Frequently Asked Questions

Generating infrasound

Can I generate sound of frequency 1Hz?

07-04-2004 01:51AM

One can produce a 1Hz signal, the issue has to do with the degree of difficulty that is acceptable. If 1 Hz is desired, a resonant source comes to mind. Resonant sources as opposed to broadband ones, can be much more efficient and, therefore, more compact. Assuming a 1Hz resonant source is acceptable, the next issue is size and energy to be delivered by the source. A standard organ pipe would have to be 1/4 to 1/2 wavelength in length which would lead to a pipe the length of a football field. Significant energy would required that the diameter be large and the frequency would suggest pretty solid walls. I have heard that there was such a tube built (folded so the length was not so great) in Russia but have no direct confirmation. An alternative is to design a Helmholtz resonator. Such a resonator consists of a large volume attached to a neck, usually a long pipe. A Helmholtz resonator can be much smaller than an organ pipe. For either resonator, a driver is required. Air flow across an edge used to excite organ pipes would work. Chopping a flow of air will work. Long throw mechanical shakers would work. NCPA has a source operating near one Hertz that uses a thermoacoustics drive. One must be carefull matching impedance to the resonator to make the efficiency acceptable but that goes back to the second characteristic of a source, the amplitude of the generated sound. Large amplitudes mean large displacements which translate into very massive devices (drivers more than resonators but small resonators can introduce losses that limit performance).

More exotic suggestions have been made regards large amplitude sources. Someone suggested that a circular pond driven with a wave machine might generate infrasound. Seems feasible but I have not looked into feasibility.

Hank

07-12-2004 07:17AM