New Applications for Loudspeakers in an Acoustical Space:
A Composer's Perspective on Alternative Loudspeaker Placement
In the early 1970's Bose marketed a model of loudspeaker that was highly unorthodox in functional design. Instead of facing the audience, these unique speakers were meant to face away from the listener and toward the corners of the listening space. Consequently the listener would be listening not to the direct sound of the system but to the indirect reflected sounds bouncing off the living room walls. At first these speakers were quite a fad. They were expensive, but they conferred status; they were "different". Today, however, it is difficult to purchase a pair anywhere, even though they are still said to be manufactured.
Why the demise of the popularity of these unique loudspeakers? One is tempted to speculate that eventually consumers got tired of having to put up with rattling picture frames, falling plaster and irate landlords. But according to audiophiles, the reason for these peculiar speakers' fall from grace is simple: the stereo image they produced was unstable, or in the terms of one salesman, "wishy-washy". If one listened to a recording of a symphonic orchestra on these speakers, it would not be unusual to get the impression that the orchestra members were wandering around a bit during the recording session.
Back in those years I had a friend at college who owned a pair and many of us would, at the end of a long day, find ourselves congregated in his room listening to electronic effects of Pink Floyd over his new fangled system. Other friends had heftier speakers and newer systems, but there was something magic about the sound of his system which at the time I couldn't put my finger on. I must have been subconsciously inspired because years later, I found myself turning quite naturally to some of the same ideas about reflected sound. What is naturally a liability for a recording of traditional music became a real bonus for me in my electronic works.
My first awakening to the creative possibilities of indirect reflected sounds came in 1983 when I was asked by Karel Paukert, organist and Curator of Music at the Cleveland Museum of Art, to write a piece for tape and church organ. PBBLS 1 would be premiered at the 1983 AK1 New Music Festival, in St. Paul's Episcopal Church in Cleveland - a fine stone chapel of rather large dimensions with renowned acoustics and an equally impressive and renowned organ. Having the use of this fabulous acoustic environment was an opportunity I had to take advantage of; I had to do something radically different that would really work for a large stone chapel. I determined that the tape portion would utilize 5 channels played through 5 loudspeakers which would be aimed not at the audience but upwards at the high vaulted stone ceiling - like the organ pipes themselves - toward God (if you will). The placement of the 5 loudspeakers, which were situated on tall pedestals flat on their backs looking at the gothic arches, was as follows:
Figure 1: Floor plan and loudspeaker placement at St. Paul's Episcopal Church, Cleveland, Ohio.
Using pre-recorded materials, I created a tape of computer processed organ sounds in a technique I called "computer concrete". Much of the tape material consists of innumerable tiny sound events, each of which issue from only one of five channels at a time. All of the materials in the tape portion are split upon to three different reels which are to be played on three different tape machines simultaneously (such a production ensures that no two performances will be identical).
Since I was planning to reflect the tape sounds off of the ceiling, I had to especially boost the treble range of the sounds on the tape. Upper frequencies are more directional than the lower frequencies which radiate with equal strength in all directions. Consequently these upper frequencies have the most energy to lose when reflected off a hard surface before reaching the ear. This I had foreseen. What I had not foreseen was the magical effect of my sounds randomly falling from the sky like a sparse hail. These brighter sounds could not be localized and seemed to originate from unpredictable shifting sources in the ceiling. My attitude towards localized sounds in space was profoundly changed at that moment.
I had an
opportunity to explore this phenomenon further when recently an older electronic
piece of mine, Homage to Mark Rothko2,
was performed in Mandeville Recital Hall - a big, square concrete room at the
University of California, San Diego. Homage is a stereo piece consisting of
thick Moog Synthesizer timbres sliding up and down. For this performance I
aimed two front speakers obliquely toward the front wall and supported them with
an additional two speakers behind and above the audience (see
Figu re 2).
Figure 2: Speaker placement to Homage for Mark Rothko
again boosted the treble range and also increased the volume of the front
speakers to such an extent that the room literally was filled with sound, like
at a rock concert. Yet, curiously, sitting in the front
row (or anywhere for that matter) was very bearable.
Once again the high timbres could not be localized at the fixed speakers.
Instead, they seemed to swoop freely around the room like screaming winged
creatures. (I vividly remember one glissando
Arthur Benade maintains that stereo imaging created by two loudspeakers facing an audience is a conceptual illusion. If one truly listens without preconceptions, what one really hears is amplitude differences between the two loudspeakers. There is no sound coming from the center. When listening to loudspeakers, "our highly developed abilities for localizing anything of a stable acoustical nature are trapped into the job of discovering the loudspeaker instead of the music it is supposed to reproduce."3 Contrary to any illusion or "imaging" intended by the composer, the loudspeaker will continue to impose its real acoustical presence.
At first this reasoning does not seem promising as the answer to my question. Nonetheless, there is a hint that I am on the right track, namely, it explains why Homage to Mark Rothko under standard conditions never sounded remotely like that recent special (spatial) performance.
