[Duke]

One technique that works well for smoothing out the in-room bass is to spread out the bass sources as much as is practical in all three dimensions.  This way each bass source interacts with the room's modes differently.  For example, suppose you have floor-standing speakers with the midwoofer about two feet above the floor, and a rear-facing port down near the floor.  Your two bass sources (woofer and port) are at a different distance from the room boundaries in two dimensions, and factor in the other speaker and now you have bass sources at different distances from the room boundaries in all three dimensions.   Toe in the speakers and you get a bit more variation in the side-to-side dimension. 

In some cases a rear-firing port can result in too much bass reinforcement due to its proximity to room boundaries.  Often it is possible to lower the tuning frequency, which can prevent boominess and extend the usable bass deeper than it otherwise would have gone.  I can describe how if anyone is interested.

Finally, in a small room early onset reflections are pretty much inevitable, and because of the way the ear/brain system works they will have more influence on perceived timbre than late-arrival reflections.  While some schools of thought advocate using a lot of absorption in a small room, I believe this arises in part from the poor spectral balance of early reflections exhibited by most speakers.  In cases where the spectral balance of the early reflections is free from major anomalies, diffusion is usually a better approach in my opinion.

Duke

[EthanH]

In some cases a rear-firing port can result in too much bass reinforcement due to its proximity to room boundaries.  Often it is possible to lower the tuning frequency, which can prevent boominess and extend the usable bass deeper than it otherwise would have gone.  I can describe how if anyone is interested.


Duke

I'd be very interested to learn how to do this.

[Duke]

Hi Ethan,

Assuming a round port, you can lengthen the port, keeping the same diameter, and this will lower the tuning frequency.  This is usually not practical.

Or you can decrease the diameter of the port, keeping the same length, and this will lower the tuning frequency.  The downside is, turbulence or "chuffing" will set in at a lower SPL, but unless you're pushing your system near its limits anyway this probably won't be a problem.

The technique I recommend is this:  Measure the internal diameter and length of the port, and see if it's practical to use a longer port while maintaining about one diameter's clearance from the woofer frame and magnet and/or inner walls of the enclosure.  Then go to Home Depot or some place that sells plastic plumbing supplies, and purchase plastic tubing that has a slightly smaller outer diameter than the port's inner diameter.  Also buy a roll or two of electrical tape. 

Cut the plastic tubing to the longest length practical, and then wrap with enough electrical tape to build up to a pretty good friction fit.  I suggest building up the tape in two bands, one near each end, but don't overshoot the length of the port in the box because that's what will be friction-fitting the inner port in place. 

Give it a listen.  If you have reduced the bass too much, shorten the inner port a bit.  If you still have too much bass, you either need a longer or narrower inner port.

Now maybe you have one speaker in a corner and the other just along the wall, in which case the speaker in the corner will be getting considerably more boundary reinforcement.   You may want to lengthen that speaker's port more than the other one's.

It is possible that the combination of driver parameters, box size, and boundary reinforcement won't allow you to tune low enough to tame the bass.  In that case, you might need to go all the way to a sealed box.

If you want to get really fancy and replace the factory port with one of your own, I recommend the modular flared Precision Ports system.  That's what I use in my commercial speakers, and I have customers using a variety of port lengths depending on their situations.  Available from Madisound and Parts Express, in 2", 3", 4", and even 6" internal diameters.  You'll probably have to cut a large enough round hole, but then you can screw the outer flare in place.  Don't put weatherstripping on the underside of the lip of the outer flare, as that will cause the plastic to crack when you tighten down the screws.  You'll get a good airtight fit if both surfaces are smooth. 

http://www.psp-inc.com/products_menu.html; port length calculator on the "tools" tab - with a little trial-and-error, you can figure out what the approximate tuning frequency of your box is to begin with.

Duke

[EthanH]

Thanks, Duke.  I'll probably try this out at some point.  I also recall seeing people stuffing drinking straws in speaker ports, and I'm wondering if this is sort of the same idea.

[Duke]

Drinking straws would also reduce the tuning frequency, especially if they're longer than the port.  You have to pack 'em in there until they deform into hexagonal shapes in order to get a good friction fit.   Doing so without pushing the straws in too far is tricky, so you'd probably have to remove the woofer during the process.

[EthanH]

I gave the drinking straws/narrower port a try and so far have mixed but mostly positive results.   Bass is deeper and less boomy, however, I think the most noticeable difference is the improvement I'm hearing in the midrange - the speakers just seem to "breathe" easier now.  It makes me want to experiment more and really dial the tuning in. 

Before trying this experiment I was getting this "hootiness" in the lower midrange that I suspected was a result of the port tuning. Stuffing the ports helped the hootiness a little but also made the midrange more forward than I prefer.  So far this experiment with lowering the tuning frequency has produced the best results, in that the hoot/resonance noise in the lower midrange is reduced while the midrange also seems more relaxed overall. 

[jimdgoulding]

My Meridians came with that in their front firing ports.  I thought it was a proprietary device to quiet chuffing or something as I hadn't seen it used before but having just looked closely at it, by joe, it does look an awfully lot like drinking straws.