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Old 9th March 2004, 11:10 PM   #1
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Location: Genk, Belgium
Default The 55 Litres challenge !!!

The aim: > Build two high end subwoofers
> F3 around 25-30Hz in a 55 Litres vented cabinet up to 80Hz max
> Prime task music
> Primary goal is quality not SPL
> Preferably No active Eq from transform circuits
> No build-in amp due to vibrating electronics
> Bryston or Meridian power amp is to be used.

The Driver: > Audio Technology 12" Flexunit http://flexunits.com

The cabinet: > Something like this: (early sketch) See picture. Inspired by Avalon

The question: Fs, Qts and the VAS of this driver can be custom configured. Which parameters should I choose to fulfill the 55 litres challenge...?

I played around with the following parameters which are from a new Scanspeak driver,
Vas 43,5 litres QTS of 0,44 and Fs of 20,5 Hz this will yield incredible results in a 55 Litres cabinet....Scanspeak 23W/4557T00 What does trouble me are the corresponding group delay figures....do I need to be worried?

Any help is very much appreciated.
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Old 10th March 2004, 12:12 AM   #2
GM is offline GM  United States
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As you already seem to have ~figured out, the easy answer is:

Vas = Vb

Fs = F3/Fb

Qts = 0.40341

This will be a very 'boomy' speaker in a typical room though due to its gain with decreasing frequency. Fine for HT, where explosions, etc., are more realistic this way, but generally speaking, an F3 of ~30Hz is about as low you normally want to go in a typical HIFI app. so you'll probably need to tune it lower to blend with the room's gain.

GD increases with decreasing Fb, but decays quicker, so the lower the Fb the better, especially since our hearing acuity falls off rapidly below ~100Hz.

GM
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Old 10th March 2004, 12:16 AM   #3
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GM.

Do you mean that your suggestions will produce a boomy speaker?

Also when you say that I need to tune it lower to blend with the room's gain, what practical steps should I take to do this?

Many thanks,

Richard
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Old 10th March 2004, 12:37 AM   #4
GM is offline GM  United States
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This is a T/S max flat alignment so not if it's in a large room well away from any walls/corners, but in a typical room near a wall/corner.......

Making the vent longer tunes the cab lower, lowering F3. I tend to tune vented subs to 16-17Hz so I can play pipe organ CDs. Since my subs are in the corners, their F3 is ~40Hz to get a ~flat in-room response to 16Hz, so don't get too hung up on having low F3.

The best thing to do is measure your room's gain and make a sub system that has a ~ inverse response. I think you'll be surprised at what you really need performance wise to get a ~flat in-room response.

GM
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Old 10th March 2004, 12:42 AM   #5
sreten is offline sreten  United Kingdom
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edit : I'll leave this for now , sreten.
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Old 10th March 2004, 10:29 PM   #6
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Default Re: The 55 Litres challenge !!!

Quote:
Originally posted by richardkrol
The aim: > Build two high end subwoofers
> F3 around 25-30Hz in a 55 Litres vented cabinet up to 80Hz max
> Prime task music
> Primary goal is quality not SPL
> Preferably No active Eq from transform circuits
> No build-in amp due to vibrating electronics
> Bryston or Meridian power amp is to be used.

If it's primarily for music, why vented? And why no active EQ? I'm going to build two subwoofers soon, and I had concluded that active EQ is very desirable, and that a vent is right out. I've got vented subwoofers now, and I'm not happy with them.
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Old 10th March 2004, 10:35 PM   #7
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Well,

I'll probably cannot rule out sealed + EQ.

It's proving extremely hard to get a decent curve from a 55 L vented sub, and EQ is probably the way forward. Other effects like group delay also bother me....I do like the skaaning drivers, I received an email from them with some possible configurations for the 12" Flexunit. See below: (Qts can be any value.)

Mms-fs-Vas (1)
60g 35 128
70g 30 150
50g 35 153
60g 30 174
50g 30 210
70g 25 215
60g 25 251
50g 25 300
70g 20 336
60g 20 392
50g 20 470
70g 15 598
60g 15 697
50g 15 836
70g 10 1345

The problem is though what curve will prove to be a good starting point for applying EQ? Perhaps you can have a play with these numbers.
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Old 11th March 2004, 12:21 AM   #8
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Hi GM..your notion with boom requires modification... speaker unit Qts of 0.4 is perfectly acceptable with smallish boxes........the results won't be as bad as so many make out it to be.........As David Weems book correctly makes out if a small box is used with units above 0.4 Qts then one has to experiment with more damping material. Just to example
I use 2x Peerless 10" 850140 units in an 110 Lit (3.88Cuft) Octet shaped cabinet with large port tuned in base to 33Hz. The driver Qts was found 10% lower than stated 0,36...an advantage as the result is no boom whatsoever with predicted system Q of 1
As I mentioned in another poster, the bass is even and very acceptable on classics. It isn't prominent for easy listening.

I have the option of blocking up the port, them a Qts of 0.66 with fb at 45Hz. The Qts is a bit low.
So many poorly made speaker cabinets and designs are themselves to blame for bad sound and internal harmonics....grotty construction and poor bracing using the 3:2:1 size concept. The oblique sides in an octet offer a much longer reflection path for harmonics, sides are self reinforcing and the result is a much smoother sound. I can recommend any cabinet design away from the dammned 3:2:1 box. It's basically an awful concept, sonically and I can recommend the labour taken in such designs.

photo shows pre lining state with store egg crate to support absorbent mat. Note minimal bracing except the very important front to back.
rich

rich
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