### Theory:

If we play two different musical instruments at the same time, the vibrations produced by the musical instruments travel through the same air medium, but we hear different sounds.

A sound has three characteristics. They are
• Loudness
• Pitch or shrillness
• Quality
Loudness:
• Loudness is a quantity that distinguishes one sound from another, even if they have the same frequency.
• The amplitude of the wave determines the loudness or softness of the sound.
• We hear a soft sound when we lightly strike a table because we produce a sound wave with less amplitude.
• We hear a louder sound if we hit the table hard. Because loudness is associated with higher energy, louder sound can travel a greater distance. From its source, a sound wave spreads outward. Its amplitude decreases as it moves away from the source, and thus its loudness decreases.
The wave shapes of a soft and loud sound of the same frequency are shown in the above figure.

The intensity of a sound wave determines its loudness. The amount of energy crossing per unit area per unit time perpendicular to the wave's propagation direction is defined as intensity.

The intensity of sound heard at a location is determined by the five factors listed below.
• Amplitude of the source
• Distance of the observer from the source
• Surface area of the source
• Density of the medium
• Frequency of the source.
The unit of intensity of sound is $$decibel$$ ($$dB$$). It is named in honour of the Scottish-born scientist Alexander Graham Bell who invented telephone.

Pitch or shrillness:
• Pitch is one of the characteristics of sound that allows us to distinguish between shrill and base sounds.
• High-pitched sounds are shrill, while low-pitched sounds are flat.
• When the vibrations of two music sounds produced by the same instrument with the same amplitude are of different frequencies, they will differ.
Quality of sound:
• Timbre is the quality that distinguishes two sounds of similar volume and pitch produced by two different instruments. A tone is a sound of a single frequency, while a note is a collection of tones. The distinguishable characteristics of a tone are then referred to as timbre.