Why are ports louder?

I recently designed and installed a system for a guy I know (cant quite consider him a friend).  It consists of two 12" alpine type r's, on a fosgate p6002, with a custom built ported box for them (built to the spec of the alpine ported box, just to fit inside his 04wrx best).  So far Ive been very impressed with it, its gotta be hitting a strong 150db.  However this guy is just never impressed and doesnt realize that for $1000 this is one of the louder setups he could have gotten that would fit in his car (yes I know some well set 15s or high excursion 12s would have been louder).  Well he doesnt really know anything about car audio, his last system was in an 03 accord, it was all from bestbuy (where he worked at the time), and it consisted of 4 12" kenwoods on some 1000w peak sony amps, all in some bestbuy proboxes.  He never wants to trust me that a ported system will give him more spl over a sealed one, I completely know that hitting a good 150 in a wrx with twelves is a pretty impressive number, and that a ported or bandpass system is the best way of doing this.  However he really wants to go buy a probox from bestbuy (sealed), and try them out, I would like to see those 1/4" boxes blow up with the stress of the type r's, but Ive been hearing some crap around school for my design of this system and my ideas of a ported box being louder than a sealed.  Can I get an explanation of exactly why a ported box is in fact louder, I know that it all has to do with it being tuned to a specific wavelength and all but Im not completely sure on the entire reason behind a ported box's loudness.

See http://www.diysubwoofers.org/ or buy the book The Loudspeaker Cookbook. Basically the, "port [] allows the passage of air in and out of the box. At low frequencies, the vent contributes substantially to the output of the system." Also, a major benefit for SPL is "better power handling within the system's passband." Which means it will get louder with less power. There are negatives to going ported. These include less forgiving design, typically worse transient reponse than sealed, and higher levels of distortion below the tuned frequency. I doubt this guy would be impressed with some slop sealed box from bestbuy. If he does go sealed make him find a nice braced and damped box. Anyway.. within the passband ported enclosures ARE louder given the same amount of power. Tell the people at school to read the sources I've given you. Also, know that not all subwoofers work the best in ported boxes. See the diysubwoofers.org page for some simple guidelines. New Project: 2003 Pathfinder

There are many technical things happening behind a vented enclosure. What you really have is called a Helmholtz resonator, but that is not really important right now. It is true that a PROPERLY constructed vented enclosure can be louder than a properly constructed sealed enclosure, but only in the band to which the enclosure is tuned. Your box/vent combination will resonate at one center frequency, with a 12dB rolloff to either side of that center frequency, ie, if your enclosure is tuned to 50Hz, at 25Hz and 100Hz, your output will be 12dB lower than the center frequency. This is the ENCLOSURE/VENT ONLY. The driver will continue to produce output at those frequencies, but at the lower frequency (25Hz), your driver will want to "unload", which means it will start to act as though it is running in free-air. This is why it is recommended that you apply a subsonic filter, to keep these lower frequencies from even being amplified... This resonance (basically the port "noise") adds to the output of the woofer, again, with most efficiency, at the center frequency (50Hz, in this example), which adds 3dB to the output of the system, with corresponding loss of additional output to either side of the resonance frequency. The reason for the additional output is because this center frequency is 360 degrees out of phase from the original inout signal. 360 degrees is ONE WAVELENGTH out of phase from the original, but due to the VERY long wavelengths you are dealing with, the interference is constructive, meaning additive. As the frequency shifts above or below, this phase changes slightly, until it reaches +180 and -180 degrees, and as we all know ±180 degrees is a complete cancellation of output, right? There is why the output rolls off... BTW, it is a complete fallicy that a 15 inch driver or a long throw driver will be louder. The maximum output of a driver is figured by multiplying Reference Efficiency (1 watt at 1 meter) by the maximum input power handling capability. 1, 2, 4, 8, 16 watts, etc, with a 3dB gain per input doubling. This is a STRONG oversimplification, there is more to it than this, but this is the general idea. If you have a 12 inch driver with an Re of 90dB, but a power handling of 512 watts, you will have a maximum out capability of 117db, but if you have a 15 inch driver with an Re of 87dB, and the SAME power handling of 512 watts, you will only end up with 114dB, a 3dB reduction in output. Also, through very complex mechanical formulae, and for reasons well above my head, you will ALWAYS exchange output for extension. This means a long-throw driver that is capable of producing low frequencies, will have a very low Re, and therefore require TREMENDOUS excursion and power handling capabilies. A perfect example is the Eclipse Ti12. With a specifed Re of 87, this is a perfect example of a driver that you have to beat the s**t out of, in order to get good output from it. I know Steven will have something to say about this statement, and I am willing to accept whatever criticism he is willing to hand out, but even the Adire stuff will NEVER have the efficiency of a JBL or PAudio professional driver. The Adire stuff is VERY capable, and has good efficiency for it's excursion capabilities, but if you look at a JBL 18, it has an Xmax of, like 1 or 2mm, but an efficency of 105dB. I do not see ANY long throw woofers (and by long throw, I mean 8-10mm or more) with an efficiency like that, and believe me, I've looked. An engineer friend of mine at Harman has tried to explain why this is, but as I said, it's over my head, and he's an ENGINEER! Oh, and muffinman, vented enclosures are ALWAYS LARGER than a sealed enclosure, so there goes your theory, eh?It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."

Bass-Reflex Enclosures      Another approach is to add the radiation from the rear of the diaphragm/cone to the radiation from the front. This is done by using the volume of air in the enclosure which acts in conjunction with the mass of air entrapped in a tuned port hole to create an in-phase, additive relationship.      This combination of the rear radiation being added in phase to the radiation off the front of the cone results in almost twice the output for a given excursion of the cone than would be expected if the speaker were mounted in an infinite baffle.      These phase-inversion or bass-reflex enclosures offer greatly increased bass response with a minimum of structural and tuning work.

I say taht you , or someone else, should build a system incorporating one of these Horn Projectors      Bass response can be substantially increased if the front of the cone is coupled to a long, expanding horn. The old Edison phonographs had a diaphragm about the size of a dime. When the large ?morning glory? horn was attached to the small diaphragm, each motion of the dime-sized surface was transformed from a small-area radiation into a large-area radiation. This means that horns act as acoustical transformers. Such transformers can take many shapes such as conical, hyperbolic, catenary, parabolic, and exponential.      In all types of horns, the effect is to present to the small diaphragm at the throat a very high but consistent acoustical impedance while transforming the high-pressure, low particle-velocity wave from the surface of the cone into a low-pressure, high particle-velocity wave with a low impedance, matching that of the air in the room as it reaches the mouth of the horn.      This consistent high impedance at the throat of the horn can be maintained as long as twice the square root of the mouth area times pi is greater than the wavelength required. If twice the square root of the mouth area times pi is less than the wavelength, then the horn does not properly ?load? the diaphragm at the throat to the air impedance at the mouth. From this it can be seen that to have a usable low-frequency horn, large dimensions are required. For 30 Hz, a mouth area of 45 square feet would be needed. Nine times five feet is a large mouth in any room.      Folded corner horns can materially reduce the enclosure dimensions required to achieve such a large mouth area by folding the horn back and forth in its own path before using the corner of the room as the mouth of the horn.      For all these reasons, a horn-loaded speaker mechanism is very efficient; i.e., it will provide more acoustical watts output per electrical watts input.

Poormanq45, are you getting that text from some site? New Project: 2003 Pathfinder