The Rotary Woofer

I first read about it more than 20 years ago, the Eminent Technology TRW-17 Rotary Woofer.  Yes, I can't afford it (not planning to open theme park).  Peter Montcrief in the International Audio Review called it the only true subwoofer.

They have a downloadable audio test CD.

They argue that there is no clearly defined low frequency limit to human hearing.  Instead, human hearing simply becomes less sensitive at lower frequencies.  At 5 Hz, it takes an astounding 115dB simply to be audible.  For cone speakers, this is extremely difficult, it takes more and more surface area to reproduce such low frequencies.  Of course, it was exactly what the TRW-17 was designed to do.

(I'm not planning go get one, they're very expensive and require custom installation.)

Dialed back 26 Hz boost to 0.1db

On Wednesday morning, I was finding it somewhat difficult to crank the volume level on Bass Ecstacy by Bass Erotica (a very low-bass-heavy album).  So then it occurred to me that after I extended the deep bass to 16 Hz this weekend by adding one subwoofer port plug on the left side and two on the right and lowering the low frequency cutoff accordingly, I may well have been adding back in much of the circa 26Hz bass the boost had been intended to restore.

So I dialed the 26 Hz EQ (in the DCX 2496 crossover) back from 3.5dB, where I had set it most recently, to 0.1dB, effectively disabling it without removing it.

Then the extreme bass became more tolerable, and I was able to raise the level by a comfortable 10dB, making the lyrics and effects much more open sounding.  I was even dancing.  But I think the bass still needs more refinement.  I believe I can still listen to this very difficult album louder in the master bedroom, where I have custom equalized the bass with parametric and graphic EQ using a Behringer DEQ 2496.

Might be time to bring back the keyboard, which was put away on Saturday Night to prepare for a party on Sunday.  I was even thinking about using a real oscillator, but might be nicer to program a frequency offset slider into my keyboard sine program.

*****

Back on Friday night, when I found another bass port plug for my left subwoofer, I measured the effects of decreasing the low frequency cutoff frequency on 16Hz playback, and the effect of adding the one plug.  Adding the plug did increase output by a few dB, as did changing the cutoff to the one-plug recommended setting (which is 18 Hz).  Adding the plug had a bigger effect than turning the cutoff to the two-plug recommended setting (which is 16Hz).  I had the numbers written down, but they did not get copied to this blog.  Now I've found a second plug for the left subwoofer, and thinking about doing a whole matrix of measurements.  But I need to replace the C cells in my Genrad 1933 SPL meter, which conveninently has a "flat" setting and 1 inch microphone.  I was measuring levels between 89 and 100dB SPL at 16-20Hz.  I believe I boosted response at 16Hz from 89 to 92dB.  The bass plug not only increased 16Hz response, it flattened the hills and valleys between 16 and 25 Hz.

SVS shows the flattest response (ruler flat) to 20Hz with no port plugs.  They show 2.5dB loss at 20Hz with one plug, but improved extension to 15 Hz before cutoff.  They show 6dB loss at 20 Hz with two plugs, with little visible knee in the curve, but lots of drooping, so response isn't actually extended until you get down to 13Hz or so.  Three plugs produces 8dB loss at 20Hz, with no indication from the curve that this offers additional frequency response extension at any frequency.  According to their graph, I think I would prefer the 1 plug modification, very little difference at 20Hz but nearly flat extension to 15Hz.  The two plug seems to sacrifice too much response at 20Hz, but seemed better when I measured on the right speaker.



*****
Here is the 

House Curve discussion

Here's a discussion (or really, an introduction to the concept) of a House Curve, what I generally call "room curve" (coming from the Tact tradition of Room Correction System preamps).

http://www.hometheatershack.com/forums/rew-forum/96-house-curve-what-why-you-need-how-do.html

He proposes an interesting way of setting curve by making 100 Hz and 30 Hz sound equally loud.  (Just due to human hearing response, the 100Hz would naturally sound louder if played at equal physical loudness, and the effect is greater at lower playback levels.  So this is obviously increasing deep bass response over the flat or natural response, a point Wayne doesn't make or belabor.)

