- TLDR: Subs tuned wrong? Sound kind of boomy and you don't feel the bass quite as much as expected, as there are 4x 18 Inch woofers (4 sub boxes in total, 1 woofer per box). Default tuning frequency is 50Hz, I calculated it for 35Hz and added some ports for that frequency, but that sounds a lot less loud. I'm wondering what the best solution would be for feeling the bass and sounding the best. Subs are hooked up in 2x2 parallel, that makes 4 Ohms. They're hooked up to QSC Powerlight 4.0's (not bridged)
Hi All!
So first of all, I'm quite new to this so I'll most definitely be making some mistakes 😅
Now onto my question,
I have 4 subwoofer cabinets which (I think) are tuned incorrectly. I'm thinking this because they sound very, boomy? And you don't really feel the bass quite as much as I would be expecting, especially since there are 4 18 inch woofers. They're hooked up in 2x2 parallel, which makes 4 ohms, to some QSC Powerlight 4.0's (not bridged)
Specs of the cabinets:
I've attached picture of them for a bit more clarification. Right port has the port calculated for 35Hz and the left port is how they are right now, tuned to 50Hz (just for example)
Woofer: B&C 18pzb46 (https://www.bcspeakers.com/media/W1siZiIsIjIwMTMvMDMvMTEvMTUvNTIvMzAvNTQyL2ZpbGUiXV0)
Volume (internal): 185L (calculated BEFORE the woofer was inserted, don't know if that's correct?)
outer dimensions: 64x52x72cm
ports: 2; slotted; depth 2.8cm ;9x12cm; rounded eges; surface area = 90.6cm2, same as 10.74cm diam. Circle
Now I've been playing around with these specs on some calculator websites to try and tune these subwoofer boxes
Tuning frequency calculation:
First I calculated the tuning frequency, which I thought was necessary, using this website:
https://www.vcalc.com/wiki/subwoofer-tuning-frequency
Values I used:
Air compliance: 155L (Vas in woofer datasheet, is that correct value to use?)
Enclosure volume: 185L
Speaker resonant frequency: 37Hz
These values returned a tuning frequency of ~35Hz
Port length calculation:
I then entered these values in the following website:
http://www.mh-audio.nl/Calculators/WVC.html
I used the top most calculator to calculate the porth lenght.
values I used are:
Dv: 10.24cm (i'm going to 3D print the port, which means that the port will be a little smaller because of the 3D prints wall)
Np: 2
Vb: 185L
fb: 35Hz
k: 0.732
this resulted in a porth lengt of 14.31cm, I 3D printed this, installed it into the subwoofer and sure enough, It sounds quite a lot better on the low end!
But then I noticed that around 50Hz, the sub is quite a lot less loud.
WinISD:
So I went looking for speaker response curve programms, and found one called WinISD. I entered the subs paramaters here and tuned it to 50Hz, 40Hz (in between 50Hz and 35) and 35Hz
The 50 Hz is how it's tuned withouth any additional port, since the front wood is 2.8cm thick.
These are the 3 waveforms in WinISD
Red = 50Hz
Green = 40Hz
Blue = 35Hz
The 50Hz has a way higher peak than the other 2 waveforms, which explains why it's such a lot louder at 50Hz, but under 45 Hz it sounds just terrible.
Best advice?
What would your advice be? I'd like to feel the bass as I said, and also for it not to be boomy. Tune it to 40Hz so It's the best of both worlds? Or tune it to 35Hz and would that be loud enough? Or don't tune them at all so they're at 50Hz and just have a really steep high pass filter at around 35-40Hz, so those bad sounding and power consuming low frequencies are cut off, which also would make it so that the amp has a bit more power to spare?
I'd like to hear from you!
Thanks in advance,
-Daan
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Hi,
Well, this is sort of all over the place. You control the tuning frequency. But it seems you're not sure the tuning frequency is correct? You can test that without any special anything by just sweeping some test tones near the frequency you think you're tuned at; as you get closer to the port tuning frequency the driver will not have high excursion and just vibrate and the air velocity from the port will increase so it will get windy; as you move away from the tuning frequency the driver will have higher excursion and the air velocity from the port will diminish heavily. You can at least see that you're in the ball park of what you tuned to.
As for "feeling the bass" that's entirely based on the room even with a near field setup. Make sure you're not sitting right in a nasty null at the frequency you "feel" when you're testing bass. Do you have a calibrated microphone? If not, get one, this is your first tool to always have when you're wondering what your room (ie, sub in the room) is doing.
As above; if you seal the sub, there's no more port problems to figure out and you can just evaluate the driver in its enclosure.
But again I would get a calibrated microphone at this point. No guessing. See what's happening.
Very best,
Well, this is sort of all over the place. You control the tuning frequency. But it seems you're not sure the tuning frequency is correct? You can test that without any special anything by just sweeping some test tones near the frequency you think you're tuned at; as you get closer to the port tuning frequency the driver will not have high excursion and just vibrate and the air velocity from the port will increase so it will get windy; as you move away from the tuning frequency the driver will have higher excursion and the air velocity from the port will diminish heavily. You can at least see that you're in the ball park of what you tuned to.
As for "feeling the bass" that's entirely based on the room even with a near field setup. Make sure you're not sitting right in a nasty null at the frequency you "feel" when you're testing bass. Do you have a calibrated microphone? If not, get one, this is your first tool to always have when you're wondering what your room (ie, sub in the room) is doing.
As above; if you seal the sub, there's no more port problems to figure out and you can just evaluate the driver in its enclosure.
But again I would get a calibrated microphone at this point. No guessing. See what's happening.
Very best,
Not particularly useful if this is a PA application.
Daan32, do you have any measurement equipment?
