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Old 29th January 2013, 01:04 AM   #1
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Default Small vented subs tuned low

We all know that for any given tuning frequency, the smaller the box gets, the longer the port needs to be.

What happens when the port volume approaches a significant fraction of the box volume, whatever that is? Could you keep increasing the port volume without negative effect? It would obviously start looking like a transmission line...
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Old 29th January 2013, 01:31 AM   #2
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usually that means it's time for a passive radiator............
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Old 29th January 2013, 06:06 AM   #3
GM is offline GM  United States
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Quote:
Originally Posted by 454Casull View Post
What happens when the port volume approaches a significant fraction of the box volume, whatever that is? Could you keep increasing the port volume without negative effect? It would obviously start looking like a transmission line...
Yes, though of course the vent will have increasingly strong resonances 'coloring' the driver's mids output if not damped, so at some point it's best overall to morph the cab, vent into an inverse tapered TL.

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Old 29th January 2013, 06:55 AM   #4
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Originally Posted by 454Casull View Post
What happens when the port volume approaches a significant fraction of the box volume, whatever that is? Could you keep increasing the port volume without negative effect? It would obviously start looking like a transmission line...
Hi 454Casull,

Simulating in Hornresp using the Loudspeaker Wizard will give you the answers.

The following related post may also be of interest:

Hornresp

Kind regards,

David
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Old 29th January 2013, 11:02 AM   #5
Juhazi is offline Juhazi  Finland
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And here is a guide to using Hornresp! (Sorry David it was very hard for me to get started with HR)

Hornresp for Dum... hmm... Everyone - Home Theater Forum and Systems - HomeTheaterShack.com
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Old 29th January 2013, 02:43 PM   #6
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Quote:
Originally Posted by 454Casull View Post
We all know that for any given tuning frequency, the smaller the box gets, the longer the port needs to be.

What happens when the port volume approaches a significant fraction of the box volume, whatever that is? Could you keep increasing the port volume without negative effect? It would obviously start looking like a transmission line...
I recently designed and built a box that has port volume at about 20% of net box volume. Seems to work fine, however it has a fairly high tune (35hz) and not all woofers will work well configured like this. Roughly 2.5 cubes net and effectively a 15"x2.5" by 27.5" long port.

If you were to attempt to tune low, say 20hz, in a 2-3 ft3 box, you need to be very careful that your port cross sectional area can keep up with your woofer's max output. Air speed needs to preferably be around 25 m/s or below at max output at tuning. With more powerful woofers the area needed increases, making the port length too long for a small box.

I've found that the best way to cram the most port you can into a small box is with a slot port on the longest dimension of the box with 90 degree turn on the end up the back wall, this gives you a bit more effective port length without extra complexity. Use a roundover and 45 deg slat to improve air flow on the inside, and roundover the outside end. Assume about 2" effective extra port beyond the actual measured length. See here for good discussion on end correction:

Ultra compact ported 15" Dayton Ref HO

Inside end of port:

Click the image to open in full size.

Looking in from the outside:

Click the image to open in full size.

Last edited by turbodawg; 29th January 2013 at 02:51 PM.
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Old 29th January 2013, 06:55 PM   #7
OscarS is offline OscarS  United States
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turbodawg, did you actually measure the true Fb? Reason I'm asking is because I've been doing subs like this for a while (not recently though), but none the less, when I use a slot port like yours where the slot ends up using 3 enclosure walls for its walls, I end up with an effective port length of ~66% longer than what it actually is (roughly 40% reduction in required port length, equivalently stated). This I verified with a WT3/DATS.
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Old 29th January 2013, 07:26 PM   #8
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Originally Posted by OscarS View Post
turbodawg, did you actually measure the true Fb? Reason I'm asking is because I've been doing subs like this for a while (not recently though), but none the less, when I use a slot port like yours where the slot ends up using 3 enclosure walls for its walls, I end up with an effective port length of ~66% longer than what it actually is (roughly 40% reduction in required port length, equivalently stated). This I verified with a WT3/DATS.
Hi, I ran some test tones at fairly high output and the frequency with the least cone movement was about 35hz or a hair above that. My original model has a 24" port length and 37.5hz tune. The box dimension on the port side is 22", so the actual port length with the bend is around 23-24". Also, my port is not exactly 2.5" tall, it's around 2-15/32", so that drives down the tune a hair.

Modeling with a 35hz tune and a not particularly accurate net volume around 2.5-2.75 cubes I get around a 2.5x15x27.5" port. So figure an additional 2" of end correction, vs. my actual, is about right.

I was expecting to end up with something tuned lower than the model, so I am pleased.
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Old 29th January 2013, 08:24 PM   #9
OscarS is offline OscarS  United States
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Originally Posted by turbodawg View Post
Hi, I ran some test tones at fairly high output and the frequency with the least cone movement was about 35hz or a hair above that. My original model has a 24" port length and 37.5hz tune. The box dimension on the port side is 22", so the actual port length with the bend is around 23-24". Also, my port is not exactly 2.5" tall, it's around 2-15/32", so that drives down the tune a hair.

Modeling with a 35hz tune and a not particularly accurate net volume around 2.5-2.75 cubes I get around a 2.5x15x27.5" port. So figure an additional 2" of end correction, vs. my actual, is about right.

I was expecting to end up with something tuned lower than the model, so I am pleased.
That's interesting indeed, because with a worst-case 2.5 ft³ and a 2.5"*15"*24" physical port gives me a standard tuning of 35hz. 27.5" gives me 33 Hz tuning. I am using Akabak to model. I would actually expect to see your box with no higher than 30Hz Fb. Would be interesting to see an actual impedance sweep of your box.
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Old 29th January 2013, 09:11 PM   #10
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One solution for when ports become too large for the enclosure:

Long port options for subwoofers
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