1) Introduction to Sound
Sound is a form of energy produced by vibrating bodies. Whenever an object vibrates, it disturbs the particles of the surrounding medium, and this disturbance travels in the form of waves. Our ear receives these waves and interprets them as sound.
Sound is not just a sensation; it is a physical phenomenon associated with vibration, energy transfer, wave motion, and hearing. It is one of the most important topics in physics because it connects mechanics, waves, and acoustics.
How sound is produced
Sound is produced only when a body vibrates. For example, a tuning fork, a stretched string, a drum membrane, or vocal cords all produce sound because they vibrate. If the vibration stops, the sound also stops.
2) Nature of sound waves
Sound is a mechanical wave because it needs a material medium to travel. It cannot travel through vacuum. It is also a longitudinal wave in air because the particles of air vibrate parallel to the direction of wave travel.
Why sound cannot travel in vacuum
In vacuum, there are no particles to carry the disturbance from one place to another. Since sound transfers energy through particle vibration, it cannot move in empty space. That is why astronauts in space cannot hear each other directly without communication devices.
Longitudinal nature of sound
In a longitudinal wave, the particles of the medium move to and fro in the same direction in which the wave moves. This creates alternate regions of compression and rarefaction.
- Compression: region where particles are close together, pressure is high, and density is high.
- Rarefaction: region where particles are farther apart, pressure is low, and density is low.
Sound wave in air = longitudinal wave
Particle motion ∥ wave direction
3) Important wave quantities
A sound wave is described by amplitude, frequency, wavelength, time period, and speed. These quantities are very frequently used in numerical problems.
| Quantity | Meaning | Unit |
|---|---|---|
| Amplitude | Maximum displacement from mean position | metre |
| Frequency | Number of vibrations per second | hertz (Hz) |
| Time period | Time taken for one complete vibration | second |
| Wavelength | Distance between two successive compressions or rarefactions | metre |
| Speed | Distance travelled by the wave in one second | m/s |
v = fλ
T = 1 / f
Here, \(v\) is the speed of sound, \(f\) is frequency, \(\lambda\) is wavelength, and \(T\) is time period. These relations are essential for solving sound-wave numericals.
Example of wave relation
If the frequency of a sound wave is 500 Hz and wavelength is 0.68 m, then speed of sound is:
v = fλ = 500 × 0.68 = 340 m/s
4) Reflection of sound
Sound, like light, can reflect from surfaces. When sound waves strike a hard surface and return back into the same medium, the phenomenon is called reflection of sound. It is the basis of echo, reverberation, and many useful instruments.
Laws of reflection of sound
- The angle of incidence is equal to the angle of reflection.
- The incident sound wave, reflected sound wave, and the normal at the point of incidence lie in the same plane.
These laws are the same as the laws of reflection of light.
5) Echo
An echo is the distinct repetition of a sound caused by its reflection from a distant obstacle. The reflected sound reaches the ear after the original sound has stopped, so we hear it as a separate sound.
Condition for hearing an echo
The reflected sound must reach the ear after at least 0.1 second. Since sound travels approximately 340 m/s in air, the reflecting surface should generally be at least 17 m away.
2d = vt
d = (vt) / 2
For echo, t ≥ 0.1 s
Examples of echo
- Shouting in a mountain valley.
- Clapping in a large empty hall.
- Sound reflected from a tall building.
6) Reverberation
Reverberation is the persistence of sound in an enclosed space after the source has stopped. It is caused by multiple reflections of sound from walls, ceiling, and floor.
Unlike echo, reverberation does not sound like a separate repeat. Instead, the sound seems to continue or overlap with itself.
Difference between echo and reverberation
| Echo | Reverberation |
|---|---|
| Distinct repetition of sound | Prolongation of sound |
| Usually heard after 0.1 s or more | Occurs when reflected sound returns quickly |
| Single reflection is enough | Multiple reflections are involved |
| Heard separately | Mixed with original sound |
Why excessive reverberation is harmful
Too much reverberation makes speech unclear and unpleasant. It becomes difficult to hear each word separately in auditoriums, halls, or rooms.
How reverberation is reduced
- Using curtains and carpets.
- Using acoustically designed walls.
- Using false ceilings with absorbent material.
- Using cushioned seats in halls and theatres.
7) Uses of reflection of sound
Reflection of sound is used in many useful devices and daily-life applications. These devices direct, concentrate, or transmit sound more efficiently.
| Device | Use of reflection |
|---|---|
| Megaphone | Directs sound in one direction to make speech louder and farther |
| Stethoscope | Uses multiple reflections to carry heartbeat and lung sounds to the doctor |
| Sound board | Reflects sound toward the audience in halls and auditoriums |
| Ear trumpet | Collects and directs sound into the ear |
8) Vibrations
Vibration is a to-and-fro motion about the mean position. Sound is always associated with vibration, so understanding vibrations is necessary to understand sound.
