Soundscience Rockus 3D | 2.1 speaker system

Audio|Video|Misc
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TESTS IN THE ANECHOIC CHAMBER

So far, this review has not been that different from other subjective reviews in tech hardware review sites. Impressions, descriptions of perceived sounds, subjective assessments of the listening experience. Is that enough for a review of a $250 speaker system for the PC or TV?

Consider — There are probably more 3-piece stereo satellites plus woofer speaker combinations on the markets than anyone really knows. Such 3-piece speaker sets have been around for home audio for at least three decades, long before they became popular for computers. Now, with PC-oriented systems also in the picture, the numbers have to be astronomical, especially if you also include simple stereo speakers that don't use a discrete center bass driver. Every audio component brand and just about every computer component brand has some kind of speaker on offer.

So how can anyone make a valid, useful assessment of one of these systems in reasonable context? This is always a challenge with audio gear — especially speaker systems — because even basic parameters such as frequency response cannot be tested without sophisticated, expensive equipment and a good quiet room whose acoustics characteristics can be removed from the equation. This is a major challenge for any audio equipment review, and only a tiny handful of long-established high-end audiophile publications have the right test gear to validate manufacturers' specifications. Even when such measurements can be made, the results are hard to put in perspective without running the same tests on half a dozen other similarly priced and/or configured speakers. No one does such tests in reviews of $250 speaker systems.

Fortunately, SPCR is a couple of major steps ahead of almost all other PC hardware review sites in acoustics metrology. We have...

  1. our own home-built, hemi-anechoic chamber, an environment that is extremely quiet and almostly completely without reverberation above ~150 Hz.
  2. a lab-reference calibrated, ultra-low noise microphone with ruler-straight frequency response that cost over $2,000
  3. a sophisticated PC-based sound measurement system whose capabilities have barely been scratched in the 2+ years since initial use

SPCR's audio measurement system can be employed to provide basic loudspeaker measurements such as on/off axis frequency response, maximum SPL, and total harmonic distortion. The audio measurement / spectrum analyzer system consists of...

For testing loudspeakers, a signal generator is needed to drive the speakers. As the speakers have built-in amplifiers, this was provided with software via the integrated sound card of a second PC, a silent PC with no moving parts, inside the anechoic chamber.

NOTES on MEASUREMENTS and TEST CONDITIONS

1. SPL: The sound pressure level at which measurements are done is extremely important. A common procedure is to provide the sensitivity with 1W input, and also test the frequency response at the same power input. For a typical passive speaker (one that does not have a dedicated amplifier built into it), this might be something like 90 dB/W, which means when driven with 1W input at, say, 1 kHz or with white noise, the speaker output measures 90 dB SPL one meter away. In fact, 90 dB@1m is a fairly common level for frequency response measurements.

Two considerations:

  • What is the right SPL as a baseline for PC speakers? The Rockus speakers are meant for use in a small room, or under nearfield conditions with a single listener a meter away. It has a stated maximum SPL of 95 dB, so anything close to 90 dB seems too high a level. 90 dB is much louder than you might think: Typical SPL scales suggest that 90 dB is about what you hear from a diesel truck 10m away, inside a moving subway train, or from a food processor directly in front of you.
  • What is a realistic volume for actual use? A check of SPL levels was done at the desktop playing a variety of music at various volumes. This was repeated in front of the TV (with the seated position 5~6' from the speakers), during movies, TV shows and other program material. The results are summarized below.
Measured SPL @ 1m, Typical Use w/ Rockus 3D | 2.1 speakers
At PC/desktop Pop music, background 70~75 dB
Pop music, "loud" 80~90 dB
Other music, background 65~75 dB
Other music, "loud" 75~90 dB
Various online videos 75~85 dB
With TV Drama 70~85 dB
Action 75~90 dB
Documentary 75~85 dB
*Other Music: Jazz, Classical, Folk, etc.

It was extremely rare for the volume level to exceed 90 dB@1m in any application. Even 85 dB@1m was fairly uncommon, reached only during brief peaks. This could be simply a reflection of my listening habits, but spot checks with other family members, friends and visitors more or less confirmed that this is fairly typical: Few people actually listen to music at their desktop or watch videos on their TV with higher volume than indicated above. There are always exceptions; remember, we're just trying to establish a reasonable baseline here.

