According to physics sound is a mechanical wave, a sequence of oscillating pressure waves, traveling through compressible media such as air. Sound is produced by moving and vibrating objects.
The sounds we hear — voices, music, a dog barking — are all the result of oscillating pressure waves in the air created by vibrating sources. There are also sounds we do not hear. For example, dogs can hear a dog whistle but we do not.
Similar to other waves in nature, sound waves, also known as an acoustic waves, are typically characterized by frequency, amplitude and phase.
Frequency is the rate at which a sound wave vibrates. It is the number of oscillations a sound wave makes in a second. The standard unit of frequency is the Hertz (Hz), which is defined as one cycle per second.
The human ear is capable of hearing frequencies in the range of 20 to 20,000 Hz. This is called the audible frequency range. Our ears cannot hear sounds that lie outside the range of these frequencies.
The amplitude of a sound wave determines the strength or the volume of a sound. The human ear interprets amplitude as loudness or intensity. Loudness is measured in decibels (dB), a logarithmic scale.
The decibel is a tenth of a Bel, a unit named after the inventor of the telephone Alexander Graham Bell.
The human ear is sensitive to a wide range of “loudness” — from the soft sound of a pin drop to the loud roar of a train or a jet engine. The logarithmic scale of the decibel allows meters and graphs to represent the wide range of sound volumes linearly and makes the measurement of sound practical and manageable.
Phase is a point in the cycle of a waveform measured as an angle. The phase difference is a measure of the relative timing between two comparable sound frequencies. Phase difference is measured in degrees from 0 to 360, where 0 and 360 degrees indicate that two sound waves are precisely in phase, and a phase difference of 180 degrees suggests that two sounds are completely out of phase.
When two sound waves combine they create a new composite wave. The amplitude of the composite wave is the sum of the amplitudes of the component waves as determined by their phases.
Two identical sound waves with the same frequency and phase will combine to create a louder sound — their amplitude will add to each other. On the other hand, identical sound waves that are 180 degrees out of phase will cancel each other; their amplitudes will sum to zero.
In other words, if two sound waves with equal volume and frequency but 180 degree out of phase are mixed together, the result will be silence. This is the principle behind active noise cancellation.
Active noise cancelling headphones allow us to enjoy listening to music or other broadcasts even in relatively noisy environments. Click here to check out a nice selection of noise cancelling headphones at Amazon.
The phenomenon of phase shifting (phasing) has been used by musicians to create unusual and dramatic musical effects since the introduction of electronics into music in the early 1960s. The video below demonstrates the effects of phasing on the sounds of a guitar.
Boss PH-3 Phase Shifter Demonstration
The Spectrum of Audible Sounds
Sound in the natural world consists of a multitude of frequency components all vibrating simultaneously in the same time and space. While audible sounds span a frequency range of 20 to 20,000 Hz, the sounds produced by the human vocal cords lie in the relatively narrow 250 to 4000 Hz frequency band. This phenomenon has given rise to the following classification used extensively in acoustic design.
- Bass – frequencies below the voice band
- Midrange – frequencies that lie in the voice band
- Treble – frequencies above the voice band
Not unsurprisingly, our ears are the most sensitive in the midrange frequency spectrum.
Musical sounds consist of a dominant fundamental frequency and higher softer frequencies. The fundamental frequency is known as the musical pitch. Musical pitch is expressed as notes, such as Do, Re, Fi, Ma or C, F flat, and G sharp.
The higher softer frequencies are called harmonics or overtones and they occur at regular multiples of the fundamental frequency. The ear does not hear the harmonics as distinct pitches but as a tonal variation of the pitch.
The fundamental frequency together with the harmonics gives musical sounds their characteristic timbre or tone. The pitch range and timbre are the distinguishing features of sounds produced by different musical instruments.
The scientific study of sound in physics is known as acoustics. Acoustics is also an engineering discipline. Here are some books about acoustics you may find interesting:
- Musicians Acoustics
- Master Handbook of Acoustics
- Home Recording Studio: Build It Like the Pros