# What is the frequency heard by a person driving at 15 m/s toward a blowing factory whistle (800. Hz) if the speed of sound is 340.6 m/s?

##### 1 Answer

Jun 8, 2018

**840 Hz**

(P.S. this is rounded to 2 s.f)

#### Explanation:

We use the formula

Here, '**observed frequency**, 'v' means **speed of sound in air**, '**frequency of the source** (blowing factory whistle) and '**speed of the person** moving **towards** the whistle.

Since the person is moving towards the whistle the velocities are **subtracted**.

The observed frequency is higher than the frequency of the whistle because the person is moving **closer**, so the observed wavelength is **smaller**. Wavelength is **inversely proportional** to frequency based on this formula (**v=λf**). So frequency should be **higher**.

By substituting the values, you should be able to find the answer.

**836.9 Hz**