Today, reverb is most often added to a recording using artificial reverberators, such as software plug-ins or digital reverb hardware units. But there are a lot of other approaches.
Many recording studios have used special rooms known as reverberation chambers to add reverb to a performance. Elevator shafts and stairwells (as in New York City’s Avatar Recording Studio) work well as highly reverberant rooms. The reverb can also be controlled by adding absorptive materials like curtains and rugs.
Spring reverbs are found in many guitar amplifiers and have been used in Hammond organs. The audio signal is coupled to one end of the spring by a transducer that creates waves traveling through the spring. At the far end of the spring, another transducer converts the motion of the string into an electrical signal, which is then added to the original sound. When a wave arrives at an end of the spring, part of the wave’s energy is reflected. However, these reflections have different delays and attenuations from what would be found in a natural acoustic environment, and there may be some interaction between the waves in a spring, thus this results in a slightly unusual (though not unpleasant) reverb sound.
Often several springs with different lengths and tensions are enclosed in a metal box, known as the reverb pan, and used together. This avoids uniform behavior and creates a more realistic, pseudorandom series of echoes. In most reverb units though, the spring lengths and tensions are fixed in the design process, and not left to the user to control.
The plate reverb is similar to a spring reverb, but instead of springs, the transducers are attached at several locations on a metal plate. These transducers send vibrations through the plate, and reflections are produced whenever a wave reaches the plate’s edge. The location of the transducers and the damping of the plate can be adjusted to control the reverb. However, plate reverbs are expensive and bulky, and hence not widely used.
Water tank reverberators have also been used. Here, the audio signal is modulated with an ultrasonic signal and transmitted through a tank of water. The output is then demodulated, resulting in the reverberant output sound. Other reverberators include pipes with microphones placed at various points.
These acoustic and analogue reverberators can be interesting to create and use, but they lack the simplicity and ease of use of digital reverberators. Ultimately, the choice of implementation is a matter of taste.