Second brochure : ON TEST IMF Studio loudspeaker.
Eight years ago we first described a unique concept in loudspeakers, the transmission line (cf. Newsletter 4, Sept., 1962), a new laboratory tool for evaluation of advances in record reproduction, replacing the full range electrostatics that were not capable of reproducing the vastly increased dynamic and frequency ranges of 1962's improved records.
Five years ago we first used our "Ribbon Monitor" at public demonstrations of advances in record reproduction (cf. Newsletter S). This we did despite the prevailing industry opinion that "the public isn't ready for an `honest' loudspeaker," or "the public does not like a flat frequency range." Notwithstanding, at least part of "the public" was enthusiastic about the Monitor, and seemed to be ready for "honest," "flat" loudspeakers.
The Monitor is expensive, large, and scarce. What was needed, we thought, was a moderately sized and priced loudspeaker containing the basic operating principles and performance of our laboratory tool; one that could be made readily available to the part of "the public" which obviously is ready for a standard of reproduction that the industry says "the public" is not!
Thus, after a long research and developmental program, we present our STUDIO Transmission Line Loudspeaker. We will describe it in some detail, just because it differs so markedly in concept and execution from what has heretofore been available to the home music lover.
The STUDIO is not a horn loudspeaker, though it does resemble an "inverted horn," and though its kind of bass propagation has been compared by listeners to the sound of large horns.
The STUDIO is not a free standing loudspeaker, though its design tends to produce the same "transparent" and "listen through" quality in the mid band (but avoiding the "room sensitivity" and "vague stereo" effects of such loudspeakers).
The STUDIO is a transmission line loudspeaker. It has two acoustic "lines" which enable the drivers to perform as if they were mounted in the outer walls of your home (the classically beloved "infinite baffle") and in addition to project a pure and musically cohesive low bass to the ears of the listener.
The bass "line" starts with very large cross-section at the driver and gradually tapers. It also is treated with progressively changing kinds and densities of materials, which absorb all the back waves except the very longest and lowest, these being damped and fed into the room through the opening (see arrow). We list the advantages:
There is no way of absorbing these lows unless the "line" is monstrously long; we think it better to use them than merely to let them bounce back randomly at the driver - as they do in "box" speakers.
The "line" puts them into phase with the bass driver, creating an auxiliary radiator for the very low bass, of very large equivalent acoustic cross section. Together, enormous quantities of air are moved, the driver itself barely vibrating! "Doppler" distortion is minimized (see below).
Instead of a single bass source (a "spherical source"), we have two spaced sources which produce a "plane front" of bass energy; bass is projected into the room and toward the listener, instead of curling back around the cabinet and along the walls (as it does with spherical sources). The listening effect is less "murky" and "mushy" bass, more "impact" and "clear" bass.
Eliminated is the large impedance rise at bass resonance of conventional speakers, which makes amplifiers work hard and tend to distort just where they should not. The impedance rise in a proper line is very small and pushed below the audio band. Amplifiers can put power into the speaker and speaker response is flattened way down - even in an anechoic chamber (see Figure 2) the STUDIO has bass, whereas other loudspeakers show up with "no bass at all" (which is why you don't see their response curves in their literature!).
One Way. Single cones carrying all frequencies must be "equalized" to cover the bare audio band (50 HZ 15 KHZ) ; which can be done at cost of
(a) Enormous amplifier power requirement.
(b) Accepting severe harmonic and intermodulation distortions as normal - if you force a cone beyond its normal range, you must produce these distortions.
(c) "Doppler" effects and distortion, i.e., the production of new and harmonically unrelated (therefore repugnant to trained ears) sounds from cones moving to produce lows and highs. These are very audible and measurable - leading one authority to rate speakers by a "mud index," produced by Doppler effects!
Two way - Better, but we know of no two way loud speakers which in fact cover the audio band (50 HZ 15KHZ). Problems are severe:
(a) Doppler effects persist, particularly if the bass cone acts as an unaided "spherical source." (per the above, the more the motion, the more the "mud").
(b) The bass cone must be used in the mid band, where it is a very poor reproducer on several counts; and there it must be crossed over to a tweeter, right where the ear is most sensitive - the wrong place ( see below)
Three way. Better. The bass cone can be cut off low enough, the mid range system can be good enough, the tweeter good enough to meet modern "good commercial" standards, covering adequately the audio band.
But - The STUDIO is by definition a definitive reproducer, and "adequate" coverage is not enough. We found some time ago that, to cover the audio band - a loudspeaker must reproduce smoothly one octave beyond both ends; i.e., it must be capable of handling 2S HZ and 30 KHZ! Any speaker which resonates or cuts-off before these limits is not a "state of the art" loudspeaker! What is worse, the effects of limiting response are quite audible to all keen listeners - showing up as "coarseness," "roughness," "muddiness," "Lack of definition," etc.
