How spring reverbs work

To understand how a spring can produce a reverberant sound we must first look at what reverberation is. When a waveform is produced within a space the sound waves reflect off the surfaces in the room and as the waveform is diffused by a surface the energy of the waveform is split into multiple waveforms which again reflect around the room. Each time a waveform reflects some energy is absorbed by the surface and each time a waveform is diffused the energy is divided among the reflections and so over time the amplitude of reverberation decays until it is inaudible. As sound takes time to travel around a room (344meters per second at room temperature) each reflection arrives at the listener later than the last reflection and so the reverberation is audible for a longer period of time than the direct original sound. Diagram 1 below shows the decay of reverberation overtime.

The time taken for the sound to reach the listener directly determines the pre-delay. The early reflections are sound waves that have reflected directly of 1-3 surfaces without much diffusion and so have amplitudes that are only slightly less than the direct sound. Our brain analyses the early reflections to determine the perceived size of a space. Longer delays are perceived at being larger spaces and shorter delays are perceived as smaller spaces.

The reverberant field is made up of multiple diffused and delayed reflections. The timbre of the reverberant field helps us determine the perceived surfaces within a room for instance a church produces a bright sparkly sound because of the reflective properties of the marble surfaces and a living room produces a dull sound sue to soft furnishing absorbing the high frequencies more quickly.

A spring reverb works by using the recoil action of a spring to produce reverberant field type echoes.
First the input audio signal is converted into physical movement by an inductor which moves forward and backwards in relation to the frequency of the input signal. The movement takes time to travel down the spring to reach the 2nd transducer which converts the movement of the spring back into an electrical audio signal. The time taken for the waveform to travel down the spring determines the pre-delay and so the longer the spring the longer the pre-delay and vice versa

As the waveform reaches the end of the spring it compresses and moves back to the other end of the spring and then compresses again and moves back again and again until the energy of the waveform has been absorbed by the springs tension and mass and so the echoes decay gradually over time.
Diagram 2 shows how a spring produces a reverberant field

By using two springs with slightly different mass and tension a more diffused reverberation is obtained. This is due the change of speed in which a waveform travels up and down a spring caused by the different mass and tension of the two springs.

Because the time delay for a waveform to travel down a spring is so fast (1-5ms) a spring unit is perceived as only producing the reverberant field part of an echo unless you use a spring that is several feet long which becomes impractical. And so therefore a spring reverb produces a small room type echo due to the short pre-delay. Producers like Joe meek used a tape delay unit set to have a single slap back type delay before the spring unit in the signal chain to obtain a pre-delay effect and so give the impression of a larger space.