Principles of Microphone Placement.

What is the main goal for each sound producer? First of all – to achieve natural sound. This means that when you are listening to the recording the sense of the presence in the room should appear.

Nowadays this problem is not solved for a hundred percent, because you can’t neglect the timbre and locational distortions, which are inevitable with the existing play systems. In addition, when you are listening to the recording there is no visuals, which allows you to ignore some of the shortcomings of “live” performance at the concert. For example, poor acoustics of the hall or the wrong balance of the ensemble: if we see the artist, it is easy for us to localize and distinguish particular instrument among others. During the recording, to achieve the same intelligibility and transparency, it is necessary to reach quality of sound that will be much more prominent. Modern recording techniques allow us achieve this.

By changing the distance between performer and microphone, sound-engineer has the ability to control the acoustic balance, i.e. locational characteristics of the signal. The most significant picture of the room where the recording take place, is where the presence in the recorded signal is both direct and reflected. For example, the record produced with microphone placed far from source, gets hollow, hardly seen. The information of the distance to the performers at the same time is unclear, and is only defined as “far.” A recording made only by closely placed microphone, also deprives the listener ability to determine its “location” in the hall. The sound in this case is close, instruments rush forward from the base line between the speakers, the ensemble becomes fragmented, as there is no unifying hall atmosphere.

It is important to note that the balance must be created by acoustic sound engineer, based on the style of music being recorded. No particular recipes will help you. Just taste and experience of engineer can tell him the correct sound plan, the width of the stereo panorama, unity, or, conversely, the spatial diversity of the ensemble of components. In the art of music there are too many factors that determine its impact that could come from any one as a determinant.

Theoretically it is possible to find a point in the room that will provide the best sound, and install there a microphone for recording. There are some enthusiasts who believe that only by using a microphone or a stereo pair, you can achieve the most natural sound. Indeed, the record made by one microphone (the so-called ONE POINT system), distinguishes the naturalness, vivacity sound. It has many features of binaural recording when you are listening through headphones.

However, in order to implement such recording, it is necessary to consider too many, often unrealistic stipulations.

Firstly, top-class orchestra playing on high-quality instruments and tools of one group should have a similar tone, wind instruments of one firm preferably. The performance of the orchestra should have good balance in all the nuances. Secondly, you need a room with outstanding acoustic qualities, suitable, among other things, to a particular genre. You will also need a scene where the band should accommodate in the form of amphitheater. And, of course, the microphone must have the highest quality characteristics, particularly, the right correction for diffuse field.

Therefore, to obtain natural sounding sound engineers typically use a special technique. For example, microphones are often located in more than one point but in two or three zones considering a sound character of the room.

There are three types of zones:

  1. An area dominated by direct signals;
  2. The zone where the acoustic ratio of reflected and direct signals is equal to one
  3. The zone of diffuse fields located behind buzz radius (remember that “buzz radius” is the distance from the sound source to the point where the reflected signal level directly related to the level equal to one).

According to this, we will consider three microphones location area.

The first zone – the zone near placed microphone. This type of microphone is sometimes called an individual or “backlighted”. Nearly placed microphone is used for bright, beautiful tone of a particular instrument or group, and for correction of the balance in ensemble. Signal from this microphone can also be individually processed – served on the reverb, equalization, delay, etc. For near placed microphone ones with cardioid characteristic or a high gain is usually chosen in order to obtain only the desired signal, without unwanted sounds.

This is followed by a general mic zone – at a distance of buzz radius. The microphone placed in this area, will give the most accurate picture of the sound. As a general the coincidence stereo compatible type is commonly used. XY or MS type, to allocate each instrument in sound base. Stereo-system AB is sometimes used as a general mic, but requires close attention, because the great danger of a “hole” in the middle, or an acoustic anti-phase with poor localization. A very decent results are made by using the “Three AB» system, i.e. AB system with a central microphone.

The microphone is placed in a diffuse-field is called distant. Far microphone must receive only reverb signals. Its signal gives an idea of the size of the room and its acoustic qualities. As a further microphone AB stereo pair with a broad base is used primarily, because signals the diffuse field must be non-localizable and volumetric, filling the entire base.

Mixing signals of microphones different areas, sound-engineer reaches the desired sound characteristics. On practice, the use of all three microphones installation is not always necessary. For example, if the concert is being recorded, the use of long-range microphone does not make sense, because together with the diffuse field, we get a close-up of all the noise made the public. The same situation is if noises reaches into the room from the outside. And while recording organ near microphones will be in a general middle place.

In an extreme case, you can only use near places microphones, creating reverbed signals with the help of artificial reverberation. This dramatically reduces the influence of the acoustic characteristics of the room on the sound quality and it is possible to make beautiful, spectacular record in unfavorable areas, although in this case and broken sound fusion may appear.

Let’s have a look on the features of the use of microphones in these areas in more detail, starting with the far-field microphone.

Far Microphone (“Air Microphone, Distant Microphone”)

The diffuse field theory implies that the level of the sound field in an ideal chamber is the same throughout the entire space. However, in the real room the sound field is not the same over the frequency and time characteristics. It makes a sound engineer looking for a place for far-microphone setup thoroughly.

Since the differences of intensity in good diffuse field are mild, