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DECAY TIME / RT60 / RELATIVE FREQUENCY

DECAY TIME / RT60

If an impulse source of sound is introduced into a room, a typical time response such as shown in FIGURE #1 (below) will result. This time response shows the reverberation effect. The response can be described by thinking of the reverberation as being composed of two parts. The first part, called early reflections, are the sounds that come from possibly only one reflection off a boundary surface. These reflections shape the listeners conception of the room size. The number, or density, of reflections increases rapidly with time when the second or clutter phase is entered. The individual echoes of the first phase merge into an almost continuous sound field. This part of the reverberation effect display the color and liveliness of the room, surrounding the listener with a diffuse sound field.






The characteristics of the room's reverberation are described in terms of two parameters. The length of time required for a decrease in level by a certain amount is called the DECAY TIME. The decay time is usually defined as the time it takes a sound to decrease by 60 db. Decay time is determined by the acoustical properties of the reflective surfaces. The time required for the arrival of the first reflections is called the delay time. The delay is related to the volume of the room.

Electro-mechanical reverberation devices provide the necessary properties of delay and decay time to simulate natural ambience. Changing the delay time will change the apparent size of the listening area. Lengthening the decay time adds additional liveliness required by some musical instruments. When the reverberation system is matched carefully to the musical instruments or sound system, creating the proper room characteristics, the effects are enriching.

Spring reverberation devices are used successfully in a number of musical instruments and accessories providing an added dimension to the sound. Organs, guitar amplifiers and mixers make extensive use of reverb devices. They are being applied to public address systems to control room acoustics. There are numerous applications for reverbs in audio systems, since they provide a low cost solution to the problem of room acoustics, sound creation and enhancement.

Accutronics' electromechanical reverberation devices consist of input and output transducers and a set of transmission springs. These components are mounted on an aluminum channel, which is usually supported by four springs to an outer case or channel. Briefly, the device operates as follows: an electrical signal applied to the input transducer is translated by the magnetic circuit to a torsional force on two small cylindrical magnets. The magnets are mechanically coupled to a pair of transmission springs and the torsional motion is propagated slowly along the spring path. At the output, a replica of the original input signal is produced by the action of the magnets again coupled to the transmission springs. The signal has now been delayed by some period, determined by the diameter, wire gauge and length of the transmission springs. Additionally, most of the torsional energy is reflected back to the input transducer, which in turn reflects it again to the output transducer. The result is a multitude of reflections each smaller in amplitude than the last. The delay time of each additional reflection is 2T since the torsional motion must travel the spring path twice to reach the output transducer.

FIGURE #2 (below) shows how a single spring reverb would ideally respond to an input pulse. FIGURE #3 (below) shows the frequency response of this single spring device. Combining the outputs of two such units, each with a different delay time, improves the reverb characteristics. The use of two lines with different delay times also more closely simulates the ideal room or concert hall. As the walls of a concert hall are not the same distance from the listener, the delay will vary from different reflecting surfaces. The two different delay times provide this variation. Combining the outputs improves the frequency response also, with one response also, with one response - with proper delay time selection - filling voids or holes in the other. The audible annoyances associated with the single spring line such as flutter, coloration and the metallic spring sound are greatly reduced using multiple paths. 






The approach of using a multiple number of delay paths is used in all Accutronics' reverberation products. Types 1 & 4 use two paths; types 8 & 9 use three spring delay paths. Delay times have been selected to give the most pleasing over-all effect within the size constraints of a particular type.

The proper decay time must be selected by application, and is therefore factory adjustable. For example, in vocal applications reverberation with a short decay time will enhance the voice; however, lengthening the decay time beyond a certain point will make speech unintelligible. The specification sheet for each type describes the options available. As a guide to selection we have found that most customers with organ products prefer about 2 seconds decay; guitar amplifier manufacturers prefer 3 seconds or longer; for vocal applications, most prefer the shortest decay possible.

To allow the most design flexibility, Accutronics supplies a variety of input and output impedances (impedance measured @ 1kHz). Selection of a particular impedance is dependant on drive circuit type employed, supply voltage and current capabilities. These criteria will be described in more detail in the next section. A list of popular transducer impedance is given in INPUT / OUTPUT TRANSDUCER CHART.