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480 lines
16 KiB
HTML
480 lines
16 KiB
HTML
<!DOCTYPE html>
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<html lang="en"><!-- #BeginTemplate "/Templates/Advanced.dwt" --><!-- DW6 -->
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<article id="content" role="main">
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<!-- #BeginEditable "Body" -->
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<h1 class="center">Sound</h1>
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<p class="center"><img src="images/bark.jpg" alt="bark" height="184" width="151"><br>
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<span class="larger">Arrow the Dog barks!</span></p>
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<p>His bark makes the air go between <b>compressed</b> and <b>rarefied</b>:</p>
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<p class="center"><img src="images/compression-rarefaction.svg" alt="compression rarefaction" height="108" width="272"></p>
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<p>The air molecules bounce back and forth a bit but don't really travel anywhere.</p>
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<p>We call that type of wave <a href="waves-introduction.html">longitudinal</a>, like this:</p>
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<div class="script" style="height: 190px;">
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images/waves.js
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</div>
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<p> </p>
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<p class="center"><img src="images/sound-wave.jpg" alt="sound wave" height="102" width="360"><br>
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<b>But</b> people show sound as "up-down" waves<br>
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because it is <b>easier</b> to show that way.</p>
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<p> </p>
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<h2>Audio Spectrum</h2>
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<p style="float:right; margin: 0 0 5px 10px;"><img src="images/frequency-hi-lo.svg" alt="frequency high and low" height="214" width="216"></p>
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<p>A low <b>frequency</b> of vibration has a low <b>pitch</b> and makes a deep sound, like a growl.</p>
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<p>A high frequency has a high pitch, like a whistle.</p>
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<p>Sound frequency is often measured in "Hertz" (Hz) which is how many vibrations per second.</p>
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<div class="example">
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<h3>Example: 50 Hertz means 50 times per second</h3>
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</div>
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<p>Humans can hear sounds between about 20 Hz and 20,000 Hz (depending on the human!).</p>
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<p> </p>
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<p>You can try it yourself:</p>
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<script>audiospectrumMain();</script>
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<p> </p>
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<p>Below 20 Hz is called <b>infrasound</b> ("infra" means below), and above 20,000 Hz is <b>ultrasound</b> ("ultra" means beyond).</p>
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<p>We are most sensitive to sounds between 1,000 and 4,000 Hz:</p>
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<p class="center" ><img style="background-color:white;" src="images/loudness.gif" alt="loudness dB vs frequency graph" height="192" width="360"><br>
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<b>A Typical Hearing Sensitivity Curve</b><br>
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(dB explained below)</p>
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<p>As we get older we are less sensitive to higher frequency sounds (a limit around 12,000 Hz is normal for an adult).</p>
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<h2>Sound Intensity</h2>
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<p style="float:left; margin: 0 10px 5px 0;"><img style="background-color:white;" src="images/sound-intensity.svg" alt="sound intensity" height="122" width="208"></p>
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<p> </p>
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<p>Sound Intensity is the amount of <b>power per unit area</b></p>
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<p>Usually measured as Watts per square meter (W/m<sup>2</sup>)</p>
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<p>(Note: 1 W/m<sup>2</sup> is very loud, like a chain saw up close.)</p>
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<div style="clear:both"></div>
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<h2>Loudness and Decibels</h2>
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<p><b>Loudness</b> is how powerful a sound seems to us.</p>
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<p>For a sound to seem <b>twice as loud</b> needs about <b>ten times the intensity</b>.</p>
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<div class="example">
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<p style="float:right; margin: 0 0 5px 10px;"><img src="images/speaker.jpg" alt="speaker" height="152" width="150"></p>
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<h3>Example: you feed 1 Watt of power into a speaker.</h3>
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<p>Your friend says "twice as loud please!"</p>
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<p>You need to use about 10 watts to make them agree it now sounds twice as loud.</p>
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</div>
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<p>So we use decibels (dB) to measure loudness:</p>
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<ul>
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<li>+10 dB means 10× the intensity</li>
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<li>+20 dB means 100× the intensity</li>
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<li>+30 dB means 1000× the intensity</li>
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<li>etc</li>
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</ul>
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<p>The scale for sound starts at 0 dB, the quietest sound humans can hear, and goes up from there:</p>
