Microphone pickup patterns(Letto 50 volte)

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By the pickup field of a source (e.g. an acoustic guitar), we mean the degree of proximity of a microphone to the source, also referred to as “presence”.

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Microphone pickup patterns

To define a microphone pickup field, various factors must be considered, such as:

• the side of the source from which the shot is taken (front, side, rear)
• the width of the instrument's sound front (small for a woodwind instrument, medium for a flute or guitar, large for drums or a grand piano, very large for a pipe organ, a large choir or a symphony orchestra)
• the distance of the shooting point from the source
• the directional characteristics of the microphone, i.e. its polar pattern, which enable it to “cover” a more or less wide angle of recording

That said, let's begin by defining what, in sound engineering practice, are the six main recording fields, which can in turn be organised into two distinct categories:

Main fields

1. internal field
2. proximity field (or presence field)
3. resonant field (or body field, or field of corporeality, or quality field)

Secondary fields

4. long field
5. field set
6. reverberation field

Main fields

Inner field

It will not always be possible to use the Internal Field because it assumes that the microphone is inserted inside the instrument.

The resulting sound pressure will be extremely high, as will the isolation from other sources, but the quality of the recording will never be excellent because the natural sound will be severely compromised by various very strong dominant resonances due to the microphone's extreme proximity to some of the sound elements emitted by the instrument compared to others.

Some components of sound will tend to dominate others, so it will be easier to use this technique with fixed-pitch percussion instruments, such as drums, provided that you work carefully with the equaliser during the tuning phase to restore an optimal balance between the aforementioned elements that make up the totality of the sound, sometimes achieving very good results even though they are quite different from the sound experienced when listening to the instrument acoustically.

As is well known, the internal field pickup is usually used as standard for rock-pop drum kits, inserting the microphone through the hole in the back skin.

The same filming technique, which was very popular in the 1970s and 1980s, can be used with certain types of tom-toms that do not have a bottom skin.

Some less common models of snare drums and tom-toms have a hole in the shell, into which a small microphone can be inserted, positioned halfway between the two opposing drumheads.

As an alternative to the above, it is worth noting that there are mechanisms available that can be mounted inside the drums of all drum models; these allow any type of microphone to be fixed internally (for example, consider the special kits from Randall May International), achieving a highly practical result that not everyone appreciates in terms of sound quality.

Of course, it is also possible to insert a microphone inside the guitar sound hole or inside the lid (even when closed) of a grand or upright piano; despite careful use of the equaliser, which does improve the sound, it is almost impossible to achieve excellent results, although it is possible to optimise the result by using special microphones designed specifically for this extreme use.

Beyond the controversial opinions, below is an objective summary of the pros and cons of using this filming technique:

Pro

• significant reduction in sensitivity to’microphone trigger (Larsen) caused by the return of the PA and/or stage monitors during amplified concerts, which makes it particularly favoured in live performances, as does the following characteristic
• extreme isolation of the microphone from other very close sources (for example, other parts of a drum kit, including cymbals),
• almost total isolation from environmental influences such as noise and room reverberation
• Very high sound pressure level, capable of optimising the signal-to-noise ratio to the maximum extent during amplification (live) or recording.

Against

• distortion of the tonal spectrum and, to some extent, of the dynamic balance of the sound; these problems are easier to resolve on instruments with constant pitch, such as percussion instruments, while they are more difficult to resolve on instruments with variable pitch; the technique must be combined with highly sophisticated equalisation to restore a sufficient level of tonal linearity to the instrument
• risk of mechanical distortion of the microphone diaphragm (or, worse, its breakage) when using microphones with low mechanical resistance; for example, a microphone positioned inside a snare drum could pick up sound pressures exceeding 150 dB
• risk of electroacoustic signal distortion and/or digital clipping if there is no PAD attenuation filter on the GAIN of the preamplifier or mixer

Proximity field

The Proximity Field and the Field Resonance, which we will consider shortly, are both, and rightly so, the most widely used in most applications, both live and in the studio.

