Understanding One-Word Substitution for Speed

The question asks for a single word that describes something moving at a speed greater than the speed of sound. This is a common type of vocabulary question designed to test your knowledge of specific terms.

Analyzing the Options for Speed Definitions

Let's look at each option provided and determine its meaning:

• cacophonic: This word relates to sound, but not speed. It means a harsh, jarring mixture of sounds. For example, traffic noise can be cacophonic.
• supersonic: This word is specifically used to describe speed. It means travelling at a speed greater than the speed of sound. Aircraft that fly faster than sound are called supersonic jets.
• harmonic: This term is related to music and sound waves. It refers to overtones or frequencies that are integer multiples of the fundamental frequency. It does not describe speed.
• subsonic: This word is also used to describe speed relative to the speed of sound, but it means travelling at a speed less than the speed of sound. Most commercial airplanes fly at subsonic speeds.

Evaluating the Correct Term for Speed Greater Than Sound

Based on the analysis of the options:

• 'Cacophonic' describes unpleasant sound quality.
• 'Harmonic' describes a relationship between sound frequencies.
• 'Subsonic' describes speed slower than sound.
• 'Supersonic' describes speed faster than sound.

Therefore, the word that means "speed greater than that of sound" is supersonic.

Revision Table: Understanding Speed Terms

Additional Information: Speed Regimes

Speed relative to the speed of sound is often classified into different regimes. The speed of sound itself varies depending on the medium (like air, water, or solids) and conditions like temperature.

• Subsonic: Speed is less than the speed of sound (Mach < 1).
• Transonic: Speed is around the speed of sound (Mach ≈ 1), where airflow can be both subsonic and supersonic in different places around an object.
• Supersonic: Speed is greater than the speed of sound (Mach > 1).
• Hypersonic: Speed is significantly greater than the speed of sound, typically five times the speed of sound or more (Mach > 5).

The Mach number ($\text{M}$) is the ratio of the speed of an object through a medium and the speed of sound in that medium:

\begin{equation*} \text{M} = \frac{\text{Speed of Object}}{\text{Speed of Sound}} \end{equation*}

So, supersonic speed means M > 1.