Hi efficient speaker, bass problems

"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

Isn't Bose doing the same thing with his 901, he uses the walls to great effect.

regards,

Duke, as you say "...but sealed boxes sound just plain have more natural-sounding bass than vented boxes!" Now I'm guessing (and really ALL this is just guessing on my part,) this is because after the bass driver emits, the vent lags after driver, then the room lags after the driver, then the room lags after the vent. That's a lot lagging. Then of course, unless your using separate subs, the loudspeakers when placed near the walls will have early reflections from midrange on up, smearing that all important range. Now if you do have separate subs, in order to keep any semblance of time coherence your going to have to introduce some sort of time manipulation. Since your already going to have to manipulate the signal, you might as well just consider bass eq from the full range loudspeakers out from the walls in the first place. Which should have the added benefit of better driver integration and more free floor space.

Personally and absolutely speaking, I have never heard a sealed enclosure sound better than vented, now if you are running tube gear then there is some advantage in using a sealed enclosure.