NTi Audio Acoustilyzer AL1 User Manual

Page 46

Application: FFT and White Noise

White noise is most often used as a test signal when measuring the

performance of acoustical systems. The FFT shows a flat spectrum with

this input signal. Feeding white noise into an electro-acoustic system and

measuring the FFT using a measurement microphone shows the frequency

response of the system with very high resolution.

Please note that the frequency response seen is most often a combination

of two effects:

a) The “real” frequency response of the electronics and speakers
b) Interference effects (resulting in comb filtering)

With a high resolution FFT, interference effects can easily be seen - while

they are nearly impossible to notice on a 1/3 octave resolution RTA.

With the Zoom FFT of the AL1 you can “Zoom In” at any frequency point to

a resolution of up to 0.73 Hz.

4 x zoom →

level reduces

by 6 dB

White Noise and Zooming

The energy of “White Noise” is equally

distributed over the linear frequency axis.

Let’s assume we have a White Noise signal

with a level of 0 dB measured using a

bandwidth of 20 kHz.

What happens if we decrease the

measurement bandwidth to 10 kHz?

→ We only measure one half of the energy –

resulting in a level of -3 dB (10*log10(0.5)).

This is exactly what happens if we

“Zoom In” using an FFT. Every time we

“Zoom In” the measurement bandwidth is

halved – resulting in a 3 dB lower level in

every bin.