In Reply to: Thanks Dave posted by 3db on November 21, 2016 at 15:48:55:
Since I've never been able to compare subs with different group delay but otherwise equivalent performance, I can't really say for sure.
A rising group delay at low frequencies can smear bass transients because the lower frequencies are delayed in time relative to the higher frequencies. But like you said, below a certain frequency there are probably diminishing returns. The lowest fundamental musical note that most audiophiles care about is A0 on a piano (27.5Hz), so any bass transients with frequency content below that are either synthesized or recorded room noise. So my intuition says I shouldn't care much about rising group delay below that.
When I was trying to select a sub, the frequency range that I was paying attention to on group delay plots is 30-50 Hz. In that range, you can see some significant differences. Sealed is generally better than ported, and some brands are better than others. The Rythmik F12G measurements (which I can't seem to find now) showed group delay staying below 10ms down to 30 Hz. JL Audio was similar. Only Rel was lower.
From what I've read, Rel's approach in their sealed subs is to keep the pass band of the sub below Fs (system resonance) as much as possible to make the output minimally resonant. That produces an acoustic 12 dB/octave slope through the pass band, which they partially compensate for with a simple low pass filter. The corner frequency of the filter is chosen such that the 12 dB slope below the corner frequency matches up with the expected room gain. This approach yields a free field response that looks like a hump, but an in-room response should be reasonably flat in the pressure region, assuming you're using the sub in a room of the appropriate size.
Most other manufacturers take advantage of the system resonance to increase output and also use a bandpass EQ to boost the response below Fs to extend the response. In addition to lowering the -3dB point, the EQ sharply cuts off the bottom end and causes the rising group delay.
You can think of Rel's approach as optimizing for best time domain performance at the expense of output, which is a bad tradeoff for HT but might be OK for music. The bigger problems with Rel in my opinion are the high levels of distortion in their smaller subs, and the poor value for money.
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Follow Ups
- More thoughts - Dave_K 14:01:12 11/22/16 (2)
- Thanks again n.t - 3db 10:21:52 11/24/16 (0)
- RE: More thoughts - Utley1 21:17:18 11/22/16 (0)