Overporting: definition & effects?

[Svante]

Hard to believe the industry went all those years having the closed box solved, but not having a clue about the ported box. They knew all about Qtc and such.

The closed box was considered supreme for hifi until 1980 or so. Before then, the ported box was considered a boom box, largely due to the mistuned units being sold at the time.

Novak's paper came out in the mid fifties. And you would think that once you had the closed box figured out, someone would just go down the line with various closed box Qtc's and port them to see what sounded good. Yet they went for years and years before the ported box was finally considered fully understood.

I think this publication from 1966 is pretty impressive and rather complete in the understanding. It did not get very well spread, but it was one of the first.

http://www.speech.kth.se/prod/publications/files/qpsr/1966/1966_7_3_007-022.pdf

 

[PeteMcK]

Re. EBS alignment, This might be useful:

http://www.44bx.com/tannoy/lowq.html

 

[Mos Fetish]

Below is a graph for two Vifa M22 WR-09 in a 65 liter box, sealed and ported to 31 Hz. The Qtc is 0.54. The other Vifa M22's WinISD listed were far to high in Qts, (0.7), for it to be them.

Yeah, the WinISD M22 is the 6ohm model; mine's the 8ohm. But the rest of the parameters are identical.

So, with a Qtc of .54, closed, I'm right on the money. WinISD vent length for the ported is pretty close as well. LspCAD calculated the port length slightly shorter, which is what I went for.

 

[pjpoes]

Ok to my earlier post on Bass traps, do some searching on the internet for bass traps to see what they are. Speaker placement and such will not change room modes enough to get rid of them. Thats not to say that placing your speakers incorrectly wont excite certain frequencies making it boomy, but some modes have to do with the room dimensions, and no matter where you place the speakers it wont fully get rid of them. This is where Bass traps are good.

I'm not sure if I understood what you meant by placing them the same distance from the side walls as the corner, but that might actually be correct. One of the idea's for room placement is to place the speakers using a rule of thirds. It was based on certain average dimensions of rooms, and so isn't always ideal, but its a good start. In a room that fits this average dimension, its not uncommon for the distance from the sidewalls to be about the same as the distance from the rear walls. I believe RPG, the room acoustics company, has a calculator you can use to come up with the best placement. I always tune by ear afterwords, but its good to find out what room modes you will get with your placement.

The main reason to try placing them farther out into your room is that it will help get rid of some of the corner reinforcement and boundery effects. This isn't actually ideal, its just to see if possibly they are what is causing the problem. If they are, a bass trap will even out the response, while still letting you have the benefits of lower extension from boundery effects.