Quick bass reflex port question...

[revans]

Hey guys, I'm just putting together my first diy pair of speakers and came to a small halt due to a slight confusion.

I got a port length of 90mm from Win ISD and went to cut some pvc pipe to size when I wondered... does 90mm refer to the literal length of the pipe (i.e. only 78mm would protude into the enclosure as 12mm would fit into the plywood front) OR does it refer to the length of the port that protudes into the enclosure (i.e. the total length of the pipe would then be 90mm + 12mm = 102mm)?

Thanks to anyone who can clarify this. I realise to most people on here this may be a really basic and stupid question, but a quick answer would be muchly appreciated.

[Geek]

The whole thing.

You have to figure the baffle in too. So if you are say gluing cardboard tube on the back of 12mm baffle, make your port 78mm for a total of 90mm

If you're using solid wood for the baffle, a 1/4" roundover router bit makes a great flare.

Cheers!

[AndrewT]

Hi,
try starting at 120mm long (total length including thickness of rounded over baffle).

Measure and listen. It might sound quite nice.
Try shortening in 10mm increments. Or have a set of plug in tubes that you can swap between (that's 5pieces of drain pipe from 80mm to 120mm).

[jol50]

Port length is always the total length of the tube the sound flows through. However you only count part of a flare. Also make a smooth transition or you could get noise from a bad seam/step in the port.

[revans]

Thanks guys for clearing that up.

@ jol50: Are you saying I should flare the opening of the port? At the moment the port is just flush with the baffle with a 90 degree exit. Should I change this?

[AndrewT]

Originally posted by revans
Thanks guys for clearing that up.

@ jol50: Are you saying I should flare the opening of the port? At the moment the port is just flush with the baffle with a 90 degree exit. Should I change this?

If the port area approaches 50% of the driver Sd then you can manage without a flare.

Smaller ports start making noise when driven hard. These benefit from a flare.

Very small ports chuff even at low SPL and must have a flare, preferably at both ends.

The flare reduces the effective length of the port.

[revans]

@ Andrew: Sorry for my ignorance but what exactly is a speaker's Sd? Also just to check, by port area I assume you mean the cross-sectional area of the pipe (i.e. pi x radius squared)?

[AndrewT]

Sd = (total) cone area, (if multiple drivers).
Pipe/port area = Pi * r^2 = Pi * d^2 / 4

ps.
you cannot design a speaker box without knowing Sd.

[jol50]

Yes, the smaller the port is in relation to the cone size the higher the air speed in the port...larger probability of air noises. Think of the port volume as a trapped column of air moving like a speaker cone or piston if you will, tuned with the weight of that much air. It is really the same thing as a passive radiator, but the PR is so much heavier it does not need the depth the port does. Air will 'vibrate' in the port but starts to get more movement at low frequency, a whole lot under tuning when it unloads. Anyway, for me it helps to understand what is going on there.

[revans]

OK, so going by what AndrewT said I should be flaring my port as the port area is nowhere near 50% of the Sd.

However, this presents a bit of a problem for me, so I was wondering how necessary flaring would actually be relative to my specifications...

The speaker I'm using is a 4"Tang Band full range driver similar to this: http://www.tb-speaker.com/detail/1230_04/w4-1320sd.htm The port has a diameter of 38mm and needs an effective length of 92mm. From WinISD I got an air speed of 0.08 Mach.

I tried looking at Flare-It (port velocity and turbulence calculator) and saw that it only even considered frequencies of 35Hz and below. The TB starts dropping away below ~75-80Hz so it probably won't be producing any big bass response anyway.

Is it possible that I would be able to disregard a flare and just have 90 degree exit?