What is a disadvantage of omnidirectional mics?

Pick Up Unwanted Ambient Noise

Omnidirectional mics pick up sound evenly from all directions, including unwanted ambient noise from the surrounding environment. This can muddy and distort the desired audio source. Since they do not discriminate directionally, omnis will capture extraneous sounds like street noise, room reverberation, and background chatter that unidirectional microphones would otherwise reject. The result can be a washed-out, unfocused recording with distracting elements across the stereo field. As this source notes, omni mics are quite sensitive to ambient noise, making them problematic in noisy environments.

Lack Directionality

Omnidirectional microphones lack directionality, meaning they do not focus on capturing sound from one particular direction like cardioid or hypercardioid mics. As explained on the Shure website, “unidirectional mics are most sensitive to sound arriving from directly in front – the angle referred to as 0 degrees – and less sensitive in other directions.”

In contrast, omnidirectional mics pick up sound equally from all directions. According to Synco Audio, “an omni directional microphone captures sound equally from all directions.” This lack of directionality makes omnis unsuitable when trying to isolate a specific sound source like a vocalist or instrument on stage.

The non-focused pickup pattern means omnis will capture unwanted ambient noise from all around. This makes them a poor choice when isolation and directionality are needed in recording or live sound situations.

Prone to Feedback

Their wide pick-up pattern makes omnidirectional mics more prone to audio feedback compared to directional mics. They pick up output from monitors/speakers and create feedback loops.

“Everything else being equal, a cardioid microphone is less likely to feed back than an omni microphone because it is less sensitive as you move off axis,” according to Sweetwater (https://www.sweetwater.com/insync/eliminating-mic-feedback/).

“Keep the microphone behind the main loudspeakers to minimize the sound that can reenter the microphone. If the microphone is in front of the speakers, then sound from the speakers can hit the mic and produce feedback,” notes Audio-Technica (https://www.audio-technica.com/en-us/support/audio-solutions-question-week-prevent-microphone-feedback/).

Shure adds, “It is generally true that an omnidirectional microphone will provide less gain before feedback than a similar quality unidirectional microphone” (https://service.shure.com/s/article/why-should-i-use-an-omnidirectional-microphone).

Less Gain Before Feedback

One of the key downsides of omnidirectional microphones compared to directional types is that they provide less gain before feedback occurs. As Shure explains, “It is generally true that an omnidirectional microphone will provide less gain before feedback than a similar quality unidirectional microphone.”

The maximum level of gain that can be used before creating feedback is lower with omnidirectional mics compared to directional types. This can limit how loud a source can be amplified before feedback starts happening. For any live sound reinforcement application where high volume is needed, the reduced gain before feedback of an omni mic can cause major issues.

According to audio expert suggestions on LinkedIn, using lower gain levels and keeping the mic stable can help reduce feedback problems. However, the reduced gain ceiling remains an inherent disadvantage of omnidirectional mics that audio engineers and performers need to contend with.

Not Ideal for Recording Quiet Sources

Their sensitivity to ambient noise makes omni mics a poor choice when trying to record quieter acoustic instruments or voices, as background noise competes. Omnidirectional mics pick up sound equally from all directions, making them prone to also capturing unwanted ambient noise in a space. This makes it challenging to get a clean recording of soft or distant sources like vocals, strings, orchestras and choirs.

More directional mics like cardioids or hypercardioids are usually preferred when isolation is needed for delicate acoustic instruments, quiet singers or distant sound sources. Their tighter pick up patterns allow them to reject more of the ambient noise compared to omnis. While omnis can sometimes work for quiet sources in treated, low-noise environments, their sensitivity leaves them at a disadvantage compared to directional mics.

Problematic for Live Settings

Omnidirectional microphones are less than ideal for live performances where there is significant ambient noise that can be picked up, such as from stage volume, PA systems, and crowd noise. Their ambient pickup pattern means they will capture sound equally from all directions ¹. This makes it challenging to isolate the desired audio source, like a vocalist or instrumentalist, from the surrounding noise. The ambient sound can easily overpower the direct source when using an omnidirectional mic for live sound.

