A Rotary Subwoofer as an Infrasonic Source J. Park and M. Garces Infrasound Laboratory of the University of Hawaii Infrasound Technology Workshop, Bermuda, November 2008

Rotary Subwoofer : Kona Infrasound Generator (KING) Compact, portable infrasonic source

KING Installation Matched amplifier, fitted baffle, ISLA as back volume

Spherical spreading Amplitude decay is consistent with spherical spreading. Lack of pressure fluctuations at ranges less than λ suggests that there are no near-field effects as expected for ka<<0.15. The source is compact.

Sin Wave 2007273 Beampatterns measured from 0-90o at a range of 1m. 6 2 3 4 5 Beampatterns measured from 0-90o at a range of 1m. Note that the on-axis (0o) response is ~2 dB less than the off-axis maximum. At ka<<0.15, we expect strictly monopole radiation….

Sin Wave 2007260 No Circuit Isolation Function Generator Output Voltage at Speaker Input 1m 3 2 1 5 4 6 No Circuit Isolation With fan motor off and no amplifier gain, function generator output/speaker input are decoupled (blue). With fan motor on, no amplifier gain (green), fan components are observed at the speaker input terminals. With fan motor on and amplifier gain applied, there is an impressive coupling of power back to the speaker inputs, presumably from lack of isolation between the motor, speaker electromotive coil and motor controller. Note that the ‘pure sin wave’ frequencies (1, 4, 8, 16 Hz) output by the function generator are not tonal, i.e. the frequency is not stable with the feedback. Amplitude fluctuations were also observed in the input timeseries. Noise Floor Fan Off Noise Floor Fan On 1 Hz Sin 3 & 1 VRMS 1 Hz Sin 2 VRMS 4 Hz Sin 3 VRMS 8 Hz Sin 4 VRMS 16 Hz Sin 5 VRMS

Fan Speed: Ω=17 Hz (1020 RPM) Sin Wave 2007305 031o 1820m Speaker Axis 032o

1.8 km Range Test 16 Hz 12 Hz 8 Hz 4 Hz

1.8 km Range Test ~135 dB @ source! 22 dB

12 & 16 Hz at 1820 m appear to be amplified… Is the KING driving a building resonance? Axial mode Eigenfrequencies are: ωn = 9.4 Hz (Lx=18.3m) ωm=18.8 Hz (Ly= 9.1m) Driven Modes at ω are: Amn – Source Qmn – Quality Factor Boundary Losses Amplification Factor (α) ω =12 Hz; ωn = 9.4 Hz, αn= 2.03 ω =16 Hz; ωm=18.8 Hz, αm= 3.19 Eigenmodes are amplified by factors of 2 & 3 at ω =12,16 Hz Atmospheric changes can also contribute.

A Simple Source Particle Velocity Model Blade dynamics can express an effective particle velocity: Which can be resolved into surface normal velocity components: Assume that with Ω > ω, a coherent annular ring of area π(Ro2 - Ri2) with an effective particle velocity Vn is produced Evaluate Rayleigh’s Integral for pressure at field point x:

Model Results (on-axis) Good Results: Ω/ω > 2 Bad Results: Ω/ω < 2

Preliminary Model Results Beampatterns showed on-axis (0o) response is ~2 dB less than the off-axis maximum. At ka<<0.15, we expect strictly monopole radiation…. Model results with the addition of a plane wave from the building wall/baffle indicates a pressure reduction on-axis. The amplitude of reduction is less than the ~2 dB observed.

Array Calibration Test I59US sonification with prototype system

Array Calibration Test I59US sonification with prototype system

Model Result Hypothesis KING has a non-zero radiation resistance (real part of radiation impedence) even though ka<<1. Radiation impedance changes from reactance dominated (mass loading) to resistance dominated (fluid displacement) by virtue of the stream particle velocities. High transduction efficiency: The electric motor that drives the blades at frequency Ω is the primary accelerator of particle velocity. Structural backvolume eigenmodes can be driven and may contribute to radiation. Building wall/baffle plane wave may affect pressure distribution. The effective particle velocity model breaks down when Ω/ω < 2.

Synopsis No circuit isolation between fan motor and electromotive coil that modulates fan blades induces feedback to the speaker input. Shielding the fan motor reduces EMI by an order of magnitude. Shielding the input and measurement cables reduces EMI. Sound intensity decay is approximately 1/r2 even at r << . The source is compact. Transfer function values (dB re Pa/V) are > -5dB, the signal to acoustic power efficiency is better than a conventional voice-coil driven diaphragm. SPL is in range of 120 dB re 20 µPa from 4-16 Hz. Power levels are dependent on fan speed. Higher fan speed creates higher particle velocities. High SPL and efficiency are a result of fluid displacement Radiation Impedence matching from the high particle velocities. An effective velocity source model is good for Ω/ω > 2. Above 4 Hz and at a range of 1.82 km, SNR was consistently in excess of 12 dB for wind shielded sensors. At a range of ~5km (I59US), SNR was >6 dB. New prototype in design stage.