Noise Measurement and Indices Questions and Answers

Answer: C

The resultant level is 59 dB(A).

Decibels add logarithmically. For n equal sources each of level L:

\[ \begin{aligned} L_{total} &= L + 10\log_{10} n \\ &= 50 + 10\log_{10} 8 \\ &= 50 + 9.03 \approx 59\ \text{dB(A)} \end{aligned} \]

Each doubling of identical sources adds 3 dB: two sources give 53, four give 56 and eight give 59. So 53 and 56 dB(A) are the levels for two and four sources, and 60 dB(A) would need ten.

Answer: D

The combined level is 66 dB(A).

Decibels are logarithmic, so sound levels add as energies, not as numbers. For n equal sources each at level L:

\[ \begin{aligned} L_{total} &= L + 10\log_{10} n \\ &= 60 + 10\log_{10} 4 \\ &= 60 + 6.02 \approx 66\ \text{dB(A)} \end{aligned} \]

Each doubling of identical sources adds 3 dB: two machines give 63 dB(A) and four give 66 dB(A). 120 dB(A) is what adding the decibels arithmetically gives for just two machines, and 90 dB(A) also treats the decibel scale as linear; both overstate the increase. 63 dB(A) is the answer for only two machines.

Answer: A

The correct statement is that measurement of sound is a function of electroacoustic method.

A sound level meter works electroacoustically: a microphone converts the pressure fluctuations of sound into an electrical signal, which is amplified, passed through frequency-weighting networks (A, C) and time-weighting circuits, and displayed in decibels. Noise dosimeters and analysers work the same way.

Electrochemical methods measure chemical species, such as dissolved oxygen or gases with electrochemical sensors. 'Biophysical' and 'electrophysical' are not the recognised names of the sound measurement principle.

Answer: C

Two equal sounds together are 3 dB(A) louder.

Decibels are logarithmic, so sound levels add through their intensities, not directly. Doubling the intensity raises the level by:

\[ \begin{aligned} \Delta L &= 10 \log_{10} \frac{2I}{I} \\ &= 10 \log_{10} 2 \approx 3\ \text{dB} \end{aligned} \]

So 60 dB plus 60 dB gives about 63 dB, not 120 dB. 6 dB is the rise for doubling the sound pressure (or for four equal sources), and 2 and 5 dB match no doubling rule.

Answer: C

Vibration sensors work on the piezoelectric effect.

Most vibration sensors are piezoelectric accelerometers. A piezoelectric crystal or ceramic, such as quartz or lead zirconate titanate, produces an electric charge proportional to the force on it, so as the sensor vibrates the mass inside squeezes the crystal and the output voltage follows the acceleration.

Magnetic induction is used in some velocity pickups but is not the standard principle. The photoelectric effect is the basis of light sensors and the thermoelectric effect of thermocouples.