The branch of phonetics that studies the physical parametres of speech sounds is called acoustic phonetics. It is the most “technical” of all disciplines concerned with the study of verbal communication and one of the fundamental questions acoustic phonetics answers is the question of: "What is sound"?
Several types of events in the world produce the sensation of sounds. Just think of door slamming, violins, wind, and human voices. All these examples involve, when you think about it, movement of some sort. Now, these movements cause pressure fluctuations in the surrounding air (or some other medium, because sound can travel not only through air, but also water, wood, metal, or any other material. In fact the only place in which sound cannot travel is a vacuum). When pressure fluctuations or in other words, vibrations, reach our eardrum, they cause it to move, and our auditory system translates these movements into neural impulses, which we experience as sound. Thus, sound is produced when pressure fluctuations impinge upon the eardrum.
Sound can travel across relatively long distances and different frequencies can move more easily and faster through certain substances than others. Approaching a concert, for example, you may well hear the thumping of the bass drum before all else. This is because a sound produced at one place, say a loudspeaker, sets up a sound wave that travels through the acoustic medium. A sound wave is travelling pressure fluctuation that propagates through any medium that is elastic enough to allow molecules to crowd together and move apart.
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There are four main properties of a sound wave, that also linguists make use of, when they analyse speech sounds: Wavelength, period, amplitude, and frequency.
The wavelength is the distance between crests of a waveform. That means that the wavelength is the horizontal length of one cycle of the wave.
The period of a wave is the time required for one complete cycle of the wave to pass by a point. So, the period is the amount of time it takes for a wave to travel a distance of one wavelength.
The amplitude of a sound is represented by the height of the wave. When there is a loud sound, the wave is high and the amplitude is large. Conversely, a smaller amplitude represents a softer sound. A decibel is a scientific unit that measures the intensity of sounds. The softest sound that a human can hear is the zero point. When the sound is twice as loud, the decibel level goes up by six. Humans speak normally at 60 decibels.
The frequency of a wave is the number of cycles that pass a set point in a second, and is measured in Hertz (Hz). Frequency is intimately connected to pitch, although they are not exactly synonymous; the A above middle C is a vibration at a rate of 440 Hz. Lower frequency vibrations are perceived as being lower in pitch, and higher frequencies seem higher in pitch.
How the brain interprets the frequency of an emitted sound is called the pitch. We already know that the number of sound waves passing a point per second is the frequency. The faster the vibrations the emitted sound makes (or the higher the frequency), the higher the pitch. Therefore, when the frequency is low, the sound is lower.
Terms and concepts such as amplitude, frequency and pitch will be useful in your further studies on phonetics and phonology, so make sure to acquire a basic understanding of these concepts before you move on. The following exercises will help you to get a better understanding of the physical properties of sound, and you can also use the "read more" button, to extend your knowledge.