Designing for Sound Performance
How to reduce appliance noise without reducing functionality.
Functionality will always be critical to the commercial success of small and large appliances. Consumers want their blenders to blend. They want their clothes dryers to dry. They want their dishwashers to wash dishes. Today, they expect all that—and much more.
In recent years, designers have done an outstanding job of creating products that perform well while providing great aesthetics and innovative connectivity capabilities. In this mature market, it is more challenging than ever to compete, differentiate and add value.
Many designers and brand owners are discovering opportunities by focusing on sound.
Consumers place a high value on quiet appliances
Reducing noise and improving sound quality have a very positive effect on consumer experience—and consumers are willing to pay for it.
In an industry report in 2007, 50% of survey participants were willing to pay 10% more for a product that was quieter if it had the same performance.1 A more recent survey of the dishwasher market showed that the appliance price correlated with a $71 increase for every decibel (dB) reduction of sound emission.2
On larger scale, environmental noise can impact customer satisfaction, brand loyalty—and even health. One article reported that 39% of consumers will avoid using an appliance at certain times because of the noise it produced.3 In another survey, 62% said that noise from appliances adversely affected the enjoyment of their home life to some extent.4
Health organizations agree. A WHO publication stated that “Exposure to excessive noise makes it second only to air pollution as an environmental cause of ill health.”5
When “sound” becomes “noise”
Normally, the sound emission of appliances ranges from 50 dB to 96 dB, compared with the 85 dB threshold that OSHA sets for occupational safety. Some appliances, like blenders and food processors, emit a high level of sound (up to 96 dB) for a short time. Others, like refrigerators or dryers, emit less sound (as low as 50 dB) over a longer period of time.
Appliance sound becomes irritating noise when the sound emission level combines with undesirable sound quality—determined by the frequency of the noise that is emitted.
The healthy human ear hears sounds within a frequency range of approximately 20 to 20,000 hertz (Hz). Within that range, most people find sounds between 2,500 and 5,000 Hz particularly irritating. This range includes such disruptive sounds as a baby’s cry or a car alarm.
These and similar noises can be annoying even if produced at a relatively low dB level.
The source of sound emission and noise in appliances
To design these unpleasant noises out of appliances, it’s good to understand the origins of these sounds.
A variety of mechanisms are responsible for producing noise in an appliance. Transmitted sound from the motor, air movement noise and even movements of gears can all add to the total noise levels produced. But the biggest—and most consistent—source of sound in a motorized appliance is the vibration from the motor resonating throughout the appliance. When you mount a vibrating motor to a solid surface, sound is emitted as the vibration turns the appliance into a resonator. As the vibration moves through the structure, it becomes a virtual speaker that radiates the vibration from the structure as audible sound.
Designing quiet appliances is a complex process because it’s very difficult to predict and address the sources of vibration and its transmission from the motor to the housing. Typically, there are three ways to reduce noise emission:
First, you can reduce vibration at the source. This usually involves using less powerful motors, or specifying high-end motors that are a more expensive option.
Second, you can isolate the motor vibration from the housing. Rubber bushings and other vibration absorbers can help, but compression during installation combined with chronic fatigue can reduce their sound reduction benefits.
Third, you can dampen the vibration by choosing materials with sound damping properties.
The third option for reducing the amplitude of resonant vibration is the most practical—and most cost effective—solution for a designer who wants to achieve a substantial and noticeable reduction in the noise level of an appliance.
Comparing the sound damping properties of materials
Engineering materials have very different abilities to reduce resonating sound. Some of their effects may be surprising. For example, most people think glass is effective at blocking sound. It is, in fact, good at blocking transmitted sound, but for glass resonating sound can vibrate up to 5,000 Hz—all the way through the most irritating sound range.
This is because glass is brittle and lacks what is called a viscoelastic response—the ability to absorb and dissipate resonant vibration and reduce noise emissions.
Polymers provide much greater viscoelastic response than glass, but differences between them can result in sound levels that vary by as much as 10 dB—and different polymers can also vary greatly in their range of vibration frequency and sound quality.
NOTE: the dB scale is a logarithmic relationship, so a 10 dB increase is perceived as twice as loud.
Plastics with a higher viscoelastic response factor are better at absorbing or dissipating vibrations that are propagated throughout the structure. They also effectively do a better job of reducing the speed of the vibration and reducing the pitch (Hz) of the vibration, thereby improving both the intensity and the quality of the sound that is emitted.
To measure and compare the force, speed and frequency of resonating waves moving through a material, engineers use a specialized tool and some complex physics to identify a damping loss factor for each material. A center impedance generator, attached to an accelerometer generates frequencies throughout the human hearing range (20 to 20,000 Hz) and measures the resulting characteristics of the sound produced.
When the results are plotted on a graph (see Graph 1), the damping loss factor provides a signature of the material’s response to mechanical vibration, which can be combined with decibel measurements of parts and enclosures for designers to use when selecting the best material for an appliance.
Material thickness and dB reduction
Once you have selected the most effective sound damping material, consider how the thickness of the material can further reduce sound emission and improve sound quality. Once again, different polymers have different responses to thickness, with some having little added effect and others further reducing noise level by up to 6 dB—perceived as less than half as loud.
An example of this effect with one engineering polymer is presented in Table 1. Nine brands of blenders were compared in tests where the only variable was the thickness of the material used in its enclosure—ranging from 0.125” to 0.675” (1/8 inch to 5/8 inch).
For this polymer, doubling the enclosure thickness from 0.125” to 0.25” reduced the emitted sound by an average of 6 dB—approximately half as loud—across all brands tested. Increasing thickness from 0.25” to 0.50” yielded similar results.
In a noisy world, consumers appreciate quieter appliances. Today, designers are mitigating sound levels and noise frequencies of large and small appliances by using sound damping materials in their designs.
Designing for sound performance starts with knowledge of the viscoelastic properties and sound damping performance of materials. This information can allow designers and engineers to reduce noise, use more powerful motors, improve customer satisfaction and create a potential for higher margins in the competitive appliance market.
1 The AEG – Electrolux Noise Report, 2010.
2 Eastman Internal Analysis, 2015.
3 Are your home appliances making you sick? Study links household noise levels to increased blood pressure and heart disease” Daily Mail.com. 17 Mar 2014.
4 Quiet Mark and John Lewis, 2014 Consumer Survey. Available at:
5 Kim, R (ed). Burden of disease from environmental noise: Practical guidance (report on a working group meeting.) WHO Publication, 14-15 Oct. 2010. Available at: http://www.euro.who.int/__data/assets/pdf_file/0004/131809/e94731.pdf. Accessed 6 Nov. 2016.
OR “Burden of disease from environmental noise. Quantification of healthy life years lost in Europe.” WHO Publication: 2011. Available at http://www.euro.who.int/__data/assets/pdf_file/0008/136466/e94888.pdf. Accessed 6 Nov. 2016.