Physics Chapter – 5 : Sound

Introduction: Sound

Centripetal force

Sound energy is a form of energy because of which our ears are able to hear something.

One cannot create sound or destroy it. But one can transform one form of energy into sound energy. For instance, when a cell phone rings, the sound is produced by converting electrical energy into sound energy.

A sound is produced when an object vibrates, that is they move in a ‘to-and-fro’ motion.

For instance,

When we strike a tuning fork or a stretched rubber band, it vibrates and produces sound.

The human voice is produced because of the vibration of the vocal cords.

String instruments produce sound as their strings vibrate.

When a bird flaps its wings a sound is produced.

A flute produces sound because the air column of the flute vibrates as air passes through it.

In order to propagate, sound requires a medium through which it can travel. This medium could be a gas, liquid or solid.

Sound propagates in a medium as the particles of the medium vibrate from a starting point. This means that sound always has a starting point and an ending point.

For instance, while you talk to a friend, as you speak, the particles in the air get displaced due to the pressure caused by the sound you produce. They then displace the adjacent particles and so on. In this way, sound travels from your place to your friend’s ears.

Therefore, we can say that the particles of a medium do not travel from one point one another in order to propagate sound. Sound propagates because of the disturbance caused by a source of sound in the medium.

A sound wave is characterized by three factors:

1.Amplitude
2.Frequency
3.Speed

Waves

A wave is a disturbance in a medium which moves from one point to another and carries energy without a net movement of particles. It may take the form of elastic deformation or a variation of pressure.

Mechanical waves

A wave that is produced when objects of the medium oscillate is called Mechanical Wave. The sound waves are therefore, mechanical waves.

Sound cannot travel through the vacuum as it always needs a medium to propagate. The vacuum contains no air hence no particles can propagate sound.

Longitudinal waves

Any wave that vibrates in the direction of the motion is called a Longitudinal Wave. Sound waves are longitudinal because the particles of the medium vibrate in the direction which is parallel to the direction of the propagation of the sound waves. The particles in the medium oscillate to and fro in the case of longitudinal waves.

Transverse Waves

A transverse wave is produced when the particles of the medium oscillate in a direction which is perpendicular to the direction of the propagation of the wave. The particles in a transverse wave oscillate in an up and down motion. For Example, light waves are transverse in nature.

Characteristics of Sound

Compression (C)	The compression region is represented by the upper part of the wave curve. It is a region where particles cluster together. The density, as well as pressure, is always high in this region.
Refraction (R)	A refraction is represented by the lower part of the wave curve. It is a region where the particles separate out. Refraction region always has lower pressure.
Crest	It is the peak of the curve
Trough	It is the crust of the curve

Wavelength (λ)	The distance between two consecutive compressions or refractions is called Wavelength. SI unit: metre (m)
Frequency (f)	The number of oscillations per unit time is called the Frequency of a Wave (Number of compressions + the number of refractions per unit time) SI unit: Hertz (Hz)
Time Period (T)	The time taken between two consecutive compressions or refractions to cross a fixed point is called Time Period of the Wave. In other words, the time taken for one complete oscillation through a medium is called a Time Period. SI unit: second (s)
The relationship between frequency and time period	f = 1/T

Pitch	Pitch of a sound depends upon: the frequency of the sound size of the object producing the sound type of the object producing the sound
Amplitude	The value of the maximum or minimum disturbance caused in the medium is called the Amplitude of the Sound. Amplitude defines if the sound is loud or soft.
Timber	The timbre or quality of sound is a characteristic with which we can differentiate between different sounds even if they have same pitch and amplitude

Tone	The sound which has single frequency throughout is called a Tone.
Note	A sound with more than one frequency is called a Note. It is pleasant to listen
Noise	It is an unpleasant Sound.
Music	It is a sound which is pleasant and has rich quality

The Speed of sound (v)	The distance by which a compression or refraction of a wave travels per unit t time is called as Sound’s Speed. SI unit: metres/seconds v = wavelength / time = λ/T = λ*F Speed of Sound in air = 333 m/s
Intensity	The amount of sound energy that passes through a unit area per second is called its intensity
Loudness	It is how our ears respond to a sound. Two sounds with same intensity can vary in loudness only because we can detect one sound easier than the other.

Laws of Reflection of Sound

The incident sound wave, the reflected sound wave and the normal, all lie in the same plane.

The angle of incident of incident sound wave is equal to the angle of reflection formed by the reflected sound wave, that is, i = r

Echo

When we hear the same sound again and again in a medium it is called Echo. The sound or echo persists in our brain for 0.1 seconds. This means that the difference between sound and its echo should be at least 0.1 seconds. It is produced as a result of reflection of sound through a medium. If sound reflects more than once we may hear multiple echoes.

Reverberation

It is the persistence of a sound after a sound is produced. A reverberation is created when a sound signal is reflected multiple of times until it reaches a sound wave that cannot be heard by human ears. Auditoriums and big halls often have to deal with reverberation. That is why the roofs are made up of soundproof materials like Flipboard and the chairs in the halls are also made up of fabrics that can absorb sound.