5.2. Desempeño docente
5.2.9. Dimensiones del desempeño docente
Chapter 3
Aversiveness of artificial sounds: Behavioural
responses depend on psycho-physiology and
motivational state
Introduction
Behavioural responses to sounds that have no biological meaning are likely to be influenced by psycho-physiological factors and motivational processes. On the perceptual side, perceived loudness and pleasantness are of crucial importance. As mentioned in chapter 2 cetacean and seal hearing is not equally sensitive over a range of different frequencies. Psychophysical experiments on humans showed that the contours of perceived equal loudness are roughly parallel to the hearing threshold within the most sensitive hearing range but are compressed at the high and low frequency edge of the hearing range (Robinson & Dadson, 1956; Fletcher & Munson, 1933). A rough approximation is therefore to assume that sound pressure levels that exceed the hearing threshold by a similar amount cause similar perceived loudness. These so called sensation levels are expressed as sound pressure level in dB above the hearing threshold. While perceived loudness depends in part on stimulus amplitude it is important to note that the physical composition of a sound does also contribute to perceived loudness (Fletcher & Munson 1933). For instance, the perceived loudness of a group of pure tones or filtered noise increases rapidly if the bandwidth of a stimulus exceeds the cochlear filter bandwidth (critical bandwidth) at a given frequency while the source level is being kept constant. In contrast perceived loudness remains almost constant if the bandwidth of the stimulus stays within a critical band (Zwicker et al., 1957). In addition duration of a sound influences its loudness: for stimuli close to the auditory threshold perceived loudness increases with increasing stimulus duration up to a maximum of 200ms. For louder sounds a continuous increase up to a duration of 100ms was found (Zwislocki, 1969).
Perceived pleasantness has mainly been studied in humans. Here, a variety of psycho-physical features of a sound influence its pleasantness. Zwicker & Fastl (1990) developed a model to describe what makes sound pleasant or unpleasant for humans. The relevant psychophysical parameters are sharpness, roughness, tonality and loudness. Roughness can be maximised by using strong frequency or amplitude
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modulation with maximum roughness caused by modulation frequencies of about 70 Hz and frequency modulation contributing more to roughness than amplitude modulation (Zwicker & Fastl, 1990). Interestingly, high loudness contributes with the lowest loading to unpleasantness for low and medium–intensity stimuli. Sharpness is mainly correlated with higher centre-frequencies within the species hearing range and tonality depends on the waveform of the sound being highest for pure-tones and low for square wave sounds. Furthermore in humans complex sounds that consist of partial tones that are related by certain frequency ratios (musical intervals) are perceived as unpleasant (dissonant) while others are perceived as pleasant (consonant). Modern classical composers (e.g. Arnold Schönberg) assumed that this is a result of culture but physiologists like von Hemholtz (1853) expected more general properties of the auditory system to be responsible. Von Helmholtz (1853) hypothesised that consonance depends on how many harmonics of two complex tones match. Non-matching harmonics result in “beating” phenomena causing a sensation of roughness. In spite of strong criticism e.g. by musical psychologists (Stumpf, 1883) consecutive research gave support for Helmholtz’s notion that interference of adjacent partials is important (see Plomp, 1965). Plomp & Levelt (1965) showed that in musically untrained subjects transition from consonance to dissonance perception depends on the cochlear filter bandwidth (critical bandwidth). The strongest perception of this so called “tonal dissonance” is caused by two partials that fall within 25 % of the critical bandwidth. This critical band theory can also explain roughness perception based on frequency or amplitude modulation (Zwicker & Fastl 1990). Given that the phenomenon of unpleasantness seems to be associated with factors as basic as auditory filter bandwidth one would expect animals to have similar sensations. However, experimental evidence, at least for musical consonance perception in animals is still equivocal. A two-alternative-forced choice experiment revealed clear preferences for consonant musical intervals in rats (Borchgrevink, 1975). However, neither consonance preference nor preference of white noise over “screeching sounds” was found in place preference experiments with monkeys (McDermott & Hauser, 2004). Further experiments on non-human primates revealed preference for slow tempos over fast but also a general dislike of music (McDermott & Hauser, 2007). In contrast experiments on Japanese song sparrows (Padda oryzivora) gave limited evidence that some birds which preferred music over silence in a first experiment also showed preference for tonal music (e.g. Bach) compared to modern atonal music (e.g. Schönberg) (Watanabe, 1998). Although one might assume that atonal music contained more dissonant intervals, music preference experiments in animals are notoriously difficult to interpret because
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it is impossible to discern the influence of different features in sounds that are as complex as music.
