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<ul class="navigation"><li class="prev"><a accesskey="p" class="prev" href="WSB5A58FE6-36EE-4d28-BAE8-E39F185FAF23a.html" title="Digital audio fundamentals"><img src="images/blank.gif" alt="Previous" width="17" height="17" /></a></li><li class="next"><a accesskey="n" class="next" href="WS41FBD92E-39EA-4eda-B490-EDE8EA1175C8a.html" title="Digitizing audio"><img src="images/blank.gif" alt="Next" width="17" height="17" /></a></li></ul><div class="hierarchy" id="hierarchy"><a href="WS3323129D-B2F3-43c0-B401-8D1EFE022412.html"><b>Home Page</b></a> / <a href="WS3323129D-B2F3-43c0-B401-8D1EFE022412.html"><b>Using Soundbooth CS5</b></a> / <a href="WSB5A58FE6-36EE-4d28-BAE8-E39F185FAF23a.html"><b>Digital audio fundamentals</b></a> 

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        <h1>

          Understanding sound

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        <div id="minitoc"><div class="t"><div class="b"><div><ul id="minitoc-links"><li><p><a href="#WS8D56CEB3-B638-4591-B75B-485C0B921836a"><h1 class="topictitle1">Sound waves</h1>

</a></p></li><li><p><a href="#WS3984B3F9-EBDC-4f8e-936D-AC9B35943D68a"><h1 class="topictitle1">Waveform measurements</h1>

</a></p></li><li><p><a href="#WS499E46E3-923E-405c-B077-B218BCB0DE61a"><h1 class="topictitle1">How sound waves interact</h1>

</a></p></li></ul></div></div></div></div><div><ul class="navlinklist"></ul>

</div>

<div id="WS8D56CEB3-B638-4591-B75B-485C0B921836a" class="nochunk"><a name="WS8D56CEB3-B638-4591-B75B-485C0B921836a"><!-- --></a><h2 class="topictitle2">Sound waves</h2><div><p>Sound starts with vibrations in the air, like

those produced by guitar strings, vocal cords, or speaker cones.

These vibrations push nearby air molecules together, raising the

air pressure slightly. The air molecules under pressure then push

on the air molecules surrounding them, which push on the next set

of molecules, and so on. As high-pressure areas move through the

air, they leave low-pressure areas behind them. When these waves

of pressure changes reach us, they vibrate the receptors in our

ears, and we hear the vibrations as sound.</p>

<p>When you see a visual waveform that represents audio, it reflects

these waves of air pressure. The zero line in the waveform is the

pressure of air at rest. When the line swings up to a peak, it represents

higher pressure; when the line swings down to a trough, it represents

lower pressure.</p>

<div xmlns:fn="http://www.w3.org/2005/xpath-functions" xmlns:fo="http://www.w3.org/1999/XSL/Format" xmlns:xs="http://www.w3.org/2001/XMLSchema" class="fignone"><img alt="" id="da01" src="images/da01.png" /><div class="figdesc">A sound wave represented as a visual waveform</div><br /><dl><dt class="dlterm">A.</dt>

<dd>Zero line</dd><p class="dlseparator"></p><dt class="dlterm">B.</dt>

<dd>Low-pressure

area</dd><p class="dlseparator"></p><dt class="dlterm">C.</dt>

<dd>High-pressure area</dd><p class="dlseparator"></p></dl>

</div></div></div>

<div id="WS3984B3F9-EBDC-4f8e-936D-AC9B35943D68a" class="nochunk"><a name="WS3984B3F9-EBDC-4f8e-936D-AC9B35943D68a"><!-- --></a><h2 class="topictitle2">Waveform measurements</h2><div><p>Several

measurements describe sound waveforms:</p>

<dl><dt class="dlterm">Amplitude</dt>

<dd xmlns:fn="http://www.w3.org/2005/xpath-functions" xmlns:fo="http://www.w3.org/1999/XSL/Format" xmlns:xs="http://www.w3.org/2001/XMLSchema">Reflects the change in pressure from the peak of the waveform

to the trough. High-amplitude waveforms are loud; low-amplitude

waveforms are quiet.</dd><p class="dlseparator"></p><dt class="dlterm">Cycle</dt>

<dd xmlns:fn="http://www.w3.org/2005/xpath-functions" xmlns:fo="http://www.w3.org/1999/XSL/Format" xmlns:xs="http://www.w3.org/2001/XMLSchema">Describes a single, repeated sequence of pressure changes,

from zero pressure, to high pressure, to low pressure, and back

to zero.</dd><p class="dlseparator"></p><dt class="dlterm">Frequency</dt>

<dd xmlns:fn="http://www.w3.org/2005/xpath-functions" xmlns:fo="http://www.w3.org/1999/XSL/Format" xmlns:xs="http://www.w3.org/2001/XMLSchema">Measured in hertz (Hz), describes the number of cycles per

second. (For example, a 1000-Hz waveform has 1000 cycles per second.)

