Detection Threshold in Animal Hearing

Detection thresholds are often considered the threshold of hearing and are measured in very quiet conditions, thereby providing the best estimate of hearing sensitivity. An audiogram is defined as the detection thresholds measured for pure tones within the animal’s hearing range. Human hearing is usually tested in a soundproof booth in order to minimize interference from extraneous sound.

Estimates of the hearing thresholds for some marine mammals. The y-axis (vertical) for the hearing thresholds is relative intensity in underwater dB. The x-axis (horizontal) is the frequency of a sound on a logarithmic scale. Figure courtesy of Darlene Ketten, Woods Hole Oceanographic Institution.

Of course, completely quiet conditions rarely exist in nature. There is always some amount of interfering ambient sound (noise). Thresholds measured with background noise are higher than thresholds measured under quiet conditions. This interference can be quantified with a signal-to-noise ratio (SNR). In aquatic environments, this is the difference between the intensity level of the signal and the ambient noise level expressed in underwater dB (see Introduction to Decibels for more information).

Hearing in Noise

Scientists recognize four SNR levels with respect to perception and communication in humans and birds^[1]Dooling, R. J., & Leek, M. R. (2018). Communication Masking by Man-Made Noise. In H. Slabbekoorn, R. J. Dooling, A. N. Popper, & R. R. Fay (Eds.), Effects of Anthropogenic Noise on Animals (Vol. 66, pp. 23–46). Springer New York. https://doi.org/10.1007/978-1-4939-8574-6_2^[2]Dooling, R. J. (2019). The Impact of Urban and Traffic Noise on Birds. Acoustics Today, 15(3), 19. https://doi.org/10.1121/AT.2019.15.3.19.

• Reception/Detection – the lowest SNR at which a listener can detect the presence of a sound 50% of the time (the detection threshold).
• Discrimination – the lowest SNR at which a listener is able to discriminate between two different sounds 50% of the time – this is a SNR 3 dB greater than the detection level SNR.
• Recognition – the lowest  SNR at which a  listener is able to recognize a specific sound – this is an SNR 3 dB greater than the discrimination level SNR.
• Comfortable communication/comprehension –  a signal level about 15 dB above the background noise is sufficient to allow comfortable speech communication.

To date, limited studies have been conducted for some of these criteria for aquatic organisms. Comfortable communication studies have only been conducted for humans.

In the following animation, the level of human speech or bird vocalization is kept constant, but the level of the traffic noise is varied to demonstrate demonstrates the varying signal-to-noise ratios corresponding to the four levels of “hearing” a sound:

Acoustic Masking

Acoustic masking is a reduction in the ability to detect, recognize, or understand sounds of interest because of interference by other sounds. When acoustic (auditory?) masking is studied in the lab, scientists typically use pure tones as the signal and broadband noise as the background. The signal-to-noise ratio at the threshold of hearing at a given test frequency is called the critical ratio. Critical ratios have been determined for humans and many land and aquatic vertebrates.

Studies with captive animals have investigated acoustic masking using pure tones and broadband background noise, but also background sounds that are encountered in the ocean (see Marine Mammal Masking)

During acoustic masking studies, scientists use a “staircase method” to determine when an animal can detect a signal in the presence of noise. With this method, the scientist increases the signal by a set decibel level until a detection occurs. The scientist then decreases the signal by a set decibel level until the signal is not detected. By increasing and decreasing the signal level multiple times according to the animal’s responses, the scientist hones in on the SNR (detection threshold) at which the animal can detect the signal with a predetermined probability, typically 50%^[3]Erbe, C., & Farmer, D. M. (1998). Masked hearing thresholds of a beluga whale (Delphinapterus leucas) in icebreaker noise. Deep Sea Research Part II: Topical Studies in Oceanography, 45(7), 1373–1388. https://doi.org/10.1016/S0967-0645(98)00027-7^[4]Southall, B. L., Schusterman, R. J., & Kastak, D. (2000). Masking in three pinnipeds: Underwater, low-frequency critical ratios. The Journal of the Acoustical Society of America, 108(3), 1322. https://doi.org/10.1121/1.1288409.

Additional Links on DOSITS

• Animals > Potential Effects of Sound > Masking in Mammals
• Animals > Potential Effects fo Sound> Hearing Loss in Mammals
• Science > Advanced Topics > Introduction to Decibels
• Science > Mesurement > What sounds can animals hear?
• Science > Measurement > What sounds can people hear?

References

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• Dooling, R. J. (2019). The Impact of Urban and Traffic Noise on Birds. Acoustics Today, 15(3), 19. https://doi.org/10.1121/AT.2019.15.3.19
• Dooling, R. J., & Leek, M. R. (2018). Communication Masking by Man-Made Noise. In H. Slabbekoorn, R. J. Dooling, A. N. Popper, & R. R. Fay (Eds.), Effects of Anthropogenic Noise on Animals (Vol. 66, pp. 23–46). Springer New York. https://doi.org/10.1007/978-1-4939-8574-6_2
• Erbe, C., & Farmer, D. M. (1998). Masked hearing thresholds of a beluga whale (Delphinapterus leucas) in icebreaker noise. Deep Sea Research Part II: Topical Studies in Oceanography, 45(7), 1373–1388. https://doi.org/10.1016/S0967-0645(98)00027-7
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