Quick bass reflex port question...

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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. :D
 
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!
 
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).
 
revans said:
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.
 
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.
 
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?
 
Hey thanks for that suggestion infinia. I went and played around with Unibox and it seems a fair bit more in depth than WinISD.

I chucked in all the correct values and got this port speed graph which I've attached (although it wouldn't let me change the port length to the 9.2cm that I'm using).

It seems that air speed only exceeds bad levels at about 72Hz and below. Is this a problem? Would I perhaps want to put in a filter to block anything below 70Hz?
 

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revans said:
It seems that air speed only exceeds bad levels at about 72Hz and below. Is this a problem? Would I perhaps want to put in a filter to block anything below 70Hz?

Yes.
HP filter would be advised if you plan on pushing SPL's and keeping lower distortion and/or even prevent damage.
Look at a sealed box for a 4" if using a subwoofer.



revans said:
What about flaring the internal end of the port? That would be better than no flare at all, correct?


Yes
Unibox lets you model that as well.
Also check the box (button) for the port secondary resonance to see.
 
OK thanks for that infinia. I checked port resonance btw, though I'm not sure what difference it made to the graphs.

Anyway though, just to summarise... will the speakers be fine with regard to port noise if I: 1) put in a high-pass filter around 70Hz; 2) flare the internal end of the port; 3) leave the other visible end of the port at a 90 degree exit?
 
revans said:
OK thanks for that infinia. I checked port resonance btw, though I'm not sure what difference it made to the graphs.

Anyway though, just to summarise... will the speakers be fine with regard to port noise if I: 1) put in a high-pass filter around 70Hz; 2) flare the internal end of the port; 3) leave the other visible end of the port at a 90 degree exit?

Check the Xmax vs frequency chart at max power to decide where the filter cutoff should go.
I don't think you will have any problem with air turbulance.
 
Wow... I looked at the "Cone Excursion vs. Frequency at 25W" [it's attached] and it seems a bit crazy. I'm guessing that according to that graph I should cut it off at about 175Hz... isn't that a little high? The Tang Bands should be able to handle at least down to 100Hz if not 80Hz, so what's up with that?
 

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Your tunings way way off. Go back to WinISD and start with the nominal box Vb and Fb, then plug those back in Unibox these will be the lowest tuning you should use.
Now adjust Pin so the top of the graph peak just kisses the Xmax limit line. Looking the lowest point of the graph dip= fb and the area below it is were the port "unloads" the driver. Below that the graph will cross Xmax steeply.
 
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