Hi efficient speaker, bass problems

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.

Unsound, I don't think the ear can hear all of these lags in the bass region.

That's one of the reasons bass is perceived as omnidirectional inside a room: Our time-domain resolution is so poor down there that the ear/brain system cannot tell the difference between the first-arrival sound and the reflections.

In contrast (so I've been told by a psychoacoustics researcher; never tried this myself), outdoors the ear is able to discern the direction of low frequency energy down to a much lower frequency than in a room. This is because there are no reflections to confuse the ear-brain system by arriving before it has resolved the initial impulse. We can hear direction at very low frequencies, as long as it's not masked by reflections arriving before our ear/brain system can isolate the first-arrival sound. All of those lags you mention arrive before the ear/brain can isolate the first arrival sound, so perceptually they just get blended in with it. And the room's effects on the frequency response are what dominates the perceptual modification of what would otherwise have been a clean first-arrival impulse.

I used to believe that the superior time-domain performance of a good sealed system was the reason for its "tight, natural" sound, but after reading Earl Geddes and Floyd Toole on the subject, I now believe that the in-room frequency response is the dominant factor by far.

Come to think of it, for many years the most natural-sounding bass I'd ever heard was from an IMF transmission line. A transmission line would inherently have poor time-domain behavior but might well have not only a room-gain-complementary native frequency response, but also significant physical displacement of its two bass sources (woofer and line terminus), with ensuing dissimilar room-mode-interaction from the two along with its smoothing effect. This would have addressed the two main problems of in-room bass reproduction: Gain from boundary reinforcement, and peaks-and-dips imposed by room modes (actually just different manifestations of the same problem). But no one really thought in those terms back then.

Duke