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How does a Bansuri (bamboo flute) produce sound

The flute is a relatively simple instrument in terms of its design which is primarily a long tube with holes. Technically the flute belongs to the woodwind family of instruments. According to wikipedia – A woodwind instrument is a wind instrument in which sound is produced by blowing through a mouthpiece against an edge or by a vibrating reed, and in which the pitch is varied by opening or closing holes in the body of the instrument. As the name suggests these instruments were made of originally made of wood. In order to understand how a flute works we must first understand a few things about sound.

What is sound?
By sound, we commonly mean the vibrations that travel through air and can be heard by humans. Sound is perceived through the sense of hearing. Humans and many animals use their ears to hear sound, but loud sounds and low frequency sounds can be perceived by other parts of the body through the sense of touch. Sounds are used in several ways, most notably for communication through speech or, for example, music.

Sound is a disturbance of mechanical energy that propagates through matter as a wave. When an object vibrates in air, it will vibrate air molecules around it. Those air molecules will vibrate other molecules around them and so on. This way a mechanical wave is formed. When this wave reaches our ear it vibrates our ear drum slightly and these vibrations are interpreted by the brain. This is how we perceive sound.

Not all sounds are perceived as musical. So what is the difference between an ordinary sound and a musical note? The sounds that are perceived as musical are generally ones that are dominated by periodic or regular vibrations rather than non-periodic ones (called a definite pitch). In a very simple case, the sound of a sine wave, which is considered to be the most basic model of a sound waveform, causes the air pressure to increase and decrease in a regular fashion. Sound waves that are close to ideal sine waves can be produced by tuning forks or whistling.

Characteristics of sound
Any simple sound, such as a musical note, may be completely described by specifying three perceptual characteristics: pitch, loudness (or intensity), and quality (or timbre). These characteristics correspond exactly to three physical characteristics: frequency, amplitude, and harmonic constitution or waveform, respectively.

• Frequency and Pitch
When a sound wave travels, particles in the medium are displaced by the wave and oscillate. The number of oscillations of a particle per second is known as the frequency (f) of the wave. It is measured in cycles per seconds or Hertz (Hz).

Pitch is a subjective quality, often described as highness or lowness, and is something perceived by the human ear. This is almost entirely determined by the frequency of the sound waves. So a sound with higher frequency is perceived to have a higher pitch whereas a sound with lower frequency is perceived to have a lower pitch. However, the relationship between the frequency of a wave and the perceived pitch is not linear.

• Amplitude and Loudness
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.

• Color/Timbre and Harmonic constitution
In music, timbre or color is the quality of a musical note or sound which distinguishes different types of sound production or musical instruments.

You might ask yourself why the sound of the flute is different from the sound of say a violin and it is very easy to distinguish between the two sounds even when both the instruments are playing the same note, for the same duration and with the same loudness. This is because the color of their sound is different.

It is not possible to produce a perfect sine wave or a sound wave that has only one frequency. In fact the sounds of all musical instruments have multiple frequencies but there is one dominant frequency. So the color of any instrument is its characteristic set of frequencies and their harmonics. This depends on many factors but mainly on the material of the instrument and its shape.

How the flute produces sound
Producing sound from a flute is similar to playing a note by blowing over the top of a bottle. A flute produces sound when a stream of air directed across the top of a hole bounces in and out of the hole. The stream beats against the air in a resonator, usually a tube. The player changes the pitch of the flute by changing the effective length of the resonator. This is done either by closing and opening holes. The sketch below shows a cross section of the embouchure.

The flute is an open organ pipe. This means that air can flow in and out from both the ends of a flute. The regular vibrations produced in a flute are due to resonance caused inside its body.
The characteristic sound of the flute

A lot of people believe that the characteristic sound of the flute is also affected by the material from which the instrument is made. For instance, instruments made of wood are usually less bright than metal instruments. Different metals are also thought to influence the tone. But this is debatable and has not been proved scientifically.

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