Bass reflex port "redesign"

Khron · 31st January 2006, 11:02 AM

I hope i'll be able to afford to start work on some bookshelf speakers ASAP. The thing is that, from what i've simulated in WinISD, most of the 6.5" midwoofers work best in ported boxes. Given that i'd want to make the boxes as small as possible (~7-8 litres), i don't really have room for round ports on the baffle. (Plus, AFAIK, they're supposed to be placed at least one diameter away from the closest wall).

So, i was thinking of doing a rectangular port, like i saw on some bookshelves, except that i want to place the opening towards the front, not towards the back. I'm attaching a picture with the two variants i had in mind.

I'm not sure of the equivalation between the round and rectangular port... I'm hoping to get some useful and constructive input from you guys...

gfinlayson · 31st January 2006, 11:09 AM

Whether the port is round or rectangular, the same rules apply. Just make the rectangular port the same cross sectional area as a round port and make it the same length. I've often heard it said that front porting works better on small speakers as the port output will contain a fair amount of upper bass/lower midrange.

Khron · 31st January 2006, 11:15 AM

I was suspecting something like that.

Also, I read that port air velocity should be kept below 20m/s...

mrlots2do · 31st January 2006, 11:40 AM

The right design will not allow close driver centers. Of the two designs, I would use the left design.

You could put the slot port on the side(s)

Khron · 31st January 2006, 11:59 AM

Slots on the side is a bit even more awkward than my idea

But i'll take that into consideration, nonetheless.

One more question though: if i'm going to biamp my speakers, would Zobel networks on the woofer and tweeter be of any REAL use? (i'm not a fan of doing things just for the hell of it...)

richie00boy · 31st January 2006, 12:05 PM

I would use the version on the right, but below the woofer instead. I would also utilise the bottom panel of the box to form one side of the slot vent.

gfinlayson · 31st January 2006, 12:22 PM

Here's an example of what richie00boy suggested.

Khron · 31st January 2006, 12:34 PM

That's pretty much what i'd want, thanks a lot, gfinlayson

Any ideas on the Zobel thing? (considering active filters and bi-amping..)

gfinlayson · 31st January 2006, 12:48 PM

With active filters and bi-amping you don't need zobels. Zobels are primarily used on woofers to keep a constant impedance, without them the woofer impedance increases with frequency and hinders the woofer's roll-off in the crossover. With active circuits, it's not an issue, unless you are using tube amps, where they would be sensible from an output impedance matching viewpoint.

Khron · 31st January 2006, 02:12 PM

I'm far from having (or wanting, for that matter) the funds for tube amps. I intend to use some gainclone pairs for each speaker, powered from some modified PC PSUs

31st January 2006, 12:05 PM	#6
richie00boy Did it Himself diyAudio Member Join Date: Nov 2003 Location: Gloucestershire, England, UK	I would use the version on the right, but below the woofer instead. I would also utilise the bottom panel of the box to form one side of the slot vent. __________________ www.readresearch.co.uk my website for UK diy audio people - designs, PCBs, kits and more

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31st January 2006, 11:09 AM	#2
gfinlayson diyAudio Member Join Date: Jan 2006 Location: Aylesford, Kent, England	Whether the port is round or rectangular, the same rules apply. Just make the rectangular port the same cross sectional area as a round port and make it the same length. I've often heard it said that front porting works better on small speakers as the port output will contain a fair amount of upper bass/lower midrange.

31st January 2006, 11:15 AM	#3
Khron diyAudio Member Join Date: Jul 2005 Location: Finland	I was suspecting something like that. Also, I read that port air velocity should be kept below 20m/s...

31st January 2006, 11:40 AM	#4
mrlots2do diyAudio Member Join Date: May 2004 Location: Philadelpia	The right design will not allow close driver centers. Of the two designs, I would use the left design. You could put the slot port on the side(s)

31st January 2006, 11:59 AM	#5
Khron diyAudio Member Join Date: Jul 2005 Location: Finland	Slots on the side is a bit even more awkward than my idea But i'll take that into consideration, nonetheless. One more question though: if i'm going to biamp my speakers, would Zobel networks on the woofer and tweeter be of any REAL use? (i'm not a fan of doing things just for the hell of it...)

31st January 2006, 12:22 PM	#7
gfinlayson diyAudio Member Join Date: Jan 2006 Location: Aylesford, Kent, England	Here's an example of what richie00boy suggested.

31st January 2006, 12:34 PM	#8
Khron diyAudio Member Join Date: Jul 2005 Location: Finland	That's pretty much what i'd want, thanks a lot, gfinlayson Any ideas on the Zobel thing? (considering active filters and bi-amping..)

31st January 2006, 12:48 PM	#9
gfinlayson diyAudio Member Join Date: Jan 2006 Location: Aylesford, Kent, England	With active filters and bi-amping you don't need zobels. Zobels are primarily used on woofers to keep a constant impedance, without them the woofer impedance increases with frequency and hinders the woofer's roll-off in the crossover. With active circuits, it's not an issue, unless you are using tube amps, where they would be sensible from an output impedance matching viewpoint.

31st January 2006, 02:12 PM	#10
Khron diyAudio Member Join Date: Jul 2005 Location: Finland	I'm far from having (or wanting, for that matter) the funds for tube amps. I intend to use some gainclone pairs for each speaker, powered from some modified PC PSUs

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