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Enclosure Design & Construction
SEALED vs PORTED (basic info.)
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<blockquote data-quote="geolemon" data-source="post: 534519" data-attributes="member: 547749"><p>Awesome info, BASS OUTLAW!</p><p></p><p>If you don't mind, let me take a step backwards...</p><p></p><p>You explained what the benefits/tradeoffs commonly are.... I want to touch on the fundamentals just a tad, and that might bridge the gap as to "why do certain enclosures offer certain benefits/tradeoffs?" //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif</p><p></p><p><strong>Why do you need an enclosure at all?</strong></p><p></p><p>Picture a sub moving. The cone moves in and out, rapidly - and it's that simple.</p><p></p><p>Picture the cone moving out, in slow motion. It compresses the air in front of the cone, and rarefies the air behind the cone.</p><p></p><p>IF the sub were just sitting there, the compressed air would largely simply bleed into the rarefied vacuum behind the cone, and most of the sound would simply cancel out as a result... leaving you with not much sound at all, if any.</p><p></p><p><strong>Sealed box:</strong></p><p></p><p>The sealed box simply contains the rear-of-cone energy, so that it can't escape, to cause the cancellations with the front-of-cone energy. The front-of-cone energy is what you ultimately end up hearing.</p><p></p><p>The sealed box also, inherently, ends up affecting cone motion - because the trapped air inside the box acts as a spring, essentially. It's more difficult for the cone to move in and out in an enclosure, because the air is preventing the cone from moving in and out, it's resisting being squished and stretched.</p><p></p><p>As a result, if you put a sub in a tiny enclosure, it inherently will always be inefficient - requiring lots and lots of power to even get moderately loud - because you are trying to brute-force your cone motion... a large enclosure is much more efficient, requiring less power to get as loud.</p><p></p><p>But maybe you don't want to waste that rear-of-cone energy anyways?</p><p></p><p><strong>Ported box:</strong></p><p></p><p>In this case, a vent is installed in the enclosure. Might be round, might be rectangular, might be any shape... ultimately it doesn't matter - what does matter, is that the port opening area and length of the port have to be sized such that they provide just a little resistance against air passing through it. The amount of resistance is critical to the performance of the enclosure.</p><p></p><p>In this case, essentially you have the same properties of the sealed enclosure, but the vent does something interesting...</p><p></p><p>Again, picture the sub in slow motion.</p><p></p><p>Picture it moving in, compressing the air behind the cone, inside the enclosure.</p><p></p><p>Naturally, this air tries to escape out the port, having suddenly been pressurized.</p><p></p><p>...and it does, but the port slows it's escape. As the air is travelling out the port, the sub has changed direction, and is now heading out again.</p><p></p><p>By now, the compressed air has travelled out the port - just in time for the cone to move out again - and now we have a situation where the front-of-cone energy is creating a pressure node in the listening space, and the rear-of-cone energy is creating a pressure node in the listening space, thanks to our friend, the port.</p><p></p><p>...and continuing the thought, the cone has moved out, rarefying the air behind the cone, creating a vacuum effect inside the enclosure, ******* air in the port... by the time the air can be ****** in through the slightly resistive port, creating a vacuum outside, the cone has again moved in, creating a vacuum node itself.</p><p></p><p>One common misconception is that since it has a hole, the pressures must be lower inside a ported box, compared to a sealed box.</p><p></p><p>The opposite is true, however!</p><p></p><p>Again, thinking about what is happening in slow motion - with the sealed box, you simply have the sub compressing and decompressing the air trapped inside the enclosure.</p><p></p><p>With a ported box, however, you have not only the sub, but the momentum of the air in the port pressurizing and depressurizing the air inside the enclosure.</p><p></p><p>As such, pressures are actually HIGHER inside a ported enclosure than a sealed one.</p><p></p><p>Things aren't perfect in "ported land" however. //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif</p><p></p><p>The port is a static thing, and as such, it's air resistance - and therefore it's delay, are a static value.</p><p></p><p>Wavelengths, however, are different for every frequency, increasing in length as the frequencies decrease. As a result, one wave takes more time to pass for a low frequency, than for a high frequency.</p><p></p><p>As such, you end up building a port that's "ideal" at one particular frequency - but as you move away from that frequency, you either overcompensate for the frequency being played, or undercompensate for it - but either way, the phase relationship is degraded, and you again move in the direction of being more or less like a sub that isn't even in an enclosure at all.</p><p></p><p>...In fact, if you operate a subwoofer too far below its tuning frequency, the air moving in the port is cycling too slow to increase, or even equal the pressures inside the enclosure - and it acts like a simple hole - resulting in little or no air 'spring' value inside the enclosure - making it very easy to bottom out your sub with even less than the RMS powerhandling limit of the sub being sent to it.</p><p></p><p>...well, that covers ported and sealed boxes... which is a good start anyway.</p><p></p><p>Infinite baffle is much like a sealed box, but with no "air spring" factor, since there's no trapped air involved.</p><p></p><p>Outside of that, other types of enclosures get more complicated...</p><p></p><p>...although it's fundamentally interesting to note that 4th order, or "single vented" bandpass enclosures utilize one sealed chamber (the one that defines the low end response), making it somewhat synonymous with the behaviors of a sealed box, and compatible with subs that are compatible with sealed enclosures.</p><p></p><p>Similarly, the 6th order, or "double vented" bandpass enclosure utilizes ports in both chambers, making it somewhat synonymous with the properties of a ported box, and compatible with subs that are compatible with ported enclosures.</p><p></p><p>Hopefully that helps augment what BASS OUTLAW started with. //content.invisioncic.com/y282845/emoticons/nerd.gif.c6fa51ddf7ff75f1c0371fbc648f70ae.gif</p></blockquote><p></p>
