The rotary subwoofer: A controllable infrasound source
The rotary subwoofer consists of a hub of radius rotating at a fixed rate of with blades of length and width modulated in pitch by signal with frequency .
Photograph of the installed source. (a) view inside the building showing the function generator, amplifier, motor controller, and motor and (b) view from the exterior showing the main rotor hub and blades.
Acoustic power spectra measured at range of with fan rotation frequency of . (a) shows signal frequencies of and and (b) and .
SPL from measurements at with versus signal frequency . rms voltage amplitude of the drive signal is denoted next to each measurement.
SPL from measurements at 1 and with versus signal frequency . Solid symbols were recorded at a range of ; open symbols were recorded at a range of and corrected to . rms voltage amplitude of the drive signal is denoted next to each measurement.
Composite signal transfer function with values of coherence .
Angular response measured at a range of for the drive frequencies , 8, . Data are SPL in units of . The axis of the fan is aligned at 0°.
Blade velocity distributions from the blade pitch modulation frequency and the fan rotation .
Effective particle velocity normal to the blade is modeled as the sum of the normal component of fan rotational velocity with the effective velocity from the blade pitch modulation .
Blade pitch angles in degrees as a function of rms input signal voltage
Model results for a fan speed of compared to measured values (SPL shown in Fig. 5) at a range of . Data and model SPL are decibels re .
Model results for a fan speed of compared to measured values (SPL shown in Fig. 4) at a range of . Data and model SPL are decibels re .
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