In this article we will cover some fundamental acoustic concepts so that we may better understand the origins of pickleball impact noise. In a previous article we discussed the general problem of Pickleball & Community Noise. In subsequent articles, we discuss the mechanics behind Pickleball Noise Propagation and provide an example showing the effectiveness of Pickleball Sound Barriers. In future articles we will examine some of the so-called low-noise pickleball paddles and balls.
Pickleball Noise Characteristics
Pickleball noise is generated by the impact of the pickleball against the face of the pickleball paddle. The short-duration impact force causes the paddle face to vibrate in a manner similar to that of a drum skin, which excites acoustic resonances within the interior of the paddle. The interior of the paddle contains a light-weight cellular structure, typically a honeycomb which contains mostly air (see our article, Pickleball Paddle Materials). The hollow interior of the paddle behaves in a manner similar to that of a drum shell, which amplifies the acoustic resonances. The amplified impact noise then radiates through the pickleball paddle faces into the environment thereby causing the community noise problem.
Pickleball impact noise has not yet been properly characterized because it has an extremely short duration and high amplitude, which makes it difficult to measure with a standard sound level meter. In previous articles, we have estimated that the contact time between the pickleball and the paddle would be on the order of 4 milliseconds (ms). Following impact, we would expect that the paddle will vibrate or “ring”, creating sound that will propagate from the paddle over several more milliseconds.
Studies by NIOSH on impulsive noise measured the acoustic time history of a pneumatic framing nailer (Figure 1). The first large peak of 132.9 dB, which occurs at around 30 ms and has a duration of about 15 ms, is attributed to the initial contact of the hammer with the nail. The second large peak of 134.8 dB, which occurs at around 50 ms and has a duration of about 35 ms, is attributed to the hammer and nail contacting and penetrating the plywood substrate.
It is reasonable to assume that a pickleball paddle will have an acoustic time history similar to the latter part of the framing nailer time history where the hammer strikes the plywood substrate. On one hand, the framing nailer likely applies a greater force to the plywood than the ball applies to the pickleball paddle, but this force is applied over a smaller area and a very short duration. On the other hand, the force applied by the ball to the pickleball paddle is of lower magnitude but over a larger area and longer duration. The impulse, which is the force multiplied by the time duration, may be equal for the framing nailer and pickleball paddle.
Sound Amplitude (or Volume)
Our brains differentiate between sounds that are loud and soft and those that are desirable and those that are not. We typically describe sound in terms of its volume and loudness, however, there are subtle differences between these terms, as described below.
We use the terms sound volume and sound pressure level to describe the objective measure of sound amplitude. The basic unit that describes the amplitude or volume of sound is the decibel or dB. It is usually not necessary to calculate sound pressure levels (SPLs), but readers should develop an appreciation for the sound levels that they encounter in everyday life (Figure 2).
Loudness
Where the sound pressure level is an objective measure, loudness is a subjective measure of how a listener perceives the strength or intensity of sound. One important factor that affects our perception of loudness is the presence of background noise. Since we are sensitive to differences in sound levels, noises that may not normally bother us during the day when background (ambient) noise levels are high, may be disturbing at night when the ambient noise levels are low. This illustrates how our ears lose hearing sensitivity in the presence of high background noise levels, and that a minimum signal-to-noise ratio is needed for us to discern music or speech over background noise.
We are all familiar with the situation where we listen to music on the car stereo while driving home, at sound levels that we consider “normal”. However, the next morning when we start up the car, we are surprised at how loud the stereo is. This illustrates how we tend to increase the volume of music and speech in the presence of high background noise that we might experience in our homes, vehicles, in restaurants, airplanes, on the street, etc. Repeated or continuous exposure to high background noise levels can lead to temporary or even permanent hearing damage.
The relationship between sound volume and loudness has been established by the loudness equation from the field of psychoacoustics:
ΔL = 33.22 log (x)
Where ΔL is the change in decibel level and x is the perceived increase in the loudness factor. This relationship is shown plotted in Figure 3.