[QUOTE="geolemon, post: 534519, member: 547749"] Awesome info, BASS OUTLAW! If you don't mind, let me take a step backwards... You explained what the benefits/tradeoffs commonly are.... I want to touch on the fundamentals just a tad, and that might bridge the gap as to "why do certain enclosures offer certain benefits/tradeoffs?" [IMG]//content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif[/IMG] [B]Why do you need an enclosure at all?[/B] Picture a sub moving. The cone moves in and out, rapidly - and it's that simple. Picture the cone moving out, in slow motion. It compresses the air in front of the cone, and rarefies the air behind the cone. IF the sub were just sitting there, the compressed air would largely simply bleed into the rarefied vacuum behind the cone, and most of the sound would simply cancel out as a result... leaving you with not much sound at all, if any. [B]Sealed box:[/B] The sealed box simply contains the rear-of-cone energy, so that it can't escape, to cause the cancellations with the front-of-cone energy. The front-of-cone energy is what you ultimately end up hearing. The sealed box also, inherently, ends up affecting cone motion - because the trapped air inside the box acts as a spring, essentially. It's more difficult for the cone to move in and out in an enclosure, because the air is preventing the cone from moving in and out, it's resisting being squished and stretched. As a result, if you put a sub in a tiny enclosure, it inherently will always be inefficient - requiring lots and lots of power to even get moderately loud - because you are trying to brute-force your cone motion... a large enclosure is much more efficient, requiring less power to get as loud. But maybe you don't want to waste that rear-of-cone energy anyways? [B]Ported box:[/B] In this case, a vent is installed in the enclosure. Might be round, might be rectangular, might be any shape... ultimately it doesn't matter - what does matter, is that the port opening area and length of the port have to be sized such that they provide just a little resistance against air passing through it. The amount of resistance is critical to the performance of the enclosure. In this case, essentially you have the same properties of the sealed enclosure, but the vent does something interesting... Again, picture the sub in slow motion. Picture it moving in, compressing the air behind the cone, inside the enclosure. Naturally, this air tries to escape out the port, having suddenly been pressurized. ...and it does, but the port slows it's escape. As the air is travelling out the port, the sub has changed direction, and is now heading out again. By now, the compressed air has travelled out the port - just in time for the cone to move out again - and now we have a situation where the front-of-cone energy is creating a pressure node in the listening space, and the rear-of-cone energy is creating a pressure node in the listening space, thanks to our friend, the port. ...and continuing the thought, the cone has moved out, rarefying the air behind the cone, creating a vacuum effect inside the enclosure, ******* air in the port... by the time the air can be ****** in through the slightly resistive port, creating a vacuum outside, the cone has again moved in, creating a vacuum node itself. One common misconception is that since it has a hole, the pressures must be lower inside a ported box, compared to a sealed box. The opposite is true, however! Again, thinking about what is happening in slow motion - with the sealed box, you simply have the sub compressing and decompressing the air trapped inside the enclosure. With a ported box, however, you have not only the sub, but the momentum of the air in the port pressurizing and depressurizing the air inside the enclosure. As such, pressures are actually HIGHER inside a ported enclosure than a sealed one. Things aren't perfect in "ported land" however. [IMG]//content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif[/IMG] The port is a static thing, and as such, it's air resistance - and therefore it's delay, are a static value. Wavelengths, however, are different for every frequency, increasing in length as the frequencies decrease. As a result, one wave takes more time to pass for a low frequency, than for a high frequency. As such, you end up building a port that's "ideal" at one particular frequency - but as you move away from that frequency, you either overcompensate for the frequency being played, or undercompensate for it - but either way, the phase relationship is degraded, and you again move in the direction of being more or less like a sub that isn't even in an enclosure at all. ...In fact, if you operate a subwoofer too far below its tuning frequency, the air moving in the port is cycling too slow to increase, or even equal the pressures inside the enclosure - and it acts like a simple hole - resulting in little or no air 'spring' value inside the enclosure - making it very easy to bottom out your sub with even less than the RMS powerhandling limit of the sub being sent to it. ...well, that covers ported and sealed boxes... which is a good start anyway. Infinite baffle is much like a sealed box, but with no "air spring" factor, since there's no trapped air involved. Outside of that, other types of enclosures get more complicated... ...although it's fundamentally interesting to note that 4th order, or "single vented" bandpass enclosures utilize one sealed chamber (the one that defines the low end response), making it somewhat synonymous with the behaviors of a sealed box, and compatible with subs that are compatible with sealed enclosures. Similarly, the 6th order, or "double vented" bandpass enclosure utilizes ports in both chambers, making it somewhat synonymous with the properties of a ported box, and compatible with subs that are compatible with ported enclosures. Hopefully that helps augment what BASS OUTLAW started with. [IMG]//content.invisioncic.com/y282845/emoticons/nerd.gif.c6fa51ddf7ff75f1c0371fbc648f70ae.gif[/IMG] [/QUOTE]
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SEALED vs PORTED (basic info.)
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