sort of port noise ALONG with something else going on.

I knocked up half of a subsonic filter tonight but ran out of board. I couldn't get zero gain filter designed with a curve I liked, but by setting 1db gain @55Hz I was able to get it flat @40hz, -3db @32hz and -6 dB @25hz so hopefully this will kill any subsonics

 

[Brian Steele]

You really believe a ~3.93" dia. vent is big enough for this tiny BP when typically a PR is spec'd?

GM

I quote from the first post = "Even with the amplifier turned right down, there's an initial noise which sounds like the sub going way past excursion limits,".

I wouldn't expect that there would be noise generated from the vent with the gain on the amp turned right down.

 

[Ron E]

Try sitting on the (fastened) removable panel while you run it. You should be able to damp any panel resonance

When you press on the cone and move it through its travel does it scratch? Maybe the coil is loose from the former or something like that. Inspect the spider to see if it is damaged.

I simulated your box and that driver should be able to handle ~140W into 2.7 ohms (~19VRMS) before the excursion peak below resonance gets you into trouble.

There is no need to angle the port toward the driver, the port works with air pressure in the box, so orientation doesn't matter.

 

[dreadnoughtstrength]

Updates - finished the subsonic filter. Played around with it in another one of these subs in a ported box and can definitely get loud before there's distortion, but it's still under 50% on the amp. Either the speakers have a very low power handling or something is still up with them. I might build another filter with a much steeper curve and see if that helps.
On this sub it has taken away a bit of the noise but not all. Guess it's time to cut the existing port out and build one externally on the box

 

[dreadnoughtstrength]

It's a very shitty photo, but I finally finished this. I ended up building a 150x150mm external port on the box, and cutting a square out of where the existing 100mm port was. There is a 45 degree baffle at the far end, to hopefully guide the air where I want it - I honestly I have no idea if it is beneficial or not, but it's built and I adjusted for the loss of port volume. Painted the whole thing in tub liner - the box absolutely soaked it up, so next time I do one I think I'll primer the whole thing. Despite that, the finish is pretty cool although not very hard wearing.

I relegated this box to my computer stereo, so I'm about to set a high pass filter exactly where I want it. With it set at 38Hz, I'm getting zero of that weird noise and power handling seems to have gone up a lot - this thing is VERY loud for a compact box. It was replacing a no-name 10" sub and 100w home theatre plate amplifier, so the improvements are huge in SPL and SQL - no more port noise from a generic box!

Couldn't be happier with how this has turned out.

 

[Ron E]

Reading your post again makes me think of a phenomena called dynamic offset or oilcanning. This is where the driver kind of jumps out (or in) to a different operating point. If this is the problem (one would expect it to do this if the box were enclosed or not), you could improve the situation by doing a clamshell (face to face) isobaric. The box volumes will probably then be too small to port so you will have to use a PR or oversize them, or abandon bandpass and go for a sealed or vented box.