Motivational factors influence behavioural responses to artificial sounds on another level. Although most reviews on the impact of noise on marine mammals mention motivational state as a potentially important factor influencing behavioural responses to sound (Richardson et al., 1995; Southall et al., 2008; Nowacek et al., 2007), no systematic experimental study attempting to discern different factors has been carried out so far. However, anecdotal evidence indicates an important role of motivation. Such evidence comes from studies that show a decreasing (Mate & Harvey, 1987) or absent effectiveness (Norberg, 1998) of acoustic deterrent devices in situations where food motivation was high.
All of these findings are also relevant for the choice of a stimulus in acoustic deterrent devices if the declared aim is to use the minimum source level that is required to elicit avoidance responses in order to minimise noise pollution and impacts on hearing (as suggested in chapter 2). Most commercially available acoustic deterrent devices use high source levels at high duty cycles (up to 50%) which are expected to cause pain if seals approach too closely (see Jeffers 1996, US Patent No. 5610876). In humans the pain threshold lies at around 120 dB re 20 µPa (Spreng, 1975) and is therefore quite close to sensation levels where short, single exposure can damage the ears of a terrestrial mammal (130 dB re 20 µPa, Henderson, 1991). At close ranges these sounds would also exceed recommended maximum sensation levels for humans if exposure lasted more than 1.5min per day (Kryter, 1994). Therefore, if these considerations about hearing damage are also true for seals then acoustic deterrent devices should ideally not produce received levels that cause pain. High duty-cycle devices operating close to the pain threshold would be particularly problematic since longer exposure will increase the risk of hearing damage (see chapter 2). However, discomfort or distress thresholds may be lower and may be used to cause a deterrence effect. Spreng (1975) showed that in humans, changes in electro-physiologically measurable parameters that are indicative of discomfort and stress occur at sensation levels as low as 70-80 dB above the hearing threshold. In the light of these considerations the sound of a high duty-cycle ADD should be below the pain threshold to avoid potential hearing damage but it should be above the discomfort threshold to cause a deterrence effect. If discomfort thresholds in seals and humans are similar (70-80dB above hearing threshold) then an 8 kHz ADD would have to produce a received levels that exceeds 134-144 dB re 1 µPa within the designated deterrence zone (hearing threshold from
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composite audiogram; see appendix 1) The biggest problems with current ADDs is that they have dramatic effects on non-target species e.g. habitat exclusion of odontocetes was shown up to distances of over a kilometre from the sound source (see review in chapter 2) When attempting to mitigate impact of ADDs a frequency band should be chosen where hearing in odontocetes is less sensitive than hearing in seals. As it was argued in chapter 2 this would be the case for frequencies between 500Hz and 2 kHz. Harbour porpoises (Phocena phocoena) have been shown to avoid an area of 645m around a commercial ADD (10kHz) which corresponds to modelled received levels of 128 dB re 1µPa (Johnston, 2002). Assuming the hearing threshold of a harbour porpoise to be 50 dB re 1µPa at 10 kHz (Kastelein et al., 2002) the received level equals a sensation level of 78 dB. Given that the harbour porpoise’ hearing threshold is at least about 30dB higher at 1 kHz received levels causing a deterrence effect can be expected to be in the order 158 dB re 1µ Pa. Assuming simple spherical spreading such a received level would be reached at 15m distance. This would mean that theoretically using the suggested frequency band could result in a dramatic reduction of deterrence ranges for odontocetes.
My study aimed to test how different factors related to psycho-physiology or motivation influence behavioural responses to noise in phocid seals. This was done by testing responses 1.) “new sounds” based on current psychophysical knowledge of what makes sounds unpleasant to humans 2.) sounds from commercially available ADDs (“seal scarers”), 3.) control sounds with assumed neutral properties. To test how motivation modifies behavioural responses to noise seals were tested in 3 different situations with a) a profitable food source b) a known but “exploited” food source c) no food source. The “new sounds” were also based on the suggested frequency band to mitigate impact onodontocetes. Zwicker and Fastl’s (1990) model was used to design the supposedly “unpleasant” sounds e.g. by applying strong frequency modulation to increase “roughness”. Therefore, the data were also expected to possibly shed some light on the perceptual (or alternatively cultural) basis of “pleasantness” of sounds in humans.
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146 146 147 147 144 147 142 144 145 143 146 145 feeding station loudspeaker Seal waiting for food Zone closer than1.5m from feeding tube Mean RL in dB (rms) re 1 micro Pa (0.8 depth) 147 147
Figure 1: Experimental setup and sound field in playback pool