Introduction
A common question our Sales and Support teams receive is, "How close does my microphone need to be to record an animal?" The answer to this question might determine how much data one might expect to collect with a single recorder, or how closely multiple recorders should be spaced to cover a large area.
Unfortunately, this is a question to which we, as an equipment manufacturer, cannot provide a simple answer. The factors that determine how close a vocalizing animal must be to a microphone in order to be recorded extend far beyond any characteristic of a given recorder or microphone.
Factors Affecting Detection Distance
Some of the most important factors in detection distance apply both when recording sounds in the human hearing range and above it.
Volume of the Target Vocalization
The louder a sound is at its source, the greater distance it will travel before it is too quiet for a microphone to record it.
Noise Level of the Surrounding Environment
A recorder deployed in a very quiet environment will be able to record a given sound from much further away than a recorder in a very noisy environment. To use an extreme example, if a Song Meter were installed next to a rushing waterfall, the call of a songbird twenty feet away might be entirely obscured by the noise of the water.
Frequency of the Target Vocalization
Lower-frequency sounds tend to travel further than high-frequency sounds, and the same is true for low- and high-frequency components of a single sound.
For example, the low-frequency vocalizations of an owl might travel further than the higher-frequency vocalizations of a song bird, even if the two vocalizations were produced at the same volume. The highest-frequency bat echolocation calls may only be detectable from less than ten meters away.
Environmental Clutter and Obstacles
Physical obstacles in the environment will inhibit a sound as it travels from a vocalizing animal to a microphone or a human listener. All else being equal, you would need to be closer to an animal to hear it in a dense forest than in an open field.
Rough Guideline for Audible Sounds
Typical human hearing is more advanced than an electronic recorder. A recorder lacks the human ability to focus on a particular sound of interest and ignore surrounding noise. If you have ever been surprised that a recording played back through headphones reveals much more ambient noise than what you noticed in person, you have experienced this difference.
That said, human hearing is subject to many of the same limitations as an audio recorder. A person standing next to a busy highway will have more difficulty hearing a distant songbird than that same person standing in a quiet field. A person will hear lower-frequency sounds from farther away than higher-frequency sounds. When you attempt to gauge how close a recorder needs to be to an animal in order to record it quickly, you can use typical human hearing as an approximate starting point, assuming the animal's vocalizations fall within the range of typical human hearing. If you know from experience that you need to be within 100 meters to hear a certain vocalization in a given environment, you will most likely need to install your microphones at least as close to the animal.
Challenges with Estimating Ultrasonic Detection Distance
The same general guidelines affect both audible and ultrasonic sound, but certain factors make it much more difficult to estimate minimum detection distance for signals like bat echolocations.
While some animal vocalizations in the human hearing range can be recorded from miles away in the right conditions, recording bats may require the microphone to be anywhere from 100 meters to less than 5 meters away. Within this limited range, differences between different types of vocalizations become much more significant.
Inability to Compare to Human Hearing
The most obvious factor is that because humans cannot hear ultrasound, we cannot use our own hearing as a guideline. A hearing person with knowledge of a given songbird is likely to know how far that species' vocalizations travel from direct experience. The same cannot be said for most bats.
Greater Significance of Frequency
The effects of frequency on how a sound travels become much more drastic in frequencies above typical human hearing. Moisture in the air causes very high ultrasound to lose energy over very short distances. If a 20-kHz signal and a 100-kHz signal are emitted with identical amplitude in identical atmospheric conditions, the 20-kHz signal might travel almost four times as far as the 100-kHz signal before it is too faint to record.
Additionally, ultrasonic microphones are not equally sensitive to all frequencies. Even if two signals reach a microphone at identical volume levels, a 100-kHz sound may be recorded more quietly than a 20-kHz sound. This effect will depend on the characteristics of a particular microphone.
Measurement Difficulties
If you know exactly how loud a sound of a given frequency is at a fixed distance from its source, it is possible to model how that sound attenuates over distance based on ambient temperature, pressure, and humidity.
However, the factors above mean it is difficult to collect precise reference data on how loudly bats of various species vocalize when exhibiting various behaviors in the wild.