What is the SI unit of intensity of sound in acoustics?

Introduction / Context:
Sound intensity is an important concept in physics and acoustics because it tells us how much sound power passes through a unit area. Many exam questions test the difference between sound intensity, sound pressure and sound level in decibels. Knowing the correct SI unit of sound intensity helps you distinguish these related but different quantities and prevents confusion between power based and logarithmic measures.

Given Data / Assumptions:
• The question asks specifically about the SI unit of intensity of sound, not about loudness or sound level.
• We are dealing with basic physics definitions according to the International System of Units.
• No numerical values of intensity are given or required, so this is a recall based conceptual question.

Concept / Approach:
Intensity of any wave, including sound, is defined as power transmitted per unit area. In symbols, intensity I is given by I = P / A, where P is power and A is area perpendicular to the direction of propagation. In the SI system, power is measured in watts and area in square metres. Therefore the SI unit of intensity is watt per square metre, written as W/m^2. The decibel is used to express sound level on a logarithmic scale, not the raw intensity in SI units, so it should not be selected when the question explicitly mentions SI unit.

Step-by-Step Solution:
Step 1: Recall the definition of intensity I as power per unit area, I = P / A. Step 2: Identify the SI unit of power P, which is the watt. Step 3: Identify the SI unit of area A, which is the square metre. Step 4: Combine these to get the unit of intensity as watt per square metre. Step 5: Among the options, only watt per square metre (W/m^2) matches this unit, so that option is correct.

Verification / Alternative check:
You can check your answer by looking at standard formulas for sound intensity. For a plane progressive wave, intensity I is related to the root mean square pressure and acoustic impedance, but whenever power per area is computed, the units always simplify to watt per square metre. Additionally, physics and engineering handbooks list W/m^2 as the standard SI unit of intensity for electromagnetic and acoustic waves. This consistency across different wave types confirms that the same unit applies here.

Why Other Options Are Wrong:
Joule per square metre: A joule is a unit of energy, so joule per square metre would represent energy per area, not power per area.
Newton per square metre: Newton per square metre is the pascal, which is the unit of pressure, not intensity.
Decibel: Decibel is a dimensionless logarithmic unit used to express ratios such as sound level, not a base SI unit for intensity.
Tesla per square metre: Tesla is the unit of magnetic flux density, so tesla per square metre is unrelated to acoustic intensity.

Common Pitfalls:
Students often mix up intensity and loudness, and because loudness is commonly quoted in decibels, they select decibel as the unit. Remember that decibel scales compare a measured intensity to a reference intensity using logarithms, but the underlying physical intensity still has units of watt per square metre. Another confusion arises between pressure and intensity; sound pressure has units of pascal, whereas intensity has units of W/m^2. Keeping track of the quantity definition and its formula is the best way to avoid such errors.

Final Answer:
The SI unit of intensity of sound is watt per square metre, written as W/m^2.