Sound

Definition of Sound Sound is a wave created by vibrating objects and moved through a medium from one location to another.

A person to hear it is not required. The medium (air) is required! If a tree falls in a forest, and there is no one there to hear it, does it make a sound? Sound is a physical disturbance in a medium. Based on our definition, there IS sound in the forest, whether a human is there to hear it or not! A person to hear it is not required. The medium (air) is required!

Review: Sound is a mechanical wave The sound wave is transported from one location to another by means of particle-to-particle interaction. If the sound wave is moving through air, then as one air particle is moved from its rest position, it exerts a push or pull on its nearest neighbors, causing them to be moved from their rest position. This requires a medium. Sound is a longitudinal mechanical wave.

Check your understanding: A sound wave is different than a light wave in that a sound wave is a. produced by a vibrating object and a light wave is not. b. not capable of traveling through a vacuum. c. not capable of diffracting and a light wave is. d. capable of existing with a variety of frequencies and a light wave has a single frequency.

When a tuning fork vibrates, it creates areas of high pressure (compressions) and low pressure (rarefactions). As the tines of the fork vibrate back and forth, they push on neighboring air particles. The forward motion of a tine pushes air molecules horizontally to the right and the backward retraction of the tine creates a low-pressure area allowing the air particles to move back to the left.

But remember – sound is actually a longitudinal wave. Graphing a Sound Wave. Rarefaction Compression The variation of pressure with distance is a useful way to represent a sound wave graphically. But remember – sound is actually a longitudinal wave.

Sound waves are longitudinal waves. What type of waves are sound waves? Sound waves are longitudinal waves.

Check your understanding A sound wave is a longitudinal wave; regions of high pressure (compressions) and low pressure (rarefactions) are established as the result of the vibrations of the sound source. These compressions and rarefactions result because sound a. is more dense than air and thus has more inertia. b. waves have a speed that is dependent only upon the properties of the medium. c. can be diffracted around obstacles. d. vibrates longitudinally; the longitudinal movement of air produces pressure changes.

Frequency of Sound Frequency is the speed of the vibration. Frequency is measured as the number of wave cycles that occur in one second. The unit of frequency measurement is Hertz (Hz for short).

The frequency of sound We hear frequencies of sound as having different pitch. A low frequency sound has a low pitch, like the rumble of a big truck. A high-frequency sound has a high pitch, like a whistle or siren.

Animation showing Frequency and Pitch

Frequency and music Certain sound waves when played (and heard) simultaneously will produce a particularly pleasant sensation when heard For example, two sound waves sound good when played together if one sound has twice the frequency of the other.

Loudness

Loudness and Intensity Loudness refers to how loud or soft a sound seems to a listener. The loudness of sound is determined, in turn, by the intensity of the sound waves. Intensity is a measure of the amount of energy in sound waves. The unit of intensity is the decibel (dB).  Intensity results from two factors: the amplitude of the sound waves and how far they have traveled from the source of the sound.

Intensity is caused by the Amplitude of the vibration The amplitude of a wave is related to the amount of energy it carries. A high amplitude wave carries a large amount of energy; a low amplitude wave carries a small amount of energy.

One important thing about loudness Loudness is Subjective! (This means it depends on the person who is hearing it.) Loudness is a personal, physical response to the intensity of sound. As intensity increases, so does loudness, but loudness also depends on the listener’s ears and brain.

The Decibel Scale: The decibel (abbreviated dB) is the unit used to measure the intensity of a sound. The decibel scale is a little odd because the human ear is incredibly sensitive. Your ears can hear everything from your fingertip brushing lightly over your skin to a loud jet engine . In terms of power, the sound of the jet engine is about 1,000,000,000,000 times more powerful than the smallest audible sound. That's a big difference!

The Decibel Scale On the decibel scale, the smallest audible sound (the threshold of hearing) is 0 dB. A sound 10 times more powerful is 10 dB. A sound 100 times more powerful than near total silence is 20 dB A sound 1,000 times more powerful than near total silence is 30 dB.

Intensity (Loudness) is measured in decibels: Source Intensity Level # of Times Greater Than TOH Threshold of Hearing 0 dB 100 Rustling Leaves 10 dB 101 Whisper 20 dB 102 Normal Conversation 60 dB 106 Busy Street Traffic 70 dB 107 Vacuum Cleaner 80 dB 108 Large Orchestra 98 dB 109.8 Walkman at Maximum Level 100 dB 1010 Front Rows of Rock Concert 110 dB 1011 Threshold of Pain 130 dB 1013 Military Jet Takeoff 140 dB 1014 Instant Perforation of Eardrum 160 dB 1016  

Check your understanding A mosquito's buzz is often rated with a decibel rating of 40 dB. Normal conversation is often rated at 60 dB. How many times more intense is normal conversation compared to a mosquito's buzz?