8" NHT woofers for midbass - diyAudio
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Old 3rd September 2009, 03:29 PM   #1
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Default 8" NHT woofers for midbass

Hi,

I am planning a to build for a separate woofer module, to be used as the woofer in budget a 3 way system. The plan is to cross from the midrange using passive line level XO, at about 500Hz, and then hand over to the sub at about 60-80 Hz.

Being a budget project, the midrange and tweeter will be using the Madisound RB kit, in a separate enclosure. I'm planning to use the NHT surplus woofer NPT-11-084-2 for the woofers. The T/S parameters are as follows.

Re 5.67
Fs 29.3
Qms 1.585
Qes 0.307
Qts 0.257
Vas 81.96
Cms 0.001081
Mms 27.3
diameter 172
BL 9.63
SPL 90.08
Zmax 34.9
Sd 0.02324
Xmax 6.0

Using WINISD, i simulated a closed box with 4 drivers (bought 8, since they are so cheap), and got a 50 litre box, with F3 at 80 Hz. Does that look about right? I'm just surprised that the F3 is so high, since the Fs of the drivers are 30 Hz.

Are these drivers suited for sealed alignment, or is bass reflex more suitable? I chose sealed for less time delay, and since i am using a subwoofer.

Thanks for comments and advice!
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Old 4th September 2009, 01:42 AM   #2
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Come on guys, any advise for an aspiring newbie on a budget?

One more question:

Should i have 4 woofers on the front baffle, like a full height floorstander, or should i have 2 forward firing, and 2 rear firing in a half height enclosure, aka brute force baffle step correction?

I think brute force BSC should have better efficiency. Since i'm crossing at about the BSC corner frequency of 500 Hz, i should gain anywhere between 2-4dB ( max of 6dB in anechoic) with this setup.

What about tactile slam? I have a feeling 4 direct firing 8 inchers should give better slam. Hmm..

Cheers!
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Old 4th September 2009, 03:21 AM   #3
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Warning Non-Expert opinion follows

I'd go for it with 4 woofers firing front and rear, but i'd be using a bigger box calculate the volume for a Qtc of 0.56 and have another look.
You are going to need a BIG subwoofer tho, 18 inch or 2 * 15 at least to keep up
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Old 4th September 2009, 06:48 AM   #4
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Moondog,

Would this 15 incher be able to keep up? I'm debating building this TL or a tapped horn for the greater efficiency. : ))
Driving a Rockford 15in DVC sub with a NAD 2600A

You mean 4 woofers, 2 front and 2 back firing? Is 80 Hz a good crossover between the woofers and the sub?

Thanks! Keep the comments flowing!
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Old 4th September 2009, 11:43 AM   #5
Rojoh is offline Rojoh  Norway
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As far as I can understand your description: You're planning to run 4 woofers from 80-500Hz? I guess such high lowpass (500) will be best suited for front-firing. At 500Hz you're in the midrange where you want clean and uniform sound for vocals.

-Johan
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Old 4th September 2009, 02:01 PM   #6
stoc005 is offline stoc005  United States
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Default Possible error in calc?? And some advice....

[QUOTE=rhapsodee;1914904]Hi,



Re 5.67
Fs 29.3
Qms 1.585
Qes 0.307
Qts 0.257
Vas 81.96
Cms 0.001081
Mms 27.3
diameter 172
BL 9.63
SPL 90.08
Zmax 34.9
Sd 0.02324
Xmax 6.0

Using WINISD, i simulated a closed box with 4 drivers (bought 8, since they are so cheap), and got a 50 litre box, with F3 at 80 Hz. Does that look about right? I'm just surprised that the F3 is so high, since the Fs of the drivers are 30 Hz.

You should have a much larger box for the 4 drivers. 50 L is too small. Just use the driver parameters from one driver and multiple the volume you come up with by 4X. Why? With 4 drivers in a common box, VAS is now 4x the value of the single driver. With a QTS of less than .34 (or so), I think these drivers will work better in a ported box. Should have an f (-3db) in the high 30s to low 40s. You will need 4 ports (or one with the same area) approx. the size calculated for the one driver if you want to avoid vent noise.

Bill
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Old 4th September 2009, 03:25 PM   #7
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Quote:
Originally Posted by Rojoh View Post
As far as I can understand your description: You're planning to run 4 woofers from 80-500Hz? I guess such high lowpass (500) will be best suited for front-firing. At 500Hz you're in the midrange where you want clean and uniform sound for vocals.

-Johan
Johan,

I was thinking about that, but i read about brute force BSC, basically a bipole system. The front 2 woofers start diffracting around the front baffle and losing 3-6dB at about 500 Hz (depending on baffle width) at the same time as the rear 2 woofers diffract around the back baffle.
http://www.t-linespeakers.org/tech/b...intro-bds.html

However, since there is no output above 500Hz due to the low pass, maybe there is no 'step' in the frequency response due the baffle step. I was however also thinking about using the woofer module as a 'stand' for the midrange+tweeter enclosure.

You are probably right about the cleaniness of forward firing for vocals, and 'slam' which is part of the reason why i chose 4 woofers.
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Old 4th September 2009, 03:45 PM   #8
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Bill,

I recalculated using AJ Sealed Designer ( used winisd previously) and got the same result for sealed - 50 litres, 80Hz F3. 4 drivers, multiplied VAS by 4...

The reason i chose sealed was for the better transient response, since i will be crossing from this woofer to a 15" TL sub at about 60-80 Hz.

Anyone know where i went wrong?
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Old 4th September 2009, 04:03 PM   #9
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Strange, i used this quick formula from DIYSUBWOOFER:
Qr = Qtc/Qts
Vr = Qr^2-1
Vb = Vas/Vr
Fb = Qr*Fs
F3 = Fb*((1/Qtc^2-2+((1/Qtc^2-2)^2+4)^0.5)/2)^0.5

And got the following :
Vb = 110l
Fb = 60Hz
F3 = 93 Hz
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Old 4th September 2009, 06:57 PM   #10
GM is offline GM  United States
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FWIW, using WinISD Pro's slightly adjusted specs, my most accurate calculator suggests an empty (net) ~42.658 Vb/0.707 Qtc/86.75 Hz F3, Fb for four which considering the low Qts seems reasonable to me.

Adding enough series resistance to double Qts trades 6 dB of efficiency for a ~43.47 Hz F3, Fb/0.707 Qtc albeit at the further expense of a 278.101 L cab.

Bottom line, a fairly large reverse tapered TL (TQWT) is typically the best trade-off between size, F3, SQ for low Qts drivers if horn loading isn't an option.

GM
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