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cv
Derby, United Kingdom
Posts 173
Joined on 09-15-2004
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1
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Post ID:
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833
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Reply to:
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833
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A slightly crazy idea for a new approach to LF
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I think we are all agreed that if time alignment and space were not issues, a bass horn would be the way to go.
Now, I'm fairly convinced that the overwhelming benefit of horns in this range is the phase response, ie flat group delay vs frequency. Most speakers, even if they have flat frequency response, will impose a 90 degree phase shift (there's a wonderful post by Tom Danley explaining this in the Joenet archive - will try and find later).
This means that at 20Hz, a bass unit will have a delay of about 13ms. At 40Hz, this delay will be half that.
This is in addition to any due to changes in frequency response/ bass alignment(sealed, reflex etc).
Upshot: an impulse spreads out in time - you can see this quite easily in published plots in eg stereophile (sorry to desecrate this site with the very mention of the name).
Now here's the idea: split the LF up into many small bands, eg:
20-25, 25-30, 30-40, 40-60, 60-120
These frequencies are chosen such that the variation in group delay in each band is +/- 1ms from the midband level.
Covering each with (probably) a sealed unit, driving each from its own amp (6 * super Melquiades anyone?), and position them at varying distances from the listening position such that the mid of each band has the same group delay. Some rough calcs indicate the above bands might achieve +/- 1ms throughout the band.
As a plus, you have "graphic equaliser" type EQ that doesn't screw the phase simply by adjusting the gain of each amp. Also there is some leeway to position each unit to minimise room modes.
Would need >18 ft or so room to do this and would need to ensure use of good, low distortion drivers...
Well, let's face it, it's probably as impractical as a bass horn, but thought I'd share the idea.
cheers
cv
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