Page 26 - Winter 2008

Fig. 2. General types of sound systems (Klepper, 1999).
The directivity of a cone loudspeaker at a given frequency depends on its diameter. The coverage (-6 dB) angle is about 90 degrees when the wavelength is equal to the cone diameter and narrower above that frequency. Home stereo sys- tems use small diameter cones to obtain wide dis- persion. Sound reinforcement systems use large area horns, line arrays, or distributed loudspeak- ers to obtain high direct field levels and good feedback control. Cone loudspeakers can now generate relatively high sound pressure levels with low distortion and excellent fidelity.
Systems used only for speech do not require large transducers. A good example is the tele- phone, where adequate intelligibility can be achieved in a modest space. Music systems, with their extended bass requirements, use larger loudspeakers, although in home systems bass drivers are rarely bigger than 10–12-inches in diameter. Distributed systems for music need 12- inch diameter loudspeakers, often coaxial, to cover the frequency range. Subwoofers, usually 15- to 18-inch cones, can be used for bass rein- forcement where organ or other low frequency instruments are supported.
Many manufacturers offer high quality cabi- net loudspeakers, with a combination of horns and cone drivers in a two- or three-way enclosure. Another approach uses a number of small two- or three-way cabinets, each having two or more cones bracketing a high frequency wave guide. These are linked together like tank treads and
hung vertically in a line or J-shaped ribbon.
In the chapel it was decided to use 12-inch diameter
coaxial loudspeakers in 2 cubic foot cylindrical enclosures. This combination offers excellent frequency response and provides enough bass for live music and choral singing. Where distributed systems are used, the bass is supported by the combined area of many loudspeakers so subwoofers may not be required. Where necessary, 18-inch drivers in individ- ual boxes can be added.
Modeling
Most designers use a computer model to assist with the loudspeaker layout. This has led to the plaintive cry, heard after installation, “But the computer said it would be OK,” which is why it is important to look behind the curtain and ask the Wizard of 0’s (and 1’s) what is in his secret code. Vendors should publish the formulas and assumptions behind their software. Otherwise designers have no clue about what they are calculating. A computer model locates and orients the loudspeakers, a three dimensional coordinate system. An individual loudspeaker is characterized mathe- matically by its sound power, as a function of frequency, and its directivity, as a function of both frequency and direction. The direct field levels are calculated at various receiver loca- tions, based on direction and distance from the loudspeakers. The power and directivity data, furnished by the loudspeak- er manufacturer, are calculated from sound pressure level measurements, taken on the surface of an imaginary sphere
Design approaches
Sound system design is handled using three basic tools: (1) loudspeaker selection, placement, and orientation, (2) electronic component selection and utilization, and (3) room acoustics. Thus, it is important for the designer to be able to control of each of these tools. There are a number of approaches to the loudspeaker layout that can be used. Several are shown in Fig. 2 and can be summarized as: (1) use one or more groups or clusters of loudspeakers to cover the room from a position above the focus of interest, (2) use mul- tiple distributed (usually overhead) loudspeakers positioned relatively close to the listeners, and (3) use a combination of 1 and 2 with electronic delays.
In this church, a cluster solution was inappropriate since it would be unsightly and inflexible when the source origination point was moved. Similarly loudspeakers mounted on the side walls were a poor choice, since it would not be possible to con- trol the perceived direction of the sound source. Thus an over- head distributed system remained the best approach. It would provide even coverage, with minimal visual intrusion, while offering enough flexibility to maintain source localization.
Sound quality
The quality of a sound system is something of an aes- thetic choice. The simplest aspects—level and frequency range—have much to do with the size and type of loudspeak- ers. Larger loudspeakers extend lower in frequency, can han- dle greater power, and provide more directional control than smaller loudspeakers. Cone loudspeakers sound more realis- tic than horns. Horns are more efficient and have better pat- tern control than cones.
24 Acoustics Today, January 2008