For the D.C. City Council's July 2 hearing on a bill to phase-out noisy, dirty gas-powered leaf blowers, and phase in quiet, clean battery-powered models, a well-known research group called Arup conducted original sound-measurement experiments.
The purpose of the tests was to see whether there was any real difference in the acoustic properties of noise from different kinds of machines. (More of about the hearing, on July 2, here; a full transcript of the proceedings is here; the testimony of Arup's witness, acoustics expert Chris Pollock, is here.)
As it turned out, indeed there is a difference. As Pollock emphasized, the main contrast is in low-frequency sound energy, which gas-powered machines produce at much more intense levels than battery-powered machines.
"Low-frequency" noise seems as if it might be more benign than a high-frequency whine, but in fact it's a much more serious public health and public nuisance problem. That is because low-frequency waves travel much further than high-frequency ones, and penetrate walls, doors, and windows. Thus two leaf blowers with the "same" loudness rating, say of 75 decibels, have vastly different noise footprints, with the gas-powered one affecting a much greater area. At the hearing, Dr. Jamie Banks, of Quiet Communities, illustrated the difference with this chart. The two blowers shown at the top are battery-powered; the two at the bottom, gas-powered. (In their response at the hearing, the two industry lobbyists said that noise and nuisance problems could be solved by training lawn crews in "considerate" use of the equipment. See for yourself.)
But here's an even simpler way to grasp the difference: just listen to the 18-second sound clip below, from the Arup experiments. It's of two blowers that officially have the "same" loudness rating, measured at the same distance.
The first ten seconds are a battery-powered machine, with a distinctive high whine -- but a sound that falls off very rapidly with distance. If this machine were running outside a building, you might not hear it inside.
The second is of a gas-powered machine.
See if you can notice the difference!