AKG LSM50 Micro Monitors.
The LSM50's single driver has to cover the whole audio spectrum, or at least cover as much of it as possible. To reproduce low frequencies audibly (ie. with any sensible efficiency), a large cone area, with a correspondingly large - and heavy - voice coil is necessary. For treble, there's a conflicting set of requirements - a small cone diameter is necessary for good dispersion, and the cone and voice coil assembly need to be lightweight, otherwise the inertia will be too great to follow fast, high frequency signals.
Obviously, any drive unit must involve trade-offs in one of these directions, and in this instance, the missing bass end of a small cone has been chosen as the lesser of the two evils. This isn't the end of the story though, because much depends on how smoothly the bass rolls off, and providing the bass harmonics are reproduced accurately your ears will synthesise the fundamental. That is, you'll hear much of the (physically) absent low bass, especially from a good, phase-coherent recording. The bass harmonics, of course, lie in the midrange zone, where excellent performance can be expected from a 5½" diameter cone. At the same time, the relatively gentle bass roll-off helps to maintain original phase relationships between any fundamental that is audible, and the upper harmonics.
Another positive aspect is that with a single (5½") drive unit, there's no crossover, so the phase and time errors that occur between multiple drivers are thankfully absent. At the same time, the small cone acts as a near point source in the midrange, and overall, these factors make near perfect stereo imaging a possibility. Last of all, the need to keep the voice coil lightweight (for a lively treble response) inevitably imposes restrictions on its power handling capabilities, and the 50 watt rating is certainly pushing today's loudspeaker technology to its limits. At the same time, don't lose sight of the fact that until a few years back, it was near impossible to achieve a 10 watt rating for a speaker of this size with any worthwhile efficiency.
Placing the LSM50s at ear level, and roughly in the horizontal position they'd be in if set at each end of a small mixing console, revealed good imaging and an intimate sound, though on some 'hot' source material, the lack of bass accentuated the forward sound to a fatiguing degree, especially at high levels. To some extent, this was due to intermodulation products (where the large movements of the cone at low frequencies interact to form dissonant 'chords' with the higher notes) but the speakers aren't entirely to blame, and one answer is to gently EQ out any forward tendencies in the music.
Bass response can be increased without EQ by placing speakers against a solid plane surface, usually walls or floors. Placing the LSM50s flush with books on a shelf is particularly recommended, but locations against a wall or floor will also prove worthy of experimentation, though I should emphasise that the bass gained won't be as smooth as that achieved with some careful equalisation. In our own tests, the best sound was attained with the units placed on the floor, about 3 feet in front of the listening seat, with a wedge under the front to angle the drivers upwards, so that the cone would be on-axis with the ears. Apart from being a little impractical for monitoring whilst sitting at a console, floor-mounting in this manner is known for its positive effects on bass 'depth' and 'space' - for a start, the ear perceives bass as more realistic when it comes from below. Anyway, the results were excellent, for whilst the bass was only mildly accentuated (where it outperformed the Tannoys reviewed in our previous issue). They possess a depth, and sheer tangibility that few 2 or 3 way speakers attain - even ones costing fifty times as much! At the same time, this potent centre-imagery was attained at the expense of hard left/right images ie. by placing the speakers relatively close together.
AKG quote 50 watts power handling capability, but providing you exhibit some sensitivity, and listen carefully for signs of stress in the speakers, it's quite safe to drive them with amplifiers of 100 watts or more. Indeed signals from an underpowered, and therefore overdriven amplifier are, if anything, more harmful and this is certainly the most common cause of failure in small speakers. Therefore, if you're using a small (say 30 watt) amp, be especially careful not to drive it into clipping.
No EQ was used in the listening tests, because the inevitable phase anomalies would tend to cancel out any advantages arising from the natural phase-correctness of the LSM50s. If this sounds unnecessarily purist, try it - you'll be surprised how accuracy in the midrange can compensate for something so seemingly unsubtle as 'no bass'. And because the bass drops off gently, you can readily and pretty accurately tidy up the bottom end with a standard Baxandall or Shelving EQ response - that's with the normal type of bass control found on budget equipment. Incidentally, it's much harder to EQ for a smooth sound on micro-monitors which exhibit a rise in the high bass region, so if you're comparing the LSM50 with Auratones (say) in a shop, make sure each is given bass EQ appropriate to its needs.
The LSM50's top-end is particularly smooth, and though the midrange might seem to benefit from a slight dip, the sound is probably better off without the phase abberations of any EQ in this critical region. Whatever EQ you use, make sure that the amplifier power is commensurate with the degree of bass boost you're applying. A 12dB lift demands a sixteen fold (x16) increase in amplifier power, and reduces the maximum SPL available before either the cone bottoms ('hits the end-stops') or the voice coil overheats. Listen out for these effects whilst setting up the EQ - and also for any signs of amplifier clipping on loud bass notes.
The exterior aesthetics are to AKG's normal standard of excellence. With any small diameter driver, bass demands large cone excursions, and the grille cloth is sensibly placed well away from the driver to reduce the likelihood of annoying flapping noises should the cone jump out of its socket - as it is wont to do on loud kick-drums.
Connection is via an XLR socket, which gains full marks, but for the fact that it's a male socket, which is, of course, the opposite of the normal standard (females for inputs, males for outputs - see September 1983 HSR). However, there's a good reason for this idiosyncrasy in that it's difficult to make a female XLR socket airtight, and the performance of the speaker depends on the absence of air leakage through the cabinet. Another reason for the XLR sex-anomaly is that the LSM50 was developed in conjunction with the BBC, who prefer to use XLRs back-to-front on amplifier outputs/speaker inputs, so as to prevent accidental connections to line level cables and equipment. Ouch!
Using an optional swing bracket, the LSM50 can be mounted on a console or a mic stand. The bracket is screwed to the cabinet via threaded bushes which are hidden underneath the veneer until you decide to use them. The bracket kit comes with a template to locate the centres of the hidden threads, and you simply punch through the veneer at this point. An adjustable wall mounting bracket, which holds the speakers with clamps is also available.
The LSM50 is impressive for its revelation of bass detail, smooth high frequency reproduction, and above all, near definitive stereo imaging. At the same time, the sound levels attainable are inevitably limited although to some extent this is compensated for psycho-acoustically viz: they sound loud.
Plainly it isn't a reference speaker, it will only suit music possessing a degree of subtlety, and you'll appreciate its strong points best with, say, an elaborate 2 or 3-way monitor as a yardstick. It's best used as an auxiliary (when you need to hear subtle detail, or to assess stereo effects from a different angle) and as such, it's an invaluable tool for anyone involved in music. Other uses are out on the road (they fit into a holdall) or as car speakers, though the 8 ohm impedance will call for a special switchmode amplifier if you're to make full use of the 50 watt rating. Oh yes, and you can also use them to mix your next Radio 1 hit on, but don't forget that the recording will be bass heavy, and that mixing solely by micro-monitor, without a valid reference, can make a serious record (or tape) unattractive to potential record buyers with decent Hi-Fi equipment.
The recommended retail price of the AKG LSM50 speakers is £56.95 including VAT (per pair).
They are distributed in the UK by AKG Acoustics Ltd, (Contact Details).
Review by Ben Duncan
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