From your Living Room to every Concert Hall
I can hardly contain myself. I am so excited. I have unlocked the puzzle that PLUTO+ presented me with. Now, ORION has taken again its top position, but it took PLUTO+ to point the way.
When I compared PLUTO+ to ORION I was stunned by the sonic similarity between these very different speakers, when properly set up in my living room, even though one is a monopole over the 8 lower octaves and the other is a dipole over 7 of those octaves, out of a total of 10 octaves for the 20 Hz to 20 kHz frequency range. I felt almost embarrassed that so much less costly drivers could give such outstanding performance, when I had labored over testing and searching for the absolute best. There even was something about human voice that made me question the correctness of the ORION in comparison to PLUTO+. Attempts to change the equalization of ORION to make pink noise and voice sound more similar on the two speakers were complete failures. The only parameter that was left to investigate was the polar response. I had already concluded that it must have been the polar response that made PLUTO+ so similar to ORION in the first place. ORION is a dipole only to about 1.5 kHz. That is over an octave less in uniformity of polar response than PLUTO+ covers, and missing an important frequency range for voice. A rear mounted tweeter on ORION to extend the dipole range might fix that. Well, it did that and much more.
A second Seas Millennium tweeter is mounted on a separate, flat baffle in the rear of the existing tweeter. The added baffle was necessary to preserve a wide and uniform polar response for the tweeter. I had played with rear mounted tweeters when I first started out with dipoles but abandoned them. The rear tweeter does not contribute to the direct sound coming from the front of the speaker over a very wide angle of +/-60 degrees off-axis. This is due to the relatively wide baffle that I use for ORION. It is also an indication that there is little diffraction effect from this wide baffle and hence the good imaging. Thus under anechoic conditions, or outdoors, one would never hear the rear tweeter from any normal listening position. In a room, therefore, anything that is heard of the tweeter, is heard via reflections off walls and objects. Like many audiophiles, I considered that problematic, especially since the commercial box speakers that used such rear tweeters, and that I had listened to at the time, only sounded more fizzy.
Yet, when I listen to ORION with added rear tweeters, calling it ORION+, I can only say that I have never heard sound reproduced as realistically in its dimensions of space, frequency and time. Sure, it is only an illusion and it all happens between the ears, but the ease with which I fall for the illusion becomes obvious when I switch back to PLUTO+. There the imaging is pin point analytical left to right in an acoustic space that has less depth and height than ORION and which by comparison to ORION+ clearly takes some effort to construct in my mind from the cues given. Sort of a re-programming after having heard ORION+.
ORION+ transports me to the performance venue. I must close my eyes so that I do not get distracted by the glaring visual incongruence between the speakers and room in front of me and the visual acoustic space before me in my mind's eye. It seems so real and in 3D. The phantom size of sources changes with volume level setting and it is more of a distance control like before. But the physical size of my living room does not seem to bound the overall phantom size. Recordings sound louder and more natural in this context, but on some recordings I find myself to further increase the volume level because it sounds more realistic and the room is not becoming overloaded. This is unlike my experience with box speakers. Even massed strings that often sound unpleasant on multi-microphone commercial recordings take on a believable character. The perception of bass also seems to be affected. Most enjoyable are the clearly audible decays of sounds in the spaces between the notes, the subtleties of the musicians' expressions, the texture, the dynamics and plain energy of playing.
If I sound excited, it's because I am. This is sound playback how it should have been all along. So what is going on when merely adding some rear tweeters can make such a remarkable audible difference at the listening position in front?
The ORION with rear tweeter added has become quite symmetrical between front and rear polar responses and has reduced output to the sides. The woofer section is fully symmetrical, the midrange not quite, but reasonably smooth in off-axis response variation, and the tweeter with its own baffle introduces only a slight tilt in the vertical polar response because it is not properly phase aligned with the midrange in its crossover region. Thus the speaker radiates essentially the same on- and off-axis sound in the rear as it does in the front. The rear radiated sound is reflected, diffused, and slightly absorbed by the surfaces behind the speaker and the greater path length, before it returns to the front and to the listener. At lower frequencies some of the sound propagates directly around the baffle and interferes with the front sound to form the dipole polar response. But in the tweeter frequency range this effect is practically non-existent and what is heard by the listener is strictly surface reflected sound.
