What Is Frequency Response?

Frequency response describes the range of sound frequencies a microphone can capture and how evenly it reproduces them. It is typically given as a range in Hertz (Hz), such as 20 Hz to 20 kHz, which corresponds to the limits of human hearing. However, the numeric range alone does not tell the full story. The shape of the frequency response curve—how much the microphone emphasizes or attenuates certain frequencies—matters just as much. A microphone’s frequency response is influenced by its diaphragm size, capsule design, acoustic porting, and the intended application.

In technical terms, frequency response is measured by playing a pure tone at varying frequencies from a calibrated source and recording the microphone’s output level. The resulting graph plots frequency (horizontal axis) versus relative output level in decibels (vertical axis). A flat line on this graph indicates that the microphone captures all frequencies equally. In reality, few microphones are perfectly flat, but many strive for near-flat response within a given tolerance (e.g., ±2 dB from 40 Hz to 18 kHz).

Why Frequency Response Differs by Microphone Type

Condenser microphones, common for field recording, often have a wider and more extended frequency response than dynamic microphones. Their lightweight diaphragm moves more freely and can respond to rapid changes in air pressure, capturing high-frequency details like insect wingbeats or bird trills. Dynamic microphones, by contrast, use a heavier coil and can have a more limited top end, making them less suitable for subtle nature cues. Yet dynamic mics can be rugged and wind-resistant, a trade-off worth considering.

How Frequency Response Is Measured

Frequency response is tested in anechoic chambers or using calibrated measurement systems. The microphone is placed at a fixed distance from a reference loudspeaker that sweeps through the audible range. The output is compared to a known standard, and deviations are noted. For field microphones, manufacturers often publish frequency response graphs that include both on-axis and off-axis response. Off-axis response is critical because in nature surroundings, sounds arrive from many angles. A microphone that colors frequencies differently when sound enters from the side can alter the perceived texture of rain, wind through trees, or ambient bird chorus.

It’s also important to know that frequency response can be affected by the proximity effect. Omnidirectional microphones are less prone to proximity bass boost, while cardioid and directional microphones exhibit increased low-frequency response when placed close to a sound source. For nature recording, where the microphone is often several feet away or more, the proximity effect is usually negligible. But for close-up insect or animal sounds, it can artificially deepen the tone.

Why Frequency Response Matters for Nature Recording

Nature sound recording aims to capture the acoustic environment as authentically as possible. Whether you are documenting bird vocalizations for ornithological research, recording a thunderstorm for a film soundtrack, or creating a library of forest ambiences, the microphone’s frequency response determines how faithfully the sounds are preserved. A microphone that rolls off the high frequencies will make bird songs sound muffled and lack sparkle. One that humps the low end can turn gentle wind noise into a distracting rumble.

Different natural sounds occupy distinct frequency regions. For example:

  • Low frequencies (20–200 Hz): Distant thunder, ocean surf, large waterfalls, low-frequency animal calls like the infrasound of elephants or the bass notes of a bullfrog.
  • Mid frequencies (200 Hz–4 kHz): Many bird songs, mammal calls (wolves, bears), rustling leaves, footsteps on dry ground, water splashing.
  • High frequencies (4 kHz–20 kHz): Insect buzzes (mosquitoes, cicadas), high-pitched bird twittering, fine gravel crunching, and the “air” of a quiet forest.

A microphone with a broad, flat frequency response preserves these relative levels, allowing the listener to hear the environment as it was. Scientists and artists alike rely on such accuracy for analysis, reproduction, and creative composition. Flattening the curve also reduces the need for heavy post-processing EQ, which can introduce noise or phase artifacts.

