North Suburban HAMMOND ORGAN Society

Although the elementary reverb unit of figure 27 would work, it would not be overly satisfactory. One of the biggest problems is uneven frequency response. There are certain frequencies at which the spring is naturally resonant. If a note of the Hammond is close in pitch to one of these frequencies, that note will be significantly emphasized. In a corresponding manner, there are some frequencies at which the spring is not very responsive. In this case, the reverberation applied to these frequencies will be significantly less. Also, if the phase of a certain mechanical wave in the spring happens to arrive at the pickup transducer exactly opposite (180 degrees out of phase) to that of the direct signal, there will be significant cancellation thereby making that particular note or pitch seem very faint.

Likewise, there will be a certain repeat echo effect which is quite noticeable as the various mechanical signals bounce back and forth from one end of the spring to the other. Also, if there is any significant mass to any of the parts, then the reverberation effect applied to the higher frequencies will be significantly less than that applied to the lower frequencies. All of these problems were present in the earliest Hammond reverberation units. But, as is true with so many other human endeavors, after we create something, we see how it performs in the real world, and then we set about to do two significant things. Make it cheaper, and/or make it better. [Regarding artificial reverb units, Hammond did both.]

necklace reverb unit diagram

This animated picture shows in simplified form how the necklace reverb unit works. The red, blue and green dots represent the vibrations in the three hanging springs that were induced there by the input transducers in response to the signal represented by the small sine wave that occurs briefly at the upper left. As you can see, the three signals from the springs arrive at different times at the output transducers. The springs, as expected, both slightly blur the initial attacks of subsequent signals as well as prolong them. Because the springs are wound from finer wire, and also because the transducers used have much less mass, these reverb units can respond adequately to the entire pitch range of a typical Hammond organ. Just as in a real room, the combination of reflected signals arriving at different times adds to the complexity of the final result, which sounds much more like real room reverb with repeats that occur randomly and are much less pronounced than those from a single spring reverb unit. Also, because the three springs have different lengths, they have different resonant frequencies from each other which makes the over-all response much flatter without pronounced peaks and cancellations that would occur with just a single spring.

This diagram is, like the previous, slowed way down so that it's easy to see and follow what happens. In a real necklace reverb, the signals even in the longest of the three springs take only about 1/7 of a second to travel from one end to the other and back again.

The earliest reverb units had two main reverb producing springs which were joined by a seesaw-like lever at their top ends. The driving transducer vibrated the seesaw, and then the mechanical waves went down both springs. At one end of one spring there was a pickup transducer, and at the end of the other spring was a short tube filled with oil. The oil was there to damp out some of the vibration in that spring. There were three other springs associated with this device also, but they were immersed for almost their entire length in tubes of oil and they were only there to stabilize the two reverb springs and also counterbalance their tension.

After a few years, Hammond designed a different type of spring reverb unit which had three springs entirely in air. They were all of different lengths. Each spring had its own transducers at each end. Because they were of different lengths, they had different transmission times, and also different resonant frequencies. The transducers were extremely small and light, so that the mass of each was much less, allowing the springs to respond to the highest frequencies that the console could put out [6kHz.] and the springs themselves were wound from much finer wire. The entire reverb unit also was not rigidly fastened to the speaker cabinet but rather suspended from a metal bracket by two short coil springs and was otherwise hanging freely. This eliminated any mechanical feedback from the vibrations of the speakers.

These reverberation units gave excellent results. A Hammond played through a speaker cabinet with one of these units sounded as though it was being played in a good hall. The three springs hung down in roughly parabolic loops, like the shape a lady's necklace makes, so this was called the necklace reverb unit. The improvement over the old oil-damped early reverberation units was profound. Here [figure 28] is a simplified diagram of the necklace reverb unit.

This reverberation unit was soon standard equipment in many different Hammond speaker cabinets. It did several things and did them quite well. First, it created the effect of playing in a large hall, even if your speaker cabinet was in a padded cell! It also smoothed out the attacks of the tones and also gave them a gradual roll-off or decay, thus eliminating or greatly reducing the otherwise telegraph-key characteristic of the instrument's tones. If a Hammond was installed in a large hall, these units were not necessary, but they contributed a great deal to the tonal effects of instruments in small rooms, such as apartments and studios. Furthermore, because the signal from the instrument was entirely electrical, and because likewise the reverb signal was electrical, these units made it possible to record a Hammond using the direct electric process without the need of a microphone. Therefore, excellent quality recordings could be made complete with the ambience of a concert hall and there was never any possibility of getting any extraneous background noises in the recordings.

As is true with many really great things, their manufacturers continue to "improve" and also to reduce costs. Eventually Hammond produced a third generation reverb unit which had only two springs instead of three. Each was wound from a different gauge of wire, and each had its own transducers at both ends. Also, these springs were under slight tension, and the entire assembly was mounted on a steel tray which was suspended in a sheet metal box by four taut springs, one at each corner. These reverb devices worked quite well, but it is the opinion of many, myself included, that they were not as good as the necklace units, sounding slightly tinny and springy. However, they were still vastly superior to the original reverb units with the springs in oil.

All spring type reverb units, even the necklace unit, suffered from a few defects. The worst defect of all was uneven frequency response, where some notes or pitches would be emphasized and some would be suppressed. It was impossible to make a reverb spring with a flat frequency response. Although not within the province of this article, I should nevertheless mention that in the recording studios, other means of generating reverberation and echo were also used. One of the most interesting is tape echo. [Click to see that article] and for our purposes here it is sufficient to say that some excellent results were obtained in some commercial recordings of Hammond organs by combining a necklace spring reverb unit with a tape echo device.

Today, modern digital signal processors can reproduce the reverberation characteristics of almost any type of room or hall and do it so realistically that it is absolutely impossible to tell on a recording that the reverberation and room ambience are artificially derived.


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