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In Reply to: Yes. They especially sound better in near-field positions at modest volumes. posted by Richard BassNut Greene on February 23, 2007 at 09:21:40:
In all the passive speakers on the market, Richard, you might agree that time coherence below 100Hz cannot be achieved, but only phase coherence at best. This is because the outputs of the woofers of the main speakers are mechanically drifting back in time as they approach their low-frequency resonance (becoming more and more delayed). The same would be said for any subwoofer, but with even more delay, down at say, 50Hz, because they have a lower tuning frequency.
Assuming a subwoofer has a properly-tuned sealed box without the built-in EQ or the variable subsonic filter or a peak limiter, the best integration for it to the main speakers is usually from using the 'THX slopes', taken (stolen) from pro-sound techniques developed in the 1980's:
The sub is crossed at 24dB/octave (4th-order Linkwitz-Reilly) and the main speakers are crossed at 12dB/octave (second-order L-W).
The crossover point for both is at the frequency of the main speakers' sealed-box resonance.
For ported main speakers, the crossover point for the sub and the main speakers would be near the frequency of the main speaker's upper-bass impedance peak, say 80Hz.
The sub would also be placed at the same tape-measure distance away from your ears as the main-speakers' woofers, or perhaps moved closer to you by 6 to 12 inches. This is because the sub usually has a lower resonant frequency, and that means it will have a little more time-delay at that not-too-much-higher crossover point.
The result is phase-coherent at the crossover point, but not time coherent. By this I mean that the main speakers' woofers and the sub are a total of 360 degrees out of sync at the crossover point (only). Which means they are 'in phase' at the crossover point, but not time-coherent. This is caused by the time delays introduced by the filter slopes in the electronic crossover. 360-degrees total could mean that the main-speakers' bass arrives to your ear a half-wave period sooner than the sub (180 degrees you could say), and that the sub finishes a half-wave period late (another 180 degrees), making that total difference of 360 degrees. It could mean that the two bass sources arrive at your ear together, if you moved the sub several feet closer to you. But then the sub finishes one FULL wavelength late. In either case, you hear a 'mumble' around an 80Hz crossover point (on string bass, plucked), and a 'stumble' at a 50Hz crossover point (on a firm kick drum). You also lose the sound of being in a large concert hall, as we tend to rely on the bass range to judge the size of the 'that room' (since there is no bass outdoors). I hope this helps.
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Follow Ups
- Re: Yes. They especially sound better in near-field positions at modest volumes. - mauimusicman 11:58:17 02/24/07 (0)
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