Group Delay Calculation

A sufficiently large and abrupt phase shift may cause the perception of time smearing at the affected frequencies. Group delay is used to quantify this phenomenon, and is essentially the derivative of phase with respect to frequency. The commonly accepted audibility threshold for group delay is between 1.0 and 1.5 cycles for any given frequency.

The PB12-Plus holds group delay below 1 cycle down to 26 Hz (18 Hz for the 16 Hz tune), and it never exceeds 1.5 cycles. In addition, the GD curves show a smooth and steady rise (normal behavior for a bass reflex subwoofer), with no abrupt changes.

Holding GD below 1 cycle across nearly the entire bass music spectrum is a good achievement for a bass reflex subwoofer, and this is due to the very deep tuning frequencies of the PB12-Plus. Enthusiasts can therefore expect “on-time” sound delivery from this subwoofer.

System Ringing and Stored Energy Release Test

The amount of system ringing and stored energy release exhibited by a subwoofer is a function of its acoustic alignment and roll-off profile. A subwoofer with a narrow F6 bandwidth and a steep roll-off will ring longer and release more stored energy than will a subwoofer with a wider F6 bandwidth and a shallower roll-off profile. Regardless, most subwoofers will ring anywhere from 25 ms - 150 ms at system resonance. Similar to group delay, the audibility of system ringing and stored energy release is frequency dependent. The deeper it occurs, the less likely it will be noticeable on music and movies.

Due to the 2nd order high pass filter (designed to help protect the woofer from over-excursion below the tuning frequency), the PB12-Plus has a fairly narrow F6 bandwidth and a 36 dB/octave roll-off slope. It rings for about 150 ms (using a –25 dB test floor) as shown below on the impulse response figure. And as illustrated in the spectral decay chart, this ringing occurs at/near system resonance, which is either 20 Hz or 16 Hz depending on the selected subwoofer tune.

Because the PB12-Plus system ringing and energy release only occurs at very deep frequencies, listeners can expect a tight sound with no overhang. Readers should bear in mind that the listening room itself has a very large influence on perceived tightness, and that room treatments and bass traps can often remedy excessive overhang.

 

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