Acoustical testing can be classified according to the test setup: Response testing involves a sound-level meter (SLM) and/ or a real-time analyzer (RTA) to measure the sound level and/ or spectrum produced by a source. Stimulus-response testing incorporates an acoustical source whose sound output is known, and measurements of the resulting sound field are made in order to determine how a room or other enclosure affects the sound. Sound-system performance, reverberation, and echo testing exemplify this type of test.
The second type of testing requires a source with a certain directivity (polar response) and frequency response. The directivity of an acoustical source depends upon its physical characteristics and size.
A source that is small compared to the wavelength of the sound being investigated is called a point source and is omnidirectional. Directivity can be represented in any of several ways.
One way is a simple graph of the beamwidth plotted versus frequency. The directivity graph of an omnidirectional source would be very boring: a horizontal line showing a beamwidth of 360° at all frequencies. Another way is to represent the radiated sound field as a balloon whose shape and coloration represent the sound pressure level SPL) radiated in each direction.
The third way is a section drawing with a vertical or horizontal cut through the balloon. Such sections often show multiple lines, illustrating the variation in directivity with frequency.
Figure 2: The horizontal (upper) and the vertical (lower) directivity of the human voice vary with frequency; the standard deviation is indicated by the colored shading. (Image Source: Christoph Pörschmann, Johannes M. Arend: "Analyzing the Directivity Patterns of Human Speakers," www.researchgate.net/publication/343344151)
Source Directivity
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