6.4 SPEECH INTERFERENCE LEVEL

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When exposed to two different sounds at the same time, the ear often

perceives only the louder sound. This phenomenon is called masking.

Masking results when the receptors in the cochlea are not available for

processing the particular sound information because they are being stimu-

Acoustic Criteria 235

Copyright © 2003 Marcel Dekker, Inc.

lated by another signal. A noise signal that is spread out over a range of

frequencies results in more masking near the center frequency than a pure

tone at this frequency (Ehmer, 1959).

If the background noise level is excessive, a person may not be able to

carry on a conversation or understand a telephone conversation. Because

noise can interfere with speech intelligibility, this noise may disrupt work

where communication is necessary.

The consonants contain much of the information conveyed in speech.

The consonants are more easily masked by background noise than are the

vowels, because the sounds of the consonants are generally weaker than

those of the vowels. Nearly all of the information in speech is contained

in the frequency range fromabout 200Hz to 6000 Hz. The understanding of

communication (speech intelligibility) is influenced by the type of communication

(technical information is less readily transmitted than ‘‘small talk’’),

whether the two people ‘‘know’’ each other well or are relative strangers,

and the length of the conversation.

One measure of the effect of background noise on speech intelligibility

is the speech interference level (LSIL). The SIL is defined as the arithmetic

average of the sound pressure levels of the interfering noise in the four

octave bands—500 Hz, 1000 Hz, 2000 Hz, and 4000Hz—rounded to the

nearest 1dB value (ANSI, 1986). These octave bands contain the frequencies

most important for communication. If an octave band analyzer is not available,

the SIL can be estimated from the A-weighted sound level reading by

the following expression:

LSIL _ LA _7dB (6-4)

The SIL values resulting in various levels of vocal effort for face-toface

communication may be estimated from the following expression

(Lazarus, 1987):

LSIL ј K _20log10r рr is in meters) (6-5)

The constant K is given in Table 6-3. The data are based on the assumption

that the information communicated is not familiar to the listener. For communication

between women and men, the data for women should be used.

Generally, the voice level used by the speaker will change as the background

noise level changes. In addition, the speaker may move closer to the

listener as the background noise level increases. The expected voice levels for

various SIL values of the background noise are given in Table 6-4.

The background noise SIL limits for telephone communication are

given in Table 6-5 (Peterson and Gross, 1972). For speakerphones, SIL

values of approximately 5 dB higher than those given in Table 6-5 may be

236 Chapter 6

Copyright © 2003 Marcel Dekker, Inc.

tolerated, if the speakerphone is not located more than 1m (39 in) from the

person.

Example 6-2. In one area of an industrial plant, the octave band sound

pressure level spectrum is given in Table 6-6. Determine the maximum

distance between the speaker and listener (both males) for communication

in a normal voice.

The speech interference level is found by averaging the sound pressure

levels in the four octave bands, 500 Hz, 1000 Hz, 2000 Hz, and 4000 Hz:

LSIL ј 1

4 р73ю69ю65ю59Ю ј 66:5dB!Use 67 dB

The distance between the people for conversation in a normal voice is given

by Eq. (6-5):

LSIL ј 67 ј 54_20 log10 r

r ј 10_р13Ю=р20Ю ј 0:224m ј 224mm р8:8 inЮ

Men would not typically carry on a ‘‘normal’’ conversation at a spacing of

only about 225mm or 83

4 in.

For conversation in a raised voice, the distance between the two people

would be as follows:

r ј 10р60_67Ю=20 ј 0:447m ј 447mm р17:6inЮ

Acoustic Criteria 237

TABLE 6-3 Background Speech Interference Levela Limits for Face-to-Face

Communication

Vocal Effort

K, dB

Women Men Comment

Normal voice 50 54 Communication is satisfactory for the

given vocal effort in this range

Raised voice 56 60

Loud voice 62 66

Very loud voice 67 71

Shouting 72 76 Communication is difficult

Maximum shouting 75 79 Communication without amplification

is impossible above this level

Limit for amplified speech 110 114 Vocal communication is impossible

above this level

aLSIL ј K _ 20 log10 r (meters)

Source: Lazarus (1987).

Copyright © 2003 Marcel Dekker, Inc.

Even this distance is somewhat close for men to carry on a conversation

comfortably in a raised voice. For conversation in a loud voice, the distance

between the two people would be as follows:

r ј 10р66_67Ю=20 ј 0:891m р35:1 inЮ

This would represent a more comfortable distance between the people.

From Table 6-5, we observe that a telephone conversation would be

difficult for the SIL of 67 dB.

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