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 * [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/a/ac/Musical_notes.svg/400px-Musical_notes.svg.png width="122" height="121"]]Chapter 26 - Sound[[image:http://upload.wikimedia.org/wikipedia/commons/thumb/a/ac/Musical_notes.svg/400px-Musical_notes.svg.png width="122" height="121"]]**


 * 26.1 - The Origin of Sound**

All sounds are produced by the vibration of some object. In a musical instrument, that is usually a vibrating string, reed, or a moving column of air in part of the instrument. The production of sound is not limited to musical instruments, any vibrating object will cause similar vibrations in the air. If the vibrations are between 20 Hz and 20,000 Hz then they are in the range that can be heard by a person with normal hearing. Any waves below that range (less than 20 Hz) are called infrasonic, and waves above that range (greater than 20,000 Hz) are called ultrasonic. Humans cannor hear infrasonic or ultrasonic waves. The frequency that we perceive for those waves we do hear is called the pitch of the wave.


 * 26.2 - Sound in Air**

The vibrating object that is the source of a sound will create a longitudinal wave composed of successive compressions (higher-pressure areas, drawn as the crest of the wave) and rarefactions (lower-pressure areas, drawn as the troughs) that will travel outward from the source.
 * 26.3 - Media That Transmit Sound**

We are used to sound traveling through the air but it will also travel through many other materials. In fact, sound will travel better and faster through solid objects and liquids than through the air.

Like any other mechanical wave, sound cannot travel in absence of a medium. A vacuum will prevent sound from passing since it contains nothing to vibrate.


 * 26.4 - Speed of sound**

Light travels much faster than sound. This is why you see a distant event before you hear the it (lightning before thunder).

Light travels fast enough that for most purposes its speed can be ignored and it can be assumed to travel instantly from one place to another.

The exact speed that sound waves travel in air varies with the temperature, barometric pressure, and humidity. About 340 meters per second at room temperature is typical.

Sound waves travel faster in warmer air and slower in cooler air.

In a more elastic material such as a solid or a liquid, the atoms are packed more closely together and have a stronger force restoring them to their original position once they are disturbed. This causes sound to travel much faster through those materials.

In water, a speed of 1,500 meters per second is typical. In steel, it is close to 5,100 meters per second.

The exact values will depend on the alloy, chemical impurities, and temperature of the material.
 * 26.5 - Loudness**

The intensity of a sound wave is proportional to the square of its amplitude. The loudness of a sound will increase with the intensity, but it will not be a proportional relationship. Instead, it will increase with the logarithm of the intensity. Each time the loudness seems to double, the intensity will have increased by a factor of ten.

The units used to measure intensity are decibels (dB). An intensity of 0 dB corresponds to the threshold of hearing, or the quietest sound that a person can typically hear. Every increase of 10 dB will cause the intensity to increase by a factor of 10. Increasing by 20 dB will increase the intensity by a factor of 100.

At the same time, since the loudness that you will perceive doubles each time the intensity increases by a factor of 10 the loudness will double with each 10 dB increase, a 20 dB increase will cause the sound to be four times as loud.
 * 26.6 - Forced Vibration**

If a small object is vibrating, than a soft sound will be heard. If the same object is played against a larger flat surface, then the larger surface will act as a sounding board and will cause more air to vibrate so a louder sound will result. This phenomenon is known as forced vibration.


 * 26.7 - Natural Frequency**

When an object that is made of an elastic material is struck, it will vibrate at a frequency (or a set of frequencies) that remains constant for that object. This is the object's natural frequency.

Consider a bell. If a bell is rung, it will always sound the same. A different bell may have a different sound, but it will also always be the same for that bell.
 * 26.8 - Resonance**

If the frequency of forced vibration matches an object's natural frequency, then the resulting sound will increase in intensity and loudness. This is called resonance.
 * 26.9 - Interference**

Since sound waves share the properties of any other wave, they also show the effects of interference when two or more waves reach the same location. If they arrive in phase, the result will be constructive interference, meaning that the resulting wave will be larger than either of the waves that arrived.

If the waves arrive out of phase, then the destructive interference that occurs will cause the resulting wave to be smaller than either of the waves that are interfering, maybe even completely cancelled out.


 * 26.10 - Beats**

If two sources emit sound waves that are close to one another in frequency but not exactly the same, then a beat will be observed. When the waves are added together, the resulting sound oscillates between loud and soft. The number of beats heard per second is equal to the absolute value of the difference between the frequencies of the two sounds.


 * Harmonics and Standing Waves**

Strings and air-filled pipes exhibit standing waves at certain frequencies. The lowest frequency able to form a standing wave is called the fundamental frequency, and those above it are harmonics of that frequency.

The frequency of each harmonic frequency is equal to the fundamental frequency multiplied by the harmonic number. This forms a harmonic series. The fundamental is also referred to as the first harmonic frequency.

Depending on the medium in which the standing waves are formed, the characteristics of the waves will vary. A pipe open at both ends has the same harmonic series as a vibrating string (producing all of the harmonics), while a pipe closed at one end will only produce odd-numbered harmonics.

The harmonics that are present from a sound source will affect the timbre or sound quality of the resulting sound.