So it was decided that for small, PC-driven speakers, 85 dB@1m would be the white noise SPL at which frequency response is measured. There are many good reasons for choosing this reference level:

  • It's on the loud side of typical music listening done with PC speakers.
  • It is probably not too loud for any speaker to achieve without overload.
  • It's not too loud for me to bear for long testing periods.
  • It's at least 40 dB above the noise floor in the chamber at any time, yet not too intrusive outside when the chamber door is closed. This means testing can done pretty much any time.

2. Frequency Response: As mentioned earlier, this is the single most widely cited specification in audio, especially with mechanical devices like loudspeakers, which traditionally have the greatest deviations from flat frequency response. It is best shown in a frequency vs sound pressure level graph. In the simplest terms, frequency response tells the ability of an audio device to reproduce sounds of different frequencies at the correct relative levels (loudness). A perfect device has a frequency response that looks like a ruler straight line; hence the term "flat" (not flat as in B-minor flat.) Alas, there are many complex issues around this much-cited parameter.

  • It is highly dependent on the acoustics of the room, the position of the speaker(s) in the room, and the position of the microphone. If this test is performed in a live room, then sophisticated calculations must be used to remove the effect of room reflections (echoes). Otherwise, it must be performed in an anechoic chamber.
  • Frequency response of a loudspeaker generally does not stay constant with loudness level. Typically, there is a range of SPL in which a speaker is most frequency-linear; go outside this range, especially above it, and the speaker will exhibit frequency non-linearities that lower fidelity.
  • Frequency response also changes with the angle of perception, both vertically and horizontally. How smoothly the frequency response changes as one moves off axis is a key to better sound loudspeakers.

Given the complexities, dozens of frequency response graphs could be plotted and posted... but their usefulness would be questionable for most readers. So... this is the procedure established for frequency response testing:

  • Place the speaker at the front edge of the 28.5" (72cm) tall table in anechoic chamber.
  • Place the microphone 1m directly in front, at the same height.
  • Set the output level to 85 dB@1m SPL using white noise.
  • Capture the frequency response graph at 1m distance, on axis, and at 30 degrees laterally off axis
  • If there are any bass or treble controls (such as bass level), engage them to show the response variations
  • Treat one satellite + center woofer as a single speaker, with bass unit directly under the satellite.

NOTE: Only a single speaker (or single speaker + center woofer) is tested, as there are many complications that arise when trying to measure a stereo pair together at the same time.

3. Harmonic Distortion: This is a relatively easy parameter to measure. A pure sine wave tone is fed into the speaker, and the spectrum analyzer sums up all the aspects of the signal that are not this pure tone, expressed in a percentage of the total signal. Harmonic distortion is not particularly important, however, as it occurs naturally in music and is thus difficult to perceive. Even 10% THD at 50 Hz is probably not audible to most people if it occurs in the context of music — and I am not talking here only about a heavy fuzz bass guitar. Amplifiers and other electronics are capable of miniscule levels of harmonic distortion (say 0.01%), but it is much higher in mechanical devices like speakers, especially when large cone excursions are involved (necessary for high volume at low frequencies). Again, the SPL at which HD is measured has a serious impact on the result, as does the frequency. Simply put, the louder and lower the test tone, the harder a speaker has to work to reproduce it. Longer cone excursions almost always result in greater signal anomalies.

After much experimentation, this is the procedure established to test for harmonic distortion:

  • Place the speaker at the front edge of the 28.5" (72cm) tall table in anechoic chamber.
  • Place the microphone 1m directly in front, at the same height.
  • Set the output level to 85 dB@1m SPL using white noise.
  • Leave the gain unchanged while running test tones at the following frequencies: 10kHz, 5kHz, 2.5kHz, 1kHz, 500Hz, 250Hz, 100Hz — and lower, to the lowest frequencies where distortion does not exceed 20%.
  • Tests were kept as short as possible: It is easy to damage speaker drivers with steady state pure tones, even at low power levels!

OTHER TESTS?

There is no shortage of tests that can be run on speaker systems. These include impulse testing, intermodulation distortion, resonance/decay testing of the enclosures, phase response, etc. With time and effort, all of these tests can probably be brought to bear on SPCR speaker reviews. The real question is, for what benefit? For the class of product the Soundscience Rockus (or AudioEngine A2, from the one other speaker review) represents, there is little benefit. Once the basics of frequency response, dynamic volume capability and distortion are covered, in-use details and good subjective descriptions should be more than adequate for a buying decision. This class of speakers will never be used by anyone to seriously replicate a live music listening experience.

Finally, keep in mind that speaker measurements are not infallible, nor do they tell the whole story. Like careful listening with the right ancillary equipment, acoustic measurements are tools by which the performance of a speaker system can be more fully understood.



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