Sound Recording in the Field

Sound Recording in the Field

This tutorial suggests recording equipment and describes some of the most common problems and errors that may occur during sound recording in the field.

Microphone Choice

There is a wide range of microphones available that could be used for recording animal sounds in the field. Well-tried and reliable microphones are available from Sennheiser. The model K6/ME66 provides a good compromise of size, sound quality and price. The more expensive Sennheiser MKH series provides slightly lower self-noise levels at low frequencies (below about 10 kHz) and is more robust in humid environments. However, the noise performance at frequencies between 15 and 20 kHz is better in the ME series (the MKH series microphones have been optimized for best noise characteristics to human listeners, while the ME series provides an equally distributed noise floor across the entire frequency range).

A parabolic reflector in conjunction with an omni-directional microphone (e.g. Sennheiser K6/ME62) would provide a significantly higher degree of directionality. Sounds originating from off-axis directions will be very effectively attenuated. This property of a parabola can dramatically improve the spectrographic analysis of recordings made in dense forest environments (a high level of short-term echoes (reverberation) from other directions can disable the meaningful analysis of rapidly modulated sounds). A parabola also provides a higher microphone output level (it is an acoustic amplifier). Unfortunately, a parabolic reflector also distorts the frequency response of the entire microphone system (see Parabolas). Due to its limited size, the reflector is only effective at wavelengths shorter than the diameter of the dish. Another drawback is its large size.

Note that all directional microphones (certainly shotgun and parabolic microphones) have a non-linear frequency response (off-axis colouration) when the subject is not located on-axis (they are more directional at higher frequencies). Consequently, these microphones should be pointed exactly at the vocalizing animal.

The following table shows the suitability of the various microphone models for various recording distances.

Further information on microphone theory and operation can be found in the book Microphones – Methods of Operation and Type Examples by Bore and Preus, Georg Neumann GmbH, Berlin.

Recorder Choice

* discontinued recorders

The column “handling” depicts the ease of use in the field. The smaller recorders have often very small buttons that are more difficult to use than the bigger ones in the larger recorders. The “ruggedness” mainly depends on the type of microphone connectors. Tiny 3.5 mm mini jack connectors are less reliable than large XLR connectors. CompactFlash recorders have no moving parts and are therefore more reliable than analog compact cassette, DAT, MiniDisk and Harddisk recorders.

The sound quality depends on the self-noise level of the preamplifier (though, in conjunction with sensitive condenser microphones (50mV/Pa), the inherent self noise level of most recorders is not an issue for practical applications in the field (see also Understanding microphone and recorder/preamplifier noise specifications and Microphone Input Noise Comparision) and artifacts caused by lossy audio compression formats (MiniDisk or MP3). The bandwidth depends on the maximum available sample rate (44.1, 96 or 192 kHz) and the recording format (ATRAC/MP3 compression). The potential artifacts that can be caused by ATRAC/MP3 compression depend on the bandwidth of the sounds to be recorded (see also Audio Compression Effects). P48 indicates whether the unit is capable to provide 48V phantom power for microphones. Phantom power is required by high-quality microphones (e.g. Sennheiser MKH series). The Sennheiser K6/ME series can also be powered via P48 (the battery is then not required).

The optionally available 24 bit recording mode that is availably in many recorders will usually not provide any advantage for common sound recordings in the field. The 16 bit format already provides a dynamic range (theoretically 96dB) that exceeds the dynamic range provided by the microphone. Even a very high sound level of 90 dB (that might occur when the microphone is placed very close to a loudly vocalizing animal) recorded with a very low-noise microphone (self-noise level of 5dBA) would only provide a dynamic range of less than 85 dB (= 90dB – 5dB).

Supported Sample Rates

Matching the recorder’s sample rate to the bandwidth of the sounds to be recorded (or to the frequency range you intend to investigate) will reduce the required storage space of the .wav files. Also note that spectrographic analysis is easier when the sample rate matches the frequency range you are interested in. The maximum signal frequency should be less than about 40% of the sample rate (except at 174.4 and 192 kHz, where most recorders provide less steep anti-alias filters).

Other Features

A pre-record buffer allows to safely capture unexpected or sporadic sound events. The boot-up time (the duration from powering the device on to ready-to-record) should be as short as possible. If stereo recording is not required, the recorder should support a mono recording mode in order to save CF card storage space. A fast computer interface (USB 2.0 or FireWire) will accelerate the data transfer to the PC (In any case, it is possible to use a separate USB 2.0 card reader interface for a quick file upload).

Clipping

Adjust the recording level (gain) in such a way that clipping does not occur (there should be a head-room for covering unexpected louder sound events). If the gain was too high, louder sounds would exceed the available dynamic range of the recorder and would be clipped. Clipping (overmodulation) can also be caused by very low-frequency but high-amplitude wind noise. Wind noise can be minimized by using a basket windshield with a hairy cover for the microphone and/or by activating a high-pass filter either on the microphone (the Sennhheiser K6 powering module has an integrated switchable high-pass filter) or on the recorder. Another way to reduce the influence of wind is to hold the microphone close to the ground, where the wind speed is lower.

Note that directional microphones are usually more sensitive to wind and handling noise than omnidirectional microphones. Handling noise can be reduced by using a suspension/pistole grip