Noise From Rotating Machinery

Humming hummingbirds, insect flight tones, and a model of animal flight sound

“…Aerodynamic effects that predominate on insect wings, such as the lift produced by wing rotation and vortex shedding, are intrinsically included in this model, as these mechanisms produce aerodynamic force and are not intrinsically different from other sources of lift and drag (Walker, 2002). For instance, periodic vortex shedding produces tonal sound, but not as a separate acoustic mechanism, because the way vortex shedding produces sound is by producing periodic cycles of lift and drag on the solid object (Blake, 2017a;Blake, 2017b, chapter 4). This mechanism provides a general explanation for why the flight tone in certain directions is, in many species, loudest (dominant) at the fundamental frequency of the wingbeat, while in other directions 2014), who used wing kinematics of a male Ruby-throated hummingbird (Archilochus colubris, body mass: 2.7 g, wingbeat frequency: 57 Hz) and a computational fluid dynamics (CFD) model to calculate normalized force components for a single flapping hummingbird wing, in the i=x, y and z directions.…”

“…The equal, opposite reaction to this aerodynamic force imparted on the wing is aerodynamic pressure, P(i,t), exerted on the surrounding air (Fig. 1B), a manifestation of Newton's 3rd law (Blake, 2017a). As the force is invariant, so is the pressure.…”

“…A spinning rotor produces three types of sources of sound: monopole-like 'unsteady source of mass' (Bodony et al, 2016), where sound is produced by a volumetric process; dipole-like 'load' or Gutin sound (Fig. 1A), where sound is produced by a force; and quadrupole-like 'broadband' sound (Blake, 2017a;Howe, 2008;Magliozzi et al, 1981;Schmitz, 1981).…”

“…Regarding the first monopole-like term, in a propeller spinning at a constant angular velocity, one type of volumetric process that generates sound is thickness noise, caused by the sudden (unsteady) volumetric displacement of air by the propeller as it passes through a control volume, where the air is displaced in proportion to the propeller's thickness. The effect of thickness noise is reduced at lower Mach numbers (Blake, 2017a;Glegg and Devenport, 2017). As animal flight is low Mach (see below), whether thickness noise could be a significant source of animal wing sounds is unclear.…”

“…Because thrust is perpendicular to the plane of the spinning propeller, while drag is parallel to this plane, a receiver in front or behind the plane of the propeller hears sound generated in reaction to the rotating sources of thrust (except exactly on the central axis of the propeller, where the distance between the center of pressure and the receiver does not fluctuate, and thus there is no sound). A receiver to the side (in plane with the propeller) instead hears sound that is the aerodynamic reaction to drag (Blake, 2017a). This Gutin sound has a property of wing tone: it is tonal and varies with orientation around a propeller.…”