Noise Pollution and Control Questions and Answers

Answer: B

The speed of sound in air at 0°C is about 331 m/s.

The speed of sound in an ideal gas is

\[ v = \sqrt{\frac{\gamma R T}{M}} \]

With \(\gamma = 1.4\), \(R = 8.314\) J/mol K, \(T = 273\) K and \(M = 0.029\) kg/mol for air, this gives about 331 m/s. It rises by roughly 0.6 m/s for each degree, to about 343 m/s at 20°C.

The other figures belong to denser media: around 1,435 to 1,500 m/s is the speed of sound in water, and about 5,100 m/s is typical of iron or steel. 1,269 m/s is far too fast for air.

Answer: B

Simple harmonic motion is vibration with a single frequency.

In SHM the restoring force is proportional to the displacement, \(F = -kx\), and the motion is a pure sine curve, \(x = A\sin(\omega t + \phi)\), with one angular frequency \(\omega = \sqrt{k/m}\). It is the most elementary vibration, and any complex periodic vibration can be built from a sum of such single-frequency components (Fourier analysis).

A vibration with two ('Duel') or multiple frequencies is a superposition of several SHMs, not SHM itself, and SHM is continuous and regular, not intermittent.

Answer: A

Normal human conversation lies mainly between 500 and 2000 Hertz.

The frequencies that carry most of the information in speech, the speech frequencies, are about 500 to 2000 Hz, which is why hearing tests and noise-induced hearing loss assessments focus on 500, 1000 and 2000 Hz. Human hearing as a whole spans roughly 20 to 20,000 Hz.

The wider bands of 10 to 10,000, 100 to 10,000 and 50 to 5,000 Hz include frequencies far outside the range in which ordinary conversation is concentrated.

Answer: B

Sound travels fastest in solid media.

The speed of sound depends on the stiffness (elastic modulus) of the medium relative to its density. Solids are far stiffer than liquids or gases, so sound moves fastest in them: about 5,000 m/s or more in steel, about 1,500 m/s in water and about 330 to 340 m/s in air.

Liquids come second, and gases and vapours, which are easily compressed, carry sound the slowest.

Answer: B

The speed of sound in air at 0 °C is 331 m/s.

In dry air at 0 °C sound travels at about 331 m/s, increasing by about 0.6 m/s for each degree rise in temperature, so it is about 343 m/s at 20 °C.

The other values belong to other media: about 1,270 m/s is the speed in hydrogen gas, about 1,400 to 1,500 m/s in water, and around 5,000 m/s or more in metals such as iron.

Answer: C

The wavelength is the distance travelled by sound per cycle.

In one complete cycle of vibration a sound wave advances by one wavelength, the distance between two successive compressions. Speed, frequency and wavelength are linked by

\[ \lambda = \frac{v}{f} \]

so in air (\(v \approx 343\) m/s) a 343 Hz tone has a wavelength of 1 m.

The number of cycles per second is the frequency, and the distance travelled per second is the speed of sound. The distance covered in two cycles is two wavelengths.