Speed of a sound — lesson. Science State Board, Class 9.

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The distance travelled by a sound wave per unit time as it propagates through an elastic medium is known as the speed of sound.

$Speed (v) = \frac{Distance}{Time}$

If one wavelength (

$λ$ ) represents the distance travelled by one wave, and one time period (

$T$ ) represents the time taken for this propagation, then

$Speed (v) = \frac{One wavelength (λ)}{One time period (T)}$

And, we know

$T = \frac{1}{v}$

By applying this in speed formula, we get

$v = n λ$

Under the same physical conditions, the speed of sound in a given medium remains nearly constant for all frequencies.

Let us solve following example for better understanding.

Example:

A sound wave has a frequency of

$2$

$kHz$ and a wavelength of

$15$

$cm$ . How much time will it take to travel

$1.5$

$km$ ?

Given data:

Frequency

$=$

$2$

$kHz$

$=$

$2000$

$Hz$

Wavelength

$=$

$15$

$cm$

$=$

$0.15$

$m$

Distance

$=$

$1.5$

$km$

$=$

$1500$

$m$

To find: Time period

Formula:

$Time = \frac{Distance}{Speed}$

We don't know the value of speed,

$v = n λ$

$\begin{array}{l} v = 2000 \times 0.15 \\ = 300 m / s \end{array}$

Now, apply in the value of speed in time formula

$\begin{array}{l} Time = \frac{1500}{300} \\ = 5 s \end{array}$

The sound will take

$5$

$s$ to travel a distance of

$1.5$

$km$ .

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6. Speed of a sound

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