Unlike the internal field, the proximity field is implemented with an external shot very close to the instrument, called close-mic (close-miking).

It is directed towards a point which, while not managing to evenly embrace the entire resonant body of the instrument, allows the latter to express “the substantial part” of its timbral content, without introducing too many acoustic defects.

Every sound source has its own ideal shooting point in order to achieve the best performance, and it will be the sound engineer's job to “find” it, positioning the microphone (or stereo microphone set) appropriately and with extreme care, knowing full well that at such a close recording distance, the slightest movement of the microphones or the performer will result in significant changes in tone and volume.

Below is a summary of the pros and cons of using this filming technique:

Pro

• great incisiveness and clarity of attack transients
• good insulation from environmental disturbances and room reverberation
• good sound pressure, with moderate risk of mechanical distortion and electroacoustic signal distortion in the case of highly dynamic sources
• good resistance to ignition (in live performances)
• good compromise between quality (in terms of sound completeness) and presence, with a particular emphasis on the latter

Against

• excessive incidence of noise from the mechanics and resonances compared to the body of the sound
• relative harmonic poverty in the timbral texture of the sound tail (which is the support of the sound audible after the transient attack)
• risk of distortion of the microphone diaphragm or preamp input when recording highly dynamic sources
• significant changes in tone and volume even in the event of slight movements of the sound sources away from the microphones during performances

Resonance Field

A proper recovery of the Resonance Field (also known as body field o field of fullness, or even quality field) is achieved when the microphone's angle of capture manages to cover the entire resonant body of the instrument fairly evenly, extending slightly beyond it.

This is achieved simply by moving the microphone away from the source as far as necessary.

This technique is widely used in recording studios as it generally allows for the best tonal response of the source compared to acoustic listening at a “close” distance. However, it requires good acoustic treatment of the recording room due to the “significant” level of penetration of acoustic reflections from the walls of the room.

This technique is often not recommended for amplifying sources during live concerts, especially in the case of many nearby sources occurring simultaneously, or high amplification volume of the PA or monitors, while it is very effective for recordings intended solely for recording, even during a live event.

The following are the pros and cons of this filming technique:

Pro

• excellent sound fidelity in relation to the original acoustic
• great tonal richness and balance
• good attack transients
• still a good compromise between presence and quality, with the emphasis on the latter
• low risk of mechanical and electroacoustic distortion during recording
• good tolerance in the event of slight movement of the sound sources relative to the microphone
• lower incidence of mechanical noise occurring during sound production

Against

• high chance of triggering (in live performances)
• excessive reverberation in rooms with poor acoustic treatment
• excessive incidence of any unwanted ambient noise in poorly insulated environments

N.B.

The fields described above are the most commonly used as a basis for live and studio acoustic recordings.

Environment fields

Long field

Compared to the resonant field, the Long Field This is achieved by moving the microphone further away from the source but keeping it perfectly aligned so as not to lose focus on it, while including a significantly higher percentage of reverberation in the recording, which, however, should not predominate but at most equal that returned by the environmental reflections (therefore, the length of the field should be commensurate with the resonance characteristics of the room).

In all cases of music production, it is essential that the recording environment is suitably treated to provide a pleasant and balanced sound, as free as possible from resonances at particular frequency bands (typically standing resonances and modal nodes of frequency cancellation).

The long field allows for very authentic recordings, similar to those heard by the ear at an acoustic concert, and is therefore suitable for recordings (typically in stereo) of large orchestras and/or choirs, or for recordings that recreate the atmosphere of an acoustic concert in a theatre (sometimes used in combination with other closer recording techniques, in sections).

This technique is often used for audio recordings intended for video production, when the aim is to obtain a sound that corresponds to the same field represented by the images.

With this technique, the sound and musical content of the performances still retain good definition, even though they are represented with less “presence”.

The long field can also be used in recording studios, in combination with the proximity field, to adjust the presence, body and ambience of the sound during mixing.

Ambient field and reverberation field

These two techniques, on their own, are used for special effects or even in videography, to suggest the idea of an “off-screen” sound or even an “external field”.

