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Old 2nd January 2006, 12:47 AM   #1
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Default speaker porting

how do i know if the ports winISD tells me to add wil work>? it tells me to put about four 8 inch long 3 inch diameter ports and that sounds ridiculous to me. Ive never heard of ports liek that and i am wondering if that means tehy are problematic at taht size? i suppose it will get the spl that the little chart shows me, but how do i know it won't sound weird?

PS it also tells me to make the ports about .5 inches long when i make them any smaller in diameter, and that is even scarier sounding
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Old 2nd January 2006, 01:51 AM   #2
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you must use a port with adequate cross section in order to minimize the velocity of moving air inside the port. This prevents chuffing(which are eddies formed by the port air moving too fast and not coupling at the port mouth with the surrounding air.

Tuning frequency is related to three things
Enclosure volume(disregarding the volume of air in the port)
Port cross section
Port length

If volume is constant, tuning is constant, and cross section increases, the port length also must increase

If volume is constant, tuning is constant, and cross section decreases, the porth length also must decrease

This is why you get such a small port length for a smaller cross section.

Look at the vent air velocity graph on winISD to determine if the port has adequate cross section.
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Old 2nd January 2006, 02:22 AM   #3
DcibeL is offline DcibeL  Canada
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When looking at the air velocity graph, it is generally accepted that the velocity should be kept below 35m/s.

If you don't trust the program, you can always calculate it yourself
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Old 2nd January 2006, 04:06 AM   #4
Collo is offline Collo  Australia
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Default Usable velocity for 3 inch ports

Hi there robotnation,

I've used WinISD to build five subs so far and found the port calcs to be fine.

As far as usable velocity is concerned, the 34 m/sec recommendation is from the earlier version of winisd. The pro version drops this to 17 m/sec.

I've done some testing of flared ports which shows that if you want to move a reasonable amount of air, you need to use port flares each end. It's been well discussed in an earlier thread:
Flared ports and turbulence
The most up to date recommendations are on page 6.

For the three inch ports you are considering, I would suggest you look at either the Precision Ports or the Lightning ports. Either brand will give you flares around 30mm in radius.
Three inch diameter with 30mm flares is good for 15.5 m/sec.

Larger ports can handle a little higher velocity. If you use 4 inch ports with 30mm flares, the 17 m/sec WinISD recommends is fine!
Also you only need a pair, which halves the cost of the flares.

ps...for those who followed the earlier thread, I'm currently building a new test box that will produce results for 6 inch ports
Sneak preview of 8 new ports to be built and tested at:
http://www.users.bigpond.com/bcollis...esting-pt2.htm

regards
Collo
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Old 3rd January 2006, 06:58 AM   #5
chops is offline chops  United States
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Default Re: speaker porting

Quote:
Originally posted by robotnation
it tells me to put about four 8 inch long 3 inch diameter ports and that sounds ridiculous to me. Ive never heard of ports liek that and i am wondering if that means tehy are problematic at taht size?
This really isn't so "ridiculous" as you might think. The SVS PB12-ISD/2 subwoofer I had had three 3" ports, but at about 14" long with flares on all six ends. And trust me when I say that this sub moved some massive amounts of air!

Click the image to open in full size.

Click the image to open in full size.
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Old 3rd January 2006, 01:56 PM   #6
chops is offline chops  United States
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Default Re: Usable velocity for 3 inch ports

Quote:
Originally posted by Collo
Hi there robotnation,

I've used WinISD to build five subs so far and found the port calcs to be fine.

As far as usable velocity is concerned, the 34 m/sec recommendation is from the earlier version of winisd. The pro version drops this to 17 m/sec.

regards
Collo
So according to WinISD Pro, my new slot ported sub design yielding a maximum port velocity of 10.5 s/m @ 12 Hz at 104dB shouldn't be a problem then, right? (7.2 m/s @ 20Hz at 111dB)
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Old 3rd January 2006, 02:06 PM   #7
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Default Re: speaker porting

Quote:
Originally posted by robotnation
how do i know if the ports winISD tells me to add wil work>? it tells me to put about four 8 inch long 3 inch diameter ports and that sounds ridiculous to me. Ive never heard of ports liek that and i am wondering if that means tehy are problematic at taht size? i suppose it will get the spl that the little chart shows me, but how do i know it won't sound weird?

