WATSON - Stereo Enhancement Loudspeakers
The formation of a sound stage, of a phantom image, of an Auditory Scene in front of me when I close my eyes and listen to two loudspeakers, is a fascinating experience of the brain at work. The brain uses memory and takes cues from the air pressure variations at the eardrums to form an auditory picture from the recording of an acoustic event. The picture has width, extending at least from left to right loudspeakers and sometimes has depth. The scene appears at and behind the two loudspeakers, which seem to define the minimum distance between me and the virtual sources. When listening with headphones the frontal part of the image tends to be located inside my head, just behind my forehead, even for binaural recordings.
When, as an experiment, the same program material is simultaneously reproduced by loudspeakers and headphones and I lift the headphones a few inches from my ears, so that I hear more of the loudspeakers, then the image moves out of the my head and assumes greater expansion in depth. But I also hear the headphones whispering. So a more full-range source than the headphones-at-a-distance seems to be needed. A pair of PLUTO loudspeakers came handy. I could confirm with them that a second pair of loudspeakers close-up, will expand the Auditory Scene, both in depth and width, created by the primary ORION loudspeakers provided that the volume from the PLUTO loudspeakers was set below the level where they become audibly dominant. Most significantly, localization accuracy of virtual sources is enhanced and intelligibility of speech and choral voices is improved. The AS is sharpened, not diffused. The expansion of the AS adds spatial separation between virtual sources. All this is, of course, very program material dependent. It works best with binaural, with sphere and with near-coincident microphone recordings. Coincident microphone recordings have clarity but are spatially less believable. Multi-microphone recordings tend to be spatial disasters, but there are exceptions. In general, I can tell a lot about the microphone placement and mix of a recording, especially when the volume level of a particular source is not consistent with its perceived location in the overall ensemble of sources. For the recording/mixing engineer such extra pair of loudspeakers might be a powerful tool to analyze and improve his stereo recording techniques. It could improve recordings for standard two loudspeaker stereo playback setup.
The volume level required from the close-by loudspeakers is relatively low so that smaller, less obtrusive speakers could be used. Also the bass response is dominated by the ORION and less capability is required from the support speakers. It even sounded occasionally that the deep bass reproduction this close to the ears shows up the nearby loudspeakers. Consequently a pair of small speakers was constructed using a SEAS FU10RB 3" driver in a 4.5" x 4.5" x 6" box, on top of a 33" long, 3" ID pipe.
Consequently a slightly larger driver with bigger motor and higher efficiency was used, the 3.5 inch SEAS W12CY003. It rests on top of a 4" to 4" pipe coupler, fires upwards and stands 34" tall on its 6.4 liter enclosure. Due to a low Qts of 0.36 its low frequency roll-off starts at a 6 dB/oct rate around 180 Hz and turns into12 dB/oct below 40 Hz. Its Sd x Xmax product is 22.5 cm3 and it is also 3 dB more efficient than the 3" driver.
Study the write-up: Creating the Gestalt of the Auditory Scene.
If this loudspeaker array were to be used for cross-talk cancellation, then the WATSON on the right should be driven by the left channel signal and vice versa. But if I do so, as an experiment, then the AS becomes narrow. If the level of the WATSONs is increased too much, then the AS reverses left and right sides, as expected. This is not correct and points out that SEL is not a CTC scheme, which requires cancellation, but it is a left and right channel signal enhancement scheme increasing D/CT and D/R. Digital Signal Processing based CTC for a single pair of loudspeakers cannot increase D/R for the listener and room. CTC actually injects cancellation sound streams into the reverberant field, which contaminate the reverberant field and which the ear picks up too. This is why CTC works best in very dead rooms or in the anechoic chamber. SEL overcomes reverberant spaces to a high degree at least for the listener in the sweet-spot.
Zero crosstalk comes automatically with headphones, but there are the issues of headphone/cup frequency response and head turning. Zero crosstalk must actually be counter-acted with cross-feed to achieve naturalness when reproducing recordings that are far from binaural recordings.
Thus I have come to call WATSON a "Stereo Enhancement Loudspeaker", SEL. An electronic time delay, an accessible volume control and a stereo power amplifier are needed for its application in a prescribed loudspeaker and listener setup.
Construction of the SE Loudspeakers is simple enough that I will not provide plans. The Schedule 40 4" ID pipe and 4.5" ID coupler, or something similar, should be available at your local hardware store. The base of the pipe is formed by a Closet Flange Hub and a 10" x 10" x 3/4" piece of plywood, which also seals the pipe. The pipe is filled lightly top to bottom with Acoustastuf to attenuate standing waves. I used Poron rubber tape with adhesive backing to form a seal and flange for the driver to sit on. The driver is not screwed in. It is heavy enough to just sit on top of the coupler. Be sure that there is air passage around the large magnet on the inside of the coupler.
The domestic acceptance factor for WATSON is probably even lower than that for PLUTO, because of its placement in the room. I look at them as being my favorite seat in the house for listening to a concert. When not in use the WATSON SEL and Tracker-Pre will be placed to the side of the room and the chair turned around for a normal living room configuration. For use at a mixing console the SEL should probably be further back or mounted to the chair in order not to interfere with the mixing desk. I think this is possible but requires investigation of the array radiation pattern, which I plan to do.
Have fun with this project and the enjoyment it will bring you!
7 Feb. 2012 - I am now using a Behringer DCX2496 to
experiment with different amounts of delay in setup B-B. I measure a signal
delay of 0.85 ms for the DCX2496 that must be added to its displayed delay in
order to get the actual delay through the unit. Gain is set to +6 dB. Volume2 is
controlled by a dual log-potentiometer connected to the output of the Behringer.