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<div class="simple">
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<table style="border: 0; margin:auto;">
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<tbody>
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<tr>
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<th align="right">decibels<br>
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(dB)</th>
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<th width="20"> </th>
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<th> </th>
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<th width="20"> </th>
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<th> Intensity<br>
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(W/m<sup>2</sup>)</th>
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</tr>
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<tr>
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<td style="text-align:right;"><b>0</b></td>
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<td> </td>
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<td>Quietest sound humans can hear</td>
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<td> </td>
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<td>10<sup>-12</sup></td>
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</tr>
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<tr>
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<td style="text-align:right;"><b>10</b></td>
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<td> </td>
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<td>Pin drop</td>
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<td> </td>
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<td>10<sup>-11</sup></td>
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</tr>
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<tr>
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<td style="text-align:right;"><b>20</b></td>
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<td> </td>
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<td>Rustling leaves</td>
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<td> </td>
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<td>10<sup>-10</sup></td>
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</tr>
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<tr>
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<td style="text-align:right;"><b>30</b></td>
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<td> </td>
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<td>Whisper</td>
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<td> </td>
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<td>10<sup>-9</sup></td>
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</tr>
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<tr>
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<td style="text-align:right;"><b>40</b></td>
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<td> </td>
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<td>Library</td>
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<td> </td>
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<td>10<sup>-8</sup></td>
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</tr>
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<tr>
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<td style="text-align:right;"><b>50</b></td>
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<td> </td>
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<td>Quiet conversation</td>
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<td> </td>
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<td>10<sup>-7</sup></td>
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</tr>
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<tr>
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<td style="text-align:right;"><b>60</b></td>
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<td> </td>
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<td>Restaurant</td>
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<td> </td>
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<td>10<sup>-6</sup></td>
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</tr>
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<tr>
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<td style="text-align:right;"><b>70</b></td>
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<td> </td>
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<td>Normal TV</td>
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<td> </td>
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<td>10<sup>-5</sup></td>
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</tr>
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<tr>
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<td style="text-align:right;"><b>80</b></td>
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<td> </td>
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<td>Truck </td>
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<td> </td>
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<td>10<sup>-4</sup></td>
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</tr>
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<tr>
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<td style="text-align:right;"><b>90</b></td>
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<td> </td>
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<td>Mower Engine</td>
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<td> </td>
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<td>10<sup>-3</sup></td>
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</tr>
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<tr>
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<td style="text-align:right;"><b>100</b></td>
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<td> </td>
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<td>Motorbike</td>
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<td> </td>
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<td>0.01</td>
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</tr>
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<tr>
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<td style="text-align:right;"><b>110</b></td>
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<td> </td>
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<td>Rock concert</td>
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<td> </td>
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<td>0.1</td>
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</tr>
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<tr>