It has cost a great deal of time and effort to develop and refine a loudspeaker that is truly "wide band," that is "homogeneous," and which can be readily used on standard home equipment - but we know of no simpler and less complex loudspeaker which in fact can be said to be definitive in performance, i.e., as good as can be made at the present state of the art!
The woofer - a heavy magnetic system drives a damped plastic diaphragm, which is infinitely superior to paper cones in rigidity, freedom from spurious resonances and breakup; much less variable in manufacture as well as with changes in temperature and humidity. It is also highly resistant to "delay waves," the sounds from inside the cabinet which come back to the cone - paper cones are virtually transparent to these, another reason why paper coned loudspeakers are "loud" or "muddy."
It is highly linear, though it hardly moves in its transmission line. One can feed 25HZ into the STUDIO, feel the room "move," and observe the woofer hardly vibrating!
The mid-range - a shallow quadruple laminate driver. The heart of the STUDIO, as of any first class loudspeaker, is its mid range system (which in the STUDIO consists of the driver and its line). The researches of the B.B.C. (British Broadcasting Company), the O.R.T.F. (French National Radio), and many, many others have established the prime importance of the mid band, because here the ear is most sensitive and intolerant to faults; and because here the entire "character" and "ambience" of the program is established.
During development of the STUDIO, we did try to use, a simpler and cheaper driver - we couldn't - too "papery," "buzzy," "unclear." We did try a little sealed box, instead of the mid line. We couldn't - ears conditioned to Monitor sound immediately complained that the sound image was "confined," "boxed-in"; that the "hall sounds" had disappeared, that the "transparency" was gone, etc.
So we ended with a replica of the mid range of the Monitor! Which is just as well - The ear will forgive many more distortions at the bottom and at the top of the musical scale, than it will in the mid range, where the critical ear hears distortions which are frankly unmeasurable by ordinary laboratory techniques. (How does one measure transparency? It can be measured in time pulses, retard times, after pulses and all that - but these are the measurements of the future, rather than "current").
High Frequency unit: A phenolic diaphragm pressure unit noted for its smoothness, wide dispersion, and very low distortion.
Super High unit: A new chemical dome which is capable of handling large amounts of power above 12KHZ. It improves "attack," makes brass sound less distorted and more "real," solidifies the stereo image. During development, we measured and tested with this unit in and out - everyone heard the difference' on all kinds o f music. It seems that, even though we
cannot hear these "highs," the ability of the system to handle them without severe phase and frequency distortions is reflected in much more faithful reproduction of what we do hear.
A thin, deep shape is ideal for even mid range dispersion. The mid and treble units are placed at the inner edge; the "left hand" speaker has them at the right edge; the "right hand" speaker is the "mirror image." Dispersion inward and toward the listener is controlled and improved - there are no "dead spots" or "hole in the middle" effects. The listener hears the desired "precedence" signal first, no matter the acoustics of the room.
The cabinet is constructed of heavy natural wood stock, with a laminate internal balancer. The back, panel is covered with a formica overlay, to render it even more rigid and acoustically opaque. The number of sections inside the cabinet, plus the damping, make the cabinet incredibly rigid and acoustically "dead." A built on pedestal elevates the speaker slightly from the floor, to eliminate excessive coupling.
The back panel has recessed input connectors and two stepped controls (mid and top). The steps are one db. only - quite audible with speakers as transparent and smooth as the STUDIO.
The STUDIO is not "room sensitive" or critical in the usual sense. It can be placed almost anywhere. Again, for monitoring use, or where absolute purity of sound image is required, it should be used in mid room, the pair canted inward. In this position the STUDIO will not be bass deficient (all other speakers are).
Evaluating the Studio
Having read this far, you have some idea of how the STUDIO performs. Unfortunately, all the phrases we have used have been used before and made more or less meaningless. Nevertheless, we will try to give you some idea of how we think you will react to the STUDIO, or, stated another way, for which kinds of listeners it will be a step toward more definitive reproduction. To be fair to you who are seriously inclined, we will try to relate listening comments to actual principles of operation as they have been incorporated into the STUDIO; and to psychoacoustic statements which have been uniformly noted by all listeners.
How does the STUDIO "sound"? The better a loudspeaker, the less it has a characteristic "sound," or "coloration." An English authority has defined a "better" loudspeaker as one which " will be found to give satisfaction in the widest variety of circumstances, with less listener fatigue and greater musical pleasure." We think this describes our STUDIO - relatively uncolored and thus able to give greater musical pleasure with less fatigue.
It is "transparent," devoid of "boxiness" and the slurring of transient effects common to simple box speakers. Its entire design is intended to remove the resonances and "after sounds" that the ear hears as "boxiness." Some people comment that the STUDIO most resembles full range electrostatics in its "open" quality - but happily without the "thin" and "electric" sound of electrostatics (their characteristic `coloration").
