Although the principle of using the Doppler Effect by means of a rotating baffle next to a speaker was Don Leslie's original idea, it has been copied by a number of manufacturers. Imitation is a very good form of flattery, of course, but perhaps speaker systems working on this principle should be referred to as 'Doppler Speakers' as they may not necessarily be genuine Leslie products.
The constructor may be assured of one fact — that making a doppler speaker from scratch is not easy. The professional product may be thought to be fairly expensive, but there is considerable technical know-how required to make such a speaker mechanically silent enough to be useful. The rotor and/or horn unit impart sound modulation at about 7Hz and thus must rotate at some 450 revs per minute. The motors employed have to be powerful enough to get the mechanism moving at full speed in a fairly short time and so vibration, mechanical hum and wind noise are factors that have to be considered in the design. Four or more motors are not unusual and these are resiliently mounted in large rubber grommets and the rotor and horn bearings also have forms of rubber suspension.
The addition of a Doppler Speaker enhances the sound of most electronic keyboard instruments. Indeed, once it has been fitted to an organ, that instrument loses its aural identity — which might even be an advantage! The rather special sound the Doppler Speaker imparts makes it difficult to tell which make of instrument is being played: without it, the expert would be able to identify many makes of organ, especially with their own electronic vibrato in operation.
There are many ways in which vibrato (and tremolo) can be applied to sounds electronically. Perhaps one of the best methods devised was the scanner-phase vibrato used in Hammond tone-wheel organs (now defunct), where the audio signal was passed down a relay line which was scanned back and forth to produce phase shift vibrato that was near perfect. Even so, you will note, Hammond also provided means for fitting a Leslie speaker: this would seem to underline my comment on sound enhancement!
The string player 'wobbles' his finger on the stopped point, making the string fractionally longer and shorter cyclicly. In electronic terms, this is frequency modulation and is usually achieved by injecting a low frequency sine wave into the master oscillator of the generator system. In practice, a set of stable generators that will stay in tune does not I ike to be modulated and in certain cases it can be difficult to obtain sufficient modulation. Occasionally one hears electronic vibrato which is noticeably lopsided — where the frequency swing is not equal either side of centre. Even when electronic perfection has been achieved, the result is often only mildly interesting.
Also called Tremulant, this is amplitude modulation of the audio signal. Arranging this effect is fairly simple compared with vibrato, as an opto-isolator (or its equivalent) and a square wave oscillator will serve admirably. Tremulant is often used in serious music, rather than vibrato, and again is only of mild interest.
In my view, what makes Leslie speakers and their imitators more interesting is that these combine both vibrato and tremolo by the nature of their operation.
The Doppler Effect is caused when a sound source is moving relative to the listener. The pitch of a police car's siren falls as it passes the listener because the frequency is travelling at the speed of sound plus the speed of the car as it approaches and at the speed of sound less the car's speed as it recedes. So the eardrum receives more pulses per second on approach than after the car has passed.
The same effect can be obtained by mounting a speaker on a large rotating baffle board and at least one manufacturer has used this method in the past for obtaining chorale. This proved to be both large and cumbersome and Don Leslie circumvented these problems by using a stationary speaker and rotor next to it, as shown in Figure 1.
Small Leslie units employ a polystyrene foam rotor, which is light enough for a relatively small motor to drive it. If built into a small organ, the unit is turned on its side compared with cabinet models, the sound emerging through slots at the side of the console. The drive from the synchronous motor is by means of pulleys and a cotton covered belt.
Larger Leslie units have heavier wooden rotors which reflect more sound than the polystyrene types. The belt drive system is similar except that a more powerful motor has to be employed. The wooden rotor is shrouded in cloth for aerodynamic reasons and thus produces less wind noise. Figure 2 shows the typical arrangement.
The speaker over the rotor may be up to 15" in diameter and is mounted either in a form of infinite baffle or with acoustic wadding behind it in the console. In either case, the sound is directed into the rotor with its opening constantly approaching and receding from the listener, so giving frequency modulation. In addition, the volume of sound alters as the rotor's aperture passes, so amplitude modulation occurs.
Some cabinet models also employ treble horn units. In this case, a crossover network feeds the lower frequencies to the rotor speaker and the higher ones to a treble driver unit. Above the driver is a pair of horns, one being a dummy for balance only, driven by separate motors, pulleys and belt.
The treble horn unit makes a great difference to the doppler speaker sound: the bass rotor alone works reasonably well but is not sufficient on its own. I would strongly recommend purchasing a treble horn unit if the doppler speaker system is at present limited to bass rotor only. This can be built into a box to sit on top of the existing cabinet.
It is usual to have two motors so that the rotor/horns can be driven at 'fast' (about 7.5Hz) or 'chorale' (about 0.5Hz) speeds according to the switching. A combination of a straight speaker signal and sound from the doppler speaker on 'chorale' gives a very full and pleasant sound to any sort of music. It takes that hard electronic edge off the instrument and, because of the slow meandering of sound, adds a cathedral-like effect to the instrument. The Leslie motors are shown in Figure 3.
It is not a good idea to feed reverberated signals through the doppler speaker as these sound rather unpleasant. If possible, a separate speaker should handle reverberation which in itself will probably give some chorale effect with the slowly turning rotor or horns.
I have already underlined the importance of the treble horn unit to give the true 'Leslie' sound. The horns should be fitted with sound diffusers, for without them the effect will tend more towards tremolo than vibrato. These are simply small 'dishes' placed about one inch from the mouth of the horn which allow the sound to be heard after the mouth of the horn has passed. If these are absent, for any reason, they can be added without too much trouble. Figure 4 shows the horns and diffusers.
The diffusers can be made by cutting out a disc of card about ¼" greater in diameter than the horn mouth. Mark out and remove a 30 degree slice and join the card with tape to make a shallow cone. Treat this card with a releasing agent and build up about half a dozen layers of fibreglass tissue on it with polyester resin. When the resin has hardened thoroughly peel off the card and trim the diffuser's edges. Drill at 3 points (120 degrees apart) and fit 1" from mouth of horns with thin dowel and Araldite.
Unless the reader happens to be a mechanical expert, it is probably best to buy complete doppler speaker units, preferably both bass and treble. As mentioned earlier, it might appear to be fairly simple to make everything from scratch, but mechanical noise will undoubtedly prove to be a major problem.
Switching, both for speakers and rotor/horn speeds should be available at the keyboard. This should allow the choice of main speaker, doppler speaker or both, whilst a further two-position switch is provided for fast and slow speeds.
The only disadvantage with doppler systems is that they are acoustic, which means that their effect is lost when using headphones for practice purposes or when making direct recordings. Those who demand these facilities plus good electronic vibrato will have to settle for bucket brigade systems.
Feature by Ken Lenton-Smith
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