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Old 3rd November 2010, 03:39 PM   #11
GM is offline GM  United States
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Quote:
Originally Posted by jschweg View Post
WinISD originally gave me an optimal port diameter of 4 inches, only reason why I changed it was because at 4 inches, the port needs to be 15.5 inches long.

This brings me to hopefully my last question.

WinISD gave me the following for the optimal box dimensions:

18W/29.25H/10.97D

Now since the depth is only 10.97, I can't fit a 15 inch vent on the same side as the driver. As long as I keep to the same box volume, can I change the dimensions around? Or maybe put the port on the side of the box where I have 18 inches? Or maybe I can do 2 port tubes and plug that into WinISD?
WinISD is wrong, it theoretically will take a much larger/longer vent to insure the vent doesn't audibly distort with the up to the 300 W peak rated power of the amp you listed. Study up on the subject if you don't believe me, but better would be to make a proof-of-concept with different size vents all tuned the same and listen/measure to find the answer same as the no doubt hundreds (at minimum) professional speaker designers and probably even more serious DIYers have done since the reflex's official inception back in 1930.

Adding to what Oliver said, think of the vent opening as a smaller driver cone with a really huge excursion capability since it takes a lot of displacement to excite the air in the room around it enough to cause a low frequency. For example, a 23 Hz WL has a diameter of approximately the speed of sound (SoS) divided by pi, so ~13,560"/pi/23 = ~187.66" diameter, i.e. bigger around than many listening rooms are wide.

With that in mind, two point sources ideally need to be < 1/4 WL apart to sum as one or ~13,560"/4/23 = < ~147.39", so in a 12 ft wide room, the woofer could be in one corner with another woofer or vent in the other and still be perceived as one big point source at 23 Hz.

Vents don't have a 'one note' bandwidth (BW) though due to pipes/ducts having a harmonic structure, so from this we see that as the frequency increases, it shrinks in size, dropping in acoustic efficiency (rolling in SPL) and at some point these two sources are far enough apart to sound as separate sources, but due to the nature of vent harmonic structure, by this point the harmonics will be rolled off enough that you won't hear them per se.

This assumes a low vent mach though, if it 'chuffs' due to being too small/short, it will usually be loud enough to notice as added distortion much higher up in frequency where our hearing acuity is better.

From all this we see that as a general rule, as long as the vent is low distortion by design, even huge subs can have the driver and vent at opposite ends and still sum as one as far as human hearing is concerned and since the room's modes and boundaries typically dominate a sub's response at the listening position, the real concern is where in the room the sub is, ergo how close both the driver and vent is to an adjacent boundary: http://www.harman.com/EN-US/OurCompa...s/multsubs.pdf

GM
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Old 3rd November 2010, 03:50 PM   #12
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Quote:
Originally Posted by jschweg View Post
WinISD originally gave me an optimal port diameter of 4 inches, only reason why I changed it was because at 4 inches, the port needs to be 15.5 inches long.

This brings me to hopefully my last question.

WinISD gave me the following for the optimal box dimensions:

18W/29.25H/10.97D

Now since the depth is only 10.97, I can't fit a 15 inch vent on the same side as the driver. As long as I keep to the same box volume, can I change the dimensions around? Or maybe put the port on the side of the box where I have 18 inches? Or maybe I can do 2 port tubes and plug that into WinISD?

The internal volume as calculated by WinISD does NOT include:
- the internal volume occupied by the driver
- the internal volume occupied by the port(s)
- the internal volume occupied by any bracing
- the internal volume occupied by a plate amp

Those volumes will have to be added to the WinISD volume to get the final volume of the enclosure.

The final dimensions H / W / D of the enclosure is not all that critical for subwoofer designs.

Ports can be placed almost anywhere you choose, just keep them at least one port diameter away from the nearest cabinet wall and also the internal port opening should not face directly at the driver.

The use of a precision port (flared at both ends) is highly recommended for your current design choice.

Last edited by Cokewithlime; 3rd November 2010 at 03:53 PM.
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Old 3rd November 2010, 03:58 PM   #13
jschweg is offline jschweg  United States
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Oh I believe you GM, no arguments there lol. I was just relying on WinISD since I'm not educated enough to know what port size/length I should be using.

Last edited by jschweg; 3rd November 2010 at 04:00 PM.
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Old 3rd November 2010, 04:48 PM   #14
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Well, if you change the vent velocity chart and use the amp's peak power rating it to guide you as I suggested, it will be good enough for HIFI/HT apps and as long as the vent length is < this rule-of-thumb you should be OK WRT vent harmonics affecting the sub's intended BW:

Lmax = 13,560"/(20*Fb)

Where

Lmax is in inches
Fb is the tuning frequency in Hz

GM
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