Which of the following scientific terms is not normally associated with the description of sound waves?

Introduction / Context:
Different branches of physics use different units and terms to quantify physical quantities. Sound waves involve quantities like frequency, intensity, and speed, while optics deals with light intensity and brightness. This question checks whether you can identify which unit is not typically linked with sound, reinforcing the correct associations between physical quantities and their units.

Given Data / Assumptions:

• We are dealing with sound waves, which are mechanical waves that propagate through a medium.
• Terms like hertz and decibel are often mentioned when discussing sound.
• Candela is a unit that students encounter in the context of light and brightness.
• Mach number appears in discussions of speeds relative to the speed of sound.
• Amplitude is a general wave property and can apply to sound, water, or light waves.

Concept / Approach:
Sound waves are longitudinal mechanical waves, so their description involves frequency, wavelength, amplitude, speed, and sound level. Frequency is measured in hertz, and sound level in decibels. Mach number compares a speed to the speed of sound and is widely used in aerodynamics. Candela, on the other hand, is the SI unit of luminous intensity and belongs to optics, not acoustics. Thus, candela is the term not associated with sound waves in standard physics usage.

Step-by-Step Solution:
Step 1: Recall that frequency of any periodic wave, including sound, is measured in hertz.Step 2: Remember that sound intensity level, particularly in hearing studies and acoustics, is measured in decibels.Step 3: Recognise that Mach number is a dimensionless quantity describing how fast an object moves relative to the speed of sound in the surrounding medium.Step 4: Identify that candela is defined as the SI unit of luminous intensity, which quantifies the perceived power of light, making it unrelated to sound wave description.

Verification / Alternative check:
Textbooks on acoustics consistently mention hertz and decibel when discussing pitch and loudness. Aircraft design and meteorology texts discuss Mach number in relation to sonic and supersonic speeds but still in the context of sound speed. Conversely, optics and photometry references define candela when describing brightness of lamps, lasers, and other light sources. Since candela appears in the SI base units list as a separate optical quantity, it clearly does not describe any property of sound waves.

Why Other Options Are Wrong:
Option a is correct for sound because frequency in hertz is a fundamental property of sound waves and determines perceived pitch. Option b is also associated with sound, as decibel is used for sound level and noise measurements. Option d is linked to sound through the definition of Mach number as a ratio to the speed of sound in a medium. Option e is a general wave property and is absolutely relevant for sound waves, since amplitude relates to sound intensity and loudness. Only candela is clearly outside the acoustics domain.

Common Pitfalls:
Students may see Mach associated with high speed aircraft and think it belongs exclusively to mechanics, forgetting that its definition uses the speed of sound. Others may think amplitude belongs only to transverse waves, but amplitude simply measures maximum displacement and is valid for longitudinal waves as well. Confusion can also arise because all the terms sound technical; carefully recalling the physical quantity each unit measures helps avoid such mistakes. The key is to remember that candela refers to light, not sound.

Final Answer:
Candela, the unit used to measure luminous intensity of light.