Global Music Resource

Sound has specific physical attributes that are helpful to know about in order to compose music, work with editing programs that graphically represent sound waves and also to work with synthesizers in creating sounds.

Sound, either spoken voice, bowing, plucking, blowing or striking an instrument, or striking an object is a source of vibrations. The vibrations produce changes in air pressure, which in turn vibrates neighboring air particles. Sound moves at the speed of 1,130 feet per second (under normal weather conditions). The vibration continues to emanate in all directions from the source and creates a series of oscillating waveforms as it moves outward. The waveform has valleys (refraction) and peaks (condensation). The sound that you hear obtains its characteristics from the wavelength, period, amplitude, timbre and frequency of the waveform.

• The wavelength is the distance between a point on one wave and the equivalent point on a successive wave.
• The amount of time required for the wave to travel actually one wave length past a fixed point is know as the period. Musical notes tend to have repeating periods.
• Amplitude is the height of the wave (peaks and valleys) measured from a midpoint. The louder the sound, the higher the wave points and the greater the amplitude. The amplitude of a sound is measured in decibels (dB). Gain is the amount of increase in the amplitude (or volume) when the wave processed through an amplifier. The range of the cycles highest to lowest amplitude is known as the dynamic range.
• The Timbre (or formant) of the sound is related to shape of the waveform. Each instrument produces a slightly different waveform which gives that instrument its identifying quality. This is how one can distinguish an individual piano, guitar, trumpet and violin while all playing the same note simultaneously. These characteristic frequencies, sometimes referred to as the harmonic content, are in turn related to the physical structure of the instrument and the resonance it may individually produce.
• The Frequency of a sound is measured in the number of wave cycles per second. The actual measurement is expressed in hertz (one cycle per second). The faster the frequency the higher the Pitch and a corresponding increase in hertz to kilohertz (kHz, thousands of cycles per second). The lower the frequency, or time between wavelengths, then the lower the pitch of the sound. Similarly, the closer the refraction (valley) and condensation (peak) in a wave length the higher the pitch of the sound. The pitch of the sound is not affected by the amplitude (volume) of the sound. As an example, the Note A just above Middle C on the piano has a frequency of 440 cycles per second. An A one octave above that note has a frequency of 880 Hz while the A one octave below it is 220 Hz. Thus, the frequency doubles per octave.

The evenly proportioned, repetitive sound or electrical waveform is known as a sine wave (the center line is 0 dB):

However, the actual waveform, or cycles of a sound, is more complex than this even, bipolar (opposite, equal height above and below the 0 amplitude line), symmetrical form. Rather, a sound is a complex composite of many frequencies in succession with various amplitude and pitch (because the length of a piano wire or violin or guitar string, or the length of an instrument, vibrates at different cycles along its entire length). Thus, the successive waveforms (sinusoid), especially that of a musical note, has various identifiable components. This is called the envelope, or the complete, entire form of the cycle. The envelope, which is also analyzed in synthesizer sound generation and recording methodology, is characterized by simple components:

• Attack or the rate the amplitude increases and the sound goes from silence to its peak
• Sustain or how long the sound maintains a level amplitude (volume)
• Decay or how quickly the sound fades out from the sustain level to silence
• Release or how quickly the sound decays once the sound source ceases to produce the sound any further.

This results in simplified, but not naturally occurring, sound cycle components that look like:

• Saw Wave which are like a sine wave but only have straight lines.
  
• Square Wave which moves sharply but evenly in amplitude.
  
• Triangle Wave which is like a sine wave but move up and down at a more abrupt rate.
  

The measurement of sound in decibels is based upon a logarithmic scale, which reflects the level of loudness of a sound that the human ear can either discern or tolerate. Humans can discern sound at just above 0 decibels and can tolerate up to 120 decibels. Frequencies can be heard from approximately 15 hertz to 20,000 hertz (20 KHz).

Even though two instruments may play the same sound note (pitch and frequency) they still sound different not only due to timbre, but also due to harmonics. The waveform contains an increase in the actual amount of hertz compared to the fundamental frequency (pure tone) of the note. Essentially, there are several frequencies (related to the lowest or fundamental frequency) combined simultaneously. Harmonics are the overtones of the fundamental tone. The timbre of the sound, or the combination of these harmonic overtones, is what gives an instrument or sound it characteristic note (definitive pitch).

When a note is played or a sound is generated, it rises to a certain amplitude (loudness) and over a period of time it declines in loudness (decay) and eventually ceases. The harmonics of the sound also decay and cease at various rates.

A note that is exactly one octave above a corresponding note is double the frequency of the note in the lower octave (440 Hz and 880 Hz).