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Old 25th May 2008, 01:25 PM   #1
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Default Bass Cab 4x10 Port Issues

I wasn't really sure whether to put this here or in Loudspeakers, this seemed appropriate.

I am currently designing a bass guitar cabinet. I am planning to use Eminence Beta10 drivers in the usual four in a cab setup. I am also planning to halve whatever volume I get for the cab and make a 2x10 using two more Beta10s.

A size of 24"x24"x18" seems decent. Using WinISD Pro Alpha, I have calculated this cabinet (going from a 136L internal size) to have an F3 of about 66Hz, which I am happy with.

My real question comes with the port. Tuned to 45Hz and with a diameter of 10.20, WinISD is telling me I need a port that is 1.41cm long, which seems wrong. Can anyone correct this? This is not necessesarily the port diameter I will end up using, but I want to ensure the sizes it is giving me are correct.

Lastly, If I halve the internal colume of the box and use half the drivers (2), will a port of the same length be tuned indentically?
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File Type: jpg beta10 - 136l.jpg (95.9 KB, 145 views)
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Old 26th May 2008, 07:20 PM   #2
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that's 1.41cm from the inside surface of the box. you might want to keep the diameter where it is, as you want to keep the air velocity under control. too much air velocity and the port will hiss with air movement (it will sound like a buzzing noise). IIRC WinISD calculates the port diameter by velocity and port depth ny desired resonant freq vs box size
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Old 28th May 2008, 10:05 AM   #3
sreten is offline sreten  United Kingdom
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Quote:
Originally posted by unclejed613
that's 1.41cm from the inside surface of the box.
Hi,

No it is not. Its total port length.
As long as the end correction is correct in the simulator (might
not be accurate for very short ports) the results should be OK.

Try a 24" x 2" slot which needs a 3.5" port length

5cuft is a very large box for a 4x10. Large boxes do not need
long ports, basically for the same Fvent and port area the
smaller the box the longer the port.
The same port in a half volume box will tune ~ 1.4 times higher.

For half the volume 24" x 1" slot needs 4" port length.

I'm not sure about any corrections for the slot at the edge e.g. :

Click the image to open in full size.

/sreten.
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Old 29th May 2008, 12:28 AM   #4
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tnx i was thinking that the port length was assumed to cut into the box volume., but you're right.... it's a "matching stub" (looking at it with antenna theory, which has many parallels to acoustics)
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Old 4th June 2008, 11:59 AM   #5
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Hi, thanks for the responses.

I now have the drivers ordered, so I got a port for it today, 110mm internal. Once I make the cabinet, I think I'll dry fit it at full length just to ensure it works as expected. I am then planning to cut it down to about 40mm, for a tuning for 42.6Hz.

The ports outer end has a chamfer of about 2mm, do I have to account for this? Should I do anything to the inner (cut) edge to avoid whoofing?

Might make a production diary of the making of the cabinet, there aren't a whole lot around with lots of pictures etc. Thoughts?

Nic.
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