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<td style="text-align:right;"><b>120</b></td>
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<td> </td>
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<td>Chain saw</td>
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<td> </td>
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<td>1</td>
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</tr>
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<tr>
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<td style="text-align:right;"><b>130</b></td>
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<td> </td>
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<td>Painful</td>
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<td> </td>
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<td>10</td>
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</tr>
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<tr>
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<td style="text-align:right;"><b>140</b></td>
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<td> </td>
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<td>Jet take off, Dangerous for ears</td>
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<td> </td>
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<td>100</td>
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</tr>
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<tr>
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<td style="text-align:right;"><b>150</b></td>
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<td> </td>
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<td>Causes deafness</td>
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<td> </td>
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<td>1000</td>
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</tr>
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<tr>
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<td style="text-align:right;"><b>194</b></td>
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<td> </td>
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<td>Loudest sound possible, can kill</td>
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<td> </td>
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<td> </td>
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</tr>
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</tbody></table>
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</div>
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<p>At 194 dB sound waves become shock waves (like a blast from an explosion).</p>
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<h2>Inverse Square</h2>
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<table style="border: 0; margin:auto;">
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<tbody>
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<tr>
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<td><span class="larger"><img src="../algebra/images/proportion-inverse-square.svg" alt="brightness decreases by the square of the distance" height="178" width="316"></span></td>
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<td class="larger"><span class="center"><b>Inverse Square</b></span>: when one value <b>decreases</b> as the square of the other value.</td>
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</tr>
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</tbody></table>
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<div class="example">
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<h3>Example: Sound and distance</h3>
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<p>The further away we are from a sound, the less intense it is.</p>
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<p class="center"><img style="background-color:white;" src="images/inverse-square-sound.svg" alt="inverse square law: distance=1 area=1 intensity=1, distance=2 area=4 intensity=0.25, distance=3 area=9 intensity=0.111..." height="184" width="422"></p>
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<p>The power per square meter decreases as the <b>square</b> of the distance.</p>
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<p>Because:</p>
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<ul>
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<li>the energy twice as far away is spread over 4 times the area</li>
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<li>the energy 3 times as far away is spread over 9 times the area</li>
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<li>etc</li>
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</ul>
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</div>
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<h2>Speed of Sound</h2>
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<p>Sound travels slowest in gases (such as air), faster in liquids (such as water) and fastest in solids.</p>
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<div class="simple">
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<table style="border: 0; margin:auto;">
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<tbody>
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<tr>
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<th>Medium</th>
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<th width="30"> </th>
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<th>Speed</th>
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</tr>
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<tr>
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<td> Air at 20°C </td>
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<td> </td>
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<td>343 m/s </td>
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</tr>
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<tr>
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<td>Air at 35°C </td>
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<td> </td>
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<td>352 m/s</td>
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</tr>
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<tr>
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<td>Water at 20°C </td>
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<td> </td>
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<td>1482 m/s</td>
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</tr>
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<tr>
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<td>Glass</td>
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<td> </td>
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<td>4540 m/s</td>
