Omnidirectional microphones are (and always have been) excellent for music recording, and are the mainstay of most classical recordings for a variety of good technical reasons. They pick up sound equally from all directions and this makes them challenging to use in live situations where directional mics are generally preferable. However, with directionality necessarily comes colouration and in designing a microphone to be directional it must cancel sound from behind and the sides in the case of a shotgun mic. This involves the use of filters and/or enclosures which will always alter the sound captured to a degree from the original (particularly at the high frequencies) and it is this change we term colouration. In a studio setting with a degree of acoustic treatment or live when unamplified (and feedback does not need to be contended with) omni-directional microphones provide a reference capture of the sound and careful use will produce fantastic results.
As the name suggests this technique involves placing the microphone(s) close to the sound source. This means that maximum sound energy is being converted to electrical energy at the mic, which has two main benefits:
- It ensures that the mic’s output is high compared to the electrical noise from the microphone
- It minimises the pick-up of reflected sound from the room’s walls and ceiling or other instruments
Use of a pop-shield is important when singing or speaking directly into an omni directional microphone to avoid problems associated with plosives (hard consonants) and sibilants (‘s’ or ‘sh’). When a person sings or speaks hard consonants like ‘p’ or ‘k’ a short, fast burst of air is created. If this rush of air contacts the diaphragm of a microphone directly the diaphragm can reach its maximum point of travel creating a popping sound which will spoil the recording. Similarly when a person pronounces an ‘s’ or ‘sh’ the hissing sound produces a very directional source of air flow which is undesirable if direct contact with the diaphragm is made.
A pop-shield breaks up this air flow without colouring the sound and protects the mic’s diaphragm. For particularly loud/expressive singers you may also need to use a foam windshield with a pop-shield for extra protection.
Stereo Miking with Omnis
Stereo miking provides a way to recreate the position of sounds on a recording. There are two basic mechanisms by which a stereo image can be created using two speakers:
- The sound can be reproduced louder from one speaker than the other.
- The sound can be reproduced at slightly different times from one speaker than the other. This time delay produces phase differences that the human ear can detect and use for location.
Narrow Spaced Omnis
A-B stereo or spaced pair miking uses two spaced omni-directional mics to capture a stereo image. With a small distance (15-60cm) between the two microphones the primary stereo effect captured is the difference in time of arrival at the two mics. (Using a wider spacing will increase the contribution of amplitude differences as one mic will be closer to some sound sources than the other).
The human ear interprets time differences between the two ears and uses them to locate low frequency sounds (25Hz – 700Hz). Reproducing these delays enables the listener to “capture the space” in a recording, and experience a stereo image. Above 700Hz the phase information becomes increasingly unreliable and the brain concentrates more on level differences and the transfer function of the pinna (the outer part of the ear) for positioning cues. For accurate phase capture the time delay must be smaller than the wavelength of the highest frequency you wish to reproduce phase information for, because phase shifts greater than 360 degrees will cause the frequency to flip from one side of the sound image to the other.
Therefore to maintain accurate phase up to 1kHz (above which the level information has a far greater precedence than phase), the maximum delay between the channels should be is 1.0ms and therefore an optimal spacing of 35cm.
There are two other factors however:
- The brain is used to interpreting phase differences based on a separation distance of a head’s width. If it identifies this separation the brain will engage an extra level of interpretation. Smaller distances comparable to a head width (15-20cm) are often used and are worth exploring, especially for binaural recording.
- At first sight time delayed stereo relies on the listener being positioned centrally between the two playback speakers, otherwise a small movement of the head will cause the phase differences to be drastically altered. This is true, but the human auditory system exhibits something called the precedence effect, which means that if the same sound signal arrives time delayed at a listener from different directions, only the direction of the first arriving sound signal is perceived. It is an important tool for filtering out echoes in everyday conversation. The precedence effect is most apparent on delays of around 14ms (5m), but a 1ms (35cm) delay will still produce approximately a 3dB perceived attenuation of the delayed signal and this helps to provide positioning cues to the brain even when the phase information is muddled.
Wide Spaced Omnis
When the mics are placed further apart, the time difference information is still captured but the frequency at which phase difference exceeds 360 degrees will lower. The stereo positioning in the recording will become increasingly effected by the proximity of the mics in relation to the sound sources. The further apart the mics are the more this comes into play. Note that level differences provide a much more solid image in stereo as the listener’s position between the speakers is not so critical. Also, the greater the spacing the greater the delay and the more precedence effect will come into play. A 2ms delay gives a 6dB precedence, which is a doubling of perceived loudness.
In general most engineers will choose a spacing of between one third and a half of the sound stage.
The reality is that the science of spaced omni recording is not fully understood. In any given set-up there will be a combination of proximity, precedence and phase effects which are difficult to anticipate. Placement is important: familiarise yourself with the information above, but above all experiment. The more techniques you try the greater a sense of intuition you’ll develop.
Generally the results are very impressive: spaced techniques allow the engineer to take advantage of the inherent quality of omnidirectional microphones in stereo recordings, particularly their extended and even low-frequency response, and their smooth off-axis pick up. The only problem to be aware of is the potential for comb-filtering effects when spaced microphones are combined in a mix, and the more the mics, the worse the effect is likely to be, although it is very hard to predict the audible results. This is not a problem so long as the mics are panned hard left and right, but attempting to mix the channels may cause problems. A good option for mono broadcast requirements is to use one channel from the recording or arrange both mics to be in the same position.
Binaural recording is a method of recording sound that uses two microphones, with the intent of capturing 3-D stereo positioning information. This effect is often created using a technique known as “dummy head recording”, which uses a mannequin head with a microphone in each ear. Binaural recording is intended for replay using headphones and will not translate properly over stereo speakers.
Binaural recordings are extremely realistic, capturing not only the attenuation effects of the head on high frequencies, but also the transfer function of the pinna. Unfortunately because replay is limited to headphones binaural recordings are of limited commercial use.
Recording with microphones will always capture a degree of the room’s response. Being omni-direction, our mics will pick-up more room than unidirectional mics. This combined with the fact that they have an extended bass response compared to most unidirectional mics means that the room’s acoustics are of particular importance.
Stereo recording techniques using omnis are best suited to anechoic/semi anechoic studio environments and rooms with a nice acoustic. If your recordings are sounding boomy on a particular note then there’s probably a room resonance at that frequency. The smaller the room the higher (and more intrusive) the resonant frequencies will be. Various acoustic treatment options are available from self-install kits with bass traps and acoustic panels to specialist firms who will test and treat a room for you. Or, there’s always the DIY option of covering the room in eggs boxes or hanging duvets on the walls!
Recording in a very echoic space (like a church) can produce results with an exaggerated room presence. This is because the ear is directional to high frequencies and therefore picks up less reflections from the rear than an omni will.