P. Cobo, C. de la Colina
Many acoustic measurement procedures require omnidirectional sources. Dodecahedra, for instance, are used as an approximation to omnidirectional sound sources for measuring room acoustic parameters. Maximum allowed deviations from source onmidirectionality are established by ISO 3382-1 standard. Deviations larger than those recommended for the ISO 3382-1 are known to cause large errors on the assessment of acoustic parameters depending on the source-receiver distance, frequency band and number of measurements.
Omnidirectional sources are also recommended for measuring ground impedance. According to the ANSI S1.18 standard, sound sources for measuring ground impedance are required to be omnidirectional to within ±1 dB within ±45º. Polack proposed another type of omnidirectional sound source, consisting of a powerful loudspeaker feeding a small aperture through a reverse horn for concentrating the acoustic energy. The proposed designs, both the open end of a pipe and the small aperture of a reverse horn, have proved to constitute a solution for omnidirectional sound sources that behave like a point source, but with a very irregular frequency response and a rather resonant structure. Strong resonances in the frequency response of the sound source can cause significant fluctuations in the measured excess attenuation curves used for inverse procedures to asses ground impedance. Fortunately, large peaks of the frequency response of a sound source can be equalised by inverse filtering.
Conventional and inverse filtered frequency responses of a point loudspeaker