Leaf Blower Noise

What is it about leaf blower noise that makes it particularly annoying, and what are the main sources of this noise? Most importantly, can anything be done about it? In this article we address these questions using a mix of psychoacoustics and physical measurements, and explore the possibility of “improving” the sound of leaf blowers.

What is it About Leaf Blower Noise that Makes it so Annoying?

When it comes to noticeability and annoyance, the sound of a leaf blower pretty much checks all the boxes. Here are a few main reasons for this:

• Unless you are operating the blower, you are not in control of the noise, and this is one of the main factors that make a sound be perceived as annoying.
• Because of changing operating speeds, including cycling periods of idling and full-on operation, leaf blower noise tends to vary in loudness and frequency, and we humans tend to readily pick up on such varying characteristics in sounds.
• The sound generated by most leaf blowers is often dominated by fairly strong tonal components which, unlike broadband noise (which they also produce), have more of a tendency to “stick out” from the background noise and be noticeable as a “whining” or “droning/buzzing” type of sound.
• Blowers, particularly gasoline-powered ones, are rich in low frequency noise, which, due to its long wavelengths, can travel effectively over long distances without being attenuated or blocked along the way as would be the case for shorter-wavelength, high frequency sounds.
• Last but not least, leaf blowers are overall very loud, with some gasoline-powered units emitting enough sound power to be heard up to a half a mile away (battery-powered units can be 10 dB or more quieter than gas-powered units).

What are the Main Sources of Leaf Blower Noise?

For gasoline-powered leaf blowers, the main sources of noise are: the blower fan, the internal combustion engine along with its exhaust, and, sometimes, the engine air intake.

Due to weight considerations, these types of leaf blowers are often powered by two-stroke engines since a two-stroke will generally have a higher power-to-weight ratio compared to a four-stroke of equivalent power. However, two-strokes tend to generate more noise than four-strokes (not to mention generating more pollutants), in part because there is a firing for every rotation of the crankshaft instead of every other one. The ability of mufflers to severely attenuate the resulting exhaust noise is hampered due to space constraints (while a much larger muffling system could be more effective, its size would make it impractical for a portable device the size of a leaf blower).

In a battery-powered leaf blower there is, of course, no engine noise, and the noise of the replacement motor can be substantially less than that of an engine. However, in electric blowers, the sound tends to be more dominated by higher frequency noise than in gas-powered units, and some people find such higher frequency content to be particularly bothersome.

In both types of leaf blowers, the impeller/fan is often the other main contributor to the overall noise. This sound usually consists of both broadband airflow noise plus discrete tonal components. The tones arise at frequencies that are multiples (harmonics) of the impeller rotation rate, and their severity depends on a number of factors. For the first few multiples, their strength and number are affected by the degree of rotational imbalance present in the impeller, its rotation rate, and the properties of various connected surfaces that may convert vibration resulting from the imbalance forces into radiated sound. A special case that is not related to imbalance, but instead to aerodynamics, can occur at the blade passage frequency – that is, at a frequency equal to the number of impeller blades times the impeller rotation rate. Since there are often many blades, the frequency of this tone will often be substantially higher than the frequencies of the tones related to imbalance. Its severity is primarily a function of geometry – if the blades encounter nearby sharp edges or other discontinuities as they rotate, then that will tend to generate a flow disturbance that can lead to a strong acoustic tone being produced at the blade passage rate (as well as, potentially, its harmonics).

The impeller will also generate airflow noise due to turbulence, which is broadband in character rather than tonal. The strength of this component is highly dependent on the airflow velocity and how close the leaf blower’s nozzle is to the ground or other surface. The latter can create additional “impingement noise,” which is similar to the increase in noise that a high-velocity hand dryer produces when your hand is placed close to the nozzle.

What Can Be Done?

Aside from wearing hearing protection, there are a couple of approaches that might be pursued to reduce exposure to leaf blower noise.

The best approach would be if leaf blower manufacturers could redesign their products utilizing cost-effective changes that would substantially reduce the noise level of their blowers (without sacrificing performance), whether through fundamental design modifications, addition of noise control materials/components, and/or active noise control techniques. One final approach is to regulate the use of leaf blowers via community noise ordinances.

Redesign

The first step in any noise reduction effort usually involves identifying and determining the degree to which various possible sources or mechanisms (e.g., actual physical components as well as various types of noise generating mechanisms such as structure-borne sound as opposed to directly radiated airborne sound, etc.) are contributing to the total audible sound, since doing so can provide an efficient path to developing the most effective noise reduction strategies. This “noise audit” step enables subsequent acoustical engineering efforts to be efficiently directed to those sources/mechanisms that are contributing significantly to the overall noise.

Depending on the specific type, size, brand/model of leaf blower, there may be one dominant source such as engine exhaust noise that needs to be reduced or there very well could be multiple sources that need attention. Depending on the dominant source mechanism(s), options for noise reduction might include redesign of exhaust and/or intake mufflers/attenuators, addition of sound absorption materials, more effective engine vibration isolation, impeller redesign and/or layout of the nearby cowling to reduce the level of blade passage tones, treatment of housing components that may be contributing via structure-borne vibration, etc. Sometimes a relatively minor design change can have a large impact on the noise produced, and implementing such a change can, in the long term, be considerably more cost effective for a manufacturer than, say, adding noise control materials to each and every unit produced.