The listener is presented with two sound pictures, F and R. The first arriving is F, the direct sound from the speaker's front and its room reflections. The later arriving R is the rear sound that is reflected predominantly from the area behind the speaker. This rear sound R has nearly the same spectral distribution from low to high frequencies as does F. But compared to the direct sound F the phase of R is highly de-correlated because of the path length being large compared to the radiated wavelength for higher frequencies. This effect is even increased because for a dipole the phase shift increases with increasing off-axis angle. Therefore R and F add on a power basis at the listener. F contains the phase and timing cues between left and right speakers to form the phantom image location. R is delayed by 12 ms in the above example. I have found in the past that speakers should be placed at least 3 feet from reflecting surfaces so that the delay is greater than 6 ms. This seems to insure that the reflection is no longer perceived as a property of the direct sound and coming from the speaker. While F and R add on a power basis and increase the perceived volume level in the short term of a few milliseconds, they are still correlated in the long term of reverberation and sound decay of the recording venue which is in the order of hundreds of milliseconds. Reverberation and sound decay give strong perceptual cues as to recording venue size and source distance. F appears to be essential to strengthen these cues in the sound field at the listener. The net result of R and F is a highly localized presentation of sources from left to right, in depth behind the speakers, and in vertical height of venue space. The sound field appears more energetic and capable of expressing a wider dynamic range without stress.
What probably has happened here is that the processor between the two ears has been presented with an auditory scene and with sound streams with which it can cope easily. It is like the familiar "cocktail party effect" of being able to tune into a different conversation while all go on at the same time. Different types of loudspeakers and setups create different auditory scenes in a room, some of which we are totally unfamiliar with from human evolution. For example, trying to generate with two loudspeakers the phantom image of a speaking person is a totally unnatural process and our brain needs help to decipher such unfamiliar auditory scene. Apparently the multiplicity of reflected sounds from the two sources, in addition to the direct sound, allows easier recognition of familiar cues provided that those reflections have specific directional properties.
The rear tweeter adds about 3 dB to the high frequency acoustic energy in the room. This gives sweetness and richness to strings, roundness to brass, vastness to applause, and texture to many sounds. It is not possible to obtain the same effect from playing with the frequency response of a forward firing tweeter, though I have often observed that really wide tweeter dispersion reduces harshness and sounds more realistic. PLUTO+ challenged me with its dispersion. I find it particularly interesting that the combined tweeter level had to be reduced only by about 1 dB after the rear tweeter had been added to the front tweeter for dipole action. The two drivers are wired in parallel and with opposite polarity. This increases combined acoustic power output of midrange and tweeters by less than 4/3 or 1.25 dB. Tweeter level adjustment is quite critical and will be room dependent but easily recognized when correct. The ORION tweeter level adjustment changes 0.5 dB between its tick marks.
It was determined by Don Barringer and confirmed shortly after by me, that reducing the rear tweeter level by about 3 dB, while leaving the front tweeter at its optimum ORION level, enhances the wall effect. We live 3000 miles apart and have very different listening spaces. Power and number wise the 3 dB reduction is identical to a 1.25 dB reduction of front and rear tweeter together. The attenuation was obtained by wiring a 2 ohm (5 W, 5%) resistor in series with the rear Millennium tweeter. The resulting 0.7 dB bathtub like frequency response of the rear tweeter might be advantageous.
Also, it is a widely held belief that reflections destroy transients and diffuse phantom images. Reflections must be avoided. Yet they are important for energy distribution in the room. This belief has been slowly changing (e.g. Moulton, Toole), but only for lateral reflections. The space behind the speakers has always been suspect for changing the sound unfavorably, and rightfully so. Almost all loudspeakers illuminate this space unevenly and sonically colored. I now also understand why flush mounted speakers sound off-the-wall to me. They completely lack rear wall reflections and thus important spatial cues.
One might postulate that the wall reflection will add a sameness to all sound playback because the listening room acoustics from reflections right close to the loudspeakers get mixed in heavily with the recorded acoustics. The exact opposite is heard. Recordings become more differentiated, the whole range from dead to wet acoustics is clearly perceived. Furthermore, what is perceived as "distortion" is more readily identified: analog tape compression (Cantate Domino, track 1, PRCD 7762), ringing of the hall (Star of Wonder, track 16, RR-21CD), or spectral discontinuity of phantom sources due to microphone mix (Soular Energy, The Ray Brown Trio, track 2, CCD-4268), etc. The rightness of a live recording really shines (Swing Live, Bucky Pizzarelli, track 7, JD218). The "Volume Control" should be relabeled as "Distance Control". I like it that I can slide quite a few rows closer to the orchestra merely with a button push on my remote volume control. It seems that the ringing of the listening room ultimately limits the maximally tolerable SPL and not necessarily the non-linear distortion of the loudspeaker.
When listening in a room and someone opens a door or window you hear outside sounds, different acoustic spaces. Their perceived character is not changed by having entered and mixed with your living room, though their physical properties are changed.
What has happened here requires a shift in the prevalent thinking about sound reproduction in a home environment, about room treatment, speaker placement, its polar response, room equalization, etc.. More than ever, the recording becomes the limiting factor. The spatial artificiality in pan-potted studio recordings, or in live recordings with too many microphones, becomes immediately apparent, though that does not necessarily detract from their enjoyment. Should it turn out that front-to-rear symmetry with symmetrical nulls in the polar response, and the smallness of the acoustic source size have to be perfected, then it is easy to imagine baffle arrangements for midrange and tweeter that would achieve this, but which are different from the ORION+. They might lead to unsymmetrical vertical responses due to driver offsets. More to investigate, but not by me until I see a need for it.
