The Reverb Tank

Often referred to as a tank, spring-reverberation devices typically incorporate several metal springs suspended in a metal case. The springs are excited using electromagnetic transducers. The principle is similar to a loudspeaker, except with a spring instead of the membrane. Sound waves travel across the spring to the opposite end, where another transducer converts the mechanical energy back into an electronic signal. A lot of the energy is reflected at both ends, but mechanical damping inherent in the system means that the the sound fades away naturally.

Several springs often share a transducer pair. In such cases, the transducer houses each spring’s cylindrical end-stop magnets, distributed evenly within the air gap (and enclosed by the laminated core, a stack of thin magnetic iron plates). Sharing a transducer pair minimizes mechanical and electric complexity, but the signals can only be treated as a whole. Tanks that can be operated in stereo require multiple transducers, and are therefore quite rare Of course you have them in Twangström! Some stereo systems had two complete tanks.

A transmission spring follows a principle physicists call the spring-mass system, and you might recall classroom experiments in which a weight was attached to a hanging spring, making it oscillate at a certain frequency. Apart from the external mass, the physical properties of the spring (material, length, wire gauge, number & diameter of coils) have a great effect on the vibration and transmission of sound. The longer the spring, the lower the frequency at which it oscillates. Longer springs also decay more slowly than short ones, as more windings result in more kinetic energy potentially stored.

Typically, reverb tanks with multiple springs are chosen for unequal delay times. Ratios based on prime numbers help in achieving dense reverb by strictly avoiding repetitive echo pattens. In a reverberation tank, the rigid suspension mounts at both ends act as a constant mass, thus damping the spring's energy. But upon vibration, some of the energy is transferred to the other parts. In a multiple-spring arrangement, all springs share a common mounting plate, so vibration becomes an interactive process. As a result, springs interchange their wave fronts and the sound becomes more washy with each reflection.

If there were no mechanical coupling involved, a minimum (static) density could be achieved by choosing suitable delay times, but there would be no buildup of echoic complexity. In real tanks a certain degree of coupling is always present, so we made it switchable in Twangström. In certain setups the echo-density buildup has another cause: the division of the spring into shorter segments. For instance, at first glance the popular Accutronics Type 4 tank appears to be two springs, but is actually four: Each sub-system comprises two springs with a small joint in between. What happens there is pretty much the same as at the suspension end of normal systems: Waves are partially reflected while most of the wavefront is passed on to the next spring. These joints seldom divide the two springs perfectly, and this fact helps spread the echos around, making the reverb tail more diffuse.