Hambric Vibroacoustic Demonstrators

Acoustic waves look quite different depending on the sound source type and how far away you are from it. Simple monopole sources are just pulsating volumes that generate sound waves that are omni-directional (they don't vary with angle around the source). Simple dipole sources are two monopole sources separated by a small distance, and out of phase with each other. Their radiated sound fields are also opposite in phase on either side of the source. At large distances from a source the wave field is nearly uniform, and resembles a plane wave, which propagates in a certain direction with no amplitude variation along its plane.

Waves propagate forever in free space, but once walls surround a source, they reflect the waves backward. When a source is surrounded entirely by walls a standing wave field is generated. For rigid walls, the standing wave looks like a simple cosine function, with peak pressures on the walls. When a space is small in one or two directions and long in the other it looks like a one-dimensional waveguide. The air or water inside pipes are 1-D waveguides, with plane waves propagating along the long direction. At a high enough frequency, however, the acoustic wavelength reduces until waves can propagate along the small direction(s) (a half wavelengh between the walls) and the wavefield begins to look multi-dimensional again.

Here, I'm showing you acoustic waves in two-dimensional space (it's tough to visualize them in 3D). There's a lot you can do with this demo! Try changing the source type as well as the frequency. See how the wavelengths decrease with increasing frequency. You can monitor the acoustic wavenumber k₀ in the title above the plot. You can also move and rotate the source location with the sliders, or move it by just clicking somewhere on the contour plot. Click the 'Animate' buttons for the contour or waveforms to cycle a wave field over several cycles. Reduce the height to see the waves become one-dimensional.

Finally, change to 'rigid walls' to examine the acoustic mode shapes within the space. Watch the title to see which acoustic mode is most dominant at your current frequency. Try to 'hone in' on the nearest resonance frequency by sliding the frequency bar and watch the mode pattern emerge. When you animate, you'll see a nearly standing wave pattern. The mode index m is in the x direction, where m = 1 is a half cosine wave, m = 2 a full cosine, m = 3 one and a half cosines, and so on (same convention for the n mode orders in the y direction). At very low frequencies the sound in the space is dominated by the so-called Helmholtz mode, which is uniform pressure over the entire space.