Port length to diameter ratio? (1 Viewer)

[Mark Tranchant]

I'm considering a sub project at the moment. I don't yet want to get bogged down in a detailed discussion of what I'm trying to do until I know it's feasible - but I will need to tune a relatively small box (1.3 cu ft) to a very low frequency (22Hz).

For a 3" diameter port, the port length will be about 36", a ratio of 12:1, and will require a couple of 90deg bends to fit in the box. I am aware that I need to allow for the port volume within the internal box size.

I've come across various suggestions for a limit to this ratio from 2.5:1 to 4:1. What is the audible or physical effect of having such a long port? Has anyone ever built such an enclosure and gotten satisfactory results?

 

[Patrick Sun]

The port will stop functioning like a slug of air (resistance of mass), and start introducing pipe resonance to the output. I try to keep port lengths to 5x-6x or less of its diameter.

 

[Jack Gilvey]

This type of project really calls for a passive radiator.

 

[Mark Tranchant]

Yes, I know that a PR would be ideal, but I don't have room for it in my high-WAF threshold design as it stands, nor do I have a good source for the radiator (UK suggestions, anyone?). The alternative would be to go electronically-assisted sealed, or possibly an isobaric design, but I'd like to avoid the extra cost.

I'm after as low a cut-off frequency as possible, combined with "musicality" and compact size (basically a box up to 2ft³). I already have a 100W amp to drive it with, and am not bothered about high output level capability.

 

[Patrick Sun]

You may as well go sealed, then, and pray that room gain will help in the bottom end.

 

[Mark Tranchant]

Aha - I've found a source for a passive radiator in the UK. That'll help - and I think I've found a way to incorporate it without drastically reducing WAF.
The plan is to use a MacAudio Mac Fire 250 subwoofer driver (available from Maplin) in a pseudo-sixth order (electronically assisted vented box) arrangement. Theory behind this is from Kaufman's "Enhanced Sound" book - I'll add details to my web site if and when I get around to this.
In theory, I should be able to get an f3 of about 22Hz in a very small (1.3ft³) enclosure.
Thanks for the suggestions and advice.

 

[Dave Poehlman]

Looks like a job for an ELF&#8482 subwoofer:

To quote a friend:

ELF is the concept of running a sealed box subwoofer strictly below its f3 and eq'ing it with a 12dB/octave low-pass at the desired f3. Yields a small sealed box woofer with deep extension, sacrificing amplifier power.

 

[Jack Gilvey]

Mark, I can't find the specs on that PR, although the woofer doesn't look all that impressive, especially the 31Hz Fs.

 

[Greg Monfort]

>For a 3" diameter port, the port length will be about 36", a ratio of 12:1, and will require a couple of 90deg bends to fit in the box.
>I've come across various suggestions for a limit to this ratio from 2.5:1 to 4:1.
====
WRT a round pipe aspect ratio, there really isn't any. All that matters is that the cross sectional area be sufficient to keep vent mach low and the length needs to be short enough to keep pipe harmonics out of the sub's BW and low in amplitude.
Awhile back this approximation was derived:
Max Length = 13560/(20*Fb)
Where:
Max Length is in inches
Fb is the tuning frequency in Hz
So for a 22Hz Fb, ~30" is the practical limit.
====
>What is the audible or physical effect of having such a long port?
====
If the pipe is long enough its harmonics will be low enough in frequency to affect the sub's output, usually in a negative way.
====
>The port will stop functioning like a slug of air (resistance of mass), and start introducing pipe resonance to the output. I try to keep port lengths to 5x-6x or less of its diameter.
====
FWIW, the bends don't stop the resonant mass, and pipe harmonics are there regardless of shape. What happens with >90deg bends is that new, higher frequency harmonics are created, increasing out of BW 'noise' that can sometimes be loud enough to comb filter with the upper BW drivers.
I've found that using 45deg elbows don't do this, even if two are joined to make a 90deg bend, at least I can't hear them and that's all I care about.
All that said, back before decent PRs were available I've built subs/speakers with far longer vents (up to 8ft long) and used acoustic resistance to damp the harmonics, but that's all ancient history.
GM

 

[Magnus Lindqvist]

That PR doesn't seem to have any spider according to the picture, it's probably difficult to mass the PR without getting the leveling problem (is leveling the right word??).

The price is almost to good to be true for us europeans!

/Magnus

 

[Greg Monfort]

'Balancing' is probably the word you're looking for, but using discs attached to the rear shouldn't tend to cock it at high excursion as long as some attempt is made to center them, and with a surround that's a significant percentage of the area I don't see the need for a spider.

GM

 

[Magnus Lindqvist]

One could make a disc that have a diameter as big as the MDF disc and this way keep the mass as close to the center as possible (keep the offset distance to minimum),good idea Greg!

/Magnus

 

[Mark Tranchant]

Dave
Yes, what I was referring to as "electronically assisted" appears to be what you describe as ELF. It can also be applied to vented boxes, although you need to change the box dimensions. See Kaufman's book.
Jack
What PR specs are needed? Moving mass and moving area only, isn't it? Could I derive the mass adjustment needed by putting it in the box and finding the resonance of the PR, perhaps at two masses (x and x+n when n is the known added mass and x is the unknown PR mass), and solving to find the required added mass?
The woofer isn't that good for straight sealed or vented use, but the combination of parameters happens to work well with the equations for the "ELF" vented box to give a deep bass in a small enclosure.
Magnus
I'd be using the PR horizontally, facing upwards, so the lack of a spider shouldn't matter so long as I place the weights symmetrically or centrally.
Greg
Thanks for the vent advice.

 

[Magnus Lindqvist]

Mark, PR's can not be used horizontally, only vertically!!

/Magnus

 

[Michael_UK]

I Have seen these in the UK(Devon), this might be what you are looking for!!!!!
http://www.iplacoustics.co.uk/ipl_sw..._subwoofer.htm
It uses equalisation in the amplifer to give low bass from a 1 foot cube.
I am getting one in the next few weeks.

 

[Mark Tranchant]

Magnus - why? I can see that the PR would end up "biased" downwards, which would reduce the excursion I could use, but is there any other reason why a PR must be vertical?

 

[Magnus Lindqvist]

Mark, yes the PR will sag heavilly because of the mass.

You could put the PR on the front and the active driver downwards (if the MacAudio driver is suitable for downfiring).

/Magnus

 

[Greg Monfort]

>One could make a disc that have a diameter as big as the MDF disc....
====
Yes, though of course you'd have to cut into pieces to install it. Chances are though that it wouldn't be the right amount of added mass, so using metal slugs or similar screwed or bolted (using a threaded insert) allows for eay adjustability. Hopefully they provide some means as an accessory to make it easy.
====
>....but is there any other reason why a PR must be vertical?
====
How can it reproduce a sinewave if there's no zero (centered) position?
GM

 

[Mark Tranchant]

If the surround behaviour is linear to its limits, it should work just as well horizontally albeit with reduced maximum excursion; the zero position would be its "rest" position. That's just theory, though...

 

[Jack Gilvey]

As always, try it and see. The sag will lead to deformation of the surround fairly quickly (if the PR is designed well, i.e. high compliance) and a reduction of usable excursion when there's probably not enough anyway (at least 2x driver Vd, more if you plan on boosting the deep bass as in an ELF) is not something I'd take lightly.

Not a wheel I'd choose to reinvent, but could make for a interesting learning experience.