Definitions [15]
Define wavelength.
For transverse waves, the distance between two neighbouring crests or troughs is known as the wavelength.
For longitudinal waves, the distance between two neighbouring compressions or rarefactions is known as the wavelength. The SI unit of wavelength is meter.
Define the term Frequency (as applied to sound waves)
The number of vibrations per second taken by a wave is called its frequency.
The return of a sound wave on striking a surface such as wall, metal sheet, plywood etc. back in the same medium is called the reflection of sound wave.
Define frequency term in relation to a wave.
The number of vibrations made by a particle of the medium in one second is called the frequency of the waves. It is also defined as the number of waves passing through a point in one second. It's S.I. unit is hertz (Hz).
Define amplitude term in relation to a wave.
The maximum displacement of the particle of medium on either side of its mean position is called the amplitude of the wave. Its S.I. unit is a meter (m).
Define wavelength term in relation to a wave.
The distance travelled by the wave in one time period of vibration of a particle of medium is called its wavelength. It's S.I. unit is metre (m).
The sound heard after reflection from a rigid obstacle (such as cliff, a hillside, a wall of a building, edge of a forest etc.), is called an echo.
Define wave velocity term in relation to a wave.
The distance travelled by a wave in one second is called its wave velocity. It's S.I. unit is metre per second (ms-1).
When two waves of the same frequency, wavelength and velocity move in the same direction, on superposition, they result in interference.
Loudness is the characteristic by virtue of which a loud sound can be distinguished from a faint one, both having the same pitch and quality.
The intensity of a sound wave at any point of the medium is measured as the amount of sound energy passing per second, normally through a unit area at that point.
Define:
Timbre
Timbre is the characteristic that distinguishes two sounds of the same loudness and pitch of two different instruments.
Define:
Intensity
Intensity is defined as the amount of energy per unit of time perpendicular to the direction of propagation of the wave.
The apparent change in frequency of sound heard by a listener due to relative motion between the source and the listener is called the Doppler effect.
Answer briefly.
What is Doppler effect?
The apparent change in the frequency of sound heard by a listener, due to relative motion between the source of sound and the listener is called Doppler effect in sound.
When the source and the observer are in relative motion with respect to each other and to the medium in which sound propagates, the frequency of the sound wave observed is different from the frequency of the source. This phenomenon is called Doppler Effect.
Theorems and Laws [1]
When two or more pulses overlap, the resultant displacement is the algebraic sum of the displacements due to each pulse.
Key Points
| Feature | Constructive Interference | Destructive Interference |
|---|---|---|
| Phase Difference (φ) | \[0,2\pi,4\pi,\ldots\] | \[\pi,3\pi,5\pi,\ldots\] |
| Path Difference | \[n\lambda\] | \[(2n+1)\frac{\lambda}{2}\] |
| Nature | Waves reinforce | Waves cancel |
| Amplitude | Maximum | Minimum |
| Intensity | Maximum (bright) | Minimum (dark) |
| Result | Crest + Crest | Crest + Trough |
- Loudness of sound depends on the amplitude of the wave, and a loud sound corresponds to a wave of large amplitude.
- Intensity of a sound wave is the amount of sound energy passing per second, usually through a unit area, and its unit is watts per square metre² (W m⁻²).
- Loudness is subjective because it depends on the listener's ear sensitivity, whereas intensity is objective.
- Loudness depends on the square of the amplitude, the distance from the source, the surface area of the vibrating body, the density of the medium, and the presence of resonant bodies.
- Sound level is expressed in decibels (dB), and noise pollution is caused by undesirable loud and harsh sounds at levels above 120 dB.
- Doppler effect is the apparent change in frequency of sound due to relative motion between the source and listener.
- If the velocity of the source and observer are not along the same line, their respective components along the line joining them must be used for the longitudinal Doppler effect.
- As the speed of light is absolute, for light waves only the relative velocity between the observer and source matters.
