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General Car Audio
Acoustic loading and wall explanation
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<blockquote data-quote="keep_hope_alive" data-source="post: 8340373" data-attributes="member: 576029"><p>you're misunderstanding acoustics.</p><p></p><p>no worry, it's not commonly discussed.</p><p></p><p>sound spreads spherically from both the sub and the port. don't think in terms of (top of the sub directs to the roof). imagine a bubble that continues to grow. as the bubble hits a surface, another bubble starts to grow, and so on. except the expansion of sound is a wave that oscillates. the oscillations are pressure changes due to air molecules vibrating.</p><p></p><p>when you measure sound you measure direct + all reflections. some reflections arrive in-phase, some out-of-phase, and some in-between. wavelengths can be easily calculated using the speed of sound (1125ft/sec) and either frequency (1/sec) or distance (ft). divide speed of sound by frequency to get wavelength. 1/2 wavelength differences cancel.</p><p></p><p>the trunk causes reflections and by putting the sub near the rear you get a reflection that is mostly in-phase with direct.</p><p></p><p>walling off the trunk eliminates losses in the trunk, so you don't waste energy. it also eliminates reflection (phase) issues in the trunk.</p><p></p><p>your port should effectively combine with the output from the sub. putting your port opening a long way away is purely trial and error and may or may not be helpful for a narrow band of frequencies. changing port tuning changes overall response.</p><p></p><p>the main reason people are louder with the windows open/cracked is because it alleviates pressure that the sub was fighting against. this allows for more excursion and thus more output. doors open also eliminate cancellation due to reflections. vehicles that are loud (155+) when sealed have hit on a specific combination of enclosure, power, vehicle configuration, and frequency.</p></blockquote><p></p>
[QUOTE="keep_hope_alive, post: 8340373, member: 576029"] you're misunderstanding acoustics. no worry, it's not commonly discussed. sound spreads spherically from both the sub and the port. don't think in terms of (top of the sub directs to the roof). imagine a bubble that continues to grow. as the bubble hits a surface, another bubble starts to grow, and so on. except the expansion of sound is a wave that oscillates. the oscillations are pressure changes due to air molecules vibrating. when you measure sound you measure direct + all reflections. some reflections arrive in-phase, some out-of-phase, and some in-between. wavelengths can be easily calculated using the speed of sound (1125ft/sec) and either frequency (1/sec) or distance (ft). divide speed of sound by frequency to get wavelength. 1/2 wavelength differences cancel. the trunk causes reflections and by putting the sub near the rear you get a reflection that is mostly in-phase with direct. walling off the trunk eliminates losses in the trunk, so you don't waste energy. it also eliminates reflection (phase) issues in the trunk. your port should effectively combine with the output from the sub. putting your port opening a long way away is purely trial and error and may or may not be helpful for a narrow band of frequencies. changing port tuning changes overall response. the main reason people are louder with the windows open/cracked is because it alleviates pressure that the sub was fighting against. this allows for more excursion and thus more output. doors open also eliminate cancellation due to reflections. vehicles that are loud (155+) when sealed have hit on a specific combination of enclosure, power, vehicle configuration, and frequency. [/QUOTE]
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Acoustic loading and wall explanation
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