July 2, 2018 Testimony of Chris Pollock, Arup

From the D.C. Council Committee of the Whole Hearing on July 2, 2018. testimony from Chris Pollock, of the research firm Arup.

Mendelson: Thank you, Mr. Johnson. Mr. Pollock?

Pollock: Thank you, Mr. Chairman and Councilmembers. I'm Chris Pollock, an acoustic consultant with Arup, with 20 years of experience measuring noise and designing buildings and spaces for suitable acoustics and noise conditions. I have a degree in mechanical engineering, and I'm a licensed professional engineer in the Commonwealth of Virginia, and I've been published in articles in the field of acoustics, and interviewed by various media outlets regarding acoustics and noise. 

Arup was asked to perform acoustical measurements, to provide technical, acoustical input and support for the testing and review of a group of leaf blowers. The questions we were asked to help answer were, "What is the difference in characteristics of the sound produced by commonly used commercial gas leaf blowers compared with commercial-grade battery blowers?" Secondly, "What are the implications for communities in which those machines operate?" We are not investigating the noise level at the operator's ears relative to exposure, but rather the impact on people in the community surrounding the leaf blowers as they are being used. 

We designed a set of tests that, in our experience, would allow us to capture side-by-side noise levels for various leaf blower types. It was arranged that seven commercial blowers would be used. Blowers were selected for comparable flow rates and decibel ratings. The leaf blowers selected, and their corresponding labels on the graphs are outlined below as part of the testimony. The battery powered blowers are labeled in blue. The Greenworks GBB 700, GBB 600, the Chevron EGO 600, and the Stihl BGA 100. The gas blowers are labeled in orange. The RedMax EBZ8500, the Stihl BR700X, and the ECHO PB-760. 

The full details of the testing sequence, equipment, and protocols will be outlines in my written report to follow. Some of the summary results are outlined below. The horizontal axis on the charts that you're looking at shows frequency, with the left side of the chart being a very low-frequency rumble sound, and the right side being a very high-frequency hissing sound. The vertical axis shows increasing sound pressure level and loudness as you go up the chart. The interesting points to note from our testing are where there are significant differences. 

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From the data above, we observe that clearly the group of gas leaf blowers all exhibit a much higher level of sound energy in the low-frequency bands. In a number of cases, this engine noise is at a peak of a 100 to 125 Hertz. This energy is quite distinctly different for the gas blowers than the battery-powered leaf blowers. This is highlighted by the two blowers, the ECHO PB-760 and the Greenworks GBB 700, both of which are measured at 66.5 decibels at 50 feet, but with dramatically different acoustic qualities and audibility at 50 feet and greater differences. 

Secondly, the audibility over distances. Based on our experience measuring sound, I witnessed that the three gas-powered leaf blowers at an 800-foot distance were audible, two being clearly audible, the RedMax EBZ8500 and the Stihl BR700X. And the third, the ECHO PB-760, being noticeable. While the battery-powered blowers at that same distance were not distinguishable above the very quiet community sound levels at that distance. 

Audibility within houses. One of the challenges with low-frequency noise is it requires heavy construction and materials to stop sound transmitting from the outside of the building to the inside. With leaf blowers, the low-frequency component of the gas leaf blowers is what is most readily transmitted, and this is clearly seen in the testing results at a 100 to 125 Hertz. The sound levels of gas leaf blowers are measured inside the house are significantly above those of the battery-powered leaf blowers, even when both of the leaf blowers are rated at the same decibel level at 50 feet. 

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Finally, how they sound. We will demonstrate for you now some of the sounds of the leaf blowers captured during our testing so you can experience the sound levels in a community with gas and battery-powered leaf blowers. These samples are calibrated to represent the measured sound levels accurately so that your experience is as close as possible to the real, measured conditions. The first sample is a comparison of a gas leaf blower at the same decibel rating based on the manufacturer's testing standards, and our testing. [Audio file played]  The important comparison is that while the overall loudness may be the same, the acoustical qualities of each are totally different. Relative to impact on a community or a specific individual, it may help to set the scene and imagine yourself in your own yard doing your own thing, reading a book, relaxing on the deck or porch, or sitting talking to your neighbor. 

This sample is the same two leaf blowers measured at 400 feet. [Audio file played]  Our final demonstration is three leaf blowers as measured inside an adjacent house. The ECHO PB-760, the GBB 700, and the RedMax 8500 all measured at 50 feet from a typical insulating glass window. [Audio file played]

In summary, our measurements indicate that the gas leaf blowers have a significantly greater low-frequency component. This low-frequency sound creates a different acoustic quality of the sound of gas leaf blowers versus battery leaf blowers. Because the low-frequency sound travels further, it is audible over greater distances, transmits most easily through the windows and glass doors of homes, and is more audible inside the home. The measured gas leaf blowers have a greater noise impact on the community than the measured battery-powered blowers. Thank you.