Bass reflex vent location?

[Davidg]

In a fairly tall bass reflex cabinet or any b/r cabinet for that matter, when the vent is in the front baffle where is the best location for it, under the driver or near the driver as in a 620 cabinet or if the cab is more of a floor standing style would you want it closer to the bottom of the cab near the floor or is near the driver better? Or is it just a matter of asthetics? Does it have any effect on sound quality or bass output?

[Davidg]

Hey Thanks for the info. RK, I've seen that publication before but I re-read it again and while there is some really good info in there they don't really talk about vent location and how it effects sound quality or bass output. I was hoping someone had experimented with this and could shed some light on it. I am currently building cabinets very similar in design to Billfort's cabinet and am trying to decide on vent location etc. Thanks again David

[GM]

Originally posted by Davidg:
In a fairly tall bass reflex cabinet or any b/r cabinet for that matter, when the vent is in the front baffle where is the best location for it, under the driver or near the driver as in a 620 cabinet or if the cab is more of a floor standing style would you want it closer to the bottom of the cab near the floor or is near the driver better? Or is it just a matter of asthetics? Does it have any effect on sound quality or bass output?

<font size="2" face="Verdana, Helvetica, sans-serif">Greets!

It can depending on the cab's aspect ratio and where the driver is located along its height. For example, consider a 42" high 9 ft^3 tower (aka MLTL) with the driver at the extreme top and a 13"^3 vent is theoretically required to tune it to a ~32 Hz Fb. If the vent is at the extreme bottom, then the close proximity to the cab's floor will acoustically make it longer, lowering the actual Fb. The cab is also long enough for 1/4 WL pipe action to occur that will further load the vent, lower Fb still further, so it will have to be made bigger if maintaining ~32 Hz is desired. As the vent moves up the baffle two things occurs, the cab's floor no longer increases the vent's effective length and now the 1/4 WL action has become 1/2 WL and complex standing waves sets up between the floor/vent and driver/vent, effectively reducung the vent's efficiency, so Fb begins rising above ~32 Hz, causing a peaking at Fb. In this case the vent will need to be either smaller in area and/or increased in length to lower it back down, though subjectively it probably won't sound as well damped (tight) as the more acoustically efficient MLTL (mass loaded transmission line).

These various standing waves will affect the BW above Fb by modulating the driver from the backside, so the most accurate reproduction comes from finding the right position for both the driver and vent along this pipeline that minimizes them. Until recently, the casual DIYer didn't have a clue about any of this and no way to sim these effects even if they did, but now it's all explained with inexpensive simming software available here: www.quarter-wave.com.

GM

[Ronald Lee]

You may want to read Altec's publication AL-6030-1 LOUDSPEAKER ENCLOSURES-Their Design and Use, maybe you can find some suggestions. But I would think it wouldn't make any difference where you put it.

[Davidg]

Hey GM, Thanks for the reply, That is exactly what I was wondering about, It only stands to reason that the wavefront is doing different things at different points in the cabinet, I was looking at my front baffle thinking about that and the relationship to the vent while trying to decide where to place it. Now I have some more studying to do! It's a never ending hobby! lol, Again thank you for your input it is greatly appreciated as is this entire board thank you all for your time and effort! David