Can electronic controls change your room's acoustical shape?
A RECENT SOLUTION to the problems of room acoustics, by attempting to "custom-tailor" the out put of a sound system to better suit the acoustical character of a particular room with reference to the sonic character of a particular speaker system, if offered by a new class of tone controls that are more elaborate and flexible than previous controls. The new type may range from a group of three controls on an amplifier or receiver (adding a midrange to the normal bass and treble controls) to a completely separate unit patched into a system and known as a room or speaker-equalizer. The basic idea is to use electronic compensation to correct for deficiencies in response that conventional controls cannot handle.
Because transducers (pickups, as well as loud speakers) are known to be the least perfect elements in any sound system, there is ample justification for using electronic equalization or compensation to help smooth their response, or make it more linear--especially since electronic compensation can introduce enough variation to counteract the undesirable effects caused by the listening room, which has its own resonances, attenuation characteristics, and reverberation peculiarities.
Controls in one form or another also are justified on purely psychological grounds. Call it "electronic musicianship," but I believe that high fidelity enthusiasts enjoy having a multitude of controls at their command-much as sports car enthusiasts delight in having additional metering facilities and optional extras in their vehicles. In my view there is nothing wrong with tailoring one's sound to one's taste-and if this means creating orchestral balances never dreamed of by a composer, may I quickly add that J. S. Bach surely never dreamed his two- or three-part inventions would be reproduced via a Moog Electronic Synthesizer either.
(Actually how far away from electronic music are the many controls to which we have access these days?) Nor, for that matter, did Berlioz--in his wildest flights of fancy--ever envisage that his Grande Messe des Morts would be captured on a plastic disc (or ribbon) and released via two big boxes (or four) in a twenty-foot living room. The amount of electronic compensation involved in this process and the attendant number and variety of knobs, levers, and switches required constitute in sum an art/science form related more to the taste of the listener than the intent of the creator.
The Conventional Controls
In recent years, as more and more stereophonic re producing equipment was manufactured, many controls, which were considered necessary at first, have all but vanished from the scene. One such was the phasing control found on the first stereo amplifiers. Today, phasing standards are so well established that once your speakers have been connected to work in phase they will continue to do so for all program sources, including the very complex stereo FM broadcasting techniques. Ergo, no more phasing control.
An early control that has not disappeared completely, but is seldom seen on current equipment, is the normal/reverse switch that enables you to interchange stereo channels, left becoming right and vice versa. Channel standardization, like phase standardization, has all but rendered the control superfluous. Still, many listeners like to have it.
Aside from , personal whimsey in perhaps flipping performers back and forth across an imaginary stereo stage, the channel-reverse control can be handy in simple trouble-shooting, like isolating a defect to a particular channel.
Associated with the normal/reverse function was a series of mode positions that enabled you to hear through both speakers the left-only signal, right-only, or a mixture of left plus right. The last of these positions, actually a monophonic mixture, has remained as the mono setting of most mode switches.
The other two settings have been dropped from most units in the interest of simplicity, though they did provide one measure of flexibility which is lost as a consequence.
Some of the first stereo amplifiers were equipped with independent left and right controls in addition to a master volume control. The theory here was that the user first set his master volume control to a suitable listening level and then, by means of the separate left and right controls, adjusted the total sound for correct stereo balance. Because most stereo users found this control system cumbersome and confusing, the separate level controls were replaced by a balance control, a single knob that adjusts left and right signal levels relative to one another. Certainly this control represents an improvement and a much needed simplification, but it does lack one possible advantage: matching signal levels to those assumed by the action of the unit's loudness compensation.
The loudness contour switches that are supposed to compensate for low listening levels by adding prescribed amounts of bass (and sometimes treble) emphasis at various settings of the volume control--more compensation for low settings, less for high settings--are now inflexibly tied to the internal amplification of the equipment. To use a loudness compensation switch correctly, one must have ac cess to separate input level controls for each signal source. In the absence of independent channel-level controls, this ability has been lost by all but a few pieces of equipment that still offer rear-panel input-level controls for external signal sources you may decide to connect with the system.
As noted earlier, the balance control seems des tined to remain a feature of all stereophonic amplifiers. So does the master volume control (or dual-ganged variant that functions as both level and balance controls). If correctly designed, it is capable of reducing or raising the levels on both channels uniformly--that is, with little tracking error over a range of more than 60 dB. In practice, however, it is often necessary to use the balance control to compensate for the poor tracking of an inferior volume control.
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Multi-band tone controls form the basis of the Sound Effect Amplifier (SEA) control system vigorously promoted by JVC. Individual SEA units are available as well as amplifiers and receivers that include the SEA feature. In addition to providing more flexible control over tonal ranges, this approach can compensate for variations in recordings, response characteristics of a system, room acoustics. In the SEA system the basic "center frequencies" are 40 Hz, 250 Hz, 1 kHz, 5 kHz, and 15 kHz. A recent use by JVC of the SEA system is its "Audio Doctor" clinics at hi-fi dealers. Listeners are advised as to their hearing sensitivities, and offered hints on balancing tone controls in a given room.
BSR, known chiefly for its turntables. recently announced a line of frequency equalizers for use in both recording and playback. The model FEW-3 shown here is a stereo unit with twenty-four sliders to cover twelve octaves on each charnel. Dual VU meter is included. A more modest unit is the BSR FEW-2 which operates in five frequency ranges with separate controls for each stereo channel. BSR also has announced a four-channel (quadriphonic) version, the model FEW-4 which is essentially a doubled-up FEW-2, offering ten controls for front and ten controls for rear controls.
Trend to multi-band tone controls is reflected in the use of five sliders on the Harman-Kardon Citation Eleven preamp. Nominal center frequencies are 60 Hz, 320 Hz, 1 kHz, 5 kHz, and 12 kHz. These controls operate on both stereo channels simultaneously.
Recent electronic units (preamps, integrated amps, and receivers) from Marantz feature multi-band control systems, with three ranges per stereo channel. In addition to separate sliders for lows, middles, and highs, Marantz system provides selectable frequency turnover points for added versatility.
Very elaborate and versatile frequency equalizers are offered by Soundcraftsmen. Model RP 10-12 (above) is professional grade single-channel unit for use in recording and in play back. Ten slider controls are used; device has VU meter and various options for installing and using. Model 20-12 (right) is stereo unit more geared to playback. It provides ten sliders on each stereo channel; comes with test record.
[Altec's Acousta-Voicette stereo equalizer utilizes twenty-four adjustments per channel to "tune" the response of a system, including the listening room itself. The work is done by a trained technician, and the unit is left installed in the system. For a detailed account of how it worked in one system, see the accompanying report. ]
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Tone controls were staples of sound equipment through the monophonic era and are still with us.
Despite the protestations of the sound purists, many experienced listeners agree that tone controls are needed to compensate for room acoustics, signal-source deficiencies, and listening tastes. By "tone controls" we mean separate knobs for bass and treble. At one time the array might also have included a presence control to boost a narrow range of mid-frequencies to give prominence to a soloist.
Presence controls largely disappeared with the ad vent of stereo, since the stereo image itself now does the job of placing the soloist in a front-center position. But the midrange control is making a come back as I shall explain further on in this article.
Basically, stereo tone controls come in three possible forms. The simplest are made of dual-ganged potentiometers that accentuate or attenuate both left and Tight channels simultaneously. A more versatile and complex tone control is the kind that involves the use of ganged but mechanically independent controls so that the pair may be operated as one-often through a friction-clutch arrangement-or as two independent knobs. The most elaborate bass and treble tone-control arrangement--found as a rule on the costliest units-offers four totally independent tone controls, i.e., separate bass and treble controls for each channel.
