Knocking Sound as Quality Sign for Household Appliances and the Evaluation of the Audio-Haptic Interaction

It has been known for a long time in the automobile industry that the first contact between the customer and a car in the showroom consists of opening the door, sitting in the car and closing the door. Therefore, the sounds of the door opening and closing are carefully designed to invoke feelings of high quality and safety in the customer. Of course, the vehicle’s operating noises are equally crucial to the perception of overall quality. The operating noises of household appliances have gained increasing importance because these noises can negatively or positively influence our daily life. When shopping, customers consider the sound power level of the household appliance provided by the manufacturers. In most cases, it is not possible to listen to the machine in operation. However, a common practice of customers is to knock the sidewalls or open and close the doors of the machine. The knocking sound carries information about the quality and solidity of the product and its material properties. The perception of the knocking sound is normally coupled to a tactile/kinesthetic impression of the knocking event. The aims of this study are to identify the perceptually important features of the knocking sound that affect the impression of quality, define the guidelines for a target sound, make suggestions regarding structural modifications to realize the target sound, and investigate the interaction between auditory and haptic stimuli in the overall product-quality assessment. To achieve these aims, experiments with unimodal and multimodal stimulus presentations were conducted. The results showed that an optimal knocking sound is dull, moderately loud, atonal, and has no distinctive long-lasting frequency components, particularly at high frequencies. A quality index was proposed based on psychoacoustic metrics. The physical coupling between the sound and the vibrations causes that both sensory cues have similar effects on perceived quality.