I'd also suggest that the ports you're using are on the small side - set the input power to a few hundred watts, and check the air velocity of the port. Ideally it'd stay under 20m/s.
Last thing - the "punch in the chest" region is more like the 70-150Hz range. What are you using above the subwoofers, and how have you handled the crossover?
Chris
So you have 2 boxes in parallel on each channel?
Did you check that all drivers are wired for the same polarity... cone moves outward with + voltage applied to the red terminal?
I agree with the others that the box tuning is probably too high but tuning lower reduces power handling capacity so you have to strike a balance. WinISD will show you where you need to go with it.
Thanks for all of your replies! I'm still learning so it means a lot! 😀
I bought these subs second hand, and the previous owner didn't do any tuning whatsoever, so it's just a hole in the front (as you can see in the attached picture, left hole, port is modeled in right hole just for example) With just this hole, which is 2.8cm deep, the tuning frequency is around 50Hz
For the crossover it's like this:
LOW: L-R 24 @ 20Hz - BUT 24 @ 90Hz
MID: BUT 24 @ 90Hz - BUT 24 @ 1070Hz
HIGH: BUT 24 @ 1070Hz - no filter @ 20KHz
the mid woofers are Thomanns the box 15LB100-8W
So I should go down from 50Hz tuning frequency to something like 40Hz or similar?
I've calculated the tuning frequency to be ~35Hz for this particular woofer-cabinet combo using the website in the original post (https://www.vcalc.com/wiki/subwoofer-tuning-frequency)
I bought these subs second hand, and the previous owner didn't do any tuning whatsoever, so it's just a hole in the front (as you can see in the attached picture, left hole, port is modeled in right hole just for example) With just this hole, which is 2.8cm deep, the tuning frequency is around 50Hz
I unfortunately don't have any measurement equipment. I've also noticed some 'air huffing' around the ports at higher volumes indeed, but would this negatively impact audio quality?
For the crossover it's like this:
LOW: L-R 24 @ 20Hz - BUT 24 @ 90Hz
MID: BUT 24 @ 90Hz - BUT 24 @ 1070Hz
HIGH: BUT 24 @ 1070Hz - no filter @ 20KHz
the mid woofers are Thomanns the box 15LB100-8W
Correct, 2 boxes in parrallel on each channel. The drivers are in phase
So I should go down from 50Hz tuning frequency to something like 40Hz or similar?
Oh that makes a difference. For that purpose protection takes precedence over all out performance. Get back into WinISD and do some more work to see how much excursion is created with the different tunings, and then set limiters in the DSP at the power level that pushed them to xmax. The lowcut filter will also need to be raised up closer to box tuning, WinISD can also show you the effect of this filter on below-tuning excursion, which again must keep the driver below xmax. In my experience if the box is tuned to 40hz for example the low cut would have to be at around 35hz.
I think I'd want a 35-40hz tuning for these, they are pretty good drivers and should perform well and be reliable... but only if you have sufficient protection in place.
A multimeter and a small resistor are all you need to measure Fb, assuming that the same device that you used to post to this forum can run REW or other software that provides a signal generator feature.
From the Loudspeaker Design Cookbook: "For a 10" woofer in a box tuned to 33.5 Hz . . . the formula's suggest a 3-4" port would be adequate. However computer simulation and empirical testing have shown that almost any practical port diameter is nonlinear. . . . which is to say that they are unable to move sufficient quantities of air at the velocity required. . . . The group delay curves get progressively worse as power increases . . . . Vent nonlinearity has always been a problem, but one which most designers were either blissfully unaware of, or not very concerned about. . . . In terms of standard plastic tube diameters . . . . 6" ports are good for 12" and 15" [woofers] "
The commentary above is more geared to home audio, where power levels are lower. An equivalent port area of about 350 cm^2 (about 21 cm diameter) would be more typical for a woofer/application like yours. Of course the port gets longer when area increases, which is why you often see slot ports in cabinets like this.
I'm not clear on whether you're interested in changing the box volume or significantly changing the port geometry.
For your woofer, Unibox suggests 121 liters tuned to 40 Hz. You lose the peak in output with a design like this, but this is also typically indicative of a better sounding alignment (extended, flat response that transitions smoothly to a linear roll-off). Getting closer to this is likely part of the improvement you heard when you got away from the 50 Hz tuning (which has an even larger peak). Your 185 liters tuned to 40 Hz looks pretty close in overall performance to the 121 liters tuned to the same frequency though, so may not be worth messing with just to decrease enclosure volume.
There's no free lunch with any of this. Only you can decide if increased output, improved f3 with a non-optimal alignment, etc. is worth degraded transient ability, increased excursion, etc. If you're mostly happy with the performance with the current box tuned to 35 Hz, just implementing the high pass filter/limiter may be good enough. If you want to get more hands-on and optimize things, there's probably some room for improvement though.
Maybe stuffing the ports isn't the ultimate solution, but it's quick and easy and might point to whether a suboptimal reflex alignment or underperforming ports are partially responsible for the poor impression of the initial bass quality.
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Oh shoot! Thanks for pointing that out, I didn't know about this at all. I always just assumed that if you match the amp power at around the speaker RMS power, everything would be fine.
I put in the paramaters in WinISP and that resulted in, for 800W of amp power:
Highpass: Linkwitz Riley 4th order @ 33Hz
Lowpass: Linkwitz Riley 4th order @ 86Hz
This keeps the woofer excursion under the XMax of 8mm
Thanks for the clarification! 🙂
I'd like to change the port geometry if possible, I think I'll use a 160mm pvc pipe, as they're standard and easy to get, especially for the length needed (28,93cm). This will also keep the air velocity at 15m/s peak, which is under the 20m/s @chris661 specified here
I also attached the new WinISD file for the 40Hz tuning, might anyone be interested in taking a look at it
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