Some equipment offers a switch that bypasses the tone controls completely. This addition is not as ludicrous as it might first appear, for there are times when you might want to return to absolutely "flat" or uniform response, and the flat settings on conventional tone controls often fail to correspond to electrically flat response. In addition, some forms of tone-control circuits introduce phase shifts at certain frequencies which, in theory at least, can partly degrade stereo separation or cause slight distortion.
An alternate to the tone-control-cancel switch is the type of tone control circuitry that drops out of the amplifier circuit when the associated knob is turned to the flat position. Yet another refinement is the stepped tone control, which makes contact with individual resistors and/or capacitors to achieve precise and repeatable settings.
Although not all tape recorders are equipped with a separate monitoring or playback head, nearly all of today's amplifiers and receivers include a tape monitor switch, ostensibly intended for just this type of tape machine. This control (usually in the form of a two-position switch) is nothing more than an interruption in the amplifying circuit to enable the insertion of a tape-monitoring signal.
Actually, this circuit-interrupt feature can be put to other uses, such as the insertion of some of the newer pieces of equalizing equipment, which typify the latest thrust in controls for stereo.
The Unconventional Controls
A relatively new form of tone control is the "spectrum contouring" control. Tailoring the over-all response curve to suit specific requirements is of course old hat in both the recording and broadcast fields. Precision equalizers capable of shaping a frequency response incrementally, as well as ex panders and compressors (or limiters) which in crease or decrease dynamic range, are widely used by professionals. Until now, however, very few home audio products sported such sophisticated controls.
In one sense, the appearance of these new controls constitutes a measure of honesty about product performance not found in any other industry.
The manufacturers of these devices are admitting, in effect, that the loudspeaker is not perfect; that your listening room is not a concert hall; and that your otherwise excellent stereo pickup cartridge may indeed have a peak at around 12,000 Hz (something you suspected all along). They're hinting too that FM broadcasters cannot transmit the full dynamic range of every musical selection and still observe the rules and regulations that cover the broadcast field.
The most predominant new control seems to be an elaborate replacement for the familiar bass and treble controls of your present stereo system. At least a half dozen products in this category are al ready on the market and more seem to be on the way. Basically, the principle involves boosting or attenuating restricted bands of frequencies, often by carefully calibrated amounts. By way of illustration, JVC-which has prominently espoused this form of control in its SEA (Sound Effect Amplifier) design-explains that the range of an ordinary tone-control combination is limited to boosting or attenuating the entire range of highs and/or lows in a prescribed, and not necessarily ideal, relation ship. Thus, if your cartridge has a peak around 12,000 Hz, turning the treble control counterclock wise, by even a small amount, will not just reduce that peak but will affect the frequency response all the way down to 1,000 Hz or even lower. Similarly, a loudspeaker which has a significant "hole" at around 200 Hz (but is otherwise low in distortion at that frequency) cannot be compensated for by the mere clockwise rotation of the bass control. Frequencies all the way up to at least 500 Hz will be boosted to some degree, altering the desired tonal balance of the most important mid-frequencies. By providing individual adjustment of seven frequency ranges, this system comes up with vastly in creased tonal compensation capabilities.
Interestingly, measurements of various listening rooms have proven that the rooms themselves may often exhibit narrow-band peaks, particularly at lower frequencies where the room behaves like a resonant chamber at a frequency of, say, 150 Hz. A conventional bass tone control rotated counter clockwise would produce only partial correction, but the use of just one compensating segment of the SEA system (or any other similar system) could offer more precise correction.
How many frequency segments are needed seems to have become a new source of controversy.
Harman-Kardon in its Citation Eleven preamplifier features five sliding controls capable of ± 13 dB of variation at center frequencies of 60, 320, 1,000, 5,000, and 12,000 Hz.
