NEET Sound Wave and its Characteristics MCQs & PYQs

The speed of longitudinal waves is given by \(v = \sqrt{\frac{B}{\rho}}\), where \(B\) is the bulk modulus. Solids and liquids have significantly higher bulk moduli compared to gases. Thus, longitudinal waves travel faster in solids and liquids. Both assertion and reason are true, and the reason correctly explains the assertion.

The velocity of sound increases with humidity because humid air is less dense than dry air. So, Assertion (A) is false. The velocity of sound absolutely depends on the properties of the medium (density, elasticity). So, Reason (R) is also false. Both (A) and (R) are false.

The speed of sound in a gas is \(v = \sqrt{\frac{\gamma RT}{M}}\). At constant temperature (\(T\)), the velocity is independent of pressure. So, Assertion (A) is false. Also, the speed of sound is not directly proportional to pressure. So, Reason (R) is false. Both (A) and (R) are false.

According to the Doppler effect, the observed frequency matches the emitted frequency only when there is no relative motion between the source and observer. So, Assertion (A) is true. The velocity of sound is a property of the medium and is non-zero in an ideal medium, irrespective of relative motion between source and observer. So, Reason (R) is false. Thus, (A) is true but (R) is false.

Assertion (A) is false. An 80  dB sound has an intensity \(10^4\) times greater than a \(40 \text{ dB}\) sound, not twice. Reason (R) is true as it correctly defines loudness in decibels.

Since (A) is false and (R) is true, and the option for 'A is false, R is true' is not provided, option (4) is selected as it states (A) is false.

Assertion (A) is false; sound travels much faster in water (approx. \(1480 \text{ m/s}\)) than in air (approx. \(343 \text{ m/s}\)). Reason (R) is true; air is indeed a rarer (less dense) medium than water. Given that Assertion (A) is false, option 4 is the only choice that fits this condition among the provided options, despite Reason (R) being true.

Assertion (A) is true because sound waves are mechanical and require a medium, whereas light waves are electromagnetic and can propagate in vacuum.

Reason (R) is true as sound waves are longitudinal and cannot be polarised, while light waves are transverse and can be polarised. However, the ability to polarise is unrelated to propagation through a vacuum.

Thus, (R) is not the correct explanation of (A).

The beat frequency is \(|240 - 300| = 60 text{ Hz}\) . For distinct beats to be heard, the beat frequency should be less than \(10 text{ Hz}\) . Thus, Assertion (A) is true. Persistence of hearing is not the reason for inability to hear distinct beats; it's the ear's inability to follow rapid amplitude changes. Thus, Reason (R) is false.

Ultrasound, a type of sound wave, is extensively used in medical imaging to view soft tissues, making Assertion (A) true. All real-world oscillations experience damping due to dissipative forces, so Reason (R) is true. However, (R) does not explain (A).

The beat frequency is \(|256 - 324| = 68 \text{ Hz}\) . Since this is much greater than \(10 \text{ Hz}\) , distinct beats cannot be heard. Thus, (A) is true. The principle of superposition holds for all sound waves irrespective of their frequency, so (R) is true. However, (R) does not explain (A).