Wind Turbine Acoustics - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 450

Figure 7-37. Sound-pressure-level gradients in a hallway excited by a pure-tone (21
Hz), constant-power loudspeaker. [Hubbard and Shepherd 1986]
Gradients and Resonances for Indoor Sound Pressure Levels
Large spatial variations in sound pressure level may occur within a house from a
uniform external noise excitation. People moving within the house could be sensitive to
these variations. Figure 7-37 illustrates the sound-pressure level gradient in a hallway with
various combinations of open and closed doors. Noise was produced by a loudspeaker at
a discrete frequency of 21 Hz. This frequency represents the low-frequency noise compo-
nents from wind turbines that would propagate efficiently through buildings. When doors
to Rooms A and B are both closed, there is a general decrease in the sound pressure level
with distance up to the end of the hallway. When doors are opened in various combina-
tions, the hallway pressure levels can be raised substantially. The changes in level that
occur when room doors are open are similar to what might occur for side-branch resonators
in a duct.
Because of the way rooms are arranged in houses, it is possible that Helmholtz resonan-
ces (cavity resonances) may be excited at certain frequencies, depending on the volumes
of the rooms and whether doors are open or closed [Davis 1957, Ingard 1953]. Hubbard
and Shepherd [1986] present results of sound-pressure-level surveys conducted inside a
room during resonance. For this condition, the inside pressures were everywhere in phase
and tended to maintain a uniform level. This is in contrast to the large gradients observed
in the excitation of normal acoustic modes in a room [Knudsen and Harris 1978]. The
latter modes are excited at frequencies for which the acoustic half-wavelengths are compar-
able to or less than the room dimensions, whereas Helmholtz resonance wavelengths are
characteristically large compared with the room dimensions. Rooms A and B in Figure 7-
37 both exhibit Helmholtz resonance behavior at an excitation frequency of 21 Hz.
Coupling Noise Fields in Adjacent Rooms
As sound-pressure-level gradients change in a hallway outside rooms according to
whether doors are open or closed (Figure 7-37), so also do the levels inside the rooms.