They are sometimes used in the studio for secondary roles, in combination with the proximity field or resonance field, in order to be able to adjust the desired amount of room reverb during mixing.

For Field Set This refers to recording from a point very far from the source, where the reflected sound clearly predominates in intensity compared to the direct sound.

For an evocative and “phased” recording of the environment, you can use the Blumlein stereo panoramic recording technique, using two bidirectional microphones with an “eight” polar pattern (which picks up sound in front of and behind the microphone but not from the two sides).

To accentuate the reflected sound compared to the direct sound, instead of the Blumlein technique, you can use the XY technique, pointing the recording axis towards the ceiling or towards a wall opposite the source.

The Reverberation Field, on the other hand, is achieved by ensuring that the stereo recording (typically made using the Blumlein technique) does not contain direct sound, but only reverberation; To achieve this, the stereo set microphones can be set up exactly as for the ambient field, but with a large acoustic screen placed between the source and the microphones (but not too close to the latter), so as to cancel out the direct sound from the source and the first reflections, thus recording only the sound of the ambient reverberation.

Mono, stereo and mixed recordings

With the exception of the internal field, all other fields of view can be captured in mono or stereo.

For stereo proximity recording and stereo resonance recording, it will be possible to use the XY technique, with the capsules either stacked or side-by-side (depending on the situation), but always offset from each other at an angle typically between 60° and 90° and arranged so that the intermediate axis of the two microphones is directed towards the source (see the figure below for stereo recording, visible on the right).

Obviously, this will allow the microphones to be placed even closer to the source due to the increased total pickup angle.

This solution is particularly interesting for the proximity field, as the dual microphones required by the stereo set will greatly reduce the acoustic distortions caused by extreme proximity.  from the source to the microphones.

In this way, thanks to the XY stereo set, the proximity field will be able to capture the entire resonance field of the instrument, while maintaining the extreme presence, brilliance and incisiveness of the transients typical of the proximity field.

In certain cases, you may decide to record the source in mono or stereo using a close-up recording and, in addition, with a second stereo set in the resonance field or long field, and then balance the two sources during mixing.

Time lag in multi-field shooting

It should be noted that, when using multiple microphones directed at the same source but positioned at different distances from it, the recording times will vary, creating tracks that are more or less out of phase in the mix, with the following result:

• attack transients that do not coincide perfectly
• changes in the tonal spectrum due to the onset of comb filtering

The issue will be of little importance when the intention is only to add a touch of ambient sound to the foreground sound, keeping the proportional acoustic volumes of the recordings unchanged (volumes which, due to the different distances from the two microphones, will be very high for the foreground recording and very moderate for the ambient recording); in this case, the shift may produce very pleasant effects, and the phase shift may be acceptable due to the lower volume of the more distant recordings compared to the closer ones.

Example 1

Recording of a guitar performed in the proximity field at a distance of 20 cm and, simultaneously, a recording of the same guitar, in axis, in the resonance field at a distance of 85 cm. At these distances, the delay in resonance recovery will be approximately 2 ms. As we know, sound pressure intensity decreases quadratically as distance doubles, so at 85 cm it will be approximately 1/16 (i.e. a natural reduction in direct sound of approximately 12 dB, partially attenuated by the reflected sound, which will have a similar volume in both microphones). A “natural” recording and mix would respect these proportions, making the time lag completely negligible.

Example 2

The opposite is true if you want the two tracks to be played at identical or very similar volumes, which can be achieved by appropriately boosting the profit of the more distant recordings (in this case equal to approximately 8-12 db); the aim in this case would be to recreate a “whole” between the two recordings, so that the volumes of the two fields can be adjusted as desired until the preferred effect is achieved. In this case, therefore, it would be advisable for the attack transients of the tracks corresponding to the two fields to be positioned in perfect synchronisation.

