A source of sound of frequency 600 Hz is placed inside water. The speed in water is 1500 m/s and in air it is 300 m/s. The frequency of sound recorded by an observer who is standing in air is? (A) 200 Hz (B) 600 Hz (C) 120 Hz (D) 3000 Hz
Hint: We will use the property of sound waves to determine the answer. We will state that the frequency of the sound waves depends on the source of the sound not on the medium they travel through.
Complete step by step solution: We are given a source of sound with frequency 600Hz. It is placed in water and the observer is in the air. Let us have a look at some definitions and properties of sound waves and its frequency. We know that the frequency of waves – like patterns such as sound, electromagnetic waves or electrical signals is the measure of the number of cycles of the repetitive waveform per second. Also, the frequency of a wave is a fundamental characteristic, thus it remains constant throughout the propagation regardless of the medium through which it travels. Here, no matter if the sound slows down while changing the medium from water to the air, the frequency will remain the same because its wavelength will vary. When a wave travels from one medium to the other, the frequency never changes. As the sound wave travels into a rarer medium, there wavelength is the only factor that changes not the frequency. So, here as well the frequency will remain constant.
Hence option(B) is correct i.e., the frequency of the sound recorded by the observer will be 600Hz.
Note: Such questions are simple though tricky. You may get confused by the data provided in the question but this is merely based upon the definitions and properties of sound wave and frequency. The number of vibrations counted per second is called frequency and it also determines the pitch of the sound. Or in other simpler words, we can say that the frequency is the measurement of the number of times a repeated event occurs per unit time.