SOUND – Formula & Equation Sheet
Essential formulas and equations from Science, tailored for Class 9 in Science.
This one-pager compiles key formulas and equations from the SOUND chapter of Science. Ideal for exam prep, quick reference, and solving time-bound numerical problems accurately.
Key concepts & formulas
Essential formulas, key terms, and important concepts for quick reference and revision.
Formulas
Speed of Sound: v = d/t
v is the speed of sound (in m/s), d is the distance traveled (in meters), and t is time taken (in seconds). This formula represents how sound travels through a medium.
Frequency: f = 1/T
f is frequency (in Hz), and T is the time period (in seconds). It indicates how many oscillations occur in one second.
Wave Speed: v = λf
v is the wave speed (in m/s), λ is the wavelength (in meters), and f is frequency (in Hz). This equation shows the relationship between the speed, wavelength, and frequency of sound.
Wavelength: λ = v/f
λ is the wavelength (in meters), v is the speed of sound (in m/s), and f is frequency (in Hz). It is used to calculate the wavelength when speed and frequency are known.
Time Period: T = 1/f
T is the time period (in seconds) and f is frequency (in Hz). This formula calculates the time for one complete wave cycle.
Loudness: L ∝ A²
L is loudness, and A is amplitude. The loudness is proportional to the square of the amplitude, indicating that a small increase in amplitude greatly increases the perceived loudness.
Intensity: I = P/A
I is intensity (in W/m²), P is power (in watts), and A is area (in square meters). This formula defines the sound intensity as power per unit area.
Echo Distance: d = vt/2
d is distance (in meters), v is the speed of sound (in m/s), and t is time taken (in seconds) for an echo. This calculates the distance to a reflecting surface.
Reverberation Time: RT = 0.1 * V/A
RT is the reverberation time (in seconds), V is the volume of the hall (in m³), and A is the total absorption (in m²). This formula estimates how long sound persists in a space.
Infrasonic Sound: f < 20 Hz
Sounds with frequencies below 20 Hz are classified as infrasonic sound, which humans cannot hear but some animals can.
Ultrasonic Sound: f > 20 kHz
Sounds with frequencies above 20 kHz are classified as ultrasonic sound, which humans cannot hear but are used in various technologies.
Equations
Speed of Sound in Air: v = 344 + 0.6T
This equation gives the speed of sound (in m/s) in air as a function of temperature (T in °C). It shows how temperature affects sound speed.
Sound Intensity Level: L = 10 log(I/I₀)
L is the sound level (in dB), I is the intensity of sound (in W/m²), and I₀ is the reference intensity (1 × 10⁻¹² W/m²). This formula is used for calculating sound intensity in decibels.
Reflection of Sound: θi = θr
θi is the angle of incidence, and θr is the angle of reflection. This law shows that the angles of incidence and reflection are equal when sound reflects off a surface.
Frequency-Wavelength Relation: f = v/λ
This equation relates frequency (f in Hz), speed of sound (v in m/s), and wavelength (λ in meters). It is essential for understanding wave behavior.
Energy of a Wave: E = hf
E is the energy (in joules) of a wave, h is Planck's constant (6.626 × 10⁻³⁴ Js), and f is frequency (in Hz). This applies to sound waves in context of quantum mechanics.
