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15" Simple Reflex

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mrspeedy View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote mrspeedy Quote  Post ReplyReply Direct Link To This Post Posted: 22 June 2012 at 3:22pm
Hi Infrasound,
So longer ports for my cabs will decrease the tuning frequency? Okie dokey! 
For a crude adjustment to see what happens, how much extra length am I looking at, double?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote infrasound Quote  Post ReplyReply Direct Link To This Post Posted: 22 June 2012 at 3:58pm
Yes, double should lower the tuning frequency appreciably.

This isn't an ideal way of doing it though - follow the instructions earlier if possible, and simulate the change in tuning frequency to look at group delay, xmax and spl.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote infrasound Quote  Post ReplyReply Direct Link To This Post Posted: 22 June 2012 at 4:00pm
If port length becomes an issue you should be able to block off the central port (simulate this)
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mrspeedy View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote mrspeedy Quote  Post ReplyReply Direct Link To This Post Posted: 22 June 2012 at 9:05pm
So blocking the central port may also reduce tuning frequency?
Ta
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Post Options Post Options   Thanks (1) Thanks(1)   Quote infrasound Quote  Post ReplyReply Direct Link To This Post Posted: 22 June 2012 at 9:07pm
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mrspeedy View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote mrspeedy Quote  Post ReplyReply Direct Link To This Post Posted: 22 June 2012 at 9:23pm
Thanks for all your help everyone, it really is much appreciated!
I will buzz off and block the 'ole as a starting point as I don't have access to instruments allowing me to find my tuning frequency at the moment
I'll do a crude check with my ears!

Thanks again

Edited by mrspeedy - 22 June 2012 at 9:23pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Tweeter_Box Quote  Post ReplyReply Direct Link To This Post Posted: 23 June 2012 at 10:03pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kevin tyler Quote  Post ReplyReply Direct Link To This Post Posted: 28 June 2012 at 5:12pm
i built similar sized 3 port boxes with 2226's in

and left the port length entirely to winisd

if they where all lets say 5mm too short or long, would it really mess things up??

sorry if its a dumb ask

kev
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Post Options Post Options   Thanks (0) Thanks(0)   Quote odc04r Quote  Post ReplyReply Direct Link To This Post Posted: 28 June 2012 at 6:16pm
It probably wouldn't mess things up totally, but your cabs might not be optimum. Without proper measurements or multiple prototypes you can never know for sure. Hence why a lot of people (including me) build one design and if they are happy with it then it doesn't change. It's a lot easier to build multiple variants of a smaller hifi box than a 15" PA enclosure.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote kevin tyler Quote  Post ReplyReply Direct Link To This Post Posted: 29 June 2012 at 11:09am
i guess its all a bit trial and error till you get it right.

cheers


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Post Options Post Options   Thanks (0) Thanks(0)   Quote odc04r Quote  Post ReplyReply Direct Link To This Post Posted: 29 June 2012 at 1:14pm
Yes, that and a bit of experience. For example you might know that WinISD tends to underestimate the tuning of a port slightly when it terminates close to another enclosure wall.

There really is no substitute for measurement once you have a cab to determine the tuning frequency. It is so easy, any soundcard, a couple of leads, a decent accuracy resistor, and some free software = an impedance chart in several seconds.

If the resistor is inaccurate then all it does is change the peak magnitude as well, the Helmholtz enclosure resonance of a ported box is still clearly seen as the minimum between 2 peaks in the electrical impedance curve. We would like this to be near the cut off frequency of the equivalent sealed enclosure - exactly where in relation to it is the alignment of the reflex box. Butterworth for flatness, Chebyshev for extended response with a bump etc.

One day I'm going to have to sit down and do all the maths with pen, paper and graphing program. You can't really beat that for knowledge of modelling.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kevin tyler Quote  Post ReplyReply Direct Link To This Post Posted: 02 July 2012 at 1:25pm
thanks for that

kevSmile
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