Bass reflex port "redesign"

[Khron]

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]

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]

I was suspecting something like that.

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

 

[mrlots2do]

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]

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]

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]

Here's an example of what richie00boy suggested.

 

[Khron]

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]

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]

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

 

[Khron]

I'm usually kind of a KISS guy (keep it simple, stupid)

One more inquiry...

I haven't yet made my mind up about the drivers i want...

I was considering Visatons at one point, but the guy that could get me some advised me against them (W130 SC 8ohm / SC 10N), instead directing my attention towards Tonsil products.

If any of you guys have some ideas of decent drivers that won't "break the bank" (ie. not Seas Excel ), here's some of my "requirements":

Woofer: 5.5", 8ohm, ~90dB/1W/1m, Fs as low as possible, mids as flat as possible, up to 2-3KHz

Tweeter: 1", 8ohm, fabric dome, ~90dB/1W/1m, Fs ~7-900Hz, as flat as possible up to 20KHz.


Looking forward to your input,
Chris

 

[richie00boy]

I bought a few Visaton SC10N on sale for use in some projects but haven't used them yet so don't know what they are like, but for the money they certainly look OK. As for Tonsil I have a friend with a pair of Opera Duetto which are very nice speakers indeed and use Tonsil bass-mid units and they are good.

 

[The golden mean]

I would say that Zobel networks can be used , primarely to prevent reflections( standing waves) in loudspeaker cables.
If used, they should be placed near the drivers.

 

[richie00boy]

Can you provide any evidence to back up this reflection theory? That's not what zobel networks usually do in this application...

 

[Khron]

Weird stuff...

I did some sims in WinISD and.. it came up with some weird results. For example, for a port height of 4cm (1.5") and width of about 20cm (8"), it spat out a required port depth of ~50cm (20"). I KNOW that can't be right.

So, if anyone has some ideas of some equations or something, to "turn" a round port into a rectangular one, I'd be most grateful.

 

[boydon_lepasci]

Well the area of a port 1.5" X 8" is very close to a 4" dia round port. Do you really need a port that large for the size and tuning frequency of your box? You could add a divider down the full length of the slot to create two smaller ports which would require less length.

 

[gfinlayson]

Yep, that sounds about right. The bigger the port, the longer it has to be to provide the same acoustic resistance. As boydon_lepasci said you need to make the port opening much smaller.

To give you a comparison, my current project is a 33 litre enclosure tuned to 33 Hz. It's bottom ported with a 102 mm ID port. The port is over 400 mm long.

According to the port size and length you've given, I reckon your cabinet is what, 8 litres? And tuned to about 60 Hz?

A 1.5 cm x 8 cm port, and 8 cm long will arrive at the same numbers as above, so you need to start again with these sorts of numbers.

There's no black magic in converting from round to rectangular ports. If you design a rectangular port with the same cross-sectional area as a round port, it will have the same length as the round port at a given tuning frequency.

 

[Khron]

Thanks a lot, gfinlayson , that sheds a lot of light upon my design

 

[Khron]

PS:

In order to get away with shorter ports, i'd have to decrease their (equivalent) diameter, right? But that may give birth to some port air velocity issues..

 

[gfinlayson]

Go for the biggest size of port that will allow the length to comfortably fit inside the cabinet, with some clearance. Also, rounding over the port outlet with a 1/2" to 3/4" roundover bit will reduce the risk of port noise.