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Old 27th March 2003, 07:04 PM   #21
SY is offline SY  United States
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You can find the formulae for port design in Loudspeaker Design Cookbook. It's a basic book that anyone building a speaker should have.
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Old 27th March 2003, 08:09 PM   #22
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If I planned on building more speakers and designing the cabinets myself then I would indeed buy the book that you suggest SY. Since I am merely building the design that is in the PDF and don't plan on building another pair of speakers maybe you could be so kind as to answer my simple question. Thanks either way.

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Old 27th March 2003, 08:28 PM   #23
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Quote:
Originally posted by G
I'm not sure. In the design it calls for a port tube that is about 3 1/4" in diameter by 3" long. I can't find a 3 1/4" ID port tube anywhere. So if I use a 3" port tube would I make it slightly longer to make up for the difference in ID?
No, I believe that you would need to make the port slightly shorter. Mind you, I'm not a tuned-port speaker man.

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Old 27th March 2003, 09:01 PM   #24
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Thank you for your response Steve.

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Old 27th March 2003, 09:04 PM   #25
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For a tubular duct, the following equation applies:

L = (2118*D^2/(Fb^2*Vb)) - 0.85*D,

where Fb is the box tuning frequency in Hz, L and D are length and diameter of the port, respectively, and Vb is the box volume in cubic feet.
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Old 27th March 2003, 09:20 PM   #26
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Which, according to my calculations means that (assuming the box volume and tuning frequency are constant):

If the length required is 3" for a 3.25" diameter port, then
the length would need to be about 2.4" for a 3" diameter port.

But, to misquote Jim Carey in The Mask:
"Somebody check me."

Steve
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Old 27th March 2003, 09:24 PM   #27
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Default Ports, dampers volume etc

This is my first ported project (now I'm scared) but I've done a couple of sealed. I do intend to measure response, and my box is oversize for exactly the reason you cite.

A question on measurement. What I have done is put in a refrence line in my spectrum analyzer of pink noise - 10 second sample, with peak hold on. This yields a frown shape dropping off in both frequency ends in a nice curve. Every speaker I've got more or less conforms to this curve (I normallize all my samples) and I can see the praks and valleys that correspond to what I can hear. So.. I can theorize about why are certain peak happens, and make a correction.

I use a ruler flat condenser in omni mode a foot or two from the speaker. As I pull back I can see the room effects leak in. (Most mikes are flatter in omni mode than cardiod, even though one would like to use a cardiod pattern for such a measurement to reduce room effects. I put the pink noise at about 85 db so the speakers are working inside their sweet spot and priximity effect is sort of in range (ie about how I would listen to them).

My question is: Am I doing this right?
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Old 27th March 2003, 09:35 PM   #28
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I'd do two things:

1. Near field. Get that mike within 1/4 inch of the cone, if you can. Likewise, when you measure the port, get that mike right at the outer surface. If you want to get a full freq response, do a vector sum of the two measurements, scaled to their relative areas. d'Appolito gives gory details in "Measuring Loudspeakers."

2. Measure impedance. Having a crosscheck to validate measurements is a really good idea.
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Old 27th March 2003, 09:39 PM   #29
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To get back to the thread (Damping Materials for Speaker Cabinets) I've got an interesting one.

I've just put together a prototype for my new speaker. It's a tall, narrow column and I designed the cabinet to require minimum damping as I believe that too much can make the speaker sound lifeless. I'm talking here about the 100Hz to 20kHz range, which I cover using four Bandor 2" units (see my link below for my original speaker using this concept).

Anyway, I think I may have overdone it. I've listened to the speaker with no damping at all and it sounds fine. Of course, I'll have to spend some time tuning by ear and also by measurement and I look forward to doing this next week (I want better than fine). But I'm curious to get other views ...

My walls are pretty dead to vibration so let's leave out panel damping and concern ourselves with:
A) Putting some stuffing in the middle of the column (wool or fibreglass wadding) and
B) Sticking something on the inner walls (carpet, wool, foam, etc.)

So guys, in your considerable collective experience, how much of A and B is too much and how much is not enough? I'm interested in anyone who has played around with this either by ear or measurement.

Steve
Oh, and Theo - I've just seen your message - if you can see discrepancies in the measurements and correlate them with what you hear, I'd say: Yes, you're doing just fine.
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Old 27th March 2003, 10:00 PM   #30
7V is offline 7V  United Kingdom
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Quote:
Originally posted by SY
d'Appolito gives gory details in "Measuring Loudspeakers."
Thanks for that SY. I've read a couple of reviews and put the book on order. It's a "must read".

Steve
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