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</tr>
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<tr>
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<td>Brass</td>
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<td> </td>
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<td>4700 m/s</td>
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</tr>
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<tr>
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<td>Steel</td>
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<td> </td>
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<td>5790 m/s</td>
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</tr>
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<tr>
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<td colspan="3" align="center"><i>Amazing! Sound travels about<br>
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17 times faster
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in Steel than Air!</i></td>
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</tr>
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</tbody></table>
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</div>
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<p> </p>
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<div class="fun">
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<h3>How Far is That Lightning Strike?</h3>
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<p class="center"><img src="images/lightning.jpg" alt="lightning" height="249" width="480"></p>
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<p>Next time there is a thunderstorm watch for lightning and start <b>counting seconds</b>.</p>
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<p>The light reaches you almost immediately (at about 300,000 km/s), but the sound takes longer.</p>
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<p><b>343 m/s</b> is about <b>3 seconds per km</b>, or 5 seconds per mile.</p>
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<table style="border: 0; margin:auto;">
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<tbody>
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<tr>
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<th>Count</th>
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<th width="20"> </th>
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<th align="center">Approx<br>
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Distance</th>
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</tr>
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<tr>
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<td>1 sec</td>
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<td> </td>
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<td style="text-align:center;">340 m</td>
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</tr>
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||
<tr>
|
||
<td>2 sec</td>
|
||
<td> </td>
|
||
<td style="text-align:center;">700 m</td>
|
||
</tr>
|
||
<tr>
|
||
<td>3 sec</td>
|
||
<td> </td>
|
||
<td style="text-align:center;">1 km</td>
|
||
</tr>
|
||
<tr>
|
||
<td>6 sec</td>
|
||
<td> </td>
|
||
<td style="text-align:center;">2 km</td>
|
||
</tr>
|
||
<tr>
|
||
<td>9 sec</td>
|
||
<td> </td>
|
||
<td style="text-align:center;">3 km</td>
|
||
</tr>
|
||
<tr>
|
||
<td>etc...</td>
|
||
<td> </td>
|
||
<td style="text-align:center;"> </td>
|
||
</tr>
|
||
</tbody></table>
|
||
<p>So if you count 15 seconds until you hear thunder, the lightning strike was 5 km (3 miles) away.</p>
|
||
</div>
|
||
|
||
|
||
<h2>Echo</h2>
|
||
|
||
<p class="words">An <b>echo</b> is a reflection, usually of sound from a hard surface.</p>
|
||
<p class="center"><img src="images/echo-clap.svg" alt="echo clap" height="247" width="355"><br>
|
||
The sound bounces off the wall (angle in matches angle out).<br>
|
||
You will hear the echo of the clap some time after you hear it from your hands.</p>
|
||
|
||
|
||
<h2>Sonar and Ultrasound</h2>
|
||
|
||
<p><b>Sonar</b> (<b>SO</b>und <b>N</b>avigation <b>A</b>nd <b>R</b>anging) is a way of listening to the <b>echo</b> of sound waves (usually underwater) to locate objects:</p>
|
||
<p class="center"><img src="images/sonar.jpg" alt="sonar" height="198" width="280"><br>
|
||
Sonar sent out ... reflects off cute fish ... sound received.</p>
|
||
<p class="center"><img src="images/sonar-shipwreck.jpg" alt="sonar shipwreck" height="261" width="300"><br>
|
||
Sonar (with computer help) can map out the sea floor and even find shipwrecks!</p>
|
||
<p>Bats use a similar method (called "echolocation"). They send out ultrasound squeaks (20 kHz to 120 kHz) to find their way at night, find insects to eat and even avoid hitting small wires!</p>
|
||
<p class="center"><img src="images/ultrasound.jpg" alt="ultrasound" height="252" width="200"><br>
|
||
And ultrasound can be used for images inside our bodies.</p>
|
||
|
||
|
||
<h2>Doppler</h2>
|
||
|
||
<p>The <a href="doppler-effect.html">Doppler Effect</a> happens when a wave's source is <b>moving</b> in relation to us:</p>
|
||
<ul>
|
||
<li>As the source approaches, the waves arrive at a higher frequency</li>
|
||
<li>And as the source moves away the waves have a lower frequency</li>
|
||
</ul>
|
||
<p>So a passing siren sounds like <i>"nee-nee-nee-nee ... woooo-woooo"</i> (or a passing race-car sounds like <i>"eeee-yoooo"):</i></p>
|
||
|
||
<table style="border: 0; margin:auto;">
|
||
<tbody>
|
||
<tr>
|
||
<td colspan="2"><img src="images/doppler-before.gif" alt="doppler approaching" height="146" width="348"></td>
|
||
<td>Approaching: higher frequency</td>
|
||
</tr>
|
||
<tr>
|
||
<td style="text-align:right;">Leaving: lower frequency </td>
|
||
<td colspan="2"><img src="images/doppler-leave.gif" alt="doppler leaving" height="146" width="347"></td>
|
||
</tr>
|
||
<tr>
|
||
<td> </td>
|
||
<td style="width:100px;"> </td>
|
||
<td> </td>
|
||
</tr>
|
||
</tbody></table>
|
||
<p>This applies to all waves, including light waves and even waves on the sea:</p>
|
||
<ul>
|
||
<li>On the sea, a boat can travel <b>with</b> the waves to make the up-and-down motion slower.</li>
|
||
<li><a href="light.html">Light</a> from stars that are moving away are "red-shifted" (a lower frequency of light) and those that move toward us are "blue-shifted"</li>
|
||
</ul>
|
||
<p> </p>
|
||
<div class="questions">17667,17669,17676,
|
||
17672,17674,17677,17679,17681,17683,17686</div>
|
||
|
||
<div class="related">
|
||
<a href="audio-spectrum-beats.html">Sound Beats</a>
|
||
<a href="waves-introduction.html">Introduction to Waves</a>
|
||
<a href="index.html">Physics Index</a>
|
||
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|
||
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