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Enclosure Design & Construction
bigger box = lower bass???
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<blockquote data-quote="helotaxi" data-source="post: 393399" data-attributes="member: 550915"><p>So based on your *vast* experience, this is the case?</p><p></p><p>It's called damping. It actually is less for a sealed box than it is for a ported box.</p><p></p><p>The ported enclosure actually exerts more control on the cone than a seled box does. This may seem counterintuitive but here's how that is so. A sealed box has a fixed quantity of air in it. As the sub moves out it creates a low pressure area inside the box that wants to "****" the cone back in. As the sub travels in, it creates a high pressure area in the box that wants to push the sub back out. Compare that to a ported box. As the sub moves out it creates a low pressure area inside the box, but since it is ported it ***** air in through the port to equalize the pressure. Now when the sub tries to move back in, it has to work against the "extra" air that just came in as well as the inertia of the air still trying to move in. As it moves back, it compresses the air in the box and the pressure again starts to equalize through the port, pushing the "extra" air as well as some of the air volume that normally fills the box. Thus as the cone moves in and out, the air moving in and out of the port creates a larger vacuum and high pressure area and exerts more control over the cone. This control is maximized at the resonant frequency of the box. At that frequency, the pressure in the box from the inward movement of the cone is just beginning to vent through the port as the cone reverses direction so the air moving out from the port adds to the pressure wave created by the sub's movement and gives a boost in output and the inertia of the air exiting the port combines with the sub moving out to create an even larger pressure gradient in the box and exert greater force on the cone limiting its movement. Because the box is basically combining the front and rear pressure wave of the sub, the port output is half a cycle behind the sub outout and this slurs the respnse a bit. Basically that loss in pure fidelity is one of the sacrifices that you make to get that 3dB more output that a ported enclosure enjoys over a sealed sub. As the frequency moves away from resonance getting higher, the box has less and less time to equalize and begins to act more like a sealed enclosure. As you begin to go below resonance, the box has too much time to equalize its pressure and the momentum of the air rushing into the box actually pushes the cone out more rather that controling its movement. This is why you get less usable output from a ported sub below tuning. Excursion goes way up and mechanical power handling goes way down. Also look at the response plots for the same sub in both a ported and sealed enclosure. As you start getting into the really low notes, the sealed generally remains linear, and will actually give you more output. The slow and gentle rolloff of a sealed system matches up well with the transfer function of the typical vehicle. The interaction between the sub and the air in the vehicle begins to amplify the bass at about 12 dB per octave starting around 100 hz. Since a sealed sub system with a Qtc of .7 rolls off at about 12dB per octave the tranfer fucntion works to keep the response of a sealed system flat down to really really low frequencies. With a ported system the response is typically flat until right around resonance at which point it begins to roll off quickly. Combine that with the transfer function of the vehicle and you get a large hump in the response of the system from around 60 hz to just below the tuning freq. Below tuning the respone rolls of quicker than the transfer function can make up for it resulting in the in-car response falling off as well.</p><p></p><p>With some tradeoffs, you can get lower bass louder by using a larger enclosure. The tradeoffs are sloppier response and a decrease in power handling from the decreased damping provided by the enclosure. The only real time that this would be of benefit is if you are trying to get really low bass from a small amp, otherwise you can usually get about the same volume at low frequencies with tighter response by just making use of the increased power handling of the smaller enclosure and giving the sub more power. Thanks to that transfer function of the car you can usually optimize the enclosure for transient response and power handling and still get really good low frequency output from a sealed system.</p></blockquote><p></p>
[QUOTE="helotaxi, post: 393399, member: 550915"] So based on your *vast* experience, this is the case? It's called damping. It actually is less for a sealed box than it is for a ported box. The ported enclosure actually exerts more control on the cone than a seled box does. This may seem counterintuitive but here's how that is so. A sealed box has a fixed quantity of air in it. As the sub moves out it creates a low pressure area inside the box that wants to "****" the cone back in. As the sub travels in, it creates a high pressure area in the box that wants to push the sub back out. Compare that to a ported box. As the sub moves out it creates a low pressure area inside the box, but since it is ported it ***** air in through the port to equalize the pressure. Now when the sub tries to move back in, it has to work against the "extra" air that just came in as well as the inertia of the air still trying to move in. As it moves back, it compresses the air in the box and the pressure again starts to equalize through the port, pushing the "extra" air as well as some of the air volume that normally fills the box. Thus as the cone moves in and out, the air moving in and out of the port creates a larger vacuum and high pressure area and exerts more control over the cone. This control is maximized at the resonant frequency of the box. At that frequency, the pressure in the box from the inward movement of the cone is just beginning to vent through the port as the cone reverses direction so the air moving out from the port adds to the pressure wave created by the sub's movement and gives a boost in output and the inertia of the air exiting the port combines with the sub moving out to create an even larger pressure gradient in the box and exert greater force on the cone limiting its movement. Because the box is basically combining the front and rear pressure wave of the sub, the port output is half a cycle behind the sub outout and this slurs the respnse a bit. Basically that loss in pure fidelity is one of the sacrifices that you make to get that 3dB more output that a ported enclosure enjoys over a sealed sub. As the frequency moves away from resonance getting higher, the box has less and less time to equalize and begins to act more like a sealed enclosure. As you begin to go below resonance, the box has too much time to equalize its pressure and the momentum of the air rushing into the box actually pushes the cone out more rather that controling its movement. This is why you get less usable output from a ported sub below tuning. Excursion goes way up and mechanical power handling goes way down. Also look at the response plots for the same sub in both a ported and sealed enclosure. As you start getting into the really low notes, the sealed generally remains linear, and will actually give you more output. The slow and gentle rolloff of a sealed system matches up well with the transfer function of the typical vehicle. The interaction between the sub and the air in the vehicle begins to amplify the bass at about 12 dB per octave starting around 100 hz. Since a sealed sub system with a Qtc of .7 rolls off at about 12dB per octave the tranfer fucntion works to keep the response of a sealed system flat down to really really low frequencies. With a ported system the response is typically flat until right around resonance at which point it begins to roll off quickly. Combine that with the transfer function of the vehicle and you get a large hump in the response of the system from around 60 hz to just below the tuning freq. Below tuning the respone rolls of quicker than the transfer function can make up for it resulting in the in-car response falling off as well. With some tradeoffs, you can get lower bass louder by using a larger enclosure. The tradeoffs are sloppier response and a decrease in power handling from the decreased damping provided by the enclosure. The only real time that this would be of benefit is if you are trying to get really low bass from a small amp, otherwise you can usually get about the same volume at low frequencies with tighter response by just making use of the increased power handling of the smaller enclosure and giving the sub more power. Thanks to that transfer function of the car you can usually optimize the enclosure for transient response and power handling and still get really good low frequency output from a sealed system. [/QUOTE]
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bigger box = lower bass???
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