Re: SEALED vs PORTED (basic info.)
You can't get the same response shape from a sealed and ported enclosure. A sealed box rolls off more gently below resonance than a ported one. A ported box is good to basically the tuning frequecny and the output quickly becomes useless below that. A sealed box can provide usable output well into the range of inaubidility and when the transfer function of the car is factored in the response is generally pretty flat well below the range of hearing.
Originally Posted by BASS OUTLAW
You can't always tune a ported enclosure low. There is an optimal combination of volume and tuning frequency for any given sub. Deviate much at all from either one and the results are anything but quality sound. The response curve will get peaky, the transient response will become poor and the group delay will become unacceptably large. If you tune an enclosure too low you lose the efficiency advantage that a ported enclosure enjoys over a sealed in the 30-50 hz range and the accuracy of the output suffers as well. You can tune a bit higher than optimal without a huge tradeoff in quality of sound (you trade efficiency for low frequency extension in this case) but go too low and you will have problems. The performance of an enclosure volume and tuning depend entirely on the sub used in the enclosure. You can't just say build a box to this volume and tuning and any sub you throw in it will work well. A larger percentage of subs will perform terribly in that enclosure than will work even marginally well. Sealed is a lot more forgiving in this respect.
To add to what geolemon said: About an octave above tuning, a ported enclosure behaves almost exactly like a sealed one. Below tuning, the pressure wave from the port not only cancels most of the output from the cone but the inertia of the air in the port acts to move the cone farther than it would in free-air. This can have rather disasterous effects on the structire of the sub. Coils jump the gap, surrounds and spiders get torn, cones get cracked/torn, fomrer strike the backplate and are cracked/deformed. None of those equate to the long life of a sub.
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