11. Characteristics of Sound

A sound wave, like any periodic wave, can be described by several fundamental physical quantities: frequency, wavelength, amplitude, and velocity (speed), and these physical quantities in turn determine the perceptual (subjective) characteristics of sound as experienced by a listener: pitch, loudness, and quality (timbre).

11.1 Frequency, Wavelength, Amplitude and Velocity

• Frequency (f or ν): the number of complete vibrations (oscillations) made by a vibrating particle, or the number of complete waves passing a fixed point, per unit time (per second). It is measured in hertz (Hz), where 1 Hz = 1 vibration per second.
• Wavelength (λ): the distance between two consecutive particles of the medium that are in exactly the same phase of vibration — for example, the distance between two successive compressions, or between two successive rarefactions, in a sound wave. It is measured in metres (m).
• Amplitude (A): the maximum displacement of a vibrating particle from its mean (rest) position. It is measured in metres, and amplitude determines the loudness of the sound — a larger amplitude corresponds to a louder sound.
• Time period (T): the time taken to complete one full vibration/oscillation; T = 1/f.
• Velocity (v): the distance travelled by the sound wave per unit time, related to frequency and wavelength by the fundamental wave equation: v = f × λ (velocity equals frequency multiplied by wavelength). This equation is one of the most frequently tested formulas in this chapter.

11.2 Pitch

Pitch is the perceptual characteristic of sound that allows a listener to judge a sound as 'high' (shrill, like a whistle or a child's voice) or 'low' (deep, like a drum or an adult male voice). Pitch is directly determined by the frequency of the sound wave: a higher frequency corresponds to a higher pitch, and a lower frequency corresponds to a lower pitch. Pitch should not be confused with loudness — a whisper and a shout can have the same pitch (frequency) but very different loudness, while a shrill whisper and a deep shout can have similar loudness but very different pitch.

11.3 Loudness

Loudness is the perceptual characteristic that allows a listener to judge a sound as 'loud' or 'soft/faint'. It is determined primarily by the amplitude of the sound wave: greater amplitude produces greater loudness. Loudness is also affected by the intensity of sound (the amount of sound energy passing per unit time through unit area, measured in watts per square metre), the distance of the listener from the source (loudness decreases as distance increases, since the same sound energy spreads out over a progressively larger area), and the sensitivity of the listener's ear. Loudness is measured on a logarithmic scale in decibels (dB), because the human ear can perceive an enormously wide range of sound intensities, and a logarithmic scale compresses this vast range into a more manageable and perceptually meaningful set of numbers.

11.4 Quality or Timbre

Quality (or timbre) is the perceptual characteristic that allows a listener to distinguish between two sounds of the same pitch and the same loudness produced by different sources or instruments — for instance, distinguishing a note played on a violin from the same note (same pitch, same loudness) played on a flute or sung by the human voice. This distinguishing characteristic arises because a real musical note is rarely a 'pure' single-frequency tone; it is actually a combination of a fundamental frequency (the lowest and usually the loudest frequency present, which determines the perceived pitch) along with a number of overtones or harmonics (frequencies that are whole-number multiples of the fundamental), and it is the particular mixture and relative strengths of these harmonics that gives each instrument or voice its characteristic, recognisable quality or 'timbre'.

11.5 Summary: Physical Quantity vs. Perceived Characteristic