An elaborately divided spectrum shaper is the Acousta-Voicette Model 729A, introduced by Altec Lansing, as an outgrowth of its Acousta-Voicing technique developed some years ago to help improve the performance of sound-reproducing or sound-reinforcing equipment in large rooms and halls. After measuring such annoyances as standing waves, sonic holes, and resonances, the engineer would insert compensating networks or filters in the amplifiers to be used. The results have been de scribed as productive of smooth, natural sound in places that hitherto were notoriously poor for sound. The present Acousta-Voicette is essentially a simplified and less costly way of accomplishing similar results in home music systems. It divides the total audio spectrum into twenty-four adjustable segments of one-third octaves (on each channel) and offers up to 12 dB of attenuation on each segment. The main targets of Altec Lansing's-device are room acoustics and speaker response peaks; by correcting these factors the company feels it is achieving true "environmental equalizing." A detailed report on the "Acousta-Voicette" accompanies this article.
Those audio designers who are opposed to such elaborate tonal compensation devices. argue that the average home user is totally unequipped to do a real equalizing job on his living room and/or sound system inasmuch as he lacks the requisite sound-level meters and other sophisticated test equipment used by professionals. In experimenting with two of these new devices, I can state that the combinations of sonic effects made possible by arbitrary or random placement of all the tempting sliding levers is' endless and that when used without discretion, the audible results can be most unmusical and in fact devastating. Still, is this any reason to discourage experimentation? I think not. I believe that we may be witnessing the emergence of two different but parallel goals.
On the one hand we have the conservatives, whose quest remains the reproduction in the home of sound that most nearly approximates the live concert hall listening experience. Their radical counterparts, on the other hand, contend that con cert hall simulation is irrelevant to the home listening experience, particularly in light of new recording/production techniques that make little or no attempt to duplicate the concert hall experience.
From this standpoint it can be argued that controls may be used creatively, somewhat akin to electronic music, and that the more flexible the control setup, the more creative the listener can become.
These listeners tend to feel as involved in the creative process as were the composer, conductor, and musicians.
And yet both groups may well accept the new controls. The conservative will use them to make minute and subtle refinements in his stereo system (listening room included), while the radicals can experiment to their hearts' content-and if they up set the levers so badly that the music seems to be coining over a telephone receiver placed inside a beer barrel, aren't they entitled to do so? The "active" (i.e., amplified) equalizer associated with a speaker system such as the Bose represents yet another approach to tonal equalization in that the equalizer is designed to overcome deviations from acoustically flat response by the speaker systems as well as from varying types of input sources, including recordings and pickups. The active equalizer offers a total of twenty combinations of response and so it too provides increased equalization positions compared to the conventional tone-control arrangement.
Yet another type of speaker control, which is not an equalizer in the usual sense and can be used with frequency contouring or not, is the dimension device just brought out by David Beatty, a prominent Kansas City, Missouri audio dealer and installer. This unit permits installing two stereo pairs of speakers.
In one room and then varying the apparent stereo spread among them from a relatively narrow aural focus to a greater sense of breadth.
Danger Signs--and Hope
At best, the new controls can add an audible improvement to most stereo systems by overcoming problems of room acoustics, speaker system imbalance, or deficiencies in program material. Conceivably they can, with judicious use, virtually change the acoustic shape of the listening room-or at least improve the performance of speakers in that room.
Obviously, the apparent duplication of control functions will require a new kind of orientation on the part of stereo listeners. Just as obviously it's going to take some practice and restraint to use the new controls effectively. If the spectrum equalizers are intended to replace conventional tone controls, then those tone controls should be left in the flat position. Just because some spectrum contouring devices afford ranges of ± 12 dB or more on each of their many levers is no reason to suppose that a pat tern of hills and valleys should toe set on the levers when one is trying to equalize one's surroundings or speakers. Usually, moderate amounts of compensation at specific audio ranges will do the job beautifully unless, of course, you just want to have some fun and create your own kind of music-one totally unrelated to the real thing.
