Does Sound Frequency Change With Medium: Why Not

Speed of sound in air is 330 m/s, while that in water is 1480 m/s under typical conditions. But does sound frequency change with medium? Let’s discuss in detail.

Frequency of sound remains the same but wavelength and velocity changes in different media. Frequency is dependent only on the source of the sound wave, whereas velocity and wavelength are the parameters that vary upon travelling through different media as they depend upon the medium in which the wave is travelling.

Why does frequency not change in different mediums?

Sound waves comes under the category of longitudinal waves and require a material medium for propagation.

Number of waves progressing through a fixed point of the medium in a second or the number of vibrations of a particle in the medium as the wave travels is its frequency. As a particle in the medium vibrates with a particular frequency, its neighbouring particle also starts vibrating with the same frequency. Frequency is independent of the medium whereas velocity of sound waves depends on elasticity and density of the medium.

v=√E/ρ

Frequency of the sound wave changes only when there is a change in the source of the wave and is independent of the rigidity of the material medium.

That is why; the velocity of sound is different in solids and liquids.

We know, v = fλ

Where, v is the velocity, f is the frequency, λ is the wavelength of the sound wave respectively.

Velocity and wavelength of sound waves change in such a way so as to keep frequency constant. Hence sound waves moving with high velocity would have large wavelengths so that frequency does not change.

Few instances/examples on Does sound frequency change with medium

Our brains perceive sound wave frequency as pitch. So change in pitch of a sound is equivalent to change in its frequency. Here are a few examples analyzing whether sound frequency change with medium:

The frequency of sound from any musical instrument
The chirping of birds
The sound produced by ringing of bells

The frequency of sound from any musical instrument

The frequency of sound produced from a musical instrument does not change with medium as it depends only upon its source.

For example, the frequency of sound produced from a guitar does not change as it propagates through different media. Its frequency changes only when the musician changes the notes. The strings of the guitar are the source of frequency of musical sound produced from the guitar. The change in frequency is perceived by the listeners as the change in pitch.

The chirping of birds

The source of the frequency of sound produced by the chirping of birds is their vocal chord. Whenever the vibrations of the vocal chords change, the frequency changes but it does not change with medium.

The sound produced by the ringing of bells

To explain more crudely, assume that a bell is made to vibrate 4 times per second. This implies that the frequency is 4 Hz. The surrounding air molecules start vibrating with the same frequency and is transferred to its neighbouring molecules. No other agent can change the frequency of vibration even when the medium is changed.

Frequently Asked Questions: Does Sound Frequency Change With Medium

Is the sound wave transverse or longitudinal? Explain each one of them.

Sound wave is an example for a longitudinal wave.

A longitudinal wave can be described as the kind of wave where vibration of the particles in the medium is along the direction of propagation of the wave. On the contrary, a wave where the vibration of particles in a medium is perpendicular to the direction of propagation of the wave is defined as a transverse wave.

15278413843 492b15d5e6 w 2 — Londitudinal and Transverse Wave
Image Credits: Flickr

Light wave, an electromagnetic radiation, is an example for a transverse wave where the vibrations of electric field, vibrations of magnetic field and the propagation direction are mutually perpendicular to each other.

What is Doppler Effect in sounds?

Doppler Effect in sound waves is the apparent change in frequency of sound as perceived by the observer when either the source or the observer or both approaches or moves away from each other.

A simple demonstration for Doppler effect is when an object producing sound moves towards us, the frequency of sound increases or we hear sound with high pitch. And when the source moves away from us, the frequency of sound decreases or we hear low pitch sound.

The most commonly observed example that demonstrates this effect is the change in pitch of the sound heard from an ambulance when it moves faster with its siren. When the ambulance approaches us, we hear high pitch sound and when it moves away, low pitch sound is heard.

Dopplerlkm effect — Doppler Effect in sound waves
Image Credits: Wikimedia Commons

The formula for finding the observed frequency of sound in Doppler effect is given by

f’=f (v’ v₀)/(v’ v_s)

Where f’ is the observed frequency

f is the actual frequency

v is velocity of sound waves

v₀ is the velocity of the observer

v_s is the velocity of the source

Depending on whether the source and observer approaches or moves away, and the velocity of the source and the observer, the above formula will slightly vary in different situations. There are four different cases:

When source approaches the observer who is at rest

Since the observer is at rest, v₀ = 0. Hence the equation becomes

f’=f (v/v-v_s)

When the source recedes from an observer who is at rest

Here, v₀=0 and the direction of velocity of source is opposite, hence negative.

So, f’=f v/v-(-v_s)

When observer approaches a stationary source

Here v_s= 0. The equation becomes

f’=f{v+v₀/v}

When observer recedes from a stationary source

Here, since the observer is moving away, direction is opposite

f’=f {v-v₀/v}

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Deeksha Dinesh

Hello, I am Deeksha Dinesh, currently pursuing post-graduation in Physics with a specialization in the field of Astrophysics. I like to deliver concepts in a simpler way for the readers.