Sound is produced by mechanical waves in a material that vibrates from 20 to 20,000 Hz. Sound requires a medium/material to propagate its motion. The wave disturbance in this frequency range is detected by the ear and translated into sound by the ear. Sound waves with frequencies below 20 Hz are termed infrasonic and those above 20,000 Hz are termed ultrasonic. Rhinoceroses use infrasonic waves of about 5 Hz to signal to each other and bats use ultrasonic frequencies of around 100,000 Hz to navigate. Sound is a compressional or longitudinal wave. All vibration is parallel to the direction of propagation of the wave. Sound spreads along a spherical front so that the intensity of sound is proportional to 1/d^². Intensity is determined by the amplitude of the wave and the power output of the source. Sound travels at 331 m/s in air at 0 degrees Celsius. For temperatures "T" above 0 degrees, the equation v = 331 + 0.6 T describes the speed of sound in air. The wave equation v = f l calculates the speed of sound as related to frequency and wavelength.

THE SPEED OF SOUND ON A STRING--

The properties of the medium will always determine the speed of the wave such as sound. Velocity is constant as long as the medium remains unchanged. The wave equation applies and means that if the frequency increases then the wavelength must decrease. A low frequency sound has a low pitch as heard by the ear and a high frequency has a high pitch. When sound travels through another medium such as a string (remember the toy telephones you made as a child), the velocity varies based on the equation shown below. Force (in newtons) is represented by F and m/L is mass per unit length of the string.

Sound waves are longitudinal.

A region of increased pressure is condensation; deceased pressure is rarefaction... a form of Simple Harmonic Motion.

Frequency is the number of cycles per second (Hertz).

A sound with a single frequency is called a pure tone. The ability to hear high frequencies decreases with age. A listener's perception of the frequency of sound is called pitch.

Pressure amplitude of a sound wave measures maximum change in pressure due to the wave disturbance.

Pressure fluctuations are usually very small. Our ear is remarkable in its ability to detect these small changes. Loudness depends on pressure amplitude. The larger the amplitude the louder the sound detected by the ear.

Sound travels through gases, liquids and solids at very different speeds.

Sound travels more than four times faster in water and more than seventeen times faster in steel than in air. In general, sound is slowest in gases, faster in liquids and fastest in solids .

The Intensity Level of Hearing (IL) compares the intensity of a given sound to the threshold of hearing.

The ear hears based on a logarithmic function. A comparison to the threshold of hearing ( I_o = 10 ^-12 W/m²).

Resonance occurs when a force can transmit large amounts of energy to a vibrating object. The frequency of the force must match the natural frequency of the object. The Tacoma Narrows Bridge is in resonance mode.

The Doppler Effect occurs because the source of a sound has a speed relative to the speed of sound... In other words, either the source of sound is moving or the receiver is moving or both are moving to create a frequency change!

The apparent shift of frequency for a moving source is caused by the spacing between wave fronts.