Sound SystemsTypes of Sound Systems
Sound System Requirements
Sound systems with centered, zonal, and distributed layout of the loudspeakers are designed to meet many acoustical and electronic requirements. The electronic considerations are the same for any electronic system of the signal amplification including frequency response, distortions, dynamic range, and many others. The main acoustical requirement and characteristics of a good system design are:
Minimum and Maximum Sound Levels
The minimum (Lmin) and maximum level (Lmax) of the direct sound pressure over the covered area (at the elevation of the listener's ear) when the loudspeakers are driven by the nominal power. Lmin and Lmax values are usually given in guidelines or standards for different frequency ranges and for specific application. For example, 90 dB Lmin value is recommended for symphony music (the level at 10-12 row in the symphony hall) and 55-60 dB Lmax value is recommended for background music in a store or exhibit.
There are many objective parameters that have been used for the evaluation of speech intelligibility over the covered area. The most critical locations for speech intelligibility are the coverage areas of minimum direct sound level and maximum noise level. The main objective parameter is the articulation losses of consonants in speech (ALcons). ALcons is the ratio of correctly heard consonants to all consonants transmitted (Losses of other speech components maybe used such as word losses and the results can be related to ALcons). The engineer usually designs the system for values of ALcons less than 15%.
C50 is also another parameter that can be used for speech intelligibility. C50 represents the ratio (in decibel) of the sound energy before and after 50 ms. The value of 0 or higher is generally considered acceptable.
Sound Level Deviation
Sound level deviation is ΔL=Lmax-Lmin; where Lmax and Lmin are considered for all of the listening area. Direct sound levels or total sound levels, which include the reverberant sound level, can be specified for ΔL. Values less than 6 dB is usually considered acceptable.
Frequency response is the frequency characteristics of the sound field over the coverage area. It is determined by the frequency response of all of electronic equipment of the system and the acoustical properties of the space. The combined frequency response of the system should be wide and as constant as possible over all frequencies.
Noise can be from many sources such as the listeners themselves, the equipment, and the outside noise. There are standards and guidelines for the noise levels at different frequencies and in different spaces as shown in Noise Contol page.
Localization of the Sound Source
It is recommended that the virtual sound is localized in the direction of the natural sound source. Signal delay, signal level balance and system layout are among the factors that are usually studied carefully to achieve a good localization.
Echo is generated in the space from delayed reflections with significant levels. There are graphics and database that determine the delay and level values that cause echo. Another type of echo is called Flutter echo, generated from many repeated reflections between parallel reflecting surfaces. There are many tools available to the designer to eliminate echo. These tools include space layout, sound diffusing panels, absorption panels and electronic signal processing.
Many other characteristics are used for objective evaluation of sound systems and room acoustics. These characteristics include (not limited to) EDT-Early Decay Time (RT for 10 dB decay), C7 (similar to C50, sound before and after 7 ms), C80 (similar to C50, sound before and after 80 ms), ITDG-Initial-Time-Delay Gap (Time difference between the direct sound and the first reflection), BR-Base Ratio (Ratio of RT on low frequency to RT on mid-frequency), and IACC-Interaural Crosscorelation Coefficient (Time correlation between sounds at both ears).
Equipment of Sound Systems
The basic equipment generally used in sound systems (not surround systems) is illustrated in the following drawing: