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mikee55 22nd March 2007 10:04 AM

Box type for driver, how do you choose?
Hello all,

Recently I measured and built a sub with which I wanted to use a Linkwitz Transform . I had to measure the parameters etc. I never fully understood what was happening with Vas and Qts etc and decided its time for another go at making a subwoofer. This time, its just a driver in a box and it runs straight off an amp via the remote speaker (A and B selector), so no crossover, for my son's PC. So this time, no electronics, just a box to design.

I have a Longmill 10" woofer and found the following spec

Re = 3.6 ohms
Zmax =50.3 ohms
Fs = 27Hz
Qms = 4.3690
Qes = 0.3368
Qts = 0.3127
fc= 77.5hz
Vas = 99.94 L
E.B.P. = 80.1662

How can I tell which enclosure to use?

Cheers mikee55:)

richie00boy 22nd March 2007 10:16 AM

Generally, Qts above 0.4 means a sealed box. EBP is also used as an indicator, with 1-50 being sealed and 51-100 being vented.

That driver looks like it will be well suited to a vented box and give great response, but the box will be reasonably large - at a guess off the top of my head about 70 litres.

You CAN use a sealed box, but it's response will be curtailed, so you would have to use an LT on it.

mikee55 22nd March 2007 10:36 AM

Thankyou, sir.
Hi Rich,
Glad you replied. The last design sounds good so I know I can trust what you say, to make this one sound good. Last time you replied, I didn't keep the information. This time I'm keeping everything. Allways on scraps of paper though, ah well, I'll keep it safe.
Hows your PC? And hows the Amplifier doing? I've give my LT sub loads of ear time, and I'm itching to turn it up! I've now got it in bridge mode and it sounds better until the low stuff comes in. Damn squeaks! The power light dims when that happens,(Hee,hee:D ).

I just want to say though, going back to the 10", I thought FS at 27hz was quite low, but QTS-CALC gave its answers without questioning my numbers. Seeing as this is a car driver wouldn't a 70 litre box be massive seeing as it would be a reflex, I imagine a little carpeted box, probably wedge shaped with a Halfords price tag claiming a million watts of power.What do you think, sir?

Cheers Mike :)

mikee55 22nd March 2007 11:19 AM

2 Attachment(s)
Just plugged data into Winlsd Pro alpha, and with an allignment(super boom box). How does that look?

Cheers Mike :)

richie00boy 22nd March 2007 11:32 AM

In a car, it will be suited to a box of the kind you describe, but a car has usable cabin gain starting relatively high in frequency.

My 70 litres was a pure guess out of my head based on Qts and Vas. The design you plotted looks OK. I'll do an alignment myself tonight if I get time.

Fs is quite low for a 10 inch driver, but that's no bad thing. This sub should perform quite well given it's cost I think.

The computer is now all up and running again. I've been trying to enquire with various companies around the world to get some PCBs made for the amp at reasonable cost.

mikee55 22nd March 2007 11:44 AM

Thanks for replying
Hello.Thanks again sir. So I won't be cutting any Timber today, good job too seeing as its wet.:) If you get time to do an alignment tonight, will you be emailing me?

Good luck with the PCB enquiry. Can't wait to purchase a Kit from you. The wife's getting annoyed with the squeeking. Its suprising how much low content is in telly adverts, lost count the amount of ads where the cone flaps and squeaks. I hear they're finaly going to regulate it and get them to lower the volume.

Anyway, cheers Rich


bjorno 22nd March 2007 03:13 PM

2 Attachment(s)
Hi Mike,

You've already got good advises from richie00boy,but here is little more to consider....


Seeing as this is a car driver wouldn't a 70 litre box be massive seeing as it would be a reflex…
No, if you want to port this driver the minimum req. volume would be about 40 L but preferably larger and if you aim at quality LF extension the starting point would be at about 83L empty or about 10-15% less if lined with damping material, thus 70 L is in the ballpark.

If your driver numbers is accurate, you probably already have built a closed box with a volume about 13.8 L that gives the fc= 77.5 and want to apply LT. See pic. Longmill.gif.

You can further minimize your box volume to: See Longmill-max.gif, but this volume must be checked not to be to low for this driver.

Driver max swept volume = effective cone area x 2 x X-max is req. to be 5% of speaker volume or less in order to avoid excess load on the driver cone when played at high RMS input powers.



bjorno 22nd March 2007 03:14 PM

2 Attachment(s)

richie00boy 22nd March 2007 06:57 PM

Have a look at this Mike. The frequency response graph on page 6 is the one you want to look at.

I made some assumptions on Xmax, power handling and cone diameter, so don't get too excited about how much SPL this thing appears to be able to offer.

mikee55 22nd March 2007 10:53 PM

I thankyou

bjorno, Thanks for going to the trouble to reply and doing Transforms, but at the end of the day, all I wanted is a correctly sized box for the driver. I was hoping to achieve a quality bass of sorts rather than just the crap box its put in. The box is an insult to the audio world, not to mention any half decent chippy. It was buit in less than an hour, the only measurement was for it to take the drivers dimension of 10". It made a noise when my son watches Jurrasic Park and Starwars etc. He's 8 years old and just started getting into music and film. With a bit of luck he might get a taste for hi fi rather than just volume.

richie00boy, Thankyou sir. If I built a 40 litre box and put a length of pipe, whatever wide and long that WQinlsd says, can I get a nice sound? Subsonics is not an issue or extreme depth, just a natural bass guitar would go down well me thinks.

Thankyou mikee55 :)

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