PS it also tells me to make the ports about .5 inches long when i make them any smaller in diameter, and that is even scarier sounding
It sounds to me as though you either have a very large box, or you are wanting to tune to a fairly high frequency (above 40Hz maybe). If it's the latter it would be quite bad for a subwoofer as the cone will become unloaded below the tuning frequency and slam its insides to bits.
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Old 3rd January 2006, 08:03 PM   #8
Collo is offline Collo  Australia
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Default Re: Re: Usable velocity for 3 inch ports

Quote:
Originally posted by chops


So according to WinISD Pro, my new slot ported sub design yielding a maximum port velocity of 10.5 s/m @ 12 Hz at 104dB shouldn't be a problem then, right? (7.2 m/s @ 20Hz at 111dB)
Hi chops,

once you get down around 10 m/sec life gets easier. At 7 m/sec you can get away with all manner of sins - unflared ends, 90 degree bends. Just what you need for bent slot ports. I would still run a rollover bit around the opening on each end, but I would say you should have no problems.

BTW, have you actually experienced any content at 12hz, or are you planning on using one of those sub-harmonic generators....

ps...thanks for posting the pic of the SVS internals. Haven't seen that shot before!

regards
Collo
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Old 4th January 2006, 12:58 AM   #9
chops is offline chops  United States
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Default Re: Re: Re: Usable velocity for 3 inch ports

Quote:
Originally posted by Collo


Hi chops,

once you get down around 10 m/sec life gets easier. At 7 m/sec you can get away with all manner of sins - unflared ends, 90 degree bends. Just what you need for bent slot ports. I would still run a rollover bit around the opening on each end, but I would say you should have no problems.

BTW, have you actually experienced any content at 12hz, or are you planning on using one of those sub-harmonic generators....

ps...thanks for posting the pic of the SVS internals. Haven't seen that shot before!

regards
Collo

Hey Collo!

Thanks for the reassurance on my new sub design. It will definately be cheaper and easier than going with PRs.

And yes, I have experienced 12Hz before, actually it was 8Hz. My father used to work for a local pipe organ company back in the 1980's (which has now gone out of business). Anyway, one summer when I was out of school, he took me on a job down in Miami where we worked on a 100+ rank organ in a huge church.

That organ had a 64' Resultant Bass which was the 32' Double Open Bass which played two note 1/5 of an octave apart "resulting" in a tone one octave lower than the standard single note 32' Double Open Bass. I will NEVER forget hearing and feeling that organ live. It was truely a moving experience to say the least!

If you ever make it over to Liverpool on holiday or something, go check out the Liverpool Cathedral. The organ there also has a 64' Resultant Bass.

And no, I'm not going to use any sub-harmonic generators. I'm not even sure if those Bash amps will be able to play that low, but at least I'll have decent cone control at sub-20Hz.

BTW, sorry. I didn't mean to "hijack" this thread or anything.
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Old 9th January 2006, 11:27 PM   #10
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hijack? oh thats fine

PS i built my speakers and they sucked, then i recalculated t/s parameters for the executioner and realized due to a big mistake i got my ports the wrong size. The rumble box was fine after recalculation, but the speaker I was using for kick (the one tuned higher than 40) was totally wrong
the eight 3 inch ports should be 1.5 inches long instead of 4.6 inches. I am just using pvc (i really can't afford anything better, i know its poor stuff) so it cant really be flared (corrct me if im wrong, i do have sandpaper and lots of time.) But yea i think ican fix it.

oh yea btw... Just where should i get my spike to be if i want to have my speaker do well on techno / house bassdrums ? i am thinking around 80 hz is what it should be, but correct me if it needs to be lower. i want this box to punch you in the chest every time theres a bassdrum. I need help so i can calculate port length once and for all. Right now it peaks around 59 and sounds really bad.
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