Superposition of wave

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Physics Notes: Sound & Musical Instruments

Physics Class Notes

1. Applications of Beats (6.9.2)

  • Tuning Musical Instruments: Artists tune instruments by sounding them alongside a reference frequency and adjusting until the beat frequency drops to zero (in unison).
  • Determining Speed (Doppler RADAR / SONAR):
    • RADAR: Microwaves reflected off moving targets produce beats via Doppler shift to determine vehicle/aircraft speed.
    • SONAR: Uses reflected acoustic waves underwater to determine submarine speed.
  • Medical Diagnostics: Doppler ultrasonography and echocardiograms use phase shifts and beat principles to measure blood flow velocity in arteries and veins.
  • Determining Unknown Frequency: Sounding an unknown frequency alongside a known variable source and adjusting until beats vanish gives the exact frequency.
Audibility Limit: The human ear can only distinguish individual beats if the frequency difference (Δn = n1 - n2) is less than 6 to 7 Hz.

2. Characteristics of Sound (6.10)

A. Loudness

  • Definition: The human perception of sound intensity.
  • Dependence: Directly proportional to the square of amplitude (I ∝ A2), surface area of the vibrating body, and medium density.
  • Unit of Loudness: Phon.
Sound Level (β) = 10 log10 (I / I0) dB

(where I0 = 10-12 W/m2 is the minimum reference intensity)

B. Pitch

  • Definition: The sensation that distinguishes between a high-frequency note and a low-frequency note.
  • Dependence: Depends directly on frequency (Higher frequency = Higher pitch).
  • Example: A female voice generally has a higher pitch than a male voice.

C. Quality (Timbre)

  • Definition: The characteristic enabling us to distinguish between two sounds having the same pitch and loudness.
  • Dependence: Depends on the number and relative intensities of overtones present.

3. Classification of Musical Instruments (6.11)

Type Vibrating Element Sub-Types & Examples
Stringed Stretched strings • Plucked: Sitar, Guitar, Tanpura, Veena
• Bowed: Violin, Sarangi
• Struck: Santoor, Piano
Wind Air columns in a pipe • Free-wind: Harmonium, Mouth Organ
• Edge-type: Flute
• Reed-pipes: Saxophone, Clarinet, Bugle
Percussion Stretched membranes or plates • Membranes: Tabla, Drum, Mridangam
• Metal Plates: Cymbals (Jhanj), Xylophone

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