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<blockquote data-quote="tinmanchris217" data-source="post: 6559679" data-attributes="member: 610505">The speaker&#039;s cone has 2 sides.&nbsp; When it pushes out, it creates a positive pressure in front, and vacuum pressure behind it.&nbsp; Since speakers vibrate, it creates a &quot;front&quot; and &quot;back&quot;&nbsp; pressure wave.Porting allows the &quot;back&quot; wave of the speaker to be useful.&nbsp; In sealed, the sound is killed, and you only see the &quot;front&quot;.As mentioned above, the &quot;back&quot; wave is opposite (or out of phase) of the front wave, so if you run free-air, expect little to no output (unless you know what you&#039;re doing).For a port to be useful, you have to convert the out-of-phase back wave in-to-phase.&nbsp; This is known as constructive interference, when the 2 waves work together.&nbsp; To convert the back wave, we use a resonance tube.&nbsp; I can&#039;t explain fully how or why it works (without a text book), but it works.The port has a certain resonance frequency, that centers the added response.&nbsp; When you play below this frequency, the woofer extends further than normal, and above, it looks doesn&#039;t move much.&nbsp; This is because the box and port interact, changing the amount of air that &quot;stiffens&quot; the system.You can tune the port to get lower bass than sealed, or flatten the response, or add more output, or make the response peak heavily.&nbsp; Generally for car audio you pick a frequency that combines the extra output and low frequency extension.&nbsp; SPL junkies do peaks, and SQ guys go flat and deep.Hope this helps, and I haven&#039;t butchered the theory in simplification.</blockquote>

[QUOTE="tinmanchris217, post: 6559679, member: 610505"] The speaker's cone has 2 sides. When it pushes out, it creates a positive pressure in front, and vacuum pressure behind it. Since speakers vibrate, it creates a "front" and "back" pressure wave. Porting allows the "back" wave of the speaker to be useful. In sealed, the sound is killed, and you only see the "front". As mentioned above, the "back" wave is opposite (or out of phase) of the front wave, so if you run free-air, expect little to no output (unless you know what you're doing). For a port to be useful, you have to convert the out-of-phase back wave in-to-phase. This is known as constructive interference, when the 2 waves work together. To convert the back wave, we use a resonance tube. I can't explain fully how or why it works (without a text book), but it works. The port has a certain resonance frequency, that centers the added response. When you play below this frequency, the woofer extends further than normal, and above, it looks doesn't move much. This is because the box and port interact, changing the amount of air that "stiffens" the system. You can tune the port to get lower bass than sealed, or flatten the response, or add more output, or make the response peak heavily. Generally for car audio you pick a frequency that combines the extra output and low frequency extension. SPL junkies do peaks, and SQ guys go flat and deep. Hope this helps, and I haven't butchered the theory in simplification. [/QUOTE]

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