Musical instruments

Classification

Audible waves originate in vibrating strings, vibrating air columns, and vibrating plates and membranes. Musical instruments are classified accordingly into three main types.

Stringed instruments

These consist of stretched strings; sound is produced by setting the strings vibrating, and they are tuned to particular frequencies by adjusting the tension in them. Three sub-types:

• Plucked string type: the string is plucked by the fingers — tanpura, sitar, guitar, veena.
• Bowed string type: the string is played by bowing — violin, sarangi.
• Struck string type: the string is struck by a stick or hammer — santoor, piano.

Wind instruments

These consist of an air column; sound is produced by setting the air column vibrating. Three sub-types:

• Free-reed type: free brass reeds are vibrated by air that is either blown or compressed — mouth organ, harmonium.
• Edge type: air is blown against an edge — flute.
• Reed pipes: with a single reed (saxophone, clarinet), a double reed (bassoon), or without a reed (bugle).

Percussion instruments

Sound is produced by setting up vibrations in a stretched membrane — tabla, drum, dhol, mridangam, sambal, daphali. Some percussion instruments instead use metal plates that produce sound when struck against each other or with a beater — cymbals (jhanja), xylophone.

How the Notes Are Produced

• A blow on a membrane or plate, or the plucking of a string, produces vibrations with one fundamental and many overtones. The superposition of several natural modes of oscillation, with different amplitudes and hence different intensities, is what characterises a particular instrument, and is why we can distinguish instruments by their sound.
• The production of different notes by a musical instrument depends on the creation of stationary waves.
• In a stringed instrument such as a guitar or a sitar the two ends of the string are fixed. Depending on where the string is plucked, stationary waves of various modes are produced; plucking at the midpoint produces the minimum frequency, the fundamental mode of vibration.
• In wind instruments the air column is made to vibrate by blowing, and the note is changed by changing the length of the air column. In a flute, uncovering the holes changes the vibrations of the air column and so changes the pattern of nodes and antinodes.
• In practice the sound produced is made up of several stationary waves with different patterns of nodes and antinodes. The musician's skill lies in stimulating the string or air column so as to produce the desired mixture of frequencies.

Quality of Musical Sound

Quality, or timbre, is the characteristic that lets us distinguish two sounds of the same pitch and loudness — the reason we recognise the voice of a particular person or a particular instrument. It depends on the number of overtones present in the sound along with the fundamental frequency. This is also why an open pipe, which sounds all harmonics, has a richer quality than a closed pipe of the same length, which sounds only the odd ones.

A sound that produces a pleasing sensation is a musical sound; it is produced by regular and periodic vibrations without any sudden change in loudness, and has well-defined frequencies with sizeable amplitude, normally harmonics of a fundamental. A mixture of sounds of different frequencies bearing no relation to each other is noise.

A sequence of frequencies which have a specific relationship with each other is called a musical scale. In both Indian and Western classical music eight frequencies in specific ratio form an octave, each frequency denoting a note — sa re ga ma pa dha ni sa, or Do Re Mi Fa So La Ti Do. An example set of values is $240$, $270$, $300$, $320$, $360$, $400$, $450$, $480$ Hz.

Standard Results

• Three classes: stringed (plucked, bowed, struck), wind (free-reed, edge, reed pipe), percussion (membrane, plates).
• All of them work by setting up stationary waves; the note is set by the boundary conditions and the mixture of overtones sets the quality.
• Sir C. V. Raman researched Indian classical instruments such as the mridangam and the tabla.