## 3 Artificial Audio Effect Technologies
### Electromechanical Effects  
There are several audio effect devices combining analogue electronics with mechanical systems. Electromechanical effects include different approaches to artificial reverberation as well as other lesser known implementations, such as echo and tremolo. A special class of electromechanical audio effects are those based on recording and playback technology, most importantly the tape player, which we discussed in the previous section. Research in the field of long-distance communication in the early 20 th century led to technologcal innovations that form the basis for the development of the first electromechanical audio effects. In an effort to develop energy-storage delay lines, Bell Telephone Laboratories built the first electromechanical delay line using helical springs [84] in order to simulate the delay occurring in long-distance telephone calls. In 1939, [PERSON] filed a patent for a reverberation system based on this technique [85], which became an integrated part of the model A-100 Hammond organ first sold in 1959. The system creates the effect by sending the signal to a transducer exciting a spring. The resulting mechanical energy is converted into an electrical signal by a transducer at the other end and is added to the original input signal on the output. These spring reverb units usually consist of two or more springs, characterised by their wire gauge, coil diameter, and metal composition, as well as their tension and length. A damping mechanism may be added to adjust the decay tie of the reverberated signal. Although (or as a result of) not producing a natural sounding room or hall simulation, spring reverberation became a typical sound in music of the 60s and 70s, particularly its use with the electric guitar. Even today, it is still a sought after audio effect. However, there are techniques to produce more natural-sounding spring reverberation. For instance, Fidi [86] used helix springs with long time delays in which transfer characteristics were changed statistically by etching the wire surfaces and filtered out residual correlated signals [12]. Based on the same principle, German company Elektro-Mess-Technik (EMT) introduced the first plate-reverberator in 1957. Instead of a spring, a thin metal plate was suspended under tension with a transducer attached to its centre. Due to the more complex vibration pattern, the plate reverb produces a denser, more natural-sounding reverberation effect. Plates of different materials were used, with steel plates [87] preceding thinner gold plates [88], that allowed smaller designs with improved high-frequency response. Plate reverberators also often included a damping system to control the reverberation time. Improving the sound of the [PERSON] organ was the motivation behind the development of the rotating speaker first developed by [PERSON] in 1937. In a [PERSON] rotating speaker system, the amplified signal is routed to rotating horns for the higher frequency part of the spectrum and a bass speaker is used for the lower frequency components. The bass speaker faces down into a rotating baffle chamber (_drum_), directing the sound outwards. The result is an effect likened to tremolo, which can be explained by the Doppler effect that occurs when a sound source moves with respect to the observer. A sound source moving towards the observer results in an upward shift in frequency and moving away from the observer results in a downward shift. [PERSON] introduced the first commercially available rotating speaker model. Somewhat related, [PERSON] invented an apparatus using a rotating speaker to produce continuous movement of a sound source between the audio channels in his electroacoustic compositions. It consisted of a round table of which the axis is mounted with a ball bearing. A speaker is placed on the table projecting sound outwards. Four microphones are positioned around the table and record the signal on one channel each [89].