Reflection, Refraction, Diffraction and Interference of Sound Waves

Reflection

Sound waves are reflected from the surfaces like light waves, obeying laws of reflection; angle of incident is equal to the angle of reflection. This can be observed in a simple experiment in which a source of sound such as a transmitter sends sound at a certain angle to the normal to a wall and a higher sound is received from reflection at the same angle on another side of the normal.

The regular reflection occurs from a surface if a whole part of an incident wavefront is reflected uniformly. Since the sound wave in speech has a wavelength of various meters, they are reflected from rough surfaces such as cliffs, walls etc. Reverberation occurs after incident rays undergo multiple reflections.

Refraction

Soundwave bends when the parts of wave fronts travel at different speeds. This occurs in uneven winds or when sound is travelling through an air of uneven temperatures. On a warm day, the air near the ground becomes warmer than the rest of the air and the speed of sound near the ground increases. So, sound waves then tend to bend away from the ground resulting in a sound that does not seem to travel well as shown in the figure. Due to the effect, sounds are easier to hear at night than during the day times as shown in the figure. Different speeds of sound produce refraction.

Diffraction

Diffraction is the phenomenon of spreading of waves around the corners of an obstacles or apertures. It is a wave phenomenon and occurs if the wavelength of the wave is of the same order as the dimension of a diffracting obstacle. Due to diffraction of sound through the doors and windows, a sound is heard inside the room from an outside source. This is due to the wavelength of a sound wave is nearly equal to the dimension of doors and windows. However, light having small wavelength can be diffracted only through a very sharp object such as the edge of a blade and diffracting grating. Diffraction of water waves in a ripple tank through a small aperture is shown in the figure. It is observed that smaller the width of the aperture, greater is the spreading of the waves.

Interference

When two frequency and amplitude overlap, this produces interference of the waves. It is characteristics of all wave motion, whether the waves are sound, light or water waves. The interference pattern consists of a region where intensity is maximum at certain points and minimum at certain points. The positions of maximum intensity are called maxima and the superposition is called constructive interference while the positions of minimum intensity are called minima and the superposition is called destructive interference.

Undamped and Damped Oscillations

A system executing simple harmonic motion is called a harmonic oscillator. A harmonic oscillator produces sinusoidal oscillations. The sinusoidal oscillations can be of two types. They are damped and undamped oscillation.

Undamped Oscillations

The oscillations whose amplitude remains constant with time are called undamped oscillations. Such oscillations can occur if frictional forces on the oscillating systems are absent. For example, if the bob of a simple pendulum is displaced in a vacuum and then released, the bob executed simple harmonic motion with constant amplitude.

Damped Oscillation

The oscillations whose amplitude goes on decreasing with time are called damped oscillations. In a real oscillating system, forces like friction are always present that dissipate the energy of the oscillator. Unless energy somehow is added to the system, dissipation eventually brings the system to rest i.e. in equilibrium.

Free, Forced and Resonant Oscillations

When a body capable of oscillation is displaced from its equilibrium position and then left free, it begins to oscillate with a definite amplitude frequency. If the body is not resisted by any kind of friction, the motion continues. Such oscillation is called free oscillation. The frequency of vibration depends on the intrinsic properties (shape, elasticity etc.) of the body which is as the natural frequency. The force acting on the system is restoring force. For example, when a simple pendulum is displaced from its mean position and then left free, it executes free oscillations. The frequency of the simple pendulum is \(f = (\frac 12 \pi)\sqrt {1/g} \).

Forced Oscillation

When a body is maintained in a state of oscillation by an external periodic force of frequency other than the natural frequency of the body, the oscillation is called the forced vibration. In such oscillation, the frequency of oscillation is equal to the frequency of the periodic force. The externally applied force on the body is called the driver and the body set into oscillation is called driven oscillation.

Resonant Oscillation

When a body is maintained in a state of oscillations by a periodic force having the same frequency as the natural frequency of the body, the oscillations are called resonant oscillations. The phenomenon of producing resonant oscillations is called resonance. Note that the resonance is a particular case of forced oscillations in which the two frequencies are equal.