AMP-ED: The Reverberation Effect

Guest post by Kurt Prange. Kurt is the Sales Engineer for Amplified Parts in Tempe, AZ. Kurt began playing guitar at the age of nine in Kalamazoo, MI. He is a guitar DIY’er and tube amp designer who enjoys helping other musicians along in the endless pursuit of tone.

    Photo by Fern Richardson

The Reverberation Effect

A listener standing some distance from a sound source will perceive sound that is actually a combination of direct sound and indirect sound that has been reflected from the boundaries of the listening area. The reflections are referred to as reverberation. Reverb can enhance the perceived sound from a source by adding depth, color and liveliness.

Reverb can be thought of as being composed of two parts:

1) Early reflections – shape the listener’s conception of room size
2) Cluttered reflections – convey the liveliness of a room

Imagine you are inside a large hall and you clap your hands once. The length of time required for the arrival of the very first reflections is called the delay time (usually on the order of tens of milliseconds, e.g. 33 ms) and is related to the volume of the room (or distance of the reflective surfaces from the listener). The number and density of reflections increases rapidly with time and they become cluttered while simultaneously decreasing in level until they are no longer audible. The length of time required for a sound to decrease in level by 60 dB is called the decay time (usually on the order of a few seconds, e.g. 3 s) and is related to the acoustical properties of the reflective surfaces in the listening area. For example, poured concrete walls will reflect more (absorb less) acoustic energy than drywall.

Electro-Mechanical Reverberation Devices: The Reverb Tank

Historical use in Musical Instruments

Laurens Hammond of Illinois popularized the use of artificial reverberation devices through his church organs in the 1940’s and 1950’s. “The early (pre-B-3®) Hammond® organs were sold to churches on the principle that organ music is greatly enhanced by reverberation, but the minister’s speech in the church is hampered by reverberation. Therefore, churches were designed to be acoustically dead, and the Hammond® organ had to have its own artificial reverberation.” “[Reverberation] made its debut in the Fender® line as a separate item, using a spring [unit] bought from Hammond®, [in] 1961. It was first incorporated in a Fender® amplifier with the Vibroverb® of 1963 and then spread widely throughout the amp line, just as vibrato/tremolo had before it.”

Spring Reverb Construction and Operation

The main components used to produce the spring reverb effect are:

  • Input and Output Transducers

Each transducer consists of a coil centered around a magnetic lamination and
small cylindrical magnets centered in the air gap of the lamination.

  • Transmission Springs

These components are mounted on an inner aluminum channel, which is connected
by four small support springs to an outer steel chassis (or channel).

An electrical signal applied to the input transducer coil generates an alternating magnetic field which moves the transducer magnets. The magnets are mechanically coupled to transmission springs. The signal is reflected back and forth through the transmission springs with an amount of delay determined by each spring’s diameter, wire gauge and length. The moving magnets of the output transducer generate an alternating magnetic field which induces an electrical signal in the output transducer coil.

The Significance of Multiple Transmission Springs

The use of multiple transmission springs helps to improve the reverb characteristics. A listener in a large hall with natural reverberation is not usually standing the same distance from each reflective surface. Naturally, there will be reflections from different surfaces having different delay times. The use of multiple transmission springs with different delay times serves to simulate a more natural ambiance, as well as improving the overall frequency response because one spring’s response will fill voids or holes in the other spring’s response.


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