I'm building a Metronome for a friend. The table on the Metronome web-page calls for a port that is 4 inches in diameter by 3 inches long. I mistakenly cut the hole for a 3 inch diameter port. Plus, I have a lot of 3 inch diameter tube. My question is, before I cut a new hole, what would the length need to be if I kept the port at 3 inch diameter. And.... is it unwise to do so. I can cut the new hole, but the box is already assembled, so I'm looking for the simplest way that still keeps the speaker as designed as much as possible.
NOTE- this is the same build I posted about earlier being built with MDF. He bought enough sonic barrier foam from Parts Express to cover the complete inside, so hopefully, any issues will be minimized. Any words of wisdom, other than not building it out of MDF, will be greatly appreciated.
Thanks,
Mike
NOTE- this is the same build I posted about earlier being built with MDF. He bought enough sonic barrier foam from Parts Express to cover the complete inside, so hopefully, any issues will be minimized. Any words of wisdom, other than not building it out of MDF, will be greatly appreciated.
Thanks,
Mike
Mike,
1) Port length and depth and cabinet internal volume combine to a box tuning, Fb (Frequency of Box) .
Without the cabinet volume, Fb can't be determined. If you figure the box volume, you can determine what length a 3" diameter port needs to be for the same Fb as 4 x 3"using various on-line calculators.
2) Reducing the port diameter will also require a shorter port for the same Fb. The lesser port volume will be "blown out" at a lower sound pressure than the port as designed. Better to cut the proper size hole and use the original port size.
Size matters.
I had to laught but in fact, it does.
If it's too small, you'll get huffing sounds and compression, if it's too big you'll get mid-garbage and it becomes too long and takes up too much enclosure volume (yes, it does not count towards the volume!). Besides that, a huge port looks impressive. Well, not everywhere, there are certain places where you do not want huge holes. 
That only gives the port changes, not which port size is appropriate. There are a lot of things to consider, from air velocity (critical at small ports) over to port length resonances and port surface losses to port length limitations and volume issues. And not to forget the advantages (and disadvantages) of passive radiators and flared resonators (hornreflex).
Thanks for all the replies. I couldn't let myself not build this "to spec," so I cut the proper size hole and am using a 4 inch port. If it was for me, I might have tried to use the 3 inch port. Building it for someone else.... I had to do it right. It only took me a few minutes, so it was no big deal.
Thanks again,
Mike
Thanks again,
Mike
NOOOOOO!!!! When I built my first Met, I made the mistake of covering all interior surfaces w/ foam. It sounded horrible! Had to cut open the bottom to get inside & remove foam from all but the back side. From that time on, I also built all my Mets w/ removable bottoms just in case I needed to get inside. Also: I put the binding posts on the bottom, rather than the back.
What do you recommend as far as lining or filling them? I made the back removable to be able to adjust lining/filling/vent size to taste.
Thanks,
Mike
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