Spring Reverberation - Theory of Operation
When it comes to musical amplification and sound reinforcement,
Spring Reverberation has continued to offer
the greatest value among any comparable technology.
To achieve the best performance possible when
incorporating Spring Reverb into an amplifier
design, it's helpful to be aware of a few of its
significant characteristics.
The acoustical effect of REVERBERATION can be thought of as
the complex interactions of delayed sound. It can be heard
in nearly any listening environment, taking place when sound
waves traveling through the air are reflected by surfaces
such as walls, ceilings, and floors.
The properties of reverberant sound are most noticeable in the
case of a single abrupt sound impulse, such as a hand clap or
a door being slammed in a large room. At first, it is possible
for the ear to hear individual echoes, referred to as EARLY
REFLECTIONS. The time between the initial sound taking place
and the arrival of a reflection of this sound is referred to
as the DELAY TIME. These initial delays give the listener a
sense of the overall size of the listening space.
After the delayed impulses have traveled back and forth through
the listening space several times, the individual delayed sounds
increasingly overlap one another. This blending of echoes into
a random breath-like effect is called CLUTTER, and gives the
listener a sense of the the acoustical reflective quality,
or 'liveliness,' of the listening environment.
The combined length of these phases is called the DECAY TIME.
The standard used in architectural acoustics is the time required
for the overall reverberant sound to decay to a measured value of
60 decibels below the level of the original "dry" sound.
This term is often abbreviated as RT60.
Throughout the history of audio amplification, numerous
techniques have been developed to simulate the natural effect
of reverberation. These have included acoustic chambers and
pipes (reflective spaces into which a speaker and microphone
were placed), large steel plates (the Gotham Audio EMT series),
and recirculating delay mechanisms based on magnetic tape (the
Maestro "Echo-Plex"), or rotating electrostatic elements (the
Tel-Ray "Organ Tone," later licensed to Fender Musical
Instruments). Developments in solid-state electronics yielded
the analog 'bucket brigade' (capacitive charge-coupled) delay,
and most recently, digital delay technology.
The use of coiled springs to delay audio signals was originated
by early telecommunications engineers to simulate the effects
of long-distance telephone lines. While the prolonged sequence
of echoes was undesirable for their purposes, it seemed most
useful to an inventor named Laurens Hammond, who promptly
appropriated the concept for use with the keyboard
instrument which bears his name.
The coiled spring mechanisms underwent a continuing process
of refinement. In the early 1960's, a Hammond engineer named
Alan C. Young developed a design which proved to be practical for
portable use. Since then Spring Reverb has found its way into
applications ranging from High Fidelity audio to car radios.
Spring Reverberation units use a combination of electromagnetic
and mechanical elements to simulate paths of delayed sound.
an audio signal drives the COIL of the INPUT TRANSDUCER,
which applies a twisting force to miniature cylindrical MAGNETS
attached to a set of precision stainless steel TRANSMISSION
SPRINGS. The twisting motion travels as a wave impulse down the length of the springs until it is rebounded by the OUTPUT
TRANSDUCER, which also uses magnetic components to generate a
delayed output signal.
In addition to the coil and magnets, the transducers also contain
DAMPER DISCS within the BRASS SLEEVE assemblies. The SUPPORT
WIRES, to which the magnets are mounted, pass through the discs,
which results in a controlled application of friction, or
'braking' action, on the twisting spring assemblies. The DECAY
TIME performance specification of a reverb unit is dependent on
the choice of material type and thickness of the DAMPER DISC;
the TRANSMISSION SPRINGS are the same for all the available decay
time ranges for any given model.
If the input of a single-spring reverb assembly was driven with a
single pulse-like signal (an amplified 'snap'), a series of
delayed output pulses would result. The first delay would
represent the amount of time the sound required to travel through
an equivalent of the uncoiled length of the spring. Following
this, the pulse would be reflected from the output side, travel
back through the spring, be reflected by the input side, and
return through to the output side. This results in subsequent
delays having a length of twice the initial DELAY TIME.
When the spring is driven with a sustained audio signal,
the delayed reflections overlap the incoming sound. Depending
upon the frequencies found in the incoming signal, the reflected
sound can interact to result in a series of RESONANCES.
To avoid this unevenness over the frequency response range
in an actual reverb unit, two or three spring delay paths
are used, each with a different delay time rating.
The Type 4 (full-sized two-spring) reverb unit, for example,
provides DELAY TIMES of 34 and 41 milliseconds, chosen
for the fullest and most even overlapping of delayed sound.
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