The Role of Sub-20 Hz and Ultrasonic Frequencies

Some field microphones extend below 20 Hz into the infrasound region. While not directly audible, subsonic vibrations can be felt by animals and are present in seismic events, avalanches, or large weather systems. For specialized research, microphones with response down to 5 Hz or even DC (via electret designs) are used. Similarly, ultrasonic microphones capture frequencies above 20 kHz, common in bat echolocation, rodent communication, and certain insect sounds. If your interests include these realms, seek microphones rated for extended range. For general nature recording, the standard 20 Hz–20 kHz range suffices.

Flat vs. Colored Frequency Response

A flat frequency response means the microphone reproduces all frequencies within its range at the same relative level. This is the benchmark for accurate, transparent recordings. Field recordists aiming for natural soundscapes overwhelmingly prefer flat microphones. A flat response also makes it easier to compare recordings from different locations or times, as there is no microphone-induced coloration.

A colored frequency response deliberately shapes the audio. Some microphones have a slight high-frequency boost to add “presence” or a low-frequency cut to reduce handling noise. While colored microphones can be useful in certain artistic contexts—for example, giving a rugged intimacy to a campfire recording—they generally are avoided for nature documentation because they impose the microphone’s character on the environment. The best approach is to capture a clean, flat recording and apply EQ in post-production if needed, rather than being locked into a fixed color.

Practical Examples in Nature Recording

Imagine recording a dawn chorus with a microphone that has a 3 dB peak at 5 kHz. The bird songs that fall near that peak will sound unnaturally loud compared to the background. Similarly, a microphone with a low-frequency roll-off starting at 80 Hz will lose the deep rumble of distant thunder, making a storm sound thin. Conversely, a microphone that exaggerates low frequencies could make quiet wind noise overwhelming. Field recordists often test microphones against known reference material (e.g., pink noise, a calibration tone, or a familiar outdoor environment) to judge neutrality.

Understanding Frequency Response Graphs

Manufacturers publish frequency response graphs with frequency on the x-axis (logarithmic scale) and relative output in dB on the y-axis. A line across the graph shows how the microphone’s output varies. Ideally, the line is as flat as possible over most of the range. Look for graphs that show tolerance bands, such as “±2 dB from 40 Hz to 18 kHz.” The narrower the tolerance, the more accurate the microphone.

Pay attention to the low-frequency slope. Many directional microphones exhibit a gentle roll-off below 100 Hz, which can reduce wind and rumble but may sacrifice deep bass for certain environments. For omnidirectional microphones, the low end often extends lower and flatter. Also examine high-frequency extension: a microphone that stays flat to 20 kHz or beyond will capture the shimmer of water and high insect harmonics. Beware of microphones with sharp peaks in the high-frequency region—they can sound harsh and unnatural.

Off-axis response graphs show how the microphone behaves for sounds arriving from the sides or rear. For nature recording, where sounds come from all directions (especially in stereo or binaural setups), a consistent off-axis response is important. A microphone that drastically colors off-axis sounds will create a false sense of space. Some manufacturers provide polar plots at several frequencies, giving a complete picture.

Choosing a Field Microphone

Selecting a microphone for nature recording involves balancing frequency response with other factors like self-noise, dynamic range, sensitivity, and convenience. Here are key specifications to review:

  • Frequency response range: Look for at least 20 Hz–20 kHz for general use; 10 Hz–40 kHz for extended range or for researchers covering infrasound/ultrasound.
  • Flatness: Aim for ±3 dB or better over the main range. Pro-level field microphones often advertise ±1 dB.
  • Self-noise: Low self-noise (e.g., <15 dBA) ensures quiet natural sounds are not buried in hiss.
  • Maximum SPL: For loud sources (thunder, waterfalls, animal roars), a mic that can handle 125 dB SPL or more is wise to avoid distortion.
  • Polar pattern: Omnidirectional microphones are often favored for ambience because they capture a 360° field and have a flatter low-end response. Cardioid or shotgun patterns help isolate a specific source but can color off-axis sounds.
  • Wind protection: Essential for outdoor use; even the best frequency response is useless if wind noise masks the recording. Use blimps, windscreens, or furry covers.

