User`s manual

11.5 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 tenth
plus THD and allows the display of each one separately via the selection drop-down.
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.11.8 shows two
curves, the first (red) is obtained with the Max Hold function, the second (black)
with Min Hold.
Audiomatica Srl FFT 02/07/2001 14.37.00
CH A dBSPL 51.2kHz 32768 Rectangular File: minnoise.fft
100 1k 10k 20k20 Hz
80.0
dBSPL
60.0
40.0
20.0
0.0
-20.0
CLIO
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
150 11 SINUSOIDAL