Examples of vibrating bodies
- A plucked string of a guitar.
- A drum membrane struck by a stick.
- The metal prongs of a tuning fork.
- The vocal cords in the human larynx.
9) Natural vibration
Natural vibration is the vibration of a body at its own frequency when it is disturbed and then left free. Every body has a natural frequency depending on its size, shape, tension, mass, and elasticity.
When a tuning fork is struck and allowed to vibrate freely, it vibrates at its natural frequency. This is natural vibration.
10) Damped vibration
In real situations, vibrations do not continue forever. They gradually lose energy due to air resistance, friction, and other resistive forces. This decrease in amplitude with time is called damped vibration.
Examples of damped vibration
- A swinging pendulum in air gradually stops.
- A tuning fork stops after some time.
- A plucked string gradually becomes silent.
In damped vibration, the amplitude keeps decreasing until the body finally comes to rest.
11) Forced vibration
When a body vibrates due to an external periodic force, it is said to undergo forced vibration. In forced vibration, the external force keeps supplying energy to the body.
Examples of forced vibration
- A table top vibrating due to a loudspeaker placed on it.
- A window pane vibrating because of loud music.
- A stretched membrane vibrating under external periodic pressure.
12) Resonance
Resonance is a special case of forced vibration. It occurs when the frequency of the external periodic force becomes equal to the natural frequency of the body. At resonance, the amplitude becomes maximum.
Resonance occurs when external frequency = natural frequency
Amplitude becomes maximum at resonance
Examples of resonance
- A child on a swing goes higher when pushed at the correct rhythm.
- One tuning fork makes another tuning fork of the same frequency vibrate strongly.
- Air columns in musical instruments resonate to produce loud sound.
- A glass may break if a strong sound of matching frequency is applied.
Importance of resonance
Resonance is useful in musical instruments where sound is strengthened. But it can also be harmful if unwanted resonance produces excessive vibrations.
13) Characteristics of sound
The three main characteristics of sound are pitch, loudness, and quality. For AMU and ICSE exam questions, pitch and loudness are the most frequently asked.
Pitch
Pitch is the characteristic by which a sound is identified as shrill or deep. A high-pitched sound has high frequency, while a low-pitched sound has low frequency.
- High frequency = high pitch
- Low frequency = low pitch
Loudness
Loudness is the sensation by which a sound appears loud or soft to the ear. It depends mainly on the amplitude of vibration and also on the intensity of sound.
- Large amplitude = louder sound
- Small amplitude = softer sound
Intensity
Intensity is the amount of sound energy passing per unit area per second. It is a physical quantity and can be measured objectively. Loudness, on the other hand, is a sensation experienced by the ear.
I ∝ A²
This means intensity is proportional to the square of amplitude. If amplitude increases, intensity increases much faster.
Relation between loudness and intensity
Loudness increases with intensity, but not in a directly proportional way. Human hearing responds approximately on a logarithmic scale.
L = K log10 I
Here, \(L\) is loudness level, \(I\) is intensity, and \(K\) is a constant. This formula shows that loudness changes logarithmically with intensity.
Factors affecting loudness
- Amplitude of vibration.
- Distance from the source.
- Size and shape of vibrating body.
- Sensitivity of the ear.
- Presence of reflecting surfaces.
- Nature of the surrounding medium.
14) Difference between pitch, loudness and intensity
| Characteristic | Depends on | Meaning |
|---|---|---|
| Pitch | Frequency | High or low sound |
| Loudness | Amplitude | Loud or soft sound |
| Intensity | Energy per unit area per second | Physical measure of sound strength |
15) Important formulas
v = fλ
T = 1 / f
2d = vt
I ∝ A²
L = K log10 I
16) Quick revision points
- Sound is produced by vibrating bodies.
- Sound is a mechanical longitudinal wave in air.
- Sound cannot travel in vacuum.
- Reflection of sound gives echo and reverberation.
- Echo is a distinct repetition, reverberation is prolonged sound.
- Natural vibration occurs at a body’s own frequency.
- Damped vibration loses amplitude with time.
- Forced vibration is caused by an external periodic force.
- Resonance occurs when external frequency equals natural frequency.
- Pitch depends on frequency.
- Loudness depends mainly on amplitude.
- Intensity is proportional to the square of amplitude.