User`s manual

9.6 AVERAGING
Averaging plays a very important role in FFT analysis. It is vital when analyzing
signals buried with noise. It is also important when taking spatially averaged
measurements. CLIO has flexible averaging capabilities. Averaging basically means
adding and dividing for the number of additions made. To start an averaged FFT
measurement you need to set a number bigger than one in the Target Average drop
down; otherwise you have a continuously refreshing single measurement that will
continue until the Stop button is pressed. It is possible to choose between linear
and exponential averaging. The instrument behaves differently in the two
averaging modes. In linear averaging the measurement is continued until the target
is reached, then it automatically stops. What you get is exactly what we have just
explained. In exponential averaging the measurement never stops. When the target
is reached the averaging continues relying on a mathematical formula which
discards the oldest acquisitions and gives more and more importance to newer
ones. The exponential averaging is the default one.
As an example Fig. 9.4 compares a single 1kHz sinusoid FFT analysis with a 100
averages one.
Audiomatica Srl FFT 10/07/01 17.07.38
CH A dBV 51.2kHz 4096 Rectangular File:
0.00 2000 4000 60 00 80 00 10000 12000 14000 16000 18000 20000Hz
0.0
dBV
-30.0
-60.0
-90.0
-120.0
-150.0
CLIO
Audiomatica Srl FFT 10/07/01 17.07.24
CH A dBV 51.2kHz 4096 Rectangular File:
0.00 2000 4000 6 000 8000 10000 12000 1400 0 1600 0 18000 200 00Hz
0.0
dBV
-30.0
-60.0
-90.0
-120.0
-150.0
CL IO
Figure 9.4
This is a classical example of signal buried with noise: the sinusoid's 9th harmonic
is clearly visible after 100 averages, but invisible for a single acquisition.
Another important feature, when averaging, is the possibility of continuing
averaging after a measurement is stopped, has reached its target or a previous
measurement has been loaded from disk. CLIO has this capability when selecting
«continue» in the drop down menu available beside the Go button (Fig.9.5).
Figure 9.5
Selecting the continue option allows for example, for spatially averaged
measurements. Fig. 9.6 shows two 1/3 octave RTA measurements of a small HT
satellite at listening position: the black one is a single 10 averages measurement
taken on axis; the red one is, instead, built using the continue option, adding a
total of eleven 10 averages measurements taken moving from -25 degrees left to
+25 right of the speaker itself.
9 FFT, RTA AND “LIVE” TRANSFER FUNCTION 107