The higher the frequency, the higher the musical pitch.</dd><p class="dlseparator"></p><dt class="dlterm">Phase</dt>

<dd xmlns:fn="http://www.w3.org/2005/xpath-functions" xmlns:fo="http://www.w3.org/1999/XSL/Format" xmlns:xs="http://www.w3.org/2001/XMLSchema">Measured in 360 degrees, indicates the position of a waveform

in a cycle. Zero degrees is the start point, followed by 90º at

high pressure, 180º at the halfway point, 270º at low pressure,

and 360º at the end point.</dd><p class="dlseparator"></p><dt class="dlterm">Wavelength</dt>

<dd xmlns:fn="http://www.w3.org/2005/xpath-functions" xmlns:fo="http://www.w3.org/1999/XSL/Format" xmlns:xs="http://www.w3.org/2001/XMLSchema">Measured in units such as inches or centimeters, is the distance between

two points with the same degree of phase. As frequency increases, wavelength

decreases.</dd><p class="dlseparator"></p></dl>

<div xmlns:fn="http://www.w3.org/2005/xpath-functions" xmlns:fo="http://www.w3.org/1999/XSL/Format" xmlns:xs="http://www.w3.org/2001/XMLSchema" class="fignone"><img alt="" id="da02" src="images/da02.png" /><div class="figdesc">A single cycle at left; a complete, 20-Hz waveform at right</div><br /><dl><dt class="dlterm">A.</dt>

<dd>Wavelength</dd><p class="dlseparator"></p><dt class="dlterm">B.</dt>

<dd>Degree

of phase</dd><p class="dlseparator"></p><dt class="dlterm">C.</dt>

<dd>Amplitude</dd><p class="dlseparator"></p><dt class="dlterm">D.</dt>

<dd>One second</dd><p class="dlseparator"></p></dl>

</div></div></div>

<div id="WS499E46E3-923E-405c-B077-B218BCB0DE61a" class="nochunk"><a name="WS499E46E3-923E-405c-B077-B218BCB0DE61a"><!-- --></a><h2 class="topictitle2">How sound waves interact</h2><div><p>When

two or more sound waves meet, they add to and subtract from each

other. If their peaks and troughs are perfectly <dfn class="term">in phase, </dfn>they

reinforce each other, resulting in a waveform that has higher amplitude

than either individual waveform.<div xmlns:fn="http://www.w3.org/2005/xpath-functions" xmlns:fo="http://www.w3.org/1999/XSL/Format" xmlns:xs="http://www.w3.org/2001/XMLSchema" class="fignone"><img alt="" id="da03" src="images/da03.png" /><div class="figdesc">In‑phase waves reinforce each other.</div><br /></div></p>

<p>If the peaks and troughs of two waveforms are perfectly <dfn class="term">out

of phase</dfn>, they cancel each other, resulting in no waveform

at all.<div xmlns:fn="http://www.w3.org/2005/xpath-functions" xmlns:fo="http://www.w3.org/1999/XSL/Format" xmlns:xs="http://www.w3.org/2001/XMLSchema" class="fignone"><img alt="" id="da04" src="images/da04.png" /><div class="figdesc">Out‑of‑phase waves cancel each other.</div><br /></div></p>

<div class="para">In most cases, however, waves are out of phase in varying amounts,

resulting in a combined waveform that is more complex than individual

waveforms. A complex waveform that represents music, voice, noise,

and other sounds, for example, combines the waveforms from each

sound.<div class="tip"><img src="images/tip_help.png" />Because of its unique physical structure,

a single instrument can create extremely complex waves. That’s why

a violin and a trumpet sound different even when playing the same

note. </div><div xmlns:fn="http://www.w3.org/2005/xpath-functions" xmlns:fo="http://www.w3.org/1999/XSL/Format" xmlns:xs="http://www.w3.org/2001/XMLSchema" class="fignone"><img alt="" id="da05" src="images/da05.png" /><div class="figdesc">Two simple waves combine to create a complex wave.</div><br /></div></div>

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