We would agree; The harmonic distortions of the STUDIO are very low, particularly the 3rd harmonic, which has been correlated with the pleasantness to the trained ear - the less the 3rd harmonic, the easier and more musical the listening. The bass response of the STUDIO is certainly not "thin" (see anechoic plots) ; if anything, listeners conditioned to "no bass at all" speakers sometimes say the deep bass of the STUDIO is "too much"!
But it does not sound "bassey"! If you switch over to the STUDIO from conventional loudspeakers, on most program material your first impression is "lack of bass." The reason is that conventional loudspeakers have their "bass" between 80HZ and 200 KHZ, in the form of one or a series of resonant peaks (you can see them on "environmental curves," but that is another story). The STUDIO, because of its transmission line, does not have these peaks, and ears accustomed to them suddenly miss them. So listen to the STUDIO for a few more minutes, and then you will notice that, instead of a muddy "whoosh," bass instruments sound with a very clear, "pressureey" impact. You will feel the "whack" of the bass drum, you will feel the air moving when the pedal opens the organ pipe and feel the build-up as the pipe sounds. You will hear bright little "attack" sounds on snare drums - sounds you never thought were on records. You will hear the snapping of the strings of the celli and the double basses. After a few minutes, go back to the "box" speakers; if you prefer them still, the STUDIO is not for you.
Transmission line bass is characterized by this clear, open, and natural quality; and distinguished by its dynamic quality. The text writers have all commented, suggesting that it be compared to the sound of very large horns, or batteries of drivers mounted in outer walls. Audiophiles, long conditioned to box bass (i.e., harmonic bass and muddled bass), generally find difficulty in accepting transmission line bass. Musicians, concertgoers, and classical recording engineers accept it immediately. (Rock engineers may accept it; some do not, saying it is "too much" - they tend to compare to public address loudspeakers!)
Next you will note (1) mid range detail, the little scrapings and noises and "hall sounds" that one gets close-up (and which the microphone has picked up); (2) a sense that the soloists and instrumentalists are placed in space, left-right, fore-aft; (3) on classical recordings, which are almost always made in empty, reverberant halls, a lot of "echo" (the engineers try to capture this, call it "ambience"); (4) on "rock" recordings, made in dead studios, a very bright and bloated image - "too close," most people will say.
You will find you are raising and lowering the volume control, as the program material changes, to get it "right." Interestingly, this is a function of all very smooth, "open "loud speakers - you tend to use the volume control as a "nearness" control, to bring the performers into focus and proper position!
Records you had previously thought "edgy" or "rough" now sound fine - as if their "distortion" has been wiped off (what has actually happened is that speaker distortions have been dramatically lowered). Cymbal crashes, trumpet blasts, staccato brass, soprano voice fortissimo - all these are clearer, less strained, and seem to exist in space, rather than at the edge of a speaker.
Switch to FM transmissions; you will notice incredible variations in microphones, studios, condition of phono equipment, and overall "transmitter distortion"; as you sweep the dial. Chances are your own phonograph will sound faultless by comparison!
Overall, you will by now be conscious of a new kind of listening experience. Instead of the stereo image seeming to appear on the face of the loudspeakers, and being squirted at you from two holes, it will appear as a firm, spacious, deep "stage" stretching from left of the left hand speaker to right of the right hand speaker. It is an uncanny illusion, which exists free of the two STUDIOS (reason - the polar dispersion patterns and proper phase so expensively engineered into the STUDIO). As you adapt to it, you will no longer be listening to two loudspeakers, rather to a musical presentation!
The design parameters of the STUDIO, which go far, far beyond those of the conventional speaker of today, are justified by the result - a very wide band, open, and transparently dynamic reproduction. We believe these design features and the resulting unique performance produce the `better" loudspeaker that makes for proper appreciation of the immense musical literature available to you in your home. If what you want is to hear what the microphones hear, the STUDIO loudspeaker is for you. It can bring you a superbly advanced musical presentation, and will not be obsoleted by the program sources to come. We can only suggest you judge it by the most critical professional standards, and decide whether that level of quality is for you.
Impedance: Substantially resistive, nominally 8 ohms, and ideal for amplifiers.
Power Required: Suggested 25 watts minimum. Can be safely used with high powered amplifiers on program material.
Frequency Range: Minimum per anechoic plots. Actually, response is uniform 25 HZ - 40KHZ.
Distortions: Harmonic - per anechoic measurements supplied
Doppler - minimal per the design parameters.
Transient - significantly reduced, as measured by tone bursts, including examination of "delay" waves.
Size: 35 1/2" high, 15" wide, 14" deep. Weight: 70 pounds. Finishes: Natural teak. or walnut.Guarantee: Five years against defects in operating parts or workmanship, under normal use conditions.
The I M F STUDIO Loudspeaker each ..............................$300
I M F PRODUCTS
7616 City Line Ave., Phila., Pa. 19151
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