ON TEST IMF Studio loudspeaker
This article is reproduced from June 1971 issue of HI-Fl SOUND by permission of Haymarket Press Ltd., Gillow House, 5 Winsley St., London, W.1., and printed by Calrion Photographic Services Ltd., 21 Greek Street, London, W.1.
Speaker with grille removed exposed to the outputs of hundreds of different speakers, and given the opportunity to explore the performance of at least some of these and question the motives of their designers, the critical reviewer of equipment is wise to find himself some reasonably firm guidelines-not prejudices, as a rule (though if he has any good science-based ones he should hold on to them), but rather the result of measuring and observing and reaching conclusions based on experience. Of course, the guidelines will be subject to some cautious adjustment as the art and science progress.
There is hardly a commentator who could refute the proposition that big loudspeakers, designed with an eye on the test-gear and an ear on the music, are the best. The fact that many people can neither afford them nor find space for them does not in the slightest affect this general truth. A test report in our December 1970 issue contrasted test data relating to two large and very good speakers-the IMF Monitor and the B&W 70.
If 'bigness' is a guideline, we can still extend a welcome to somewhat smaller reproducers of high quality-those medium-sized free-standing speakers of which there are all too few really good examples. Often in this field the designer does not make the best of his hardware, for reasons largely to do with economics. Otherwise there would not be so many total-enclosure models about.
IMF's contribution in this area is the Studio, a scaled-down version of their Monitor, in that it exploits the transmission-line principle used with such success in the bigger model. Thus the cost is lower-but this is decidedly not a cheap speaker. Some details of this principle will be found in the earlier report but new readers will require the reminder that the transmission-Iine, a pressure-loading arrangement, is a long folded pipe, openended and terminating near the bottom of the front baffle. This line loading is applied to the bass driver to absorb the back wave progressively with rising frequency.
The line is inversely tapered and contains graded amounts of sound-absorbent material. Acoustic mass is imposed on the bass diaphragm, increasing its effective area, and a major advantage is the generous low-frequency output obtained with relatively small diaphragm movement. Other factors being right, this approach should give firm, well controlled If performance. Indeed, on first hearing this speaker one is struck by the depth of the freely generated bass and the apparently low distortion with large inputs.
The 8in. plastics-cone bass unit starts life as a KEF driver, seen in other systems of quite different type, and is a version to suit the Studio. Handling the rest of the range are a 1/4in. tweeter and a 3/4 in. diaphragm super-tweeter to extend the response well beyond the audible limit (both these units have 'chemical' diaphragms-actually Melinex) ; and a 5in. mid-range unit backing into its own short transmission-line. Complex crossover design is involved. Enclosure construction is solid, and the material used is high-density particle board with a veneer finish.
This general description applies to what the makers call the 'export' or semiprofessional version. IMF also make at higher cost a professional version-as they do of the Monitor-and one of the specification differences is a special enclosure construction in which the board is stress laminated with a Formica skin to give even better control over colouration. Such a skin is, however, applied to the rear of the model under test. Also included on this version are level controls providing ±2dB adjustment of mid-range and hf.
B&K frequency response pen-traces are shown. These were obtained in the anechoic chamber with sine-wave input and with the microphone at one metre on axis (mid-range). Level controls were at mid-position. In interpreting these traces bear in mind that room gain comes into effect at the lower frequencies depicted. Lf output does in fact extend down to about 25Hz. At the other end the response holds up well outside the audible limit-a fact that is not accurately reflected by the B&K trace.
Separate traces are shown for clockwise and anti-clockwise rotation from axis the relevance of this is realised when it is seen that the Studio is supplied in 'mirror-image' pairs. That is, the left speaker has mid and high units towards the right of its baffle, and the right speaker has them towards the left. This is to give an inward dispersion, towards the listening area and away from nearby surfaces, minimising reflections and diffraction effects around the enclosure front.
First impressions of low distortion are substantiated by the following figures, which are but a fraction of the figures associated with some cheaper medium sized systems.
Distortion at 80dB output level
The impedance curve of Fig. 3 shows how little the impedance departs from the nominal 8 ohms and is in itself a success story. This characteristic is well in accord with the effect one is conscious of when listening: power is easily driven into the speaker. What happens to it after that is the next most important matter!
As in earlier reports I am listing figures which relate sound pressure level to input and frequency.
It can be seen that over important parts of the range about 3 watts are needed for the levels produced under these test conditions, so that around 45 watts would be required to deal with 100dB peaks. If this suggestion of a monitoring level does not appeal, remember that a 95dB level represents a large orchestra playing fairly loudly and that, as always, you will still need a sensible reserve of power if the system is to be of the highest quality. The designer's suggested driving power range is 25-60 watts (rms-based), and I strongly endorse the recommendation. For final assessment of performance I used two amplifiers, one rated at 35W per channel found adequate in a room of about 2,800 cu. ft.-and another offering a more generous reserve with its 50W rating.
Impressions gained from extensive listening tests can be summed up in this way. The sound was smooth yet lively, with good attack on transients, and-an important finding-the output was well integrated, free from disembodied effects in the upper range. Such is the smoothness of the top end that I suspect many users would want to put in the maximum power using the level controls.
With a pickup of suitably high calibre the stereo image was stable and realistic. An even stereo spread has come to be associated (through misrepresentation) with omni-directional speakers, of which there is now greater variety; but in fact front-facing speakers, if good enough (the dispersion pattern is all-important), should deal honestly with the input and sharply reveal the stereophony that is in the recording, presenting it as a consistent sound-stage and without giving the listener the impression that he is restricted in his listening area. To present stereo in a generalised way when originally it was sharp and detailed is akin to throwing the baby out with the bathwater (and is practically an invitation to recording companies not to bother).
The Studio differs from the bigger IMF Monitor in relatively subtle ways. I was able to compare it with the Monitor-the samples on which I reported in December and noted the tatter's even deeper, freely reproduced bass, and an impression of ease that goes on up into the mid-range. The Studio sounds that bit more like a loudspeaker but is still a good one in relation to price and size. Bass is tighter and deeper than from most reproducers in this middle range. This, without doubt, is the big advantage-the merit that justifies the price. With this we can associate its power handling capability.
Finally, the Studio is intended to be part of a high-grade system with the emphasis on performance rather than cost. Factors to consider are power reserve appropriate to the room; pickup quality; and turntable rumble, which must be minimal. A good speaker tends to be transparent to the quirks arising in earlier links of the chain, so pay attention to details.
Drive units: bass unit 8in., mid-range Sin., high frequency 1 3/4 in. chemical diaphragm, super-tweeter 3/4 in. chemical dome.
crossover filter: 12-element-375Hz, 3.5kHz, 13kHz.
Impedance: 8ohms nominal.
Frequency range: 25Hz to beyond audibility.
Power requirement: 25-60 watts recommended.
Dimensions: 36 x 14 x 15in.
Weight: 68lb. approx.
Finish: teak or walnut. Distributors: IMF Products (GB), 62-68 Silver Street, Reading, Berks.