Soundscience Rockus 3D | 2.1 speaker system

Audio|Video|Misc
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ANECHOIC CHAMBER AUDIO ANALYSIS

1. Frequency Response - We begin with a frequency response graph showing one satellite + center woofer combined, on axis with the satellite, and off axis by 30 degrees. This is with the bass level set to 3, for the highest bass output. As mentioned earlier, it is not useful to test the frequency response of both left and right speakers together, due to measurement complexities.

Click for large view
Frequency response graph of the Rockus 3D | 2.1 - click for larger view.

Looking first at the on-axis response: From about 100 Hz to 4 kHz, which spans over five octaves, more than the entire range of human voices, the overall response is pretty good, excepting three dips. The first, centered around 300 Hz, is most likely some kind of interaction / cancellation effect related to the woofer and satellite combination, and/or reflections from the mostly undamped floor of the hemi-anechoic chamber. The second dip, at 500~600Hz, is more difficult to explain. Finally, there is another sharp dip around 1 kHz. None of these dips correlate well to what I heard in the listening sessions, except perhaps the ~300Hz dip, which might correlate to a slight thinness sometimes heard in things like male vocals.

It is the higher frequency peaks that are more problematic: In the octave 4~8 kHz, there is a broad peak of 5~7 dB, followed by a sharp dip around 10 kHz, then another even sharper peak at 11~16 kHz. Those peaks are responsible for the over-brightness heard with some program material, especially music that has high content in those last couple of octaves. The off-axis curve shows why the Rockus generally sounds better off axis: The entire range above ~3 kHz is reduced in level so that it is closer in level to the midband, say 500~1000 Hz. The greatest change off-axis is at 10~15 kHz; with music that has a lot of constant high frequency content, moving one's head could cause audible shifts in the sound balance.

Contrast/compare the above with the frequency response of the AudioEngine A2. Yes, since they were here, and these labs tests were never done for the A2s (as the anechoic chamber was not put into service till several months after that review), they got whisked in for their time in the lab.

Click for large view
Frequency response graph of the Rockus 3D | 2.1 - click for larger view.

The overall curve is smoother, and except for narrow dips that are probably inaudible in actual use, above ~400 Hz, the only significant departure from flat response is a broad dip at 3~6 kHz. This might be what makes the speaker sound smooth, yet a bit lacking in sparkle sometimes. The top end is very well behaved on or off axis, extending out to past 15 kHz before sloping off. Then there is the artificial rise in the low end which starts around 300 Hz and peaks some 5 dB higher at 180~200 Hz. This is responsible for that over-rich mid-high bass. As you can see, output drops off pretty steeply below that... looks like around 12 dB/octave. There is little useful output below 100 Hz, so the surprising bass it does manage to convey is very much a clever illusion.

2. Effect of Bass Level Switch - This is straightforward: On-axis graphs showing the response with the bass level switch in each of the three positions. You are advised to start with level 2 and set the position up or down in accordance with your tastes, room acoustics, etc. Once set, it is usually best to leave it alone.


Rockus 3D | 2.1: Effect of Bass level switch - click for larger view.

3. Nearfield Frequency Response Measurements - This technique places the microphone within an inch or so of each driver. The other drivers are turned off if possible or muted by putting them farther away and muffling them physically. It shows us the sound emanating from each driver.

A somewhat different picture of the satellite's frequency response emerges, with a broad dip between 2 kHz and 4 kHz. This may also be responsible for some of that thinness in the sound on some music. That top octave peak remains unchanged.

The crossover in the Rockus cuts the signal to the satellite below 250Hz at a very steep slope, most likely 24 dB/octave. The high filter for the bass driver is about as steep, dropping some 20~24 dB between 110 Hz and 220 Hz. Although the crossover details are not detailed, a spec of 24 dB/oct @180 Hz would be fair. This is a very safe crossover for this type of system, as it ensure the little satellites will never be exposed to high power bass signals. The signal is already 30 dB down at just 180 Hz. It is possible the bass signal is cut off at too low a frequency; this may explain the dip around 300 Hz seen in the other frequency response curves.

The nearfield measurements also explain clearly why the Rockus satellites did not have the problem of vibrating the entire desktop; the higher excursion bass notes simply don't come from the satellites, so they don't excite the resonances in the desktop.

The tweeter and woofer in the A2 speakers are so close together that I could devise no practical way to measure each driver output separately.

4. Harmonic Distortion

As with all the other tests, the output level was set to 85 dB@1m with white noise. Sine wave tones were then run, for harmonic and intermodulation distortion to be measured with our SpectraPLUS audio analyzer. This test was doen with both the Rockus and AudioEngine A2 speakers.

Measured Distortion, Rockus 3D | 2.1 speakers
Test Tone
Rockus 3D | 2.1
AudioEngine A2
SPL (dB)
THD
SPL (dB)
THD
10 kHz
78
0.38%
78
0.15%
5 kHz
84
0.64%
84
0.36%
2.5 kHz
77
0.21%
77
0.28%
1 kHz
78
0.68%
78
0.39%
500 Hz
81
1.20%
81
0.60%
250 Hz
70
4.80%
78
0.76%
100 Hz
85
4.90%
79
5.90%
80 Hz
88
16.40%
72
8.10%
70 Hz
72
13.20%
69
12.40%

The harmonic distortion figures look pretty much as expected: They are low at the higher frequencies and increase steadily as frequency drops. In the Rockus, that the distortion is higher at 80 Hz than 70 Hz suggests the passive radiator has a bigger role at the lower frequency. Distortion at 80 Hz is lower in the A2, but note that the Rockus is a whopping 16 dB higher in level. At higher frequencies, the superiority of the A2's tweeter asserts itself. The satellite driver in the Rockus has to do a lot more, extending all the way up beyond 10 kHz.



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