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Old 1st November 2004, 07:43 PM   #1
Ryan_U is offline Ryan_U  Canada
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Default Ports

Hello,

I'm new to DIY audio and I am reading up a lot before I try a first project. I have a question regarding ports in a vented box design.
When I use a speaker design software the port length is ALWAYS longer than the depth of the ideal box shape. For example, I 'll get a port of 3 inches of diameter by 20 inches long to tune at x hz but the ideal box will be only 14 inches deep. If you reduce the size of the port you get shorter lenghts but even then only really small ports that would create distortion get close to the depth of the box (like a 1/2 inch port). The only way you could get the correct port size/diameter with an ideal box size would be to have the port facing down. Most speakers (80%) have rear facing ports. Some designs have front or side facing ports but in my design software, and I've tried two so far, the ports would always be too long. They must therefore be tuning at less then optimal frequencies in order to have the port fit. Can somebody explain? Would a bottom facing port be any good? Is it that people never really use optimal box dimensions but only use the liters a box need and then design the box? Am I right when I assume that larger port diameters = longer tube = less port noise?
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Old 1st November 2004, 08:37 PM   #2
Vikash is offline Vikash  United Kingdom
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yes, larger port diameters = longer tube = less port noise. Also flared ports can help.

What modeling software are you using. And what driver, enclosure size and tuning frequency are you aiming for? What you're experiencing does happen but is based on the above variables. You certainly shouldn't be experiencing it all the time. The port can face wherever you want so long as it has practical space to breathe. It doesn't even have to be inside the enclosure.

For example, this is common with the Peerless XLS10 where a small enclosure can be used but calculated port lengths are longer than the longest wanted dimension. Alternative in this scenario is use a passive radiator which does the same job as a port and brings its own advantages and disadvantages.

Also, if you use something like Unibox or WinISD then you can see the port air speeds at the SPL levels of intereste and it will arm you with better knowledge of what an acceptable port size is. Reduce the port dia. accordingly and you get a smaller required length.
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Old 1st November 2004, 09:08 PM   #3
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Default Re: Ports

Quote:
Originally posted by Ryan_U
Is it that people never really use optimal box dimensions but only use the liters a box need and then design the box? Am I right when I assume that larger port diameters = longer tube = less port noise?

I don't understand the first question, but I'm pretty sure the answer is no. Some speakers have suitable T/S parameters for vented boxes; Some don't. Some are suitable for use with a passive radiator. Which loudspeaker(s) are you considering?

The answer to the second question is yes, although beyond some point port noise becomes insignificant.
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Old 1st November 2004, 10:05 PM   #4
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you can bend the tube inside the speaker.
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Old 2nd November 2004, 12:33 PM   #5
Ryan_U is offline Ryan_U  Canada
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Thanks for the replies.

I am looking into using the Peerless HDS 164 as a woofer and a Seas tweeter (h1189 or h1212) These drivers seem to be a nice compromise between price and quality and I think a sinple x-over can be used (more on that later). I just can't justify buying a ss 9700 tweeter at 250$ each even if it is the best thing around.

I used Unibox and Winsid to model the box. The volume of the box comes out to around 12 liters give or take, which is a nice size for this project. If I use a 7 cm port I get a length of 30 cm. The ideal box shape is 23x38x14 cm. The vent mach is 0.07 so that is acceptable. But as you can see the only way I would be able to put the port is if it is facing up or down. I don't think having the port extend out of the box would be an ideal situation since these speakers will be 2-ways for the rear sattelites of a home theater system but it could be really cool for another project.

I don't see any problems with a port that faces down but if I use stands, they will have to leave the port opening free.

For flared ends I was thinking I could use a router to round off the openings and then use a sand paper to give it the desired trumpet like shape.

I'm going to keep lurking here untill I feel confortable I have most of the knowledge I need. Keep up the great community. I just can't wait for the winter months so that I will have the time to start building these speakers.
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Old 2nd November 2004, 02:20 PM   #6
Vikash is offline Vikash  United Kingdom
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If you're set on that driver and want to go ported then you could:

1) bend the port inside the enclosure (L shaped slot port perhaps)
2) make the external enclosure size bigger and partition the inside to give the required volume to the driver area.
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Old 2nd November 2004, 04:16 PM   #7
Puggie is offline Puggie  United Kingdom
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try a slightly larger box, as box size goes up the port length goes down for a particular tuned frequency, so if your port is 30cm in a 12l box, in a 15l box your port will be shorted, also look at bendng ports, if you was going to use PVC pipe can you get 90Deg bends for it?

How about building stands with the port running down the stand, I may do this for my rears in my surround setup, build a stand with 3 tubular legs and run the port down the central leg.

another thing to be careful of is if your port is long enough you may start to get problems with the port resonation as you can set up a higer frequency resonance in the port, usually this is found as nasty sharp ringing peaks in your mid band.
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