On the Bench

Set-Up: All objective tests were conducted outdoors, away from any reflective structures, with the microphone on the ground plane, facing the subwoofer, at a distance of 2 meters from the width and depth center point of the cabinet.

This test environment is often referred to as ½ space (2 pi steradians). In comparison, an anechoic chamber (or suspended in free air) would be considered full space (4 pi steradians). Testing a subwoofer in ½ space with a 2 meter microphone distance is equivalent (in both output and frequency response) to testing it in full space with a 1 meter microphone distance.

For the Gold PowerSub, I used the left channel 20 Hz input, the low pass filter controls were set to 250 Hz, and the phase control was set to 0⁰.

Frequency Response: An short-duration (about 0.5 seconds), digitally synthesized logarithmic sine sweep was used to evaluate the frequency response of the subwoofer. The output of the sweep was adjusted to 90 dB - 95 dB over the majority of the pass band, and 1/6 octave smoothing was applied.

The Gold PowerSub frequency response measured 18 Hz - 250 Hz ± 3 dB, characterized by a mild peak in the 120 Hz region. The frequency response was very flat over the most common bass frequencies, measuring 20 Hz - 80 Hz ± 1.5 dB.

A sealed subwoofer will naturally roll-off at 12 dB/octave unless additional high pass filtering is employed. The Gold PowerSub roll-off slope measured 12 dB/octave, indicating no additional high pass filter. According to Paul Scarpelli, this was a deliberate decision, in order to take best advantage of any room gain and maximize in-room deep extension.

Output Compression Testing: This test measures how loud the subwoofer can play before its frequency response becomes non-linear. As the signal strength is increased, all subwoofers will eventually show a reduction in output at the lowest frequencies, and this phenomenon is known as compression. Output compression can be caused by excessive heat in the woofer voice coil, exceeding the mechanical limits of the woofer or passive radiator suspension, or by exceeding port flow limits. Compression can also be caused by amplifier limiter circuits engaging to prevent clipping or woofer overload.

Output compression was evaluated with a 45 second logarithmic reverse sine sweep from 100 Hz - 10 Hz. Sweeps were conducted at progressively louder (2 dB increments) levels, and the effects of compression were noted.

As shown below, the Gold PowerSub frequency response remained linear (uncompressed) up to the orange curve. The next three higher sweeps (blue, green, purple) show progressively more output compression occurring. Beyond the purple sweep level, the Gold PowerSub started to exhibit signs of audible distress, so I terminated the test at that point. Also provided below is a graph showing the actual amount of output compression (relative to the orange curve) for the blue, green, and purple sweep levels.

Total Harmonic Distortion (THD): Harmonic distortion occurs when multiples of the fundamental signal are produced due to non-linear driver behavior. A subwoofer with low THD will sound clean and distinct, especially at the deepest frequencies. THD was evaluated with sine waves (about 5 seconds duration), and was limited to approximately 10% unless otherwise noted.

For a 15” woofer, the Gold PowerSub has modest clean output limits across the pass band, only breaking 100 dB at the highest test frequencies. This is partially due to the relatively small enclosure volume, which acts to limit output efficiency at the lowest frequencies. As stated earlier, maximum clean output was not a primary design goal for the Gold PowerSub. The medium throw woofer selected for this subwoofer exhibited the cone control and transient characteristics Triad wanted, at the expense of some output capability.

Frequency (Hz)	SPL (dB)	THD (%)
20	80.0	9.9
22	83.5	10.4
25	86.1	10.2
32	92.0	10.5
40	95.8	10.3
50	98.7	9.8
63	101.9	10.3
80	105.4	9.9

Bandwidth Linearity: Bandwidth linearity is calculated by dividing the average distortion-limited SPL by the maximum distortion-limited SPL, and expressing the result as a percentage. A score of 100% means the subwoofer exhibits perfect clean output linearity across a given bandwidth.

Bandwidth (Hz)	Average SPL (dB)	Bandwidth Linearity
20-80	92.9	88%
22-80	94.8	90%
25-80	96.7	92%

Phase Response and Group Delay: 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 was calculated at select music note frequencies. The approximate audibility thresholds are based on extrapolations of existing group delay audibility studies.

Due to its acoustic alignment and natural 2nd order roll-off, the Gold PowerSub exhibited an extremely linear phase response. Group delay was very low (virtually non-existent at 50 Hz), staying far below the approximate audibility thresholds at each test frequency.

Musical Note/Frequency	Group Delay (ms)	Approximate Audibility Threshold (ms)
F2 / 87 Hz	5.8	15
G1 / 49 Hz	<0.1	25
C1 / 33 Hz	11.4	35
A0 / 27 Hz	11.1	42

Impulse Response: A phase setting of 0⁰ on the Gold PowerSub resulted in a non-inverted impulse response. The impulse response (black line) shows very little overshoot and system ringing for only 25 ms. This is the shortest ring time I have yet measured, and is an excellent performance. Considering the roll-off profile of the Gold PowerSub, I expected this type of impulse response behavior.

Most subwoofers will ring anywhere from 25 ms - 100 ms. The amount of ringing exhibited by a subwoofer is affected by its roll-off profile. A subwoofer with an abrupt and steep roll-off will generally ring longer than a subwoofer with a more gradual and shallow roll-off. While the differences in subwoofer ring time are certainly of academic interest, readers should keep in mind that a typical untreated listening room will exhibit modal ringing for several hundred ms (sometimes even longer). This in-room modal ringing will generally act to mask most differences in subwoofer ring time.

Spectral Decay: This test shows the low level spectral decay products of the subwoofer in three dimensions along a time axis. This test can help reveal audible cabinet and woofer resonances, or loose/vibrating parts in the subwoofer. Spectral decay was evaluated to the –35 dB level, relative to the test volume.

Minor system resonances were noted above 60 Hz, but none of them exceeded –25 dB in amplitude, and the largest resonance (at 120 Hz) dropped below the –35 dB test floor after about 190 ms. None of these resonances will be audible under normal use, and in fact the overall spectral decay signature is among the best I've seen.

It was interesting to note the almost complete absence of resonances below about 60 Hz, which can be partially explained by the extremely well braced and rigid enclosure. But this exceptional performance also results from the fact that the natural (unassisted) resonance frequency of this subwoofer is about 60 Hz. While the frequency response is extended below 60 Hz with electronic equalization, there are clearly no additional system resonances below this frequency.

In-Room Frequency Response

The Gold PowerSub was placed near the front left corner of my 2,000 ft³ room. For digital bass management, the main speakers were set to “Small” with a crossover frequency of 80 Hz. The digital bass management circuit imposes a 2nd order high pass filter on the speakers, and a 4th order low pass filter on the subwoofer. The low-pass filters were set to 250 Hz to prevent cascading with the low-pass filter in the digital bass management circuit.

The in-room frequency response was measured at the primary listening position (about 11 feet from the subwoofer) with the main speakers and the subwoofer operating. Setting the subwoofer phase to 0⁰ provided the best results, with no obvious cancellation at the crossover frequency.

The in-room frequency response showed typical anomalies caused by room acoustics and the asymmetrical filters in the digital bass management circuit. I used my Rane PE-17 parametric equalizer to optimize the frequency response, taming moderate room mode peaks. Room gain was also noted starting below about 30 Hz. The net result of combining room gain with the Gold PowerSub's anechoic roll-off profile is a remarkable improvement in deep extension. The final in-room FR at the listening position was 11 Hz - 100 Hz ± 4 dB. You read that right . . . 11 Hz!

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