Secrets Benchmark Product Review
 

Canton Vento AS 850 SC Subwoofer

Part II

September, 2006

Ed Mullen

 

Objective Testing and Measurement

Set-UpAll 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 DistortionHarmonic 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. 

Click Here to Go to Part III.

© Copyright 2006 Secrets of Home Theater & High Fidelity

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