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Port Location--On Top???

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  • Port Location--On Top???

    I'm going to build a set of Overnight Sensations that unfortunately will be located in pretty much one of the worst listening spaces possible. They are intended to be computer speakers for my small desk/office setup, and I the room just doesn't have the space to correctly put them somewhere. The distance from the bottom edge of my displays to the top surface of my desk isn't large enough and there's no space to either side of my monitors either (I have 3x 24" monitors side by side). Because of this I've decided that I will mount them to the walls just above my computer displays and will point them downwards slightly so that the tweeters are directed towards my ears. I have a couple of questions given this setup:

    1) Most people seem to mount speakers or build stands such that the tweeters are level with their ears at the listening position, as opposed to simply angling the speaker up so that it points towards the ears. Is there really a difference? My gut says no, but I don't see angled mounts nearly as much.

    2) I know that putting speakers near the wall will boost bass frequencies because of boundary reinforcement. I don't think I can get around that since I'll be mounting them to the wall. I believe the general rule of thumb is to trying and keep the port at least 1-2 port diameters away from the wall. This may be possible for me, but I'm not completely sure since I haven't looked at mounts yet. I also know that front port versions of the OS have been done and seem to work okay. Just for the sake of curiosity, would there be any downside to me putting the port on top of the speaker, above the tweeter? For the sake of boundary reinforcement, the corresponding 1/4 wavelength of the 53Hz tuning frequency is a little over 5.5 feet, so I don't think moving the port anywhere on the box will help in that regard. I think from what I've read previously, people are hesitant to put ports on the front because of sound leakage and other noise that you can hear from the inside of the enclosure. I'm just wondering why the port couldn't be moved from the top instead, which would would ensure that I'm well over 2 port diameters from the wall and would also locate it in a place and orientation that I think would be less conducive to hearing sound leakage. Am I missing any reason why this would be a bad idea? Are the coupling issues with the driver when the port is oriented in a different plane than the driver?

  • #2
    Sanus WMS3B Tilt & Swivel Speaker Mount Pair Black

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    • #3
      Generally speaking a top port isn't an acoustic problem so much as the practical problem of stuff falling in.
      Francis

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      • #4
        Originally posted by mister_project View Post
        I think from what I've read previously, people are hesitant to put ports on the front because of sound leakage and other noise that you can hear from the inside of the enclosure. I'm just wondering why the port couldn't be moved from the top instead, which would would ensure that I'm well over 2 port diameters from the wall and would also locate it in a place and orientation that I think would be less conducive to hearing sound leakage. Am I missing any reason why this would be a bad idea? Are the coupling issues with the driver when the port is oriented in a different plane than the driver?
        I don't think people are that hesitant to put ports on the front, it is pretty common. The reasons against are usually (1) you may need to make the speaker taller if it is a small bookshelf speaker and the size wasn't designed for a front port and/or (2) people may not like the look of a front port.

        People don't put ports on the top because of dust and other things that might fall into the speaker. ("Other things" being highly variable if you have small children.) There are some posts with top ports where people put grill cloth or something to prevent this. Also, you would want to keep the inside opening of the port away from the back of the woofer which may be more difficult with a top port depending on the size and shape of the speaker.

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        • #5
          I'll add, front ports can be inferior to rear ports or bottom ports if much midrange escapes the port. Not always a big problem, though.
          Francis

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          • #6
            Originally posted by fpitas View Post
            I'll add, front ports can be inferior to rear ports or bottom ports if much midrange escapes the port. Not always a big problem, though.
            Ok, so let's say I'm not concerned about stuff falling into a port (or kids) or just put the port on the bottom instead of the top. Assuming that there's enough clearance between the port and the drivers/interior surfaces of the enclosure, it sounds like this arrangement could be slightly beneficial right?

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            • #7
              Originally posted by mister_project View Post

              Ok, so let's say I'm not concerned about stuff falling into a port (or kids) or just put the port on the bottom instead of the top. Assuming that there's enough clearance between the port and the drivers/interior surfaces of the enclosure, it sounds like this arrangement could be slightly beneficial right?
              Certainly nothing wrong with it, as long as (as noted above) the port entrance isn't right behind the woofer. That's generally a bad idea.
              Francis

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              • #8

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                • #9
                  Thanks for the input everyone. Chris, that photo is perfect! I also like what they did with the screen so that at least you can fish out anything that does fall in.


                  Is there are general rule of thumb for how far away the port should be from the drivers and internal walls? I thought I read somewhere to try and keep the port tube 1 port diameter away from the walls (maybe it was 1/2?) but that seems like it might be difficult. What about general rules of thumb for how far away from the woofer in the sense of impeding air movement on the backside of the cone by crowding it with the port tube? Or does that not matter just as long as the port opening on the inside of the boxis not close to the woofer? If so, I think I could just make the port opening closer to the tweeter side and I think that would solve the problem. This of course all assumes that there is enough space to get the port tube around the woofer magnet which I will check out with CAD shortly.


                  Thank you all for your help!

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                  • #10
                    Sorry, one final general question on this subject for you all as well. I occasionally see bent ports, mostly with subwoofer enclosures but occasionally with other designs too. I speculate that most of the time it's because there isn't enough physical length in the enclosure to fit the port in any other way. However, I'm wondering if a bent port like this:
                    http://techtalk.parts-express.com/fo...ght-sensations
                    also serves as somewhat of of a filter and prevents undesirable sounds from leaking out of the enclosure. Or perhaps it's just low frequencies and doesn't do much for higher frequencies.

                    If I try to think about the first order mechanisms that might drive the behavior I'm asking about, I wonder if this problem can be answered by looking at the length of each arm of the port--the length from the port opening to the corner and if certain frequencies can navigate that batter than others. So the question is, which sounds "bend" around the corner of the port. I wonder if it's really more synonymous with diffraction effects just like how you'd analyze a baffle. Perhaps you could consider the distance from the port opening to the corner analogous with the distance between a driver and the edge of the baffle. Say that from opening to corner, the port is 2.7" long. I chose that length arbitrarily because it corresponds to a 5 kHz wavelength. Treating this like a baffle then, sounds with longer wavelengths should be able to bend around, while sounds with shorter wavelengths should all reflect off the front of the baffle. In this sense, the port bend does act like a filter to at least partially reject higher frequency sounds from escaping the enclosure. On the other hand, the port is not actually a baffle and is full enclosed so perhaps all of the internal reflections would just reflect and scatter the frequencies all over, both back into the enclosure and out of the port, until all of the energy was lost. Am I even close to right on this one or can somebody explain the actual effect?

                    All this aside though, is bending ports best avoided if possible due to minor loss effects, or does it not negatively impact things at all since the fluid is just air?

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                    • #11
                      You have Port Noise Complaint. If you used a mount such as I linked, the stock rear port position would be just fine. I continually marvel at the angst that ports cause.

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                      • #12
                        bends ... - what bends?

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                        • #13
                          Never seen measurements, but it does seem likely a curved or folded port would attenuate higher frequencies.
                          Francis

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                          • #14
                            Originally posted by djg View Post
                            You have Port Noise Complaint.
                            I think you may be giving away your age.

                            dlr
                            WinPCD - Windows .NET Passive Crossover Designer

                            Dave's Speaker Pages

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                            • #15
                              Originally posted by dlr View Post
                              I think you may be giving away your age.

                              dlr
                              I've been waiting a long time to use that.

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