In the discussion above, it has been implicitly assumed that the goal is to reduce the noise level, as might be reflected in, say, the overall A-weighted dB value. However, since two completely different sounds can have numerically equal dB values, such a metric may not always adequately represent how annoying or “acceptable” a sound might be for either a bystander or the user of a product such as a leaf blower. If the goal is to increase a subjective attribute such as acceptability, it may be necessary to conduct a “sound quality” analysis in which juries of people provide judgements of different possible leaf blower sounds. Sound quality, which deals with how the ear/brain system shapes subjective response to sounds (and, by association, to the devices producing them), depends on such responses rather than on objective metrics. People’s subjective responses to the sound of a product such as a leaf blower will depend to some degree on their expectations based on the product’s function and the context of its use (including whether they are bystanders or users of the product). Factors affecting acceptability, for example, include not only the objective strength or “loudness” of the sound and its duration, but also may include subjective factors such as annoyance, amenity value, information content, etc. How all these factors and others affect the perception of an attribute like “acceptability” will usually be product specific, which is why, in our sound quality studies, we often utilize people in carefully-designed listening tests rather than just blindly relying on various sound quality metrics.

Active Noise Control

While the basic concept of active noise control (ANC) is relatively straightforward – a loudspeaker is used to produce a “mirror image” replica of the noise waveform so that when the two waveforms combine in air they sum to zero (that is, they “cancel each other out”) – in practice there are a number of considerations and limitations (some associated with fundamental physics and some with implementation) governing the actual performance that ANC can achieve. One of these has to do with being able to generate the “correct” sound that will effectively cancel out the existing sound.

Conceptually, this cancellation sound needs to be controlled continuously for amplitude, frequency and phase. In practice, this is accomplished using a microphone or some other input to provide a reference signal correlated to the disturbing sound (for a leaf blower this could conceivably be as simple as a signal related to motor/fan speed), and one or more additional “error” microphones to monitor the resulting sound. Along with these, there will be either an analog or (more commonly) a digital signal processing device that carries out feedback or feedforward control algorithms, which constantly adjust the cancellation signal sent to the cancelling loudspeaker(s) so that sound at the monitoring location is minimized. This approach enables the cancellation signal to readily adapt if the original disturbing sound changes with time in terms of its level, frequency or phase characteristics, as would be the case for operating a leaf blower at different speeds.

The loudspeaker(s) providing the cancellation sound need to be capable of actually generating a sound pressure level equal to that of the noise source itself, without introducing significant harmonic distortion elements. The overall size of leaf blowers provides a limitation on the size of these transducers, and size is one factor affecting their performance in the low frequency range, which can be an especially dominant range for some gas-powered blowers.

ANC systems also have an upper frequency limit which is generally a few hundred Hz or at most around 1000 Hz in some specialized applications such as noise-cancelling headphones. Broadly speaking, ANC is easiest to carry out when the sound pressure is oscillating relatively slowly – i.e., its frequency is “low”. As frequency increases it becomes harder for the cancellation to keep up, though this low frequency performance can often complement traditional noise control approaches that rely on materials or other means to provide sound absorption and/or sound isolation, which perform best at higher frequencies.

Another factor often limiting the usefulness of ANC is the practical need for it to exert control over an electro-acoustic system having many “degrees-of-freedom.” Each of these degrees can be thought of as an independent entity that the ANC system has to keep up with and control, which usually implies multiple sensors, multiple cancellation speakers, greater complexity and greater cost, especially if the interest is in obtaining “global” control over a large space rather than just local control at one or just a few specific locations. In this respect, lower frequencies – with their associated longer wavelengths – and smaller spaces are advantageous for ANC in that they can both serve to reduce a tendency of the sound field to change significantly from point to point.

In a prior Acentech project, we conducted a feasibility study of using ANC to reduce the noise from a battery-powered leaf blower. The results from this study indicated that, at least for the example unit considered, active noise cancellation would likely not be able to significantly reduce the overall sound levels. Part of the reason for this is that much of the sound for this electric unit resided in the frequency range above which ANC can be effective. In addition to broadband noise above about 1000 Hz, there was a strong tone at the fan blade passage rate, which was far above the frequency range where ANC could be expected to be effective (reduction of this tone might instead be accomplished by modifying parts that are in close proximity to the blades). There were also prominent tones in the noise at the fan/motor rotation frequency and its first harmonic, likely due to structure-borne vibration from the motor/fan getting into the housing components, which would likely not be as effectively attenuated by cancellation loudspeakers intended to reduce aerodynamically generated noise (though a combination of better balancing along with effective vibration isolation of the motor/fan could likely reduce these tones). Application of ANC to gasoline-powered leaf blowers, which tend to have lower frequency content, might prove to be more feasible.

Community Noise Ordinances

While some communities have implemented, or have attempted to implement, complete bans on the use of all leaf blowers (not necessarily just due to noise concerns but also to pollutant and airborne dirt/dust concerns), a more feasible approach often takes the form of restricting when leaf blowers are permitted (e.g., only during the spring and/or fall seasons, and only on certain days during certain hours). Other communities have taken to banning gas-powered leaf blowers or those above a certain noise level, while allowing quieter electric units. As with many noise regulations, enforcement is often uneven or non-existent, though having a regulation in place at least provides a mechanism to enable noise complaints to potentially be taken more seriously.

Conclusion

While there are no quick fixes for the loud noises generated by today’s leaf blowers, we remain hopeful that manufacturers will be open to engineering developments that can eventually lead to quieter units becoming available. We will continue to monitor such developments and be available to offer guidelines and advice regarding their implementation and effect on the sounds produced by this important, widely used product.