LECTURE 8THE ACOUSTIC ASPECT OF SPEECH SOUNDS

Brainstorming questions

1 Do you know physical properties of a sound?

2 How many physical properties of a sound do you know?

Any sound in nature is an acoustic phenomenon. It is a form of moving matter and energy. A sound is generated by a physical body which is set into vibration by some external force.

When an external force is applied to a physical body (such as a string, a drum, any kind of plate, etc.) the physical body begins to oscillate - to move forward and back. These alternate movements of the physical body produce сondensations and rarefactions of air which are known аs sound waves.

A speech sound is also a physical phenomenon. As it has been said before it is a product of the complex work of the speech mechanisms which regulate the air stress, thus producing condensations and rarefactions of air.

A sound has a number of physical properties: frequency, intensity, duration.

т--------- '---------- ¹-------------------- ^—-------- ^h Time in seconds Fig. 3.1. Sound waves

Frequency is the number of vibrations (or cycles) per second


Air
Pressure >	I	ел --- 1-	os	oh 1---	os ---- 1—

A man's ear may perceive the vibrations of the air when they occur at a rate of 16 to 20000 cycles per second. Frequency is measured in cycles per second (usually abbreviated tосps.).

Sound waves may be rhythmical or non-rhythmical. When the vibrations are repsated at regular intervals of timewe get rhythmical waves. They are perceived аs vowels. When the vibrations are repeated at irregular intervals of time we get non-rhythmical waves. They are perceived as consonants.

Frequency of sounds depends upon the mass, length and tension of the vibrator. For example, the vocal cords which are greater in mass produce slow vibrations; they are perceived аs low-pitched. If the vocal cords are longer they produce slow vibrations too, which are also perceived as low-pitched. If the vocal cords are less tense they produce slow vibrations which are perceived аз low-pitched too, and vice versa.

That is why man'svoice is usually lower than that of a woman. A child's voice

is usually the highest.

Our perception of the frequency is the pitch of the sound.

The second physical property of a sound is its intensity. Compare the following two waves:

Air pressure


*	♦------------ 1—	V ^ О 1	сs 1----------- 1	ом 1------------ 1--------- ,

Air pressure

Time in seconds

One and the same sound pronounced with different intensity.

Intensity of a sound depends upon the force which is applied to a physical body. The greater the force, the larger

the amplitude and vice versa.

These sound waves have the same frequency, but the amplitude of vibration is different. The first has twice of the amplitude of the second. It means that the sound of larger amplitude is louder, and the sound of smaller amplitude is less loud.

Changes in intensity are perceived as variations in the loudness of a sound. This is measured in decibels (usually abbreviated to dbs.).

The third physical property of a sound is its duration. Sounds can only exist in time.

The duration of a sound is measured in milliseconds (usually abbreviated to msecs.) - thousandths of a second. In speech there are no definite boundaries between different speech sounds: one speech sound gradually fades into another. Duration of a sound is perceived by a man as its length. All the physical properties of a sound exist simultaneously. They may be singled out and separated from one another only for purposes of acoustic analysis.

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