Audio Engineering

Measurement 75

2.25.1 Calculating the Maximum Allowable Total Harmonic Distortion in an
Arena Sound System

The most diffi cult parameter to achieve in the typical arena sound system is a suffi cient
signal-to-noise ratio ( SNR ) to ensure acceptable articulation losses for consonants in speech.
It must be at least 25 dB. In that case, the total harmonic distortion should be at least 10 d B
below the 25-dB SNR to avoid the addition of the two signals. If both signals were at the
same level, a 3-dB increase in level would occur. Therefore (  25 dB)  (  10 dB) means
that the total harmonic distortion ( THD ) should not exceed  3 5 d B.

Percentage 



100 1020

dB

(2.50)

Therefore we could calculate

100 10 1 78

2035

    



.%.

This is why carefully thought-out designs for use in heavy-duty commercial sound work
have a THD of 0.8 to 0.9%:

20

100

100

log



x%

dB change.

Since the 0.8% already represents (100  99.2), we can write

20

08

100

log 42

.

 dB.

Now, suppose an amplifi er has 0.001% distortion. What sort of dynamic range does this
represent?

20

0 001

100

log 100

.



That is a power ratio of

10 10 000 000 000

(^10010)
 ,,,.
We can conclude that if such a fi gure were achievable, it would nevertheless not be useful
in arena systems.