I find the "discussion" (actually, there isn't any discussion, except Wayne raises and addresses some issues in his own writing) rather lacking.  The rationale for house curve is very flimsy.  I suspect that HTS editors feel take house curve as a given need, didn't need much convincing.  Wayne is at his best shooting down other bogus ideas, such as that a house curve is needed because of the industry's "X curve".

Despite having this gnawing feeling that rationale for a room curve (other than flat) is at best circular reasoning, I count myself as a believer in having a house curve and Wayne's ideas (such as the 30Hz, 100Hz test) are sensible even if his arguments for them are weak.

I'm thinking the way to think about this is to consider that every room has a reflective signature, and a room curve is chosen to make music more intelligible given that reflective signature, they type of music it is, the type of speakers, etc.  Start with this as "hypothesis 1" and I think it's fairly obvious.

"Hypothesis 2" goes farther, making some specific claims.  Flat average response falls flat because it lumps together direct and reflected sound, which the brain is somewhat capable of perceiving separately.  You would think the direct sound should be the flattest, if you equalize the total sound, which includes proportionately more bass, you will make the direct sound component of it lighter in bass.

Aha, but we do have ways of mesuring or computing direct vs reflected sound.  And a system can be designed around the goal of flat direct sound.  Has been I'm sure.  And what is the result?  I don't know, hypothesis 2 could be wrong.

"Hypothesis 3" takes a different but similar view.  Instead of direct vs reflected analysis, our brains are assumed to have real-room-response correctors.  When we go into any room, we start correcting the sound to fit our perceived sense of how the room itself boosts that (in modal patterns in the bass).  Therefore, the recorded sound played back should have those same boosts.

The problem, however, with taking H3 seriously is that flat-played-back sound will indeed get the room boosts added to it.  That is what the natural room boost does to all sounds.  So from this perspective, a "house curve" would be adding to this.  But why should boost be added more to played back sound

The keyboard oscillator

For testing speakers, particularly subwoofers,  I find that nothing beats the conveninence and flexibility of a keyboard synthesizer.  Most Kurzweil "programs" (what others might call soft instruments) are very complex.  But it's not hard to cook up a sine wave oscillator useful for testing subs, building on the simple "Default Program" number 200.  Here is what I generally set up:

Tone: sine wave
Pressure sensitivity: 0
Control #8: volume
ADSR: sustain 100%, else 0%

The area between C0 and C1 (16 and 32 Hz) is interesting.  I think my EQ boost in this area helps restore strong response in this difficult region.  But each different note causes a different feature of the room to start rattling.  Only below 18Hz or so does it seem like keys do nothing.  And there I wonder if I haven't programmed my SVS PB13 to cut out too high.

*****

I can't reiterate enough how useful a keyboard oscillator is for system adjustment.  It just sits there calling me to plunk a few more notes, checking out some other aspect of the sound.  Changes I've made so far:

+5dB at 27Hz changed down to +3.5dB

Room mode cut: 44Hz -9dB Q: 2.2 (sounds all to the better)  This is the most important room mode to tame within the subwoofers operating region where it is most important to eliminate high Q resonances (the sub tends to stimulate room modes far more than the panel speakers...room resonances can almost be ignored on the Acoustats).  Back before I started using Tact correction in the Living Room (January 2010, I remember it well if not memorialized here) I had been using two tuned resonance cancellors at something like 38 and 45Hz.  The keyboard makes it easy to check these things out and be sure you haven't gone too far (I know 9dB is a lot but it is vaporized in the wind of the resonance).

