Product sound quality - from perception to design

This article on product sound quality is concerned with the relationships between the work of product designers and the perceptions of consumers regarding the acceptability of product sounds. Designers make choices regarding structure, materials and components in a product. The tools they use should allow them to anticipate the effect of these choices on sound quality. This discussion recounts the role of psychoacoustics in product design and product acceptability and notes the results of that work in metrics for sound quality and consumer/ user perceptions about the product. The successes and drawbacks of this activity are noted. Recent work on a new paradigm using "Acoustical Sensory Profiles" as an intermediary between metrics and perception is described, along with some results using the procedure. Future developments are indicated.

I have spent more than 30 years working with products that are 'noisy.' But I learned very quickly that 'noisy' might not equate with 'loud.' A shop-type vacuum cleaner had a noise problem: a slight rattling in the motor during coast down after the unit was turned off, after the roar of the unit had died away! The roar was expected, but the rattling could indicate to the owner that there was a problem with the motor. The product could not be shipped until the rattle was eliminated.

Product sounds can be objectionable, but they can also be favorable. The sound of an automobile door closing is a classic example that has been studied for many years. Compare the sounds of doors closing on 2 cars, one of which costs 3 times as much as the other (Sounds A,B).1 You can tell which one is the more expensive car. The first is an Audi A4; the second is a Ford Escort.

One GM engineer used the request of his product planners to further his education. When planners asked Don Malen to "Improve solidness of door closing sound without adding too much cost," he made it the topic of his Ph.D. thesis at the University of Michigan. Now, engineers might regard the charge as the typically vague request from a product manager, but it has the strength of saying what the product planners really wanted. Dealing with this charge required Malen to address issues in acoustical signal analysis, human perception, mechanism design and cost benefit analysis.

As long as products do not change very much, certain expectations grow with regard to their sounds. We come to expect products to have a certain type of sound. If a motor or gear sound in that product becomes too loud, we might be concerned or bothered by it, but a certain amount of sound from the motor or gearing is expected and acceptable. The vacuum cleaner rattle was unacceptable simply because it should not occur in that product.

But what about a product that is new? When the instant camera was introduced, it made a sound that was completely different from that of a 35 mm SLR camera (Sounds C,D).1 If the product is different, then experience shows that the new sound can become acceptable, particularly if there is a difference in function, as there is with an instant camera.

Truly new products such as front loading washing machines (recently reintroduced into the U.S. market) or hybrid electric/ IC engine automobiles may sound different from the products they hope to replace. Should the product manager simply try to make them as quiet as possible and hope for the best, or should the sound of these products be exploited as a product differentiation? Are there ways to help the product manager make that decision?

Since I believe a key element in dealing with these issues at present is through listening tests, let's look at the background of psychoacoustics. Psychoacoustics is the study of how people perceive sounds. A major tool of psychoacoustics is jury testing in which people are asked to listen to and judge sounds for certain characteristics.

Loudness and Annoyance

A great achievement of psychoacoustics has been the development of a scale of loudness, which is the perception of the strength of a sound. The loudness of tones was studied in the US by Fletcher and Munson2 and in the UK by Churcher and King.3 This work was then extended to the loudness of bands of noise and much more complex sounds. As a result there are computational algorithms or metrics for loudness that work extremely well for calculating and predicting how people will perceive the loudness of all kinds of sounds.

Until the 1950s, it was assumed that loudness would predict when product sounds were objectionable. But a new product, the jet air transport, shattered that idea. Jet airplanes that were equally loud as piston engine aircraft were much more annoying. Compare the sound of a piston engine aircraft (Sound E)1 and a jet airplane (Sound F).1 From the late 50s into the 60s a number of studies using listening tests were used to create a new algorithm (or metric) to predict the perceived noisiness of aircraft in PNdB (perceived noise decibels). This idea has been further developed into predicting degrees of annoyance around airports, in the U.S. called a Noise Exposure Forecast (see Figure 1).