CLIOwin 6.5 PCI User's Manual - Audiomatica Srl

11.4 DISTORTION AND SETTINGS
Sinusoidal stimuli allows CLIO to evaluated distortion in its single harmonic contribution.
If not Set in Impedance Mode, CLIO always evaluate harmonics from second to fifth and
allows display each one separately via its own push buttons. While it is straight to have
meaningful distortion figure of electrical devices, measuring Loudspeaker distortion, in
normal environments (without anechoic chamber) is not trivial. We will give only some
advise here, relaying on examples, as the topic is far beyond the scope of this User
Manual. To do this we will use CLIO’s FFT Menu in quite an advanced way. Distortion
evaluation is adversely affected by several parameters; Two are the most important.
Noise
50 dBSPL of ambience noise, a common figure, usually does not affect Amplitude
evaluation which is usually carried out at an average level of 90dBSPL. This is
particularly true using CLIO Sinusoidal Analysis capability which, by means of DSP
filtering, allows exceptional S/N Ratio. Unfortunately evaluating 1% distortion
means looking for signals that are 40dB lower than the 90dBSPL mentioned above,
in the same magnitude order of environment noise.
Gating Effects
Device settling time, not perfect delay removal and reflections arriving within the
sampling time (Meter On) seriously affect distortion measurements, creating
artifacts.
It is advisable to perform a noise evaluation using FFT Analysis. Fig.9 shows two curves,
the first (red) is obtained with the Max Hold function, the second (black) with Min Hold.
80.0
CLIO
dBSPL
60.0
40.0
20.0
0.0
-20.0
20 100 1k Hz
10k 20k
Figure 11.8
The first, a sort of worst case, should be taken as the reference lower distortion floor
once risen 10dB at least. The second is useful to identify pure tones in the noise
spectrum; these are likely to produce both distortion increase as well as cancellation
artifacts. In our case this tones are caused by more than 20 different computer fans,
spread everywhere in the room. Supposing we carry a distortion analysis at an average
90dBSPL, residues below 1% are difficult to evaluate up to 600Hz, things going much
better at higher frequencies. Remember that the frequency axis should be referred to
the harmonic we are looking for, not to the fundamental. The obvious solution to
overcome noise is increase level. One way is to put more voltage at the Loudspeaker
terminals; unfortunately this increase distortion by its own even if it provide important
information regarding the device. The second way is to narrow the Microphone to
Loudspeaker distance. The next figures, dealing with Gating Effects, refer to a
114 Chapter 11 - Sinusoidal