4/21/07 After extensive testing and extended listening it was found that the rear tweeter should be at the same output level as the front tweeter, provided that some slight, but highly critical, adjustments have been made to the ASP frequency response and tweeter level setting. For specific details see your ORION Owner Support page.
The second plus in ORION++ stands for the THOR subwoofers that I have been using on a permanent basis ever since I introduced the w-ASP and increased the crossover frequency to 50 Hz. I have not heard detrimental effects due to group delay and if anything the subwoofers reduce distortion from the ORION woofers. Now with ORION+ and my higher SPL desires I feel that the subwoofers justifiably have a place in my music playback system.
In my system the two THOR subwoofers are driven from the four amplifier channels of the AT6012 which are not used by the ORION. Amplifiers cannot be "bridged" internally to the AT6012. Instead I take two of the amplifiers and drive them out-of-phase from their RCA inputs with signals provided by the ORION/THOR crossover/equalizer. A single Thor is connected between the hot (red) output binding posts of the two amplifiers. The two amplifiers appear to deliver enough power in this push-pull mode to Thor though each amplifier sees only half the load impedance. I have not noticed problems with my power demands for the ORION+ even when the AT6012 drives the low impedance of the tweeters in parallel at the same time. The only exception so far has been subwoofer amplifier channel clipping on the door slam of the 'Trucking and Welding Shop' track on my Sound Pictures demo CD. Consequently I switched to using an AT1802 power amplifier to drive the two subwoofers. But I have found that the vast majority of my listening is completely satisfied with the ORION+ bass output volume capability. So I rarely turn on the subwoofers. They remain immediately available when a visitor wants to hear their effectiveness for a particularly demanding piece of music.
ORION was originally developed to minimize the influence of the room on the accuracy of sound reproduction and the creation of a believable auditory scene in your head. I had not understood to what degree the loudspeaker affects how we hear or not hear the room. Today I will say: the room is rarely the problem, but the loudspeakers and their setup determine the disappearance of the room from our attention. This discovery is extremely exciting and opens the door to a new level of realism in reproduced sound.
From the first reactions to this page I can see that people wonder how I can get so excited over an old hat trick like a rear tweeter. Yes, adding the rear tweeter to ORION does nice things, but that is only because of the walls! My excitement is over the wall! It can be your friend! It's the third element in sound reproduction and I have always been fighting it. For the first time I heard consciously what it can contribute and it makes sense to me. If you knew this all along, then I wonder why you kept it to yourself. This could be big! An inconvenient truth?
In October, 2007, I added a page to the Concepts collection with the title:
Room optimized stereophonic sound reproduction
It is based on my paper " Room reflections misunderstood?" that I gave at the 123rd AES Convention in New York. My working title, "It's the wall, stupid", would have been misleading. The rear tweeter ensures full spectrum characteristics to the reflections from all surfaces/objects and not just those from the wall behind the speakers. Measurements of reflections in my room show an abundance of reflections. Rather than being overwhelmed by reflections the ESP seems to use them to advantage. The "Precedence Effect" makes this plausible. The listening room can become cognitively disassociated from the acoustic scene that is presented by the playback of a recording. Certain conditions regarding the loudspeakers, their setup, and the room must be met.
When you have read this page from the top down, you might have gotten a sense of the developments in understanding the rear tweeter integration and what is happening with it perceptually. My conclusions are explained more generally in the Convention paper.
Rear tweeter level adjustment
The rear tweeter is to be wired in parallel with the front tweeter in dipole polarity. This means that the (+) terminal of the front tweeter is connected to the (-) terminal of the rear tweeter and the (-) of the front tweeter is wired to the (+) terminal of the rear tweeter. The front tweeter is wired to its power amplifier. The tweeter level is adjusted on the ASP circuit board where each tick mark on the potentiometer represents a 0.5 dB change. The total range is +/-2.5 dB around the center position.
In most loudspeaker setup situations the optimum combined front and rear tweeter level is found by fine tuning the potentiometer. It is still surprising to me how significant a change in the sonic gestalt is perceived from a 0.5 dB adjustment in level. This is not observed in an instant A-B comparison but by extended listening. You will know subconsciously when the level is right, just as you know when there is something not quite right. Of course, you will need to use a variety of recordings and not rely on the one recording you always used as a reference. Stick with classical, live jazz, choral, female vocal, etc. Stay away from heavily processed studio recordings. With the correct setting, the sound should be natural and balanced, but not bright. The high frequencies become completely integrated with their midrange fundamentals and do not stand out unless that is the characteristic of the source or the recording. There is space and air to the highs.
There are apparently setup situations, though, where the optimum tweeter level is difficult to find. In such cases the following procedure is suggested.
It may take some time to optimize your system but you will be greatly rewarded with credibility in what you are hearing spatially and in timbre.