"Tuning" a Room with Acousta-Voicette
ALTEC LANSING'S claim for its "Acousta-Voicette" process that it is "the most significant breakthrough in the improvement of home high fidelity sound since stereo" seemed to us, at first blush, not only immoderate but highly unlikely. Now after using the system in a 20- by 27-foot living room for the last five weeks its owner finds himself essentially in agreement with that claim. And even those who may balk at the strong phrase "most significant breakthrough" will have to admit that a vast improvement--in an already top-quality system-has been accomplished by the "voicing" process.
The unit itself is a direct offspring of the much more expensive and sophisticated commercial "Acousta-Voicing" system developed in 1967 by Altec Lansing to cure the often massive acoustical problems of such environments as churches, auditoriums, music centers, and recording studios. The theories involved are not difficult to understand.
Since a listening area may offer anything but flat response for all the reproduced frequencies of the musical spectrum, Altec has designed a device that compensates both for the unique problems of an individual room and for the particular manner in which loudspeakers perform in that room. Furniture, drapes, windows, pictures, books, book shelves, and the walls themselves--to name but a few possibilities--all to some degree collect, absorb, and reflect sound. The end result is erratic response caused by standing waves, holes, peaks, and valleys in the frequency spectrum. These effects not only can degrade the sound of the best reproducing equipment, but they can cause a speaker system to sound different-and often in an unpredictable way--from one room to another or even from one location to another in the same room. These effects also can mask a portion of the music, especially the inner details of heavily textured passages.
The Acousta-Voicette is a two-channel active-filter critical-band equalizer with a frequency response from 20 Hz to 20 kHz ( ± 1 dB) at less than 0.5 total harmonic distortion. The equalization center frequencies are from 63 Hz to 12.5 kHz at 1/2-octave centers.
There are forty-eight active filters (twenty-four per channel). Each is designed for a maximum cut of 14 dB. Their "skirts" (the sloping sides of their responses) cross each other at a 7-dB level down, which permits the total filter action to provide a continuous shaping effect for a smooth over-all frequency contour characteristic. To recover any signal loss due to the filtering, the 729A also provides some circuit gain. The device, by the way, will work satisfactorily if it is simply patched in between a separate preamp-control unit and a basic amplifier, but the preferred "location" is ahead of a stereo system's master gain or volume control which-in most home equipment-translates to the tape-monitor facility commonly found on receivers or amplifiers. The reason for this is to avoid any possible increase in the system's noise level.
Front-panel controls include the forty-eight sliding tuners for all the filters (twenty-four per channel), plus a gain control for each channel, and a three-position "tape/in/out" switch that serves to reinstate the tape-monitor function if it has been pre-empted by the hookup, and also permits an A/ B comparison of how the system sounds with and without Acousta-Voicing. When equalizing has been completed, and you are satisfied with the results, a glass cover may be screwed over the front panel to prevent accidental rearrangement of the critically adjusted slide controls. The rear of the unit contains stereo pairs of signal jacks for tape in put, tape output, regular input, and regular out put-in addition to the line cord and fuse holder.
While in theory you can tune your listening room without professional help, Altec doesn't recommend it and neither do we. We asked the Altec technicians to tune our room in two ways: firsts by using the Altec-supplied test record (which contains material of one minute's duration for each of the 1/2-octave bands as well as wide bands of pink and white noise) plus a $700 Hewlett-Packard sound-level meter, with built-in calibrated micro phone; next, by using a signal generator in con-junction with a real-time spectrum analyzer. Be cause of the high cost of the analyzer (a $9,000 Hewlett-Packard unit that graphically displays the full audio spectrum in 1/3 octaves) most dealers probably will use the meter method, which is a good deal slower and, unless very carefully utilized, perhaps a bit less accurate than the analyzer technique. The latter unit possesses the excellent facility of showing instantly the effect on adjacent bands when a single band is in the process of adjustment. According to A-L, a lower-cost version of the real-time analyzer is being designed now; it is expected to cost under $3,000 and may find fairly wide acceptance among many audio dealer installers.
