Handbook for Sound Engineers

1388 Chapter 36

36.4 Factors Affecting Sound System
Intelligibility

36.4.1 Primary Factors Include

• Sound system bandwidth and frequency response.


• Loudness and signal-to-noise ratio.


• Room reverberation time (RT 60 ).


• Volume and size and shape of the space.


• Distance from the listener to a loudspeaker.


• Directivity of the loudspeaker.


• The number of loudspeakers operating within the
  space.


• The direct to reverberant ratio** (this is directly
  dependent upon the previous 5 factors).
  
  
  • Talker annunciation/rate of delivery.
  
  
  • Listener acuity.
  
  
  
  

** Strictly speaking a more complex characteristic than
the simple D/R ratio should be used. Better correlation
with perceived intelligibility is obtained by using the
ratio of the direct sound and early reflected energy to
late reflected sound energy and reverberation. This
may be termed C50 or C35 depending upon the split
time used to delineate between the useful and deleteri-
ous sound arrivals.

36.4.2 Secondary Factors Include

• System distortion (e.g., harmonic or intermodula-
  tion).


• System equalization.


• Uniformity of coverage.


• Presence of very early reflections (<1–2 ms).


• Sound focusing or presence of late or isolated higher-
  level reflections (>70 ms).
  
  
  • Direction of sound arriving at the listener.
  
  
  • Direction of any interfering noise.
    
    
    • Gender of talker.
    
    
    • Vocabulary and context of speech information.
    
    
    • Talker microphone technique.
    
    
    
    
  
  
  
  

The bulleted parameters marked with a bullet (•) are

building or system related, while those marked with an

asterik (*) relate to human factors outside the direct con-

trol of the system itself.

How each of the above factors affects the potential

intelligibility of a sound system is discussed below

together with ways that a system designer can minimize

the deleterious effects and optimize the desirable char-

acteristics.

36.5 System Frequency Response and Bandwidth

Speech covers the frequency range from approximately

100 Hz–8 kHz, although there are also higher

harmonics affecting the overall sound quality and

timbre extending up to 12 kHz and above. Fig. 36-4

shows an averaged speech spectrum with the relative

frequency contributions in octave bands. Maximum

speech energy occurs over the approximate range

200–600 Hz—i.e., in the 250 Hz and 500 Hz octave

bands, and falls off rapidly at about 6 dB per octave at

higher frequencies as can be seen in Fig. 36-4.

The lower frequencies correspond to the vowel

sounds whereas the weaker upper frequencies corre-

spond to the consonants. The contributions to speech

intelligibility, however, do not follow the same pat-

tern—indeed quite the reverse. Fig. 36-5 shows the rela-

tive octave band percentage contributions to intel-

ligibility. Here we can clearly see that most intelligibil-

ity is concentrated in the 2 kHz and 4 kHz bands, these

contributing approximately 30% and 25%, respectively,

while the 1 kHz octave contributes a further 20%. These

Figure 36-3. Speech waveforms (as Fig. 36-1) in back-
ground noise. The noise masks much of the waveform
detail but the speech remains intelligible.

Input data–V

Time–ms

0.0 500 1000 1500

1.0

0.8

0.6

0.4

0.2

0.0

0.2

0.4

0.6

0.8

1.0

auto

Figure 36-4. Average speech spectrum (octave band

resolution).

30

35

40

45

50

55

60

65

dB

Typical Speech Spectrum

Frequency (Hz)

125 250 500 1k 2k 4k 8k