Microphone Types for Nature Recording

Common field microphones include:

  • Small-diaphragm condenser microphones (SDCs): Excellent transient response, wide frequency range, and low self-noise. They are the workhorses of nature recording. Examples: Sennheiser MKH 8020 (omnidirectional, flat to 60 kHz), Schoeps CMC series, DPA 4006.
  • Large-diaphragm condensers (LDCs): Often have a slightly colored midrange and are less common for field use due to bulk, but some (like the Neumann TLM 103) are used for close-up wildlife.
  • Shotgun microphones: Tightly directional, good for isolating a single bird or animal sound. However, their frequency response often has a buildup in the low mids and a dip in the low frequencies. They require careful positioning to avoid off-axis coloration.
  • Contact microphones: Used for capturing vibrations through solids (e.g., wood, rock); their response is highly uneven and not suitable for air sound.
  • Hydrophones: Specialized for underwater recording, with frequency response tailored to water acoustics.

Our own Fleet line of field microphones is designed with a near-flat frequency response from 20 Hz to 20 kHz (±1.5 dB) and extremely low self-noise, making them an excellent choice for capturing authentic natural soundscapes. For detailed specifications and comparisons, see our microphone selection guide.

Additional Considerations for Nature Recording

Wind and Environmental Noise

A microphone with an ideal frequency response is useless if wind noise contaminates the recording. Wind creates low-frequency turbulence that can overload the mic’s preamp and create deep rumbles. Use a windscreen designed for your microphone—closed-cell foam for moderate wind, and a “dead cat” fur cover for stronger gusts. Some field microphones have built-in low-cut filters that roll off frequencies below 80–100 Hz to reduce wind rumble. If your recording goal includes deep bass (such as distant thunder), use the filter judiciously or record with it off and apply a high-pass filter in post if needed.

Preamp and Recorder Impact

The microphone’s frequency response is only as good as the signal chain behind it. A high-quality preamp with a flat frequency response and low noise floor is essential. Many portable recorders (e.g., Sound Devices, Zoom F-series, MixPre) offer transparent preamps. When paired with a great field microphone, they preserve the frequency response intended by the mic’s design. Avoid using built-in preamps on consumer recorders that may color the sound with added noise or EQ.

Calibration for Scientific Recording

For research purposes, it’s important to calibrate your entire recording chain. Use a calibrator (e.g., a pistonphone or a reference sound level meter) to record a known level. This allows you to later correct for any frequency response irregularities in post. Some recordists generate a calibration file with pink noise and use software like Audition or Izotope RX to apply an inverse EQ curve. Calibrated recordings enable precise measurements of sound pressure levels and frequency content.

Stereo and Binaural Techniques

Frequency response also interacts with stereo microphone techniques. For A-B spaced pairs, you typically use two identical omnidirectional microphones with matched frequency responses to ensure consistent imaging. For mid-side (M-S) stereo, one microphone faces forward (cardioid) and another is bidirectional (figure-8). The frequency response of the bidirectional mic must be carefully matched; otherwise, the sum/difference decoding will create comb filtering and image shifts. Binaural microphones (like dummy heads) have a special frequency response shaped by the ear pinna, which adds psychoacoustic cues—this is purposely colored to replicate human hearing.

Conclusion

Frequency response is a cornerstone specification for any microphone used in nature sound recording. A broad and flat frequency response ensures that the full spectrum of natural sounds—from the lowest rumble of thunder to the faintest high-pitched insect—is captured with fidelity and detail. By understanding the concepts of flatness, tolerance, off-axis behavior, and the influence of environmental factors, you can make informed decisions when selecting a field microphone. Pair that knowledge with proper wind protection and a quality recorder, and your recordings will truly reflect the richness of the natural world.

For further reading, check out Sound On Sound’s guide to microphone frequency response and The Wildlife Sound Recording Society’s microphone recommendations.