Definition of loudness

The frequency of vibration of the vocal folds is perceived as the pitch of the voice.

In contrast, it is the amplitude of the vibrations (i.e. the size of the oscillations of the vocal folds) that affects loudness. The greater the amplitude of the vibrations, the greater the amount of energy carried by the wave, and the more intense the sound will be.

Now, intensity (often called the acoustic intensity) is perceived as the loudness of the sound, i.e.

• the greater the intensity, the louder we perceive the sound to be
• the lower the intensity, the quieter we perceive the sound to be

[NB: In lay terminology, loudness is sometimes referred to as volume, but this is not a term we will use here.]

Figure 1. Amplitude of a sound pressure wave.

Controlling intensity

Intensity is controlled mainly by the force with which the air from the lungs is allowed to pass through the larynx. You will know yourself that in order to shout you need to take a large breath and expel it forcefully: it is not really possible to shout loudly with a minimal amount of air.

Now, it is important to understand that the pitch of the voice can remain constant whilst the loudness of that particular pitch can be varied. In other words, it is possible to keep the frequency of vibration the same but to increase the amplitude of the vibration by forcing more air through the larynx. This means that the resultant speech sound wave produces greater motion in the air molecules. These, therefore, strike the ear drum of the listener more forcefully and the sound is perceived to be louder.

Variation in loudness

Vocal loudness will typically vary according to:

• context (e.g. high levels of background noise; speaker addressing an audience; singing)
• speaker’s mood
• the message content

Unit of measurement

The unit of measurement for loudness is the decibel (dB).

Depending on the vocal behavior, intensity levels can vary considerable from a whisper at around 10 dB to loud shouting which may reach up to 90 dB. Table 1 shows some typical values with environmental comparators.

Table 1. Vocal Intensity with Environmental Comparators.

[Source: various, including Hacki (1996); BBC (2004); The Physics Classroom (2011)]

Intensity profile

Figure 2 shows an example of the pitch and intensity profiles of a male and female (both 47 years of age) recorded speaking the utterance Hello, how are you? From the text accompanying the graphs, we see that the male speaker (me) has a lower habitual pitch (Speaking Fundamental Frequency, SF₀) than the female speaker (106 Hz and 202 Hz respectively). As the speakers were instructed to speak at a normal conversational level, it is perhaps not surprising to see that mean intensity is about the same for both (63 dB for the male speaker and 65 dB for the female speaker). This correlates well with the figure of 50-70 dB for a normal conversation in Table 1.

Figure 2. Pitch and Intensity Profiles for Male and Female Speakers.

References

BBC (2004) Loud Music ‘Risks Staff Hearing’ London: BBC News [WWW] http://news.bbc.co.uk/1/hi/uk/4077533.stm Accessed 15 Jun 2011.

Hacki, T. (1996) ‘Comparative speaking, shouting and singing voice range profile measurements: physiological and pathological aspects’ Logopedics Phoniatrics Vocology 21: 123-129.

The Physics Classroom (2011) Intensity and the Decibel Scale [WWW] http://www.physicsclassroom.com/Class/sound/u11l2b.cfm Accessed 15 Jun 2011.