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Traffic Noise and Birds
 Figure 3. Typical bird hearing curve (audiogram; solid line) that shows the lowest sound level an animal can hear at each frequency. From Dooling, 2002. Birds hear best around 1-5 kHz and worse at lower and higher frequencies. Most of the energy in bird vocalizations also falls in this frequency region. Gray area roughly represents the shape of the spectrum of traffic noise, showing that most energy in traffic noise is at lower frequencies and so it will not, at least at low levels, interfere too greatly with birdsong that occurs in the frequency range of best hearing for birds.
(SNR). There is an extensive literature on humans listening to speech in a noisy environment For humans, the overall SNR at which about 50% of speech may be correctly identi- fied in a steady noise is about −5 dB (i.e., the masking noise is about 5 dB greater in intensity than the speech; Festen and Plomp, 1990). For speech to be heard at a comfortable level that would allow unambiguous acoustic communication and the perception of speech would require a SNR of about 15 dB (Franklin et al., 2006). In a noisy restaurant, for example, the SNR is likely to be, perhaps, around 5 or so dB, adequate for strained conversation but making clear and easy communica- tion impossible.
How masking by noise or other sounds affects hearing has been studied extensively in both humans and birds using simple sounds such as pure tones and white noise (e.g., Dool- ing et al., 2000). These kinds of masking experiments also provide the best metric for species comparisons. In contrast to the overall noise level referred to above, in these kinds of experiments, the common unit of noise level is called the
spectrum level, defined as the intensity level of the noise within a one-hertz-wide band. A comparison of the level of the signal (i.e., a pure tone) and a noise spectrum level also provides a SNR, (measured in decibels) that is used to describe whether a signal has a level above or below the level of the noise and by how much. Either increasing the signal level or lowering the noise spectrum level will allow easier detection of the signal embedded in the noise.
The SNR concept is used to measure an important aspect of the hearing abilities of animals and humans. This “criti- cal ratio” describes an animal’s threshold as the SNR of a tone at a level that is just masked by a noise at a fixed level that occurs within a band of frequencies around the signal. Estimates of critical ratios are available for many species including humans and 16 species of birds (Dooling et al., 2000; Dooling and Leek, 2018). Critical ratios derived from pure tones and white noise are the best method of making masking comparisons across species. And the critical ratios have a practical relevance for hearing under natural condi- tions where ambient or anthropogenic noise might affect the perception of vocal signals and the range over which vocal signals may be heard. They have also been used to study the evolution of detection mechanisms.
Knowing how much noise is effective in masking a signal can, in a more ecological context, aid in understanding which anthropogenic noises may interfere with the acoustic communication of birds in their natural habitats. The data show that the measured threshold for detecting a pure tone is about 20 dB above the spectrum level of the masking noise for humans (i.e. a critical ratio of 20 dB), and it is about 26 dB above the spectrum level of the masking noise for birds (i.e., a critical ratio of 26 dB; Dooling and Leek, 2018).
In other words, humans hear better in noise than birds by 6 dB. It is important to note that signal and noise levels used for calculating the critical ratio are the levels reaching the ear (i.e., measured at the ear of the listener), not at some location distant from the listener. In other words, the same level of noise has more impact on hearing in birds than it does on humans by 6 dB. This 6 dB effect extends over the bird’s entire frequency range. Consequently, at 4 kHz, the SNR for detection for humans is about 23 dB and for birds at 4 kHz it is, on average, about 29 dB.
For those interested in determining the extent to which noise might interfere with communication among birds,
 22 | Acoustics Today | Fall 2019 | volume 15, issue 3

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