Sonic boom is produced when a source of sound travels at a speed:
greater than the speed of sound.
Understanding Sonic Booms and Sound Speed
A sonic boom is a powerful shock wave created when an object moves through the air faster than the speed of sound. Let's break down the conditions under which a sonic boom is produced.
What is the Speed of Sound?
The speed of sound is the distance traveled per unit time by a sound wave as it propagates through an elastic medium. This speed depends on the properties of the medium, such as its temperature, density, and composition. In dry air at 20 degrees Celsius (68 degrees Fahrenheit), the speed of sound is approximately 343 meters per second (about 767 miles per hour).
How Sonic Booms are Produced
When an object moves through the air, it creates pressure waves. If the object is moving slower than the speed of sound (subsonic speed), these pressure waves travel away from the object at the speed of sound, spreading out in all directions. The air ahead of the object has time to react to the approaching object.
However, when an object moves at or above the speed of sound, something different happens. The object is traveling faster than the pressure waves it creates. These waves pile up, or compress, in front of and behind the object. This compression forms a shock wave.
- If the object travels exactly at the speed of sound (transonic speed), the pressure waves accumulate just in front of it, causing a significant increase in drag (often called the "sound barrier").
- If the object travels greater than the speed of sound (supersonic speed), it continuously outruns the waves it creates. The pressure waves trail behind the object in a cone shape, similar to the wake of a boat moving through water. This cone of compressed air is a shock wave.
When this shock wave passes over an observer on the ground, the sudden change in pressure is heard as a loud "boom" or double "boom". This is the sonic boom.
Analyzing the Options
Let's consider the given options in the context of producing a sonic boom:
- greater than the speed of sound: As explained above, this is the condition where the object outruns the pressure waves it creates, leading to the formation of shock waves and a sonic boom.
- greater than the speed of light: The speed of light is vastly faster than the speed of sound. While an object moving at such speeds would certainly be supersonic, the condition for a sonic boom is specifically related to exceeding the speed of sound in the medium, not the speed of light. Reaching the speed of light is physically impossible for any object with mass according to current understanding of physics.
- lesser than the speed of sound: When an object moves slower than sound, the pressure waves propagate away freely, and no shock wave is formed. Therefore, no sonic boom is produced.
- equal to the speed of sound: While traveling exactly at the speed of sound causes significant air compression and resistance, the distinct shock cone characteristic of a sonic boom, which trails behind the object and causes the "boom" as it passes an observer, is primarily associated with supersonic speeds (greater than sound speed).
Therefore, a sonic boom is produced when a source of sound travels at a speed greater than the speed of sound.
| Speed Compared to Sound | Classification | Sonic Boom Produced? | Explanation |
|---|---|---|---|
| Less than Speed of Sound | Subsonic | No | Pressure waves spread out ahead. |
| Equal to Speed of Sound | Transonic | No (Significant compression, but not the characteristic trailing shock cone) | Pressure waves pile up just ahead. |
| Greater than Speed of Sound | Supersonic | Yes | Object outruns waves, forming a trailing shock cone. |
| Much Greater than Speed of Sound | Hypersonic | Yes | Forms strong shock waves. |
Revision Table: Sonic Boom Conditions
| Condition | Effect |
|---|---|
| Source speed < Speed of Sound | No sonic boom. Pressure waves disperse normally. |
| Source speed = Speed of Sound | High drag (sound barrier). Pressure waves accumulate ahead. |
| Source speed > Speed of Sound | Sonic boom produced. Shock waves form a cone trailing the source. |
Additional Information: Mach Number
The speed of an object relative to the speed of sound is often described using the Mach number, named after Austrian physicist Ernst Mach. The Mach number (M) is defined as the ratio of the speed of the object (v) to the speed of sound (c) in the surrounding medium:
\(M = \frac{v}{c}\)
- M < 1: Subsonic speed
- M = 1: Transonic speed (exactly the speed of sound)
- M > 1: Supersonic speed (greater than the speed of sound)
- M > 5: Hypersonic speed (significantly greater than the speed of sound)
A sonic boom occurs when the Mach number is greater than 1 (M > 1).
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