Abstract
Omnidirectional sound sources are needed to perform a large variety of tests in acoustics. Typically, they consist of conventional speaker drivers arranged in a dodecahedron. However, the directivity of the speaker drivers sharpens with frequency, which induces an intense decrease of the sound pressure levels at the edges of the dodechaedron. In this work, the problem is mitigated by building an Omnidirectional Parametric Loudspeaker (OPL), which contains hundreds of small ultrasound transducers set on a sphere. Each transducer emits an ultrasonic carrier wave modulated by an audible signal. Thanks to nonlinear propagation, the air itself demodulates the signal bringing it back to the audible range. The construction of an OPL prototype is challenging. The structure has been built by 3D-printing a set of pieces that conform to the sphere. Each piece contains the exact location of the transducers, which are aligned in parallels to facilitate the structural assembly and the wiring. The performance of the OPL has been tested in an anechoic chamber. Measurements show that the OPL has a good omnidirectional behavior for most frequencies. It clearly improves the directivity of dodechaedral sources in the high frequency range, but performs worse at low frequencies.
Highlights
Omnidirectional sound sources are needed for acoustic tests such as the measurement of the reverberation time in rooms [1], the measurement of the airborne sound insulation in buildings [2], or the measurement of the sound absorption coefficient in a reverberation room [3]
The Omnidirectional Parametric Loudspeaker (OPL) has a similar directivity pattern for frequencies above 500 Hz, presenting only some small oscillations that tend to diminish with increasing frequency
This reflects in the large oscillations of the OPL directivity pattern
Summary
Omnidirectional sound sources are needed for acoustic tests such as the measurement of the reverberation time in rooms [1], the measurement of the airborne sound insulation in buildings [2], or the measurement of the sound absorption coefficient in a reverberation room [3]. They are usually constructed by setting speaker drivers on the faces of a regular convex polyhedron so as to emit sound waves in all directions. This was proven in [5], where an analytical model was developed to predict the acoustic radiation of RPLs
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