HAMMOND ORGAN

     If you play a Hammond organ in a large, reverberant room, the room acoustics will, during the buildup of the room's natural reverberation, slightly soften the attacks of the tones, and the reverberation will provide both a sustaining action and a gradual decay, this essentially eliminating the Hammond's instant-on, instant-off characteristics. Hammond to a significant degree overcame the key click problem by designing their amplifiers and speaker systems to be effective only up to 6kHz which is the highest frequency that the instrument can produce. There is no need to go beyond that. The key clicks, on the other hand, consist of a very short burst of high frequencies going right up to the limit of human hearing. Therefore, by making the amplification and speaker system unresponsive above the highest notes or pitches that the instrument can produce, the key clicks can be made less noticeable. However, if a Hammond went to a small room rather than a large, reverberant room, the instant-on, instant-off characteristic was still a problem.
     Hammond engineers therefore developed an artificial reverberation unit whose purpose was to create a reverberation effect such as that which you would encounter in a large room or hall, and to apply this effect to the signals at the speaker cabinets, thereby overcoming what was otherwise a real problem.
     Hammond discovered that you could use coil springs to simulate reverberation. In order to do this, there has to be a transducer at each end of the spring. At one end, the electrical signal goes in and gets converted to mechanical energy, which in this system literally shakes or vibrates the spring in an exact replica of the electrical signal fed into it. At the other end, a similar transducer converts the mechanical motion of the spring back to electrical energy, thereby developing an electrical signal that copies the vibration of the spring.
     Because these mechanical vibrations travel relatively slowly in a spring, you do not need a spring more than perhaps two to three feet long to introduce a delay in the sound, similar to an acoustic echo in a room. Furthermore, at each end of the spring, the signal (mechanical vibration at this point) gets reflected back and forth along the spring, thus the spring sustains or reverberates the signal as large rooms and halls do with their multiple echoes that blend together to form a smooth reverberation effect. .
     The picture below is a diagrammatic representation of a single spring used as an artificial reverberation producer.

Figure 27, left. This diagram shows, in simp-lified form, the basic elements of an artificial reverberation unit based upon a coil spring. To be effective, the wire from which the spring is wound should be of fairly small gauge, and the individual turns of the spring must not touch each other.
     At the top of the spring, a transducer converts the incoming electrical signal from the Hammond con-sole to a mechanical sig-nal which is applied to the spring, vibrating it slightly. In actual practice, the vib-rations are very minute. You would not see any-thing moving or vibrating if you watch the unit in action.
     At the other end, a similar transducer con-verts the minute mech-anical vibrations in the spring back to an elec-trical signal which is then mixed with the original signal and the composite goes to a power amplifier and then to the speakers.
     Although seemingly simple in principle, there are a number of important criteria which must be met in order for a spring re-verb unit to be successful. The earliest Hammond reverb units were just borderline, but they were, nevertheless, a decided improvement as compared with the use of no such device in a small room.

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