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In Reply to: RE: Rear port pros and cons (not an unbiased opinion) posted by theaudiohobby on November 18, 2007 at 03:44:52
I chose not to use down-firing ports mainly because they are more difficult to access for changing port length, but otherwise have no objection to them for a floorstander. I'm not sure how practical they are for a stand-mount speaker, where you really don't have effective boundary reinforcement. For a stand-mount speaker, rear porting may be the better configuration for taking advantge of boundary reinforcement.
I don't understand what horn loading and concentric (coaxial?) designs have to do with port location. If you're saying that in some cases the port is supposed to contribute to the midrange output, then I'd disagree strongly with that design approach.
There are arguments for a rear-firing port aside from boundary reinforcement, as indicated in my previous post. If the port length is fixed (as is usually the case), then it could well be argued that a rear-ported speaker designed with boundary reinforcement in mind is particularly placement sensitive. On the other hand, a rear-ported speaker whose port length is user adjustable is extremely room-adaptable.
In my experience, the low-frequency acoustic environment changes very significantly from one room to another, so I place a high priority on the ability to make large adjustments to the low frequency characteristics of a loudspeaker. I have encountered rooms where speaker positioning alone could not give balanced-sounding bass, and significant adjustment of the speaker's output was necessary.
Duke
Follow Ups:
Provided the Speaker stand is designed with rear-port in mind, a downward-firing port gives nothing away to rear-firing port in terms of praticality.
The port length is not infinitely adjustable and there is nothing to stop to a variable length downward or front-firing ports. Therefore, I fail to see how a variable length port provides unique advantages to rear-firing ports. At any rate Kef and Tannoy have used front firing ports very effectively in their coaxial designs in both domestic and profesional applications such that I fail to see any other advantage for rear-firing ports than their ability to take of boundary reinforcement.
Music making the painting, recording it the photograph
Port adjustability is one issue, and might be a nice option to have regardless of where the port is (though a potential issues arises with a front-facing adjustable port; see below). I will concede your point that port length is not infinitely adjustable, but if you have access to a modelling program you can see how much variation in bass response is attained by changing the port length within reasonable finite limits set by the enclosure's dimensions. Offhand I can think of only one manufacturer besides myself who offers adjustable-length ports, but I'm sure there are others.
Port placement is a different issue. In my opinion either rear-firing or down-firing is usually preferable to front-firing. Apparently mine is a minority position, judging from a glance at the marketplace.
Port adjustability is not an advantage unique to rear-firing ports. In my opinion flexibility is maximized when the adjustable port is rear-firing; most of my reasons for this have already been stated but here is another:
Sometimes the room offers too much bass reinforcement, and with a rear-firing port you can reduce the boundary reinforcement by moving the speaker away from the walls. With a down-firing port, you'll still be getting a lot of boundary reinforcement from the floor wherever you place the speakers. So if the goal is maximum adaptability through a combination of speaker placement and port tuning adjustment, I think that rear-firing offers a wider range of flexibility than down-firing.
One factor that I haven't mentioned yet is the damping material inside of a reflex enclosure. For maximum bass you want no damping material, but to minimize midrange coloration through the port you want some damping material. The amount of damping material you need to acceptably minimize midrange coloration is greater with a front-firing port than with a down-firing or rear-firing port. This translates into more powerful low bass from either of these less-conventional formats because of the reduced damping material requirement.
Note that increasing the port length also increases the damping material required to keep midrange coloration acceptably low. So this is another reason why an adjustable port makes more sense on the bottom or rear of the speaker, as less damping material is required with these configurations.
Yes there are excellent speakers with front-firing ports, and probably some lousy ones with rear-firing ports. It's not the biggest issue in loudspeaker design.
Duke
I have been using a pair of Tyler Taylo monitors that are front ported. They are excellent speakers, but found myself plugging the front ports to control a bit of "boom" and to help integrate my sub. I recently picked up a pair of Merlin TSM-MMe's that are sealed and they are much easier to integrate with my sub and they sound marvelous. If you have a small or otherwise difficult room, i would consider a sealed enclosure. My 2 cents.
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