Help/Evaluation of first sub build

jschweg

Member
2010-10-26 9:58 pm
This is going to be my first build ever, and need a little advice on the enclosure and port tuning since I don't really want to do a sealed box. I'm learning as I go along here so bear with my uneducated questions.

I've decided on the Peerless XLS series for the driver, here are the specs:
http://www.parts-express.com/pdf/264-1116s.pdf

Powered by an apex senior plate amp here:
The Apex Junior Subwoofer Amp

Driver puts out 150RMS/300Max, so I figure the 250 Watt @ 8ohms from the apex senior should be good.

The driver specs say Vas is 139.2 Liters, so if I convert that to cubic feet, that's about 5. That being said, I need to build an enclosure that is equal to 5 cubic feet right?

Now concerning the port tuning, this is where I'm starting to get a little lost. This is going to basically be an average HT sub, from what I've been reading online tuning it to like 30 or 35hz would probably be the best? That being said, I need some help figuring out the port diameter and length.

Do I at least look like I'm on the right track here. Thanks in advance.
 

jschweg

Member
2010-10-26 9:58 pm
I found the application notes from the Tymphany on building a compact ported box for this sub, but 40 liter is pretty small and can easily go larger than this especially since the port length would be a big pain in a box this small.

Total volume: 40 litre
Port diameter: Ø90 mm (double flared recommended)
Port length: 510 mm
Tuning frequency: 30 Hz
 
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That being said, I need to build an enclosure that is equal to 5 cubic feet right?

Depends on who you ask and/or what tuning (Fb) you want. Lots of options with vented alignments with just as many opinions.

For HT, some movies have enough LFE output to need huge sub systems driven with kilowatts to get all the movie's sonic experience, so for a less serious HT, sealed or 4th order band-pass (BP) is theoretically the best overall.

The majority of the LFE's high power output is above 20 Hz though, so a ~14-20 Hz tuning depending on the allowable $$$/space/WAF budget is what some folks opt for. These typically require high pass filters to protect the drivers below tuning.

Drivers with a low mass:compliance ratio (low Fs, Vas) require long vents to make up for not needing much back box air compliance, so using passive radiators (preferably two to ~cancel out their strong vibrations) as a vent substitute is the norm, though morphing the box/long vent into one big, folded up reverse tapered vent (aka TQWT) has been gaining acceptance in recent years as well as its tapped variants.

This particular driver has a high inductance (Le), so has a rolled off response below ~100 Hz, so while this lower tuning is preferred overall, it pretty much needs to be corner loaded.

The higher tuning OTOH yields a worthwhile increase from ~30-60 Hz where a lot of high output LFE is due to many commercial cinemas being somewhat BW limited, so probably the best compromise for you at least for now. 40 L/18 Hz (yellow trace), 40 L/28.8 Hz tuning based on the 90 mm/510 mm vent (green trace).

This vent has such a high vent mach (friction restriction) that flaring isn't going to lower it enough to ensure it won't 'chuff' if it's 'told' to do a sustained high power LFE, so ideally either a PR should be used or step up to some form of TL loading if a ~2-4x larger cab is acceptable which also nets you either a higher dynamic headroom (peak SPL) and/or allow a lower tuning.

GM
 

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jschweg

Member
2010-10-26 9:58 pm
Thanks for your post, this is great information. I've been reading it and playing with WinISD. I've been looking at some other Peerless drivers and found this one which is better quality and 4 ohms, which will get me more power from my plate amp.

Peerless 835017 XXLS 12" Aluminum Cone 4 ohm

If I do this:

3.3 cubic foot box (18W/29.25H/10.97D)
(1) 3 inch dia, 8 inch long vent (tuned to 23hz)

The frequency response looks real flat.
 
You're welcome!

WinISD or the Pro version? The latter is more accurate. Anyway, again, the vent is way too small/short, so it may audibly chuff if much power hits it around/at tuning, not to mention it won't hit the peak SPL that the sim predicts.

I'm guessing that you're not seeing this because you're not inputting the amp's power in the SIGNAL tab which has a 1 W default setting.

Don't know about the plain version, but in the Pro version you can change the plot values, so to easily see if the vent's large enough, right click on the REAR PORT-AIR VELOCITY plot and in OPTIONS, GRAPH, change the END to 17.00 m/sec. Now, if the plot goes off screen you know it's too small.



GM
 

jschweg

Member
2010-10-26 9:58 pm
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?
 
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jschweg

Member
2010-10-26 9:58 pm
Correct on the question of dimensions. Additional space will need to be allocated for the materials/bracing/driver.

Could I take like 8 inches off the height, and just add that into the depth?

Not really sure how critical the WinISD "optimal dimensions" are if the cubic foot is the same.
 
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/OurCompany/Technologyleadership/Documents/White Papers/multsubs.pdf

GM
 
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.
 
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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