26 Feb. 2012 - Upon further listening and investigation I concluded that the new SEAS FU10RB driver has sufficient output capability for my SEL application. It need not be mounted in a small box as above for high frequency extension but can be mounted at the top of a pipe for omni-directional operation in the horizontal plane. This new SEAS driver was shown at RMAF 2011 and should become generally available soon. The small diameter and lightweight column, which holds up the driver is less obtrusive and more easily stored out of sight when not in use than previous versions of WATSON. The SEL should enhance any stereo speaker pair, not just the ORION.
3 Mar. 2012 - An in-room measurement of the pink noise SPL at the head location and in the 500 Hz octave band, indicated a WATSON SPL about 2 dB lower than the SPL of ORION. This is the correct level for many recordings, but some recordings image more believably with a lower level from WATSON. The measurement does not reflect the higher ratio of direct sound to room reflected and reverberated sound in favor of WATSON. It is a significant contributor to hearing the recording venue and not the listening room.
When WATSON is moved further away, like at arm's length with stretched out fingers, then the input level for WATSON has to be increased. This could be the situation when listening while seated on a couch. The spatial effects remain, except that the AS cannot be spread as widely to the sides. Thus, the placement of the SELs is not critical, provided they are significantly closer to the ears than the main speakers and their subtended angle is larger.
4 Mar. 2012 - An analysis of relative signal levels from ORION and WATSON in my living room provides further insight into the contribution of the Stereo Enhancement Loudspeaker to spatial perception. With a room volume of 137 m3 and a RT60 of 450 ms in the midrange, the reverberation distance, or critical distance, becomes 1.7 m for the dipolar ORION and 1 m for the omni-directional WATSON. Thus at a 2 m listening distance for position A the direct sound from ORION is 1.5 dB below its reverberated level. The direct sound from WATSON, which is placed 0.6 m in front of A, is 1.5 dB above its own reverberated level. The small circles in the graph below indicate the distance from the ORION, where direct sound and reverberated sound are equal for ORION by itself and for WATSON by itself when placed at 1.4 m and 3.4 m from ORION.
Direct and reverberated sounds combine on a power basis
due to their random phase relationship. At listening position A the total sound
level generated by ORION is about 2 dB higher than the total SPL generated by
WATSON. The two direct signals differ by only 0.5 dB. I found that many stereo
recordings were spatially enhanced at such relative sound levels.
WATSON-SEL functions by increasing the direct signal level
sound streams relative to the room reverberated sound in the 100 Hz to 3 kHz
range. This makes the recording venue space more audible, if the two stereo
tracks contain such information. The larger subtended angle of more than +/-300
increases ITD and spreads the Auditory Scene laterally, making it bigger and
asking for a proportionally larger volume level from ORION. Delay of the WATSON
drive signal is not necessary, because any combing and cancellation between the
direct sounds from WATSON and ORION is masked by the room reverberated sound and
Related material and references:
 WATSON page of the spreadsheet modes1.xls for calculating reverberation distance and direct signal levels at the listening location.
 Room Acoustics
 Brad Rakert, William M. Hartmann, "Localization of sound in rooms. V. Binaural coherence and human sensitivity to interaural time differences in noise", J. Acoust. Soc. Am., Vol. 128, No. 5, November 2010
14 March 2012 - After more experimentation and listening, also using PLUTO as the main speakers instead of ORION, I have come to some conclusions about the applicability and effectiveness of WATSON. The essential parameter appears to be the ratio of the main speaker's direct-to-reverberated sound in the room, whether from ORION or PLUTO. If the main speaker's direct signal level is more than 6 dB below the reverberated level, then WATSON will still increase spaciousness, but imaging precision suffers. Also, and even more importantly, it becomes more difficult to find a level setting for WATSON, where it is not recognized as a separate source from the main speakers, while at the same time providing spatial enhancement.
It shows the increasing difficulty to obtain reverberation times below 300 ms. An average absorption of 50% is equivalent to 50%, or half the surface area of the room behaving like open window area, where sound escapes and is not reflected.
The reverberation time T60 is best measured, for example with ARTA test software. With some experience it can also be estimated by looking at the room's furnishings, carpets and wall hangings, since V/S is easily calculated from room size measurements. A measurement of T60 in a domestic size room is meaningful because WATSON operates in the frequency range above the Schroeder frequency where the room behaves statistically. WATSON is of no use in the modal region of the room. Once T60 is known the reverberation distance and the direct-to-reverberant sound ratio can be calculated. It is interesting to note the value for the average surface absorption for future reference.
Table F on the WATSON page in modes1.xls shows whether the chosen listening distance d from the main speakers will be advantageous for WATSON.
In general, and also without WATSON, the listening distance should be less than 2*Rx to obtain the best imaging performance from stereo speakers in a given room. At greater distances the room sound begins to dominate. Conventional box loudspeakers with their directional gain variation from 1 at low frequencies to 10 and more at high frequencies, imbed this behavior into the reverberated field and are therefore not frequency neutral transducers. Between low and high frequencies the reverberation distance changes from Rm to 3.16 Rm and the direct to reverberated sound ratio increases 10 dB for flat on-axis, free-field frequency response. This makes the changing high frequency balance problematic and tends to make the loudspeaker more noticeable as such in the Auditory Scene. Frequency independent directivity, as realized with an acoustically small monopole, dipole or cardioid loudspeaker, is necessary for neutrally rendered sound under reverberant conditions. A benefit of increased directivity is either an increased listening distance for the same direct-to-reverberant sound ratio, or hearing less of the room by having a larger D/R ratio at the listening position. When listening to music in a concert hall, D/R changes all the time because different instruments have different directional characteristics. Some excite the hall's reverberation more than others. This is typical. A loudspeaker that changes D/R with frequency in the listening room acts like a musical instrument of sorts and not as a neutral transducer.