195 Noise Control Eng. J. 51 (4), 2003 Jul–Aug Measurement of the attributes of complex tonal components commonly found in product sound Aaron Hastings, a) Kyoung Hoon Lee, a) Patricia Davies, a) and Aimée M. Surprenant b) All rotating components in machinery produce sounds that contain tonal components, and the presence of these tones can signiicantly affect the quality of the product sound. Tone corrections for metrics based on weighted, average sound pressure level have been used since the late 1960s to assess annoyance due to aircraft noise and to rate climate control machinery. Much research has also been focused on measuring the strength of well separated tones in noise. Metrics such as the Prominence Ratio, the Tone-to-Noise Ratio, and variants, as well as more complex models such as the Joint Nordic Method, Auresʼ Tonalness, and Virtual Pitch, produce values that often correlate well with subjectsʼ judgments on the level of the tonal features that they perceive when listening to the sound. However, when sounds are more complex, these metrics do not always work well. Models for the tonalness of two types of tonal sounds are considered here: narrow-band random noise and tones with random frequency luctuations. The inluence of bandwidth and roll-off rate on perceived tonalness are explored for the narrow-band sounds, and the effect of the range and the rate of change of the frequency variation on perceived tonalness is explored for frequency modulated sounds. It was found that roll-off rate affected the perception of tonalness, and that when frequency variations could be tracked or were very small, metrics derived from averaged spectra produced inaccurate predictions of tonalness. Based on the results of these two investigations modiications to tonal metrics are proposed. © 2003 Institute of Noise Control Engineering. Primary subject classiication: 79; Secondary subject classiication: 61 1. INTRODUCTION A.Factors affecting the perception of tonalness Strong tonal components in a productʼs sound can detract from the productʼs acceptability. A better understanding of how tonal components are perceived is useful for designers who want to be able to improve the sound quality of their products. Although “tonality” is commonly used to describe the degree to which a sound is tonal, this term is also used in the musical community to describe the organization of all the tones and chords of a piece of music in relation to a tonic. 1 In order to avoid confusion, the term “tonalness” will be used to describe the degree to which a sound is perceived as tonal. The tonalness of a sound can be evaluated by the soundʼs ability to evoke a pitch, i.e. its pitch strength; 2 by the amount of tonal content, i.e. by the presence of prominent tones; 3 or by how similar a sound is to a pure tone, i.e. a single sinusoid. 4 Although there will be differences, for example, a tone in noise can have a strong pitch strength but will not sound like a pure tone, these methods of tonal evaluation should have similar trends for single tonal components that are narrower than a critical band. Basic factors that affect tonalness include: bandwidth, center frequency, level above threshold, and the number of tonal components present. Wiesmann and Fastl have shown that as the bandwidth increases, the perception of pitch strength decreases for bandpass noise. 2 Aures modeled the bandwidth effect as being independent of the location of the tonal componentʼs center on the critical band rate scale. 5 Aures also found that a frequency of about 700 Hz for pure tones induced the most tonal sensation. 5 Similar results were obtained by rating pitch strength, with the maximum for pure tones occurring around 1.5 kHz. 6 Provided that a tonal component is easily perceived, tonalness does not change much due to changes in the absolute level of the tonal component. 5,7 When additional tonal components are added, the tonalness should change. However, this change will depend on what question is asked during the evaluation. If the question asked is what is the pitch strength or salience, then individual components will tend to be stronger if they are not harmonics of the same fundamental. 8 If the question asked is how much tonal content the sound has, then for widely spaced components, the overall tonalness will increase equally for harmonic and inharmonic components. 9 It is presumed that when subjects are asked to evaluate how similar a sound is to a pure tone, additional components will not increase the perception of tonalness. B. Methods used to predict tonalness and tonal prominence Existing sound pressure level based noise metrics have been modiied to take into account the presence of tones by adding penalties for the presence of tones in sound. These metrics include the 1995 Standard for Sound Rating of Outdoor Unitary Equipment, 10 the United States Federal Aviationʼs regulations for Aircraft Flyovers 11,12,13 and revised noise criteria for design and rating of heating ventilation and air-conditioning systems. 14 Although these tonal penalties are useful for determining acceptability for the applications for which they were designed, it is not clear how these penalties a) The Ray W. Herrick Laboratories, School of Mechanical Engineering, Purdue University, West Lafayette, IN 47907-1077, U.S.A.; E-mail: daviesp@purdue.edu. b) Department of Psychological Sciences, Purdue University, 703 Third Street, West Lafayette, IN 47907-2081, U.S.A.; E-mail: ams@purdue.edu.