Acoustic phonetics is the study of the acoustic (physical) properties of sounds, which are determined by measuring pitch (formants and frequencies), loudness (amplitude), and quality (timbre). Sounds are measured in terms of sound waves, which are composed of harmonics, frequencies, and formants that show air pressure patterns created during phonation. The physical process of speech production can be divided into three independent elements:
The source of the sound (i.e. larynx), the filters that modify the source (i.e. the shape of the vocal tract formed by articulators), and finally, the output.
Sound Measurements and Sound Waves
A sound wave is the alternating pattern of high and low air pressure areas. Periodic sound waves repeat precisely and display a regular pattern (i.e., a musical note), whereas aperiodic sound waves are irregular and show no pattern of precise repetition (i.e.,noise), as seen in Figure 3.
The simplest sound wave, a sine wave, is generally never perceived in nature. This most basic sound wave provides a clear idea of the primary features of a sound wave, the frequency and amplitude. Here is an example of a sine wave audio. Warning: Do not turn up the volume too loud!
Frequencies refer to the rate at which a sound wave cycles from positive (crest), to negative (trough), and back to positive. Frequency is measured in hertz (Hz) which represents the number of cycles per second. The majority of human sounds falls between 250-4000 Hz thus sounds that occur out of this range can be difficult to perceive correctly, or at all. Any given sound is composed of a number of different frequencies. Different combinations of frequencies account for the difference in the sounds you recognize around you.
Amplitude is the measurement of the intensity or strength of a sound and is shown in decibels. More specifically, amplitude is the extent to which air particles are moved or altered during changes in atmospheric pressure caused by sound waves. The greater the change in the sound wave, the greater the amplitude.
Harmonics are pure tones that are common in nature; most systems vibrate in more than one mode (simultaneously). The fundamental frequency (F0), or first harmonic is the dominant tone. Superposition of harmonics upon the fundamental frequency result in a complex sound waveform. Harmonic overtones are related to the fundamental frequency mathematically (as integer multiplies). To calculate this, you simply need to know that the second harmonic is twice the duration of the fundamental tone, and the third harmonic is three times the fundamental tone., etc. We refer to these as complex sound waves as can be seen in Figure 6.
Formants are the the crest, or spectral peaks of a sound wave. Formants occur at around 1,000 Hz and correspond to a measurement of resonance produced in the vocal tract during phonation. The formant with the lowest frequency is referred to as the F1, the second F2, the third F3, etc. Formants can be easily seen in a sound image (spectrogram) as a conglomeration of dark bands. The darker the bands, the more sonorous, or audible the sound. They can be seen by the red lines in Figure 7.
More on acoustic descriptions of the consonants Standard English.
To practice your mastery of this material, click here!
Return to Phonetics Tutorials R. Aronow, K. Bannar