Characteristics of Sound

A sound wave is characterised by three factors:

1. Amplitude: It is the maximum disturbance caused in the medium from its rest position. It determines whether a sound is loud or soft. 

• Higher amplitude → Louder sound.
• Lower amplitude → Softer sound.

2. Frequency: It is the number of complete cycles (one compression + one rarefaction) produced per second.

`f="1" / "T"`

$T$: Time period (time taken for one complete cycle).

SI Unit: Hertz (Hz).

1 Hz = 1 cycle per second.

Example,

A tuning fork with a frequency of 512 Hz produces 512 compressions and rarefactions per second.

 Frequency determines the pitch of the sound.

• Higher frequency → Higher pitch (e.g., a whistle).
• Lower frequency → Lower pitch (e.g., a drum).

3. Speed: Speed of sound is the distance travelled by a compression or rarefaction in one second.

`v= "wavelength" / "time period"` = `lambda.f`

• $`v`$: Speed of sound
• λ: Wavelength
• `f`: Frequency

SI Unit: Metres per second (m/s)

Example,

Speed of sound in air = 343 m/s (at standard conditions).

Cycles of compression and rarefaction in a sound wave and change in air pressure

To observe how the length of the free part of a ruler affects the frequency and pitch of sound.

• Place a plastic ruler on a table, keeping one end pressed down while a portion extends beyond the edge.
• Press and release the free end of the ruler to make it vibrate and produce sound.
• Shorten the free part by moving more of the ruler onto the table and repeat the process.

Compare the sounds:

• A longer free part produces a lower pitch and deeper sound (lower frequency).
• A shorter free part produces a higher pitch and sharper sound (higher frequency).

Conclusion:

The length of the vibrating part of the ruler affects the frequency and pitch of the sound. Longer lengths create lower frequencies and deeper sounds, while shorter lengths create higher frequencies and sharper sounds. This experiment demonstrates how vibrations influence sound pitch.

Vibration of the ruler and the sound produced

1. Compression: The upper part of the wave curve represents compression. It is a region where particles are close together, resulting in high pressure and high density.
2. Rarefaction (R): The lower part of the wave curve represents a rarefaction. It is a region where particles are spread apart, resulting in low pressure and low density.
3. Crest: The peak of the wave, representing maximum displacement in a compression.
4. Trough: The valley of the wave, representing maximum displacement in a rarefaction.

1. Aim: To study how the length of a pendulum affects its time period and calculate its frequency.

2. Requirements: Strong thread, wooden or metal bob, support to hang the pendulum, stopwatch, ruler or measuring tape

3. Procedure

• Tie the bob to the thread and hang it from a support to create a pendulum. Measure the thread length in centimetres and record it.
• Swing the pendulum and use the stopwatch to measure the time for 20 oscillations.
• Shorten the thread by 10 cm each time and repeat the process 4-5 times.
• Record observations in a table.
• Calculate the time period (T) using the formula: T = `"1" / "20"`  (where t is the time for 20 oscillations).
• Calculate frequency (n) using the formula:
  n = `"1" / "T"`

4. Observation Table Example

5. Conclusion

• The time period increases with the length of the pendulum.
• The frequency (oscillations per second) is the inverse of the time period.
• The amplitude (how far the pendulum swings) does not affect the frequency.

1. Wavelength (λ): It is the distance between two consecutive compressions or rarefactions. SI Unit: Metre (m).
2. Time Period (T): It is the time taken for two consecutive compressions or rarefactions to pass a fixed point. Or the time for one complete oscillation. SI Unit: Second (s).
3. Pitch: Pitch is how high or low a sound seems to a listener. Factors Affecting Pitch:

• Frequency of the sound wave.
• Size of the object producing the sound.
• Type of the object producing the sound.