Using the loudness equation, an increase in sound level of 3 dB results in an increase in the perceived loudness of only 20%, which is barely perceptible. On the other hand, an increase in sound level of 5 dB increases the perceived loudness by about 40% which will be clearly perceptible. An increase in sound level of 10 dB will double the perceived loudness.
Frequency Weighting
Research in the field of psychoacoustics surveyed several individuals from several different countries and found that their hearing was less sensitive to pure tones in the low and high frequency ranges (below about 200 Hz and above about 6000 Hz). This loss of hearing sensitivity is more pronounced with lower amplitude sound (around 40 dB) verses higher amplitude sound (greater than 80 dB). Those readers who had audio systems from the “old days” will recall that these systems had a “loudness button” that boosted the amplitude of music in the low and high frequency ranges when the music was played at low volume.
In order to account for changes in hearing sensitivity in the low and high frequency ranges, audio engineers created the A- and C- weighting curves shown in Figure 4. Sound levels that are measured with these weighting functions are reported in terms of dBA and dbC. Unweighted sound levels are often reported in terms of plain dB or in dBZ. Although the A-weighting scale is widely used, critics claim that it may not be realistic because it represents hearing sensitivity at a low amplitude of 40 dB (at 1000 Hz). Use of the C-weighting curve has been proposed to correct acoustic spectra for very loud or impact noise, where the attenuation values at low frequencies is significantly less than A-weighting.
Pickleball Noise Measurements
Assuming that the pickleball impact noise is similar to that of a pneumatic framing nailer (Figure 1), we should expect that the duration of the noise will be around 35 ms. The short duration of this noise cannot be measured with sound level meters set to either “slow” or “fast” sampling rates for the following reasons:
- The “slow” sampling rate records noise in one second intervals and are designed for long duration data samples where short duration or transient impulse events can be ignored. An example of this would be an environmental sound survey over several hours where you would want to ignore events like car doors slamming.
- The “fast” sampling rate records noise in 125 ms intervals, which will allow for the capture of some transient impulse events, but the time frame is still more than four times longer than the duration of the impulse noise.
More advanced sound level meters have settings for the “impulse” sampling rate of 35 ms, which corresponds nicely with the 35 ms duration of the pickleball impact noise. As a result, the pickleball impact noise will appear to have a higher amplitude than the same noise measured with sound level meters set to “slow” or “fast” sampling rates.
Noise recordings with sound level meters set to “slow” or “fast” will underpredict the magnitude of the impact noise because it will be heavily averaged into the background or ambient noise. As an illustration, Figure 5 shows a time history of impulse noise with an overlay of how a sound level meter would sample the noise using the “slow”, “fast” and “impulse” sample rates.
As indicated, the “actual” impulse noise reaches a peak level of 117 dB at about 1.6 seconds, and the “impulse” sample rate is 15 dB lower at 102 dB. If the “fast” sample rate is used, it results in a reading that is 5 dB lower at 97 dB, and the “slow” sample rate provides a reading that is another 5 dB lower at 92 dB. Each 5 dB reduction would represent a 70% decrease in the perceived loudness level. When viewing the sound levels where there are no large impulse peaks (such as in the 0 to 0.5 second range), the “slow”, “fast”, and “impulse” sample rates yield a similar result of about 87 dB.
Surprisingly, numerous environmental sound surveys make the mistake of using an inappropriate time sampling rate. The pickleball impact sound levels may actually be higher than those measured with their sound level meters set to the “slow” or “fast” sampling rates.
In our discussion above regarding the framing nailer (Figure 1), we assumed that a pickleball striking a paddle will have the same time history as the latter peak where the hammer strikes the plywood. Using the results of Figure 4, the 135 dB peak would be reduced by 15 dB to 120 dB if the “impulse” sample rate was used to measure the sound. We will therefore assume that the impact sound of a pickleball striking a paddle is 120 dB going forward.
Future Articles
In our next article, Pickleball Noise Propagation, we will examine how sound propagates from a pickleball court to a residence and evaluate the effectiveness of sound barriers and absorbers that are commonly used to attenuate pickleball noise. In future articles we will examine the various technologies and products offered by pickleball paddle and ball manufacturers to reduce impact noise.