PB13 subwoofer tuning: I've put in two plugs into left sub ports (leaving 1 of 3 ports open) which permits me to dial back the sub low frequency cutoff (lfc) two notches (I had previously dialed back the lfc from 20Hz to 18Hz anyway.)  So now I'm choosing the 16Hz cutoff and I'm doing so within recommended usage.  Putting the lfc in in a lower-than-recommended position now buys very little difference (1dB) at 16Hz: measuring (with GR 1933 meter) I get 69 or 70dB at 16 Hz, 69 with recommended filter setting, while 70dB is about the average response level 20-80Hz, though response clibs to 72.5dB at 18Hz.  As it now stands, 70dB at 16Hz is barely barely audible (the lower setting changes this to barely audible); the strong 72.5dB at 18Hz is nicely audible.

New house curve

I started running the Tact Room Correction analyzer for measurements on Friday evening and into Sunday.  I decided that my living room system is measuring so good it does not need full system correction (and I will need to get better about pasting from measurement into target curve).  However, I made several important adjustments.

Final Adjusted Response, Both Channels, Nov 20

First I moved speakers slightly back and out by about 4 inches, the most available with current positioning of Belkin PureAV power conditioner.  This seemed to bring a slight additional improvement in image coherency,   The highs are also adjusted to be slightly hotter, with listening position just barely off the Acoustat beam, and flat response in the highest frequencies.  The previous speaker position is marked by tape.

To reduce bass blooming around 100 Hz, I backed down the subwoofer crossover from 84Hz to 71Hz, and changed the slope from 24LR to 48LR.

I lowered panel crossover to 80Hz, not wanting to make it lower for speaker durability.  I tried several crossovers, but decided I liked the thinking behind 24LR the best.  Years ago measurements showed the LR24 as having the nicer looking impulse than LR48, but I wonder about that now and think the the Tact impulse itself has multiple cycles, so I would have to do actual impulse measurement with other program..

To increase bass in the range 22-30Hz, which looked notably sucked out in both channels, I added a bandpass filter to the Behringer subwoofer outputs, 5dB of boost, center frequency 27Hz, Q 2.2.  It could use more boost in left channel than right, but crossover currently has this set to stereo mode.

I increased delay for panels relative to bass slightly.  I get the best measurments of bass impulse when turning off crossover.  Then put sub in one channel, panel for same channel in opposite channel, can get picture of bass vs panel impulse.  I also did this method of using two channels to measure one channel for checking crossover frequency response.

Resulting delay ia now 0.85mS in right channel (was 0.70) and 0.75 in the other (there is an extra 0.10 mS delay for right subwoofer as compared with left, same as before).  This is consistent with having listening position closer to the front, so relative distance to subs from listening chair has increased slightly.

I also tried dropping the bass level from -7.0 to -8.0, but quickly decided I wanted more bass.  Funny in the crossover picture the sub bass dominates, but that's the way it sounds best.

Coherent Imaging and making the speakers disappear

More than two months ago I moved my supertweeters out of the living room.  They were contributing to a gridlock which made it impossible to move the Acoustats.  Since I am now (since early this year) listening from a position much closer to the Acoustats, it was seeming like they might be too far apart for the close-up listening position.  The angle between the speakers from my head was more than 60 degrees, and while I was still getting a center image, beyond the center things seems a bit vague,

Finally on Sunday evening I started moving the Acoustats laterally in toward the center.  First about 5 inches in on either side, then a few more inches which reached the maximum point I could move them inward because of my electronic equipment.

The new position also allowed me to move the listening chair even closer in and still get a stable center image, and moving closer in gave me nicer bass.  The nicer bass is because I am closer to both the Acoustats and the subwoofers, and because I'm moving away from the center of the room where all room modes have their main cancellation and there is a big bass suckout.

But there were two problems now.  The image started to get a shrunken quality, with the soundstage no longer seeming life sized.  And now, some instruments seemed to be playing right from the speakers themselves.

To fix those problems, I moved the Acoustats slightly back and slightly to the side.  Because of electronic equipment, notably the tuner and the MSB PAD-1 which converts the tuner output to digital, I couldn't move the speakers back more than about 4 inches.