Directional microphones are generally preferred for live settings as they can be positioned to minimize ambient noise pickup. Their tighter polar patterns help reject unwanted sounds coming from the sides and rear of the microphone. This allows better isolation of the intended source. Sound engineers typically choose directional mics like cardioids or supercardioids for live performances to help control ambient noise issues.

Challenging for Studio Use

In a recording studio setting, ambient sound bleed is usually considered undesirable. Omnidirectional microphones pick up sound equally from all directions, so they tend to capture excess ambient noise in addition to the intended source. This lack of directionality makes them impractical for most close-miked recording applications where isolation and control are preferred.

For example, when recording vocals or acoustic instruments, studio engineers typically want to capture a clean, dry signal, free of room ambience. Unidirectional microphones like cardioids or figure-8 patterns are better suited for rejecting unwanted off-axis sound in studio environments. Their directional designs allow for more isolation and control over the sound source.

According to professional audio engineers, omnidirectional mics generally only work well for studio recording when used as distant room mics, well away from any specific instrument or source. Even then, the ambient bleed can make mixing and processing more difficult compared to more directional microphones (Sound on Sound).

Difficult to Position Optimally

It can be tricky to find an ideal placement that minimizes ambient bleed for omnidirectional mics. Their wide pick-up pattern provides little flexibility. Most experts recommend placing omnidirectional mics very close to sound sources, within the “critical distance” where direct sound is louder than reverberation. According to Shure, for omni mics this distance is around “no farther from the talker than 30% of Dc, e.g. if Dc is 10 feet, an omnidirectional microphone should be placed no more than 3 feet away.” This can be limiting for recording larger instruments, groups, or capturing room ambience. Proximity effect can also become an issue when positioning omnis really close. So optimal placement requires balancing distance to prevent unwanted room reverberation with getting close enough to the source. This can be challenging compared to directional mics where off-axis rejection narrows placement options.

Require Careful Mixing

One disadvantage of omnidirectional microphones is that they can require more careful gain staging and mixing to achieve isolation compared to directional microphones. Because of an omni mic’s ambient bleed pick up, additional microphones or ambient noise can easily spill into the omnidirectional mic. This can make it more difficult to achieve a clean, isolated recording.

To compensate, sound engineers need to be very meticulous with gain staging when using omni mics. Keeping levels low enough to avoid bleed while still capturing the desired source requires precision. The omni mic signal also generally requires more processing like equalization and compression to shape the tone and dynamics. In a multi-mic recording session, omni mics pose a tougher mixing challenge to seamlessly blend with other mics. Their ambient sensitivity necessitates great care to prevent a muddy, unfocused sound.

According to this Sound on Sound article, “Omnis need a bit more care to use well, but that effort is rewarded with more natural, open and airy recordings.” Their exceptional realism comes at the cost of additional mixing effort. Omnis capture more “room” than directional mics, demanding meticulous gain staging and processing.

Generally Not Preferred

Due to the disadvantages above, omnidirectional mics are generally not the first choice for either live sound or studio recording applications. Compared to directional mics like cardioids or hypercardioids, omnis pick up more ambient noise which can muddy the sound (1). They are also prone to feedback in live settings, offer less gain before feedback, and lack the directionality benefits of mics that focus on a specific sound source (2). For recording music or speech, their sensitivity to noise from all directions makes them a less ideal choice than cardioids or shotgun mics.

In the studio, omnis can be difficult to position optimally and require very careful mixing to isolate the desired source while minimizing bleed. Their lack of directionality means they cannot be easily aimed at a particular performer or instrument (3). Live sound engineers typically prefer the greater directional control of cardioids for capturing individual voices and instruments on stage. Omnis tend to get overwhelmed by stage volume and room acoustics.

While omnis can provide a natural, accurate reproduction of ambient sound, their key disadvantages mean they are generally not the first choice mic for most live or studio applications. More directional options like cardioids usually deliver superior isolation, gain before feedback, and control over the sound source.

(1) https://www.reddit.com/r/audioengineering/comments/15y3w0n/is_there_a_correct_or_preferred_orientation_for/

(2) https://www.sweetwater.com/insync/omnidirectional-microphones/

(3) https://ledgernote.com/columns/studio-recording/choosing-the-right-microphone/