Acoustic phonetics Overview

Acoustic phonetics Overview 

          “Acoustic phonetic sis the study of sound waves made by the human vocal organs for communication” (Ladefoged, 1993, p. 192).

                “The branch of science dealing with the sounds of speech in terms of their frequency, duration, intensity” (Dictionary.com).

                This chapter will discuss a small sampling of the results of applying technology to the study of speech.

Simple Sound Waves

          Simple sound wave talks about the movement of sound. One kind of movement which can set off a sound wave is a vibration such as that produced by violin string. In this kind of sound wave, a vibrating body of some sorts set the air molecules surrounding it into vibration. We have two physical phenomena resulting from this tendency for equidistance which make it possible for sound waves to move through the atmosphere. These two phenomena are compression and rarefaction. In compression, air molecules are crowded together more than they normally are. In rarefaction, air molecules are spread further apart than they normally are.

Complex Sound Waves         

            Complex sound wave is the combination of a number of simple waves in the same way. The sound wave which is produced by the vocal cords is a complex wave. Complex wave is composed of a fundamental wave which vibrates at the rate of the vocal cord vibrations, and which is the first of a set of harmonic waves which are multiplies of the fundamental.

Some Speech Sounds

            In this chapter, we will begin by discussing some sonorant sounds of English (vowels and nasals). Then we will briefly discuss the acoustic manifestation of some English obstruents (stops and fricatives)

Vowels

            In the production of vowels, the vocal tract acts as a filter which enhances some vocal frequencies and damps others. The vocal tract acts as a filter on the complex wave produced by the vocal cords. For example, when the vocal tract is positioned for the vowel {i}, harmonics at about 300 Hz and 2300 Hz are enhanced, while harmonics at other frequencies are damped.

            The peaks in the filter function are called formants and they differ depending on the quality of the vowel.

Nasals

In the production of nasal consonants, the oral tract is closed as if producing a stop consonants but the velum is lowered allowing air to escape out of the nose. In acoustic terms then the nasal passage serves as the filter for the vocal sources in addition to the oral tract. Consequently, nasal consonants all have nasal formants which formants which reflect the resonating characteristics of the nasal passage.

Stop

Most of the important information for stop consonants in English is encoded either as formants transition is vowels or as a delay in the onset of voicing of the vowels. For the sake of simplicity, we will discuss only word initial stops although much of what will be said also applies to stops in other positions.

            The main acoustic cue for voicing in English word initial stop consonants is called VOT. This stands for Voice Onset Time. In producing the voiceless stop consonants, we delay in starting the voicing of the vowel for about 50 milliseconds. In other words, we release the closure and then hold the vocal cords open for five one-hundredths’ of a second and then allow the vocal cords to start vibrating for the vowel. The measure of this delay in the onset of voicing is called voice onset time. For voiced stops in English, VOT value is very small, and voiced stops in some languages have negative VOT values which mean that voicing starts before the stop is released. While the vocal cords are being held apart in thr delay of voicing for voiceless sounds air from the lungs rushes out.

Fricatives

Fricatives involve a type of sound that we have not explicitly dealt with up to this point. Basically, the difference between the noise used in vowels and the noise used for fricatives is that the sound in vowels is periodic, (i.e., it involves the repetition of a relatively stable wave form), while the sound in fricatives is aperiodic (it involves random movement of air molecules).

            The main differences between fricatives (in acoustic terms) are relative frequency, thus {s} is higher pitched that {s}, amplitude, so {s} is louder than {f}, and duration (e.g. in English {s} is longer than {z}.

Conclusion

In this file, we have investigated the nature of simple and complex sound waves. We then noticed that vowels and nasals can be described acoustically as the result of combining the complex wave caused by the process of voicing with the filtering action of the oral (or nasal) tract. We then noticed that stop consonants are distinguished from each other by virtue of the transition from consonant position to vowel position and by their relative voice outset times. Finally, we discussed the aperiodic nature of the fricatives consonants and some acoustic dimensions which keep the different fricatives separate from each other.