But that 4 inches made a magic difference.  Now the speakers themselves were not longer clearly the source of as many instruments as before.  Instead, the position of those instruments moved forward, to the the same depth as the center of the image.  Thus the center of the image was no longer by itself, there is now a right-center and left-center, and much of the music appears to be coming from a plane about 3-10 feet back from the speakers.

So I'm glad I started these speaker moving experiments because I now think I may have some of the best imaging I've ever heard.  I plan to move some of the equipment so I can move the speakers even farther back from the listening chair, and possibly more the the side as well, for an even better, more lifesized image.

One thing very peculiar was that I need to dial in about 0.23ms of right channel delay to make the center image work.  Either that, or move physically closer to the left channel, so that it seems I am way off center.  I dial in this overall delay very conveniently using the Tact, though it can't be correct that way as it affects both subwoofers and speakers alike.  I tried muting the subwoofers, and I still needed that 0.23ms of delay.  Notably when I muted the subs, I also noticed that the notes in the bass line for Spanish Harlem began to sound equal, though at a much lower level.  That's very strange also because when I muted the subwoofers I have very little bass response below 85Hz where the subwoofer crossover is, and that is basically where all the first 3 bass fundamentals are.

The need for delay is very puzzling.  It might represent some early reflection, or some difference between the speakers, such as the fact that I replaced the 40 uF cap on the right side with a nice 630V solen film capacitor, but haven't made that same change to the left side.

***** Update

The next day, the need for a 0.23msec right channel delay disappeared.  I was listening to a Cactus Pear recording in which violin sounded slightly to the left, and piano more toward center.  I was thinking at first this was a demonstration of the wider center image (including left center and right center) I bragged about yesterday, but to be sure I tried headphones and realized violin (played by Stephanie) was supposed to be in the center.  Dialing back the delay to zero fixed the problem.  I recalled some of the songs I had listened to on Sunday night, and there too the need for right channel delay disappeared.

I have to believe this was either a temporary threshold shift or some similar problem with my own hearing.  It's true I was listening fairly loudly, I noticed I had heated up the Aragon amp to 150 degrees F.  But I noticed the need for delay before cranking up the volume.  Maybe it had to do with my cold, or a temporary earwax configuration.

The Bass Line on Spanish Harlem

One of the great mastering engineers had a suggestion for tuning the bass response of a monitoring system.

Spanish Harlem on the Rebecca Pidgeon album by Chesky.

One thing for sure it is a very appealing track I don't mind listening to over and over again.

But hearing the first 3 bass notes, I wonder "are they actually supposed to sound the same loudness"?  I've now tried all my systems and the Koss Phones.  All but my Bedroom system (which was actually tuned using Spanish Harlem...) play the notes in slightly increasing loudness, with the third being notably louder. The third note also has an especially "open string" quality.

Now I see what the notes are, they are G, B, D.  Indeed, it seems like the D is the open bass string D, and it should sound naturally louder.  Even if the notes were somehow played exactly the same level, we should hear the D louder due to increased hearing sensitivity.  Now, finally, I'm looking at the opening in Wave Editor, a nice Mac program, and I see that the three notes are clearly in increasing loudness, with the third being way louder the the previous two.  Here are the peak levels:

G1: -23.2dB (49 Hz)
B1: -17.6dB  (61.7 Hz)
D2: -14.1dB  (73.4 Hz)

Even those numbers, suggesting a 9dB increase, don't do the view in the Wave Editor window justice.  The D is way louder, also with the sustained ring that comes from being an open string.  The opening G looks pitifully small.

Could it be I've mistuned my bedroom system for this?  The bass line does sound very nice, probably the nicest, on my bedroom system, which has highly hand-tuned parametric filters intended to provide smoothly increasing room gain down to 16Hz.  (Has good response to 13Hz, by the way, with SVS 16-46 sub.)

Then again, maybe that was the idea, to get nice bass boosting room curve.  It turns out that actual flat response falls flat.  A bit of increasing low end boost, seems to sound the most natural (not to mention powerful) for some reason.

In kitchen I can play the synth bass in Garage Band and the 3 notes sound like identical loudness.