Hi efficient speaker, bass problems

Wesixas, we may be splitting hairs here, but, I agree even more with your criterion. Of course the impedance will have to be figured in as well.

"Duke, some of us are not so enamored with the sound of most loudspeakers with ported bass." - Unsound

Me neither! But remember that unless you're outdoors you never hear the bass independent of the room. The bass of most ported-box speakers isn't voiced to synergize well with gain from boundary reinforcement. However if done right, I believe that a good ported box has qualitative advantages over a good sealed box... mainly because it can be more closely tailored to what's going on in the room.

"Duke , it does seem to me that you are discussing speakers designed for corner placement, if not how would this apply when speakers are setup and voiced away from the walls." - Weseixas

Corner placement would be the extreme example, but wouldn't it make sense to approximately match the speaker's inherent bass rolloff characteristics with its acoustic environment regardless of where it is? If the speaker is up on a stand out in the room away from walls, maybe even in an open-floorplan home, then you're going to need a great deal more bass energy from the speaker in order to get acceptable tonal balance. In this scenario we'd shorten the ports and raise the tuning frequency, and once again a ported box gives us options that a sealed box does not.

I don't know why more manufacturers don't incorporate user-adjustable ports. Most of my customers have ended up happiest with a tuning frequency different from the one I shipped the speakers with. One customer is even tuning the two speakers differently based on where they're located (one's in a corner, the other isn't). I use Precision Ports, available from Parts Express and Madisound and probably others, in case anyone wants to retrofit them to their speakers. They screw into the cabinet so they can be removed, and I just use a few wraps of electrical tape to secure the sections together.

Duke

Duke,

I think your assessment of the pros and cons of ports is right on!

"Duke , it does seem to me that you are discussing speakers designed for corner placement, if not how would this apply when speakers are setup and voiced away from the walls." - Weseixas

As mentioned by Duke, the speakers can be placed just about anywhere in the room, including corners. I own a set of his Jazz Modules and I too have to wonder why other manufacturers whose speakers incorporate ports don't allow for user adjustment.

My speakers are 5' out into the room (19 x 15), 6' from side walls, and toed in approximately 45 degrees (the axis crosses about 18" in front of me). The bass port can be extended about 8", but I do not use any of the extensions. So I'm getting more bass energy over most of the bass region, but a more rapid roll off once it starts rolling off. With a 12 watt SET amp using it's 3 ohm tap (the speakers are 8 ohm minimum, 12 ohm top) I'm not suffering from lack of bass response.

What I've been talking about above might be called "room gain complementary tuning". Let's look at a couple of well-documented examples.

First up, the little Guru QM10, which defies expectations with its bass extension into the lower 30's. When placed up against the wall as recommended, the QM10 gets boundary reinforcement off the wall probably up to 200 Hz or so. As we go progressively lower in frequency, other room surfaces start to become a small enough fraction of a wavelength distant that their reflections are also in-phase, thus reinforcing the original sound. First the floor, then the ceiling, and somewhere along the way the near sidewall, and then the far sidewall. The designers apparently anticipated that this room gain would be about 3 dB per octave, and so they chose a woofer, box size, and tuning frequency that would give an approximately 3 dB per octave anechoic rolloff down to about 35 Hz. The rolloff accelerates after that, and after anticipated room gain we're -3 dB in the lower 30's.

Here are SoundStage's measurements of the Guru QM10. Notice that from about 140 Hz down to about 35 Hz (two octaves), the bass rolls off at about 3 dB per octave anechoic:

Guru QM10 measurements

If anyone has a modelling program, I challenge you to find a practical sealed box alignment that will result in a 3 dB per octave rolloff anechoic like that. A Qts = .50 sealed box still rolls off at 6 dB per octave anechoic.

The Guru is not alone in using a 3 dB per octave rolloff to take advantage of room gain. Let's look at the corner-placement-recommended Audio Note AN-E reviewed by Stereophile. From 200 Hz down to 25 Hz, the rolloff averages about 3 dB per octave:

Audio Note AN-E measurements

My point being, here are successful real-world examples of room-gain-complimentary tuning, including the all-important measurements that reveal what the designer is trying to do. If we were designing a speaker that would normally be getting less boundary reinforcement, we'd want to take that into account as well.

Now wait just a minute, you're probably thinking; all this theory is great, but sealed boxes sound just plain have more natural-sounding bass than vented boxes! Okay, here's why (imho of course):

Most sealed boxes roll off somewhere between 6 dB per octave and 12 dB per octave. Most (nearly all) vented boxes are tuned for deepest-loudest-possible bass, so they are "flat" as low as possible and then roll off very rapidly (18 to 24 dB per octave). Of the two, the sealed box's rolloff characteristics come closer to approximating the inverse of room gain, especially if it's a low-Q box, so sealed boxes will work better in a wider variety of rooms than "typical" vented boxes. And too much bass (more likely from a typical vented box) is a more heinous crime than too little bass.

But take both speakers outdoors, where boundary reinforcement is negligible, and the flat-tuned vented box will have better tonal balance than our sealed box, and much better than our room-gain-complementary tuned Guru or Audio Note. It's not just the speaker and it's not just the room (or where it is in the room) - it's both.

Duke
dealer/manufacturer/no affilication with Guru Pro Audio or Audio Note