It turns out that by eliminating the direct sound from the loudspeaker the listener is left with only indirect reflected sounds which radically alter the listener's perception of where the sound is coming from. This is due to the "precedence" effect which causes reduplicated sounds of like amplitude arriving at the ear within 35 milliseconds of each other to be heard as one sound coming from the source of that first sound.4
In Figure 3 direct sounds coming from source "S" will always reach listener "L", before the reflected sounds following path "SAL" or "SBL". Its path is the shortest. Therefore the listener will always locate the source of sounds as point "5", even though sounds are coming from points "A" and "B" as well. Take away the direct sound path "SL" and the listener will localize the sound as coming from point "A" (along the next shortest path).
Figure 3: Demonstration of the "precedence effect"
Lower frequencies will always be localized at the speaker source because these frequencies have large wave lengths and are sent out evenly with relatively equal strength in all directions (characteristic of a "simple source"). This is true of loudspeakers no matter what directions they face. However, as the frequency increases, sounds no longer dissipate evenly throughout the room, but rather into increasingly narrow beams that travel out along the axis of the speaker cone.5 The higher the frequency, the more directionally focused is that frequency (see Figure 4). Aim this cone away from the listener and the listener will hear the higher frequencies greatly reduced in amplitude, coming from a reflective surface. To compensate for the loss of energy incurred from impact with the reflective surface, these higher frequencies can be artificially amplified.
In addition to this phenomenon, highly focused upper frequencies excite the room nodes in a much more selective manner than does a simple source. Such focused beams of sound do not become weaker with distance to quite the same extent as it is observed for the uniformly radiating lower frequencies.6 This accounts for the perceived proximity of a sound. Highly focused sounds tend to "zing" out and fall into the laps of those listeners in their trajectory.
When one considers that different high frequencies project from the speaker cone at different angles and with differing degrees of concentration or focus, and that most concert loudspeakers have not one but 3 speaker cones, it becomes apparent that it is impossible for the listener to predict the proximity of the sound or from whence the sound came.
This is not to say that all this happens randomly. Given a smooth glissando from high pitch to low, the sound will most likely be localized along a continuous path. Where that path lies with respect to the listener depends entirely upon where the listener sits, and perhaps even in what direction the listener is leaning in his or her seat. Figure 5 shows a possible scenario for a two octave descending glissando (from a frequency of 4K to one of 1 K) which takes into account the various acoustic effects I have presented.
Figure 5: A somewhat unscientific depiction of what a descending glissando might look like.
The benefits I have derived from such a discovery are numerous. Aside from the obvious possibility of (1) moving sounds around and playing with their virtual source, this discovery has (2) made a virtue of a perceived and much lamented deficiency: the loudspeaker as an encased sound source. Acoustical musical instruments can not so efficiently deflect or eliminate the direct sounds they produce. in reflecting the sounds from loudspeakers, (3) the composer is no longer concerned with how a piece sounds to the "ideal listener" located at the epicenter of the room. He will seek only the most advantageous "scattering"' of sounds. Nor need the audience be concerned; every seat in the house is ideal, albeit different. in this endeavor, (4) the acoustic space of the concert hall becomes a participant in the music in a much more profound sense than before. Rooms can be "tuned" for given performances (if only by moving the speakers). Different concert spaces produce different effects, which adds to variety. But certain spaces can be custom tailored for specific concerts. Such a concert space is the Espace de Projection at IRCAM in Paris with its moveable modular walls, ceiling and even floor; each modular unit is equipped with three types of surface, from very reflective to very absorbent. Reflected sounds from loudspeakers allow for (5) increased volume without a necessary increase in loudness. This kind of volume is amplitude that sufficiently fills the nooks and crannies of a room in the sense that a liquid fills a volume. It is not pointed and injected into the listener's ear. With the ability to custom tailor the amplitudes of the higher frequencies comes the added benefit of (6) a better signal to noise ratio. Tape hiss is markedly less obtrusive when the loudspeakers are not facing the listener. And finally, (7) I find myself philosophically opposed to physically aiming a given music at an audience - it is much like training a shot gun on captives. What is the point of enforcing only one possible perspective, that of the "ideal middle man," on a given work? it is aesthetically more appropriate to encourage the audience to discover the space they share (with its many possible different vantage points) via the acoustic phenomena that travel through it.
offers other models of a more standard type of loudspeaker,
but for some reason has not given up on their revolutionary reflective
speakers. I am glad - indeed, the technology of reflective speakers has already
found its way into automobile sound systems. It does not seem to me far-fetched
in this new age of computers that one day soon it will be possible to calculate
the exact effects of a loudspeaker's cooperation with a given space and to have
the speaker automatically adjust itself to reproduce a virtual sound image of
even the most traditional music - a kind of sonic hologram. Plug in the exact
dimensions of the
1 PBBLS for Organ and Five Channel Tape, by the author, 17 minutes, 1983.
2 Homage to Mark Rothko, by the author, 12 minutes, 1979.
3 Arthur H. Benade, Fundamentals of Music Acoustics, Oxford University Press, New York, 1976, p.215.
4 Ibid. p.204.
5 Ibid. p.214.
6 Ibid., p.202. This information is extrapolated from a discussion of relative trumpet frequencies.