Objective Testing and Measurement
Set-Up: All objective tests were conducted
outside away from any reflective structures, with the subwoofer sitting on
the ground plane. The microphone was also placed on the ground plane,
facing the woofer at a distance of 2 meters from the width and depth center
point of the cabinet.
Frequency Response: A subwoofer with a flat
frequency response will sound even and linear across the entire pass band.
The deeper a subwoofer can extend, the better it will be able to
reproduce pipe organ music, industrial music with synthesized bass, or the LFE spectaculars found on many DVDs.
The frequency response was measured with a 30 second
reverse sine sweep from 200 to 10 Hz. The output of the sweep was adjusted
to 90-95 dB over the majority of the pass band. Data resolution is 1/24
octave with no smoothing applied.
The 850 SC frequency response was exceptionally flat
across the majority of the pass band in all three room compensation settings
– very impressive.
The Narrow setting measured 50 Hz - 130 Hz
± 1.5 dB, a
fairly broad F6 bandwidth, and a 24 dB/octave roll-off.
The Normal setting measured 33 Hz - 140 Hz
± 1.5 dB, a
fairly narrow F6 bandwidth, and a 36 dB/octave roll-off.
The Wide setting measured 34 Hz - 160 Hz
± 1.5 dB and
26 Hz - 190 Hz ± 3 dB with a very wide F6 bandwidth, and a 36 dB/octave
roll-off. This setting provided the deepest extension and the best match
for the acoustic transfer function of the evaluation room, and was therefore
used for all subsequent objective testing.
Shown below is a graph indicating all three room
compensation settings, and also a separate graph of each setting with more
detailed response information.
Output Compression: As playback level is
increased, any subwoofer will eventually reach its linear output limits.
These limits are usually first reached at the deepest frequencies. This
phenomenon is known as output compression, and can be caused by exceeding
the thermal, mechanical, or port flow limits of the subwoofer. Output
compression can also be caused by amplifier limiter circuits engaging to
prevent subwoofer overload.
The goal of this test is to determine how loud the
subwoofer can play before its frequency response becomes non-linear. It is
also useful to assess the extent and severity of the compression at
progressively higher sweep levels. A subwoofer which remains
compression-free at loud sweep levels will sound powerful and dynamic on
demanding music and DVD passages. Conversely, a subwoofer which exhibits
significant compression will have poor dynamics at high playback levels,
with the deepest passages lacking power and impact relative to the mid-bass
passages.
Output compression was evaluated with a 30 second
reverse sine sweep from 100 Hz - 10 Hz. Sweeps were conducted at
progressively louder (3 dB) increments. Sweep 1 (light blue) shows the
maximum uncompressed sweep level. Sweeps 2-5 (orange, dark blue, bright
green, and purple, respectively) illustrate progressively more output
compression, particularly in the 20 Hz - 30 Hz region. Sweep 5 (purple) showed
at least 2 dB of compression across the entire pass band, in addition to
very heavy compression in the 20 Hz - 30 Hz region, so the test was terminated at
this sweep level.
Harmonic Distortion: Harmonic distortion occurs
when multiples of the fundamental signal are produced by the woofer due to
non-linear (electromotive or mechanical) driver behavior. As the subwoofer
is pushed louder, it will start to create progressively higher levels of
harmonic distortion, and this will eventually manifest itself as audible
distortion during playback.
Total harmonic distortion (THD) was plotted for the
five output compression sweeps, so readers can see the interrelationship
between output compression and harmonic distortion. Note in Sweep #1
(uncompressed), THD remains low (<10% at all test frequencies). THD then
progressively and rapidly increases in Sweeps 2-5 as output compression
becomes more severe, particularly in the 20 Hz - 30 Hz bandwidth.
The audibility of harmonic distortion is a function of
the sound pressure and order of the distortion harmonics relative to the
fundamental, and the presence of any other masking sounds in the source
material. Also, odd-ordered distortion harmonics are musically dissonant, and
are usually perceived as more objectionable for this reason. While there is
debate over distortion audibility thresholds, my own experience indicates
that total harmonic distortion (THD) exceeding 10% will cause audible
doubling, a loss of clarity, and noticeable pitch shifting during normal
music and movie playback.
To quantify this observation, the 10% THD sound
pressure limits were determined at 20, 22, 25, 32, 40, 50, 63, and 80 Hz
using a short duration sine wave. A –12 dB/octave line was then applied to
each screen shot. This line starts at the test frequency, and then slopes
downward to the frequency corresponding to the 10th order
harmonic distortion component, where it then levels to horizontal.* If all
of the individual harmonic distortion components remain below this line, the
subwoofer will sound audibly clean at this frequency and sound pressure.
Conversely, if any of the individual harmonic distortion components crosses
this line, this suggests that a lower sound pressure would be needed at this
frequency in order for the subwoofer to remain audibly clean sounding.
*It should be noted that
the individual harmonic distortion limits bounded by the –12 dB/octave
down-sloping line closely parallel the stepped limits found in the CEA-2010
standard for distortion audibility in subwoofers.
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III.