Whether a real-time analyzer or a sound-level meter is used, if the test record (rather than the signal generator) is run as the source of signals you should ask the technician to first check the actual frequency response of the playback cartridge, noting its output before the signal enters the Acousta Voicette. If any serious peaks or dips are discovered (4 or more dB departure from flat response), best results for Acousta-Voicing-and indeed for playback generally--will require replacing the pickup with one that has a demonstrably smoother response. Otherwise, the Acousta-Voicing will of necessity have to compensate for significant peaks (or dips) in the cartridge, and the resultant system compensation may prove to be well off linear response for other program sources, such as a tape deck or FM tuner, you may play through the system. For that matter, and for the same reason, the signal generator must be in top working order and should be calibrated.
Incidentally, there is no standard cost for installing a Model 729A when tuning a room; dealers have been charging up to $75. The reason for this charge becomes apparent when you consider what is involved if the job is done conscientiously.
The first step is to plot right- and left-channel curves as they exist in the listening room. The "house curve" resulting from this initial plotting gives the necessary clues to where the individual filter adjustments will begin. The technician, starting with the left channel, locates the highest peak in the frequency response. He then plays that band back on the system while adjusting the filter for that frequency and watching the results on the sound meter (or real-time analyzer). The first attempts will smooth only the highest room peaks (no more than three and probably fewer than that). At this point an entirely new complete frequency response curve should be run in order to see the overall effect of the initial adjustments. This pattern of gradually setting the filters and running and rerunning the frequency-curves is followed until the desired uniformity of response (generally within plus or minus 2 dB across the musical spectrum) is reached. The process is one of painstaking touching-up, of trimming and re-trimming, with an eye on the instruments and an ear cocked for any audible signs of "protest" from the loudspeakers (the better they are, the more successful the room-tuning can be).
There is, in short, a good deal of craftsmanship call it art-involved in the work. The use of the real time analyzer simplifies and speeds the work, but still demands a high level of skill and care on the part of the technician.
Because Acousta-Voicing represents a considerable investment the question comes up: "Is it worth the $800 plus labor?" That question, it seems, is rather like asking if it is worth paying for first-class accommodations on a twelve-hour flight. Once you've tried it, if you can afford it, you wouldn't go any other way. Similarly, if you have first-class listening equipment, once your room has been tuned, the improvement in sound is absolutely dramatic; you can feel that your original setup is almost poor when you switch the Acousta-Voicette out of the system. A good deal of this effect, of course, depends on how poor the room was to begin with-but even in a good room, Acousta-Voicing makes a difference. With it, for instance, the second movement of Leinsdorfs Mozart Jupiter Symphony displays a delineation of individual instrument timbres in a way all too uncommon even in the concert hall; Bach organ fugues have a new clarity which, in at least one case, resulted in hearing a line of counter point previously well hidden; the vocal quintet toward the end of the first act of DGG's Tannhaser took on the special impact of each of the minstrels speaking separately, yet together, as never before. The balalaikas of the Red Army Band surrounded their splendid choristers just as we re member them doing in Royal Albert Hall. The Mormon Tabernacle Choir sounded as big as it should and yet the Budapest Quartet sounded as small as it should.
Obviously, then, the total listening experience is affected. The key criteria are clarity, balance, true stereo effect (think how much poor stereo effect you've heard), warmth, and--most of all--a feeling of unblemished reality. There's another important factor: with the listening room itself correctly tuned-i.e., its deleterious acoustic effects eliminated from the playback-the true acoustic quality of the program material can emerge and be more clearly perceived. In this regard too, then, Acousta Voicing enhances the realism of high fidelity reproduction.
The Altec 729A "Acousta-Voicette" measures 18 3/4 by 5 11/16 by 7 inches. Price is $875.
For additional information, write to the manufacturer.
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(High Fidelity, 1976)
Also see:
BUYER'S GUIDE TO SPEAKER SYSTEMS--A complete listing of available models from various manufacturers giving important design and performance features, sizes, prices.