10.1 - Sound is Produced by a Vibrating Body

Sound is produced when an object or body vibrates. Vibrations are rapid back-and-forth motions of an object, creating disturbances in the surrounding medium, typically air, that we perceive as sound. The type of vibration, its frequency, and amplitude determine the characteristics of the sound, such as pitch and loudness.

When an object vibrates, it pushes and pulls on the particles in the surrounding medium (usually air). These particles move back and forth, creating compressions and rarefactions. The continuous movement of these compressions and rarefactions forms a sound wave. The sound wave travels away from the vibrating body, carrying energy through the medium.

The process can be described with the following key components:

• Vibration: The initial disturbance or motion that generates the sound wave. For example, a guitar string or a tuning fork vibrates when struck.
• Medium: The substance through which sound travels. Sound can travel through solids, liquids, and gases. In air, sound travels as a longitudinal wave.
• Compression: A region of the sound wave where the particles of the medium are closer together due to the vibration of the object.
• Rarefaction: A region where the particles are spread farther apart, creating a lower pressure zone in the medium.

The frequency of vibration determines the pitch of the sound. A higher frequency results in a higher pitch, while a lower frequency produces a lower pitch. The amplitude of vibration influences the loudness of the sound. A larger amplitude corresponds to a louder sound, while a smaller amplitude results in a softer sound.

The speed at which sound travels depends on the medium through which it is moving. In air, sound travels at a speed of approximately 343 meters per second (at 20°C). Sound travels faster in denser media like water and steel compared to air. The formula for the speed of sound in a medium is given by:

$v = \sqrt{\frac{B}{\rho}}$

Where:

• v: Speed of sound
• B: Bulk modulus of the medium
• ρ: Density of the medium

Sound requires a medium to travel, so it cannot travel through a vacuum. For instance, in space, where there is no air or other medium, sound cannot propagate. This is why astronauts cannot hear each other in space.

In musical instruments, the sound is produced by the vibration of a string, air column, or membrane. For example, in a musical instrument like a drum, when the drumhead vibrates, it sets the surrounding air molecules into motion, creating sound waves that we hear. Similarly, in wind instruments like a flute or trumpet, the vibration of an air column produces sound.

The ultimate cause of sound production in all cases is the initial vibration of the object, whether it is a string, membrane, or air column. This vibration creates the necessary disturbance in the surrounding medium, which then propagates outward as a sound wave.