The alignment of attack transients can be achieved by appropriately delaying the recordings of the closest microphones using a delay plugin that can be installed on the track, after performing a precise calculation of the delay; to this end, Consider that a distance of 33-34 cm (approximately) causes a delay of about 1 ms (1 millisecond) and that each ms contains 44.1, 48 or 96 samples (respectively in digital sessions with sampling frequencies of 44.1, 48 and 96 kHz), so with this data you can easily calculate the correct compensation, adjusting the plugin delay accordingly (which is usually expressed in samples and/or ms).

For more reliable control of perfect alignment, you could perform a test recording lasting a few seconds, then check the timeline to verify the perfect alignment of the transients of the two tracks. For better monitoring of the resulting sound during recording, it would be preferable to perform the alignment before recording, but in any case, it will always be possible to refine this alignment afterwards by delaying the near-field track until it perfectly matches the other. At this point, you can freely decide on any ratio between the two fields, focusing solely on the desired sound type, without encountering serious phasing problems and ensuring perfect transient focus.

It goes without saying that the delay can be set to be exactly synchronised or kept slightly ’loose“ if this produces a more pleasant sound or is more functional in the context. To make the best choice, you should first listen with both fields set to the same perceptible volume.

The directivity of microphones in recording fields

The wider the angle of coverage of a microphone, the shorter the distance required from the sound sources to obtain the coverage fields described above.

Example

For an acoustic guitar, position a supercardioid microphone (with a useful pickup angle of approximately 70°-90°) at the right distance to obtain a correct pickup in the resonance field; by replacing the microphone with another cardioid type with a pickup angle of 90°-110°, you can get closer to the source while maintaining a similar pickup field; Similarly, using a semi-panoramic (180°) or panoramic (360°) microphone will allow you to get even closer.

With a panoramic microphone, you will obtain a slightly different effect due to the greater incidence of environmental reflections picked up by the capsule.

It will therefore be evident that, for the same desired field of recording, as the directivity angle of the microphone increases, it will be necessary to move the microphone closer to the source. Proximity will reduce the proportional incidence of ambient reverberation, also optimising the signal-to-noise ratio (important in the case of very weak sound sources). 

However, extreme proximity will emphasise the sound components of the instrument emitted directly in front of the microphone more than those further away, reducing the linearity and therefore the fidelity of the natural recording. This can be partially restored by using a semi-omnidirectional or omnidirectional microphone (which is practically free from proximity effect) or by moving the cardioid microphone further away from the source.

Choosing the recording range according to the role in the mix

A good rule of thumb is that each microphone recording should include the recording of multiple fields in different tracks, so that they can be appropriately selected or balanced in the subsequent mixing phase.

For example, if you record an acoustic guitar using both close miking and resonance miking, after precisely aligning the attack transients, it will be easy to adjust the balance between body and presence, as well as between softness and attack, simply by adjusting the volume of the two audio tracks.

In this way, the judicious use of multi-field recovery could result in balanced, rich colours that may already be satisfactory during the mixing phase, without the need for extensive use of the equaliser and artificial reverbs.

This recording method will be particularly relevant for solo sources; it will also be relevant in the case of sparse arrangements, while it will be less important with denser ones (where it will often be preferable to avoid using multi-field tracks in order to reduce the tonal thickness of the sources, rather than increase it).

In situations where a source is not predominant, it is often preferable to record using only the proximity field; in this way, the sound will be poorer and less resonant but much drier, “smaller” and more defined, and therefore more suitable for fitting into a mix consisting of numerous sources (such a sound will in fact have a reduced capacity for tonal masking; in cases of doubt, you can decide to record both main fields and then decide what to do with them).

Using environment fields

We considered how the use of ambient fields, used on their own, is of secondary usefulness, as they are limited to videographic uses, to accompany the fields already represented visually in a manner consistent with the sound element.

The long field is used when you want to acoustically represent a source that is not very present, with a field that is somewhat distant, moderately ambient, in a live acoustic theatre setting.

Alternatively, the ambient field and reverberation field can be easily simulated through the appropriate use of digital ambient processors.

As a possible alternative to the resonance field, it may be interesting to combine a long-field or ambient recording with a close-up recording, to be adjusted during mixing.

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