CLIOwin 7 PCI User's Manual - Audiomatica

11.4 DISTORTION AND SETTINGS
Sinusoidal stimuli allow CLIO to evaluated distortion in its single harmonic form. If not
Set in Impedance Mode, CLIO always evaluates harmonics from second to fifth and
allows the display of each one separately via its own push buttons. While it is simple to
obtain meaningful distortion figures of electrical devices, measuring Loudspeaker
distortion in normal environments (without anechoic chamber) is not easy. We will only
give some advice here, relying 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 of which 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 order of magnitude as environment noise.
Gating Effects
Device settling time, non 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 raised 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 these 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 are 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 to increase the level. One way is to put more voltage at the
Loudspeaker terminals; unfortunately this increases distortion by itself even if it
provided important information regarding the device. The second way is to narrow the
140 Chapter 11 - Sinusoidal