Effects of partial masking for vehicle sounds

Transcription

1 Effects of partial masking for vehicle sounds Hugo FASTL 1 ; Josef KONRADL 2 ; Stefan KERBER 3 1 AG Technische Akustik, MMK, TU München, Germany 2 now at: ithera Medical GmbH, München, Germany 3 now at: Müller-BBM Active Sound Technology GmbH, Planegg, Germany ABSTRACT In psychoacoustic assessments of vehicle sounds, the target sounds are usually presented in a quiet background. However, in practical applications, vehicle sounds are commonly superimposed by background noises. Therefore, in a pilot study, idle noises of two passenger cars with gasoline vs. Diesel engine were partially masked by a synthetic vs. a natural background noise. As synthetic background noise, uniform exciting noise (UEN) was chosen, which is similar to pink noise, but has the additional advantage that it shows the same level in each critical band. As natural background noise, a recording of a pedestrian area was used. The results reveal different partial masking for different engines as well as background noises. Effects of sound signature seem to play an important role in partial masking of real world sounds. Keywords: Partial masking, Uniform exciting noise, Pedestrian area, Diesel engine, Gasoline engine, Sound signature I-INCE Classification of Subjects Numbers: 61.5, 63.1 INTRODUCTION In psychoacoustic assessments of vehicle sounds, the target sounds are usually presented in a quiet background. However, in practical applications, vehicle sounds are commonly superimposed by background noises. Therefore, in a pilot study, idle noises of two passenger cars with gasoline vs. Diesel engine were partially masked by a synthetic vs. a natural background noise. As synthetic background noise, uniform exciting noise (UEN) was chosen which contains no audible fluctuations. As natural background noise, a recording of a pedestrian area was used, including voice sounds and clearly audible fluctuations. Classic work on partial masking (1, 2, 3) addresses the superposition of a test sound by a masking sound which, however, does not completely mask the test sound to inaudibility, but reduces its loudness. Effects of such partial masking are illustrated in figure fastl@mmk.ei.tum.de josefkonradl@web.de stefan.kerber@mbbm.com 4583

2 Figure 1 Partial masking of pure tones at 1 khz by pink noise (4) In figure 1, the perceived loudness of a 1 khz tone is plotted as a function of its level. Without masking background noise, the dashed curve is obtained. This is the classical curve showing how perceived loudness increases with increasing sound pressure level. For tone levels above 40 db it illustrates the well known relation that a level increase by 10 db increases perceived loudness by a factor of two. If in addition to the 1 khz tones pink noise with 1/3-oct band levels of 40 db (circles) or 60 db (triangles) is added, results illustrated by the respective solid lines are obtained. They show that the test tones remain inaudible until they reach a certain level (filled symbols). Increasing the level beyond this point leads to a steep increase of loudness above threshold and an approach to normal loudness values for high test tone levels. This behavior is typical for partial masking and it shows also up in persons with hearing deficits, e.g. noise induced hearing loss, where it is called recruitment. EXPERIMENTS Subjects Ten subjects with normal hearing ability (HL < 20 db) aged between 20 and 28 years (median 24 years) took part and concluded all experiments. One subject was female, nine subjects were male. From the originally 12 subjects, two were excluded, since they had problems to understand the task and gave always the same answers irrespective of the vehicle sounds presented. Obviously, they rated the background sounds instead of the vehicle sounds. Sounds and procedure Vehicle sounds were recorded in a semi-anechoic chamber 1.2 m in front of the idling vehicle. Two rather different vehicle sounds were used: one from a vehicle with a six cylinder gasoline engine, and one from a vehicle with a four cylinder Diesel engine. The original levels of the vehicle sounds were attenuated or amplified to yield levels L T of 45, 50, 55, 60, 70, and 80 db, respectively. As background noises, also two rather different sounds were chosen: The synthetic background noise was uniform exciting noise (UEN) which is rather similar to pink noise, but has the additional advantage that it shows the same level L cb in each critical band (4). For a level of L cb = 40 db in each critical band and 24 critical bands a total level of L UEN = lg 24 = 53.8 db is obtained. As natural background noise, the sound recorded in a pedestrian area with a level of L PedAr = 68 db was chosen. 4584

3 Sounds were presented in a sound attenuating booth diotically via electrodynamic headphones (Beyer DT 48) with freefield equalizer (4). Each sound was presented to each subject four times in random order; hence the median values reported are based on 40 datapoints each. The sequence of sounds is illustrated in figure 2. Figure 2 Illustration of the sequence of sounds used for the experiments According to the illustration in figure 2, first the background noise with 1 s duration was presented. After 0.3 s from the start of the background noise, the vehicle sound of 0.7 s duration was added. Then after a pause of 0.3 s duration the vehicle sound of 0.7 s duration (without background noise) was presented. The initial time gap of 0.3 s helps to distinguish between background noise and vehicle sound. The procedure used was a Method of Adjustment. The task of the subject was to vary the level of the second vehicle sound in such a way that it produced the same perceived loudness as the first vehicle sound partially masked by the background noise. The user interface to implement this Method of Adjustment is illustrated in figure 3. Figure 3 Illustration of the user interface for the method of adjustment The sequence of sounds illustrated in figure 2 was started by hitting the button PLAY indicated in figure 3. By operating the slider with the mouse, the level of the second vehicle sound was varied. This procedure was repeated until the first and the second vehicle sound elicited the same loudness. By hitting NEXT the level of the previous adjustment was stored and the next combination of vehicle sound and background noise was started. 4585

4 RESULTS The results of the present pilot study are summarized in figure 4. In each panel, the level of the respective second vehicle sound which produces the same loudness as the partially masked first vehicle sound is given as a function of the level L T of the respective first vehicle sound. Figure 4 Level of the respective second vehicle sound as a function of the corresponding first vehicle sound L T. Upper panels: Background noise of pedestrian area. Lower panels: Background noise uniform exciting noise (UEN). Left panels: Vehicle with Diesel engine. Right panels: Vehicle with gasoline engine. The solid curves connect medians of adjusted values of, the dashed curves illustrate L T = The results displayed in figure 4 suggest that at high levels L T of the vehicle sounds no partial masking occurred. This effect is indicated in figure 4 by the dotted line which represents equality of L T and. For low levels L T, the solid curves clearly deviate from the dotted lines, i.e. substantial partial masking showed up. The magnitude of the partial masking depended both on background noise and vehicle sound. In order to assess these influences in detail, the median levels are again compiled in Tables 1 and 2. For the levels L T of the vehicle sounds, the corresponding levels of the respective second sound as well as the level difference = L T are given. can be regarded as a measure of the reduction of the loudness of the vehicle sound by the background noise and hence as a measure of the magnitude of partial masking. 4586

5 Table 1 Partial masking of vehicle sounds by uniform exciting noise Diesel Gasoline L T The data displayed in Table 1 would seem to suggest that at low level L T of the vehicle sound, the partial masking is larger for the vehicle sound with gasoline engine than Diesel engine. This holds in particular for levels L T of 45 and 55 db. Because of less partial masking, the vehicle with the Diesel engine might be easier detected in the continuous UEN background due to its time-variant sound signature. If this argument of sound signature holds, the vehicle with gasoline engine should incur less partial masking in the pedestrian area, because the steady engine sound contrasts with the time varying background noise. This hypothesis can be checked by means of the data displayed in Table 2. Table 2 Partial masking of vehicle sounds by noise from a pedestrian area Diesel Gasoline L T A comparison of values in Table 2 for sounds from vehicles with Diesel vs. gasoline engines reveals that the approach to take into account the signature of the vehicle sounds would be supported by the data displayed. At all values of L T from 45 to 70 db the values for the vehicle with the Diesel engine are larger than the values for the vehicle with the gasoline engine. DISCUSSION Results of this pilot study showed that partial masking of vehicle sounds by background noise may be influenced by effects of sound signature. If in contrast to partial masking, (complete) masking of vehicle sounds by background noise is studied, sound signature seems to be of minor importance. For example, Kerber (5) in his concept of critical distance could faithfully predict the audibility of approaching vehicles in various background noises from the sound pressure of the vehicle, the vehicle speed, and the sound pressure of the background noise (6). Before giving effects of sound signature too much weight it should be mentioned that the present pilot study shows some drawbacks: Most severe may be the fact that different levels of the vehicle sounds were produced by attenuation or amplification of original levels. This inevitably leads to substantial distortions in timbre. Also, durations of vehicle sounds of only 0.7 s may be somewhat short compared to durations in real life. Although the partial masking of vehicle sounds by background noises during idling definitely is of interest, experiments with sounds from approaching vehicles as used by Kerber (7) may be even more important for practical applications. 4587

6 Despite these drawbacks the present pilot study clearly showed that experiments on partial masking are not confined to synthetic signals but can be performed by subjects also with real world sounds with comparable accuracy of measurement. For future experiments, both the sequence of sounds and the implemented method of adjustment can be recommended. REFERENCES 1. Zwicker E. Über die Lautheit von ungedrosselten und gedrosselten Schallen. Acustica 1963; 13: Scharf B. Partial masking. Acustica 1964;14: Gleiss N, Zwicker E. Loudness function in the presence of masking noise. J Acoust Soc Am. 1964;36: Fastl H, Zwicker E. Psychoacoustics, Facts and Models (3 rd edition). Berlin Heidelberg New York: Springer-Verlag; Kerber S. The Importance of Vehicle Exterior Noise Levels in Urban Traffic for Pedestrian - Vehicle Interaction. ATZ Worldwide 2006; 108(7-8): Kerber S, Fastl H. Perceptibility of approaching vehicles in urban background noise. Proc. of 36th Intern. Congress on Noise Control Engineering INTER-NOISE 2007; August 2007; Istanbul, Turkey Kerber S. Wahrnehmbarkeit von Fahrzeugaußengeräuschen in Hintergrundgeräuschen: Psychoakustische Beurteilungen und modellbasierte Prognosen. München: Verlag Dr. Hut;