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Old 10th September 2012, 09:23 PM   #11
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Have you read this discussion?

Audibility of group delay?
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Old 10th September 2012, 09:23 PM   #12
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would be hard to decide between ported and sealed..
sealed has better group delay, ported has bass extension.
on the otherhand, rolloff characteristics differ too, in favour of sealed.

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this would be my choice with a sealed sub..
but its not the simples design at all.
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Old 10th September 2012, 09:37 PM   #13
DrNick is offline DrNick  United Kingdom
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Quote:
Originally Posted by turbodawg View Post
I'm sorry, but you have either read something incorrect or have misunderstood what you have read. It is a complete myth that sealed designs (always) sound better. The designs you are currently entertaining will not sound any better than a proper ported design on music, and they will have mediocre extension for HT.

Are you performing time correction on this system? Because that is pretty much the only way to get transient correct bass.
Thanks Turbodawg, I may well not have the full picture yet, and if this thread can help me to correct any misapprehensions I have picked up before I set out to build something, I'll be very grateful to all involved!

I can delay each channel individually in the amp, but this might be more for video sync than for audio purposes.
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Old 10th September 2012, 09:44 PM   #14
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Here's a random WinISD comparision of group delay I pulled off the net:

Click the image to open in full size.

Let me assure you that the 1ms difference at 50Hz will not be audible. 5ms at 30hz will not be audible, nor any of the huge spike below that.

What will be completly audible is if the subwoofer is correctly time aligned to the mains.

I'm sorry to say, there are many poor subs and poorly integrated subs out there, many of which are sealed.

Last edited by turbodawg; 10th September 2012 at 09:51 PM.
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Old 10th September 2012, 10:05 PM   #15
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Quote:
Originally Posted by wintermute View Post

As for the question how loud will 20Hz be at 86db I can't really answer except to say, you won't be able to hear 20Hz, you can feel it (if it has enough power) but not hear it.

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Lies!
OK maybe I made an uniformed gross generalisation I was under the impression that you cannot actually "hear" 20Hz, I personally can detect it (above a certain threshold) through the unpleasent pressure I feel in my ears, but I would not be able to say ha that's a 20Hz sine wave. I can start to actually hear it when the volume is cranked up but that is various resonances (and perhaps some 40Hz second harmonic) when my crappy speakers start to rattle due to the pressure Maybe some people can actually hear a PURE 20Hz sine wave, a quick google search suggests this is so. I stand corrected!!

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Old 10th September 2012, 10:06 PM   #16
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Going for active circuitry is always a requirement with dedicated subwoofers. Adding in a 20Hz low-cut is just a case of another TL072 with some caps and resistors.

Here's some reading for you - scroll down for the stuff related more to what you're after, especially the bit about Linkwitz Transform.

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Old 10th September 2012, 11:13 PM   #17
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Quote:
Originally Posted by wintermute View Post
I was under the impression that you cannot actually "hear" 20Hz, I personally can detect it (above a certain threshold) through the unpleasent pressure I feel in my ears, but I would not be able to say ha that's a 20Hz sine wave.
My hearing is shot above about 12K, but I can certainly hear 15Hz.
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Old 11th September 2012, 12:15 AM   #18
DrNick is offline DrNick  United Kingdom
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Quote:
Originally Posted by turbodawg View Post
The designs you are currently entertaining will not sound any better than a proper ported design on music, and they will have mediocre extension for HT.
I'm looking at the curve I posted for the WinISD model of the Scanspeak 30W. This shows -3 db at 40 Hz, -6 db at 30 Hz and -12 db at 20Hz. Should I worry that this will be insufficient for my application? I'm not looking to shake stuff off the walls here, so it may not get used very loud, I just want some solidity and space down in the low end.

I have read some posts suggesting that at the low frequency limit there could be up to 12 db of room gain per octave. HTGuide Forum - Guestimating room gain Linearteam - For everyone wanting to predict "Cabin Gain". However, if this starts at below the lowest mode, 17Hz, it's not going to make a lot of difference.
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Old 11th September 2012, 12:26 AM   #19
DrNick is offline DrNick  United Kingdom
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Quote:
Originally Posted by chris661 View Post
Going for active circuitry is always a requirement with dedicated subwoofers. Adding in a 20Hz low-cut is just a case of another TL072 with some caps and resistors.

Here's some reading for you - scroll down for the stuff related more to what you're after, especially the bit about Linkwitz Transform.

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Chris
Thanks, I am waiting to hear back from Arcam if they have a high pass filter in the LF output for the amp. The bit of the circuit is here,

Click the image to open in full size.

but I have not figured it out yet. I guess if there is not then I'll have to add one.

That's a great link, thanks for that!
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Old 11th September 2012, 02:43 AM   #20
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Originally Posted by DrNick View Post
I have modelled the Tangband, Scanspeak 12" and the Dayton 15" with Le set to zero, and it does flatten all the curves out to zero on the transfer function graph at higher frequencies. Is it possible to achieve this in practize by using a Zobel network to compensate for the driver coil inductance?
The driver won't necessarily behave exactly as it does in the sim, you may find that the natural tendency to roll off due to the VC inductance actually combines with a natural tendency for the driver to be more efficient at those higher frequencies so it will even out (it might not either!)

the main reason I suggested turning it off was so that it was easy to line up the "zero db" point so you can get a direct comparison between the extension each driver has, however as you are now questioning, are you going to have enough spl at those low frequencies with 6db or more down at 20Hz...

If you do decide to remain with sealed you probably need to choose the driver with the highest power handling and do a linkwitz transform on it. but take all this I'm saying with a grain of salt as I have no experience with high output down this low!

Tony.
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