In acoustics, the decibel (dB) is a commonly used unit for measuring which of the following physical quantities?

Introduction / Context:
The decibel, written as dB, is widely seen on audio equipment, mobile phones and noise-level meters. It is a logarithmic unit that expresses relative levels of power or intensity, especially for sound. This question checks whether you can correctly associate the decibel with the physical quantity it usually measures in general science and everyday applications.

Given Data / Assumptions:

• The unit under discussion is the decibel (dB).
• Options link it to the speed of light, intensity of sound, intensity of heat or none of these.
• We consider standard school physics usage where decibels are encountered most often in connection with sound.

Concept / Approach:
Sound intensity is a measure of the power of sound per unit area. Because the human ear perceives sound across a very wide range of intensities, it is convenient to use a logarithmic scale. The decibel scale expresses sound intensity level relative to a reference intensity. Although decibels can be used in other contexts (for example, in electronics to compare power levels), their primary and most familiar use in basic physics and general knowledge is to quantify sound levels. Therefore, we need to match decibel with the intensity of sound.

Step-by-Step Solution:
Step 1: Recall that the loudness of sound is often reported as a number in decibels, such as 30 dB, 60 dB or 100 dB. Step 2: Understand that this number represents the sound intensity level compared to a reference intensity, using a logarithmic formula. Step 3: Compare the options: speed of light is measured in metres per second, and heat intensity is not commonly described in decibels in basic physics. Step 4: Recognise that everyday examples, such as noise pollution limits or safe listening levels with headphones, always use decibels for sound. Step 5: Conclude that decibel is a unit connected with the intensity of sound.

Verification / Alternative check:
Physics textbooks define sound intensity level L in decibels using a formula of the form L = 10 * log10(I / I0), where I is the sound intensity and I0 is a reference intensity. All standard tables of physical quantities list decibel under sound intensity level or audio power levels. The speed of light uses units such as metres per second, while thermal energy and heat flow are measured in joules or watts rather than decibels. This confirms that the most appropriate match in the context of this question is “intensity of sound.”

Why Other Options Are Wrong:
Speed of light: This is a velocity, measured in units such as m/s, not in decibels.
Intensity of heat: Heat or thermal energy is described in joules or watts, and although power ratios can be expressed in decibels in advanced topics, this is not the standard school-level association.
None of these: This is incorrect because decibel is definitely used in connection with sound intensity.

Common Pitfalls:
Some learners confuse decibel with other common units like hertz (Hz), which measures frequency, or they mistakenly think decibel measures loudness directly instead of intensity level. Remember that decibel is a relative logarithmic unit, and in basic physics and GK, it is almost always linked to sound intensity levels. Keeping examples like “noise in a library is about 40 dB” in mind helps anchor this association.

Final Answer:
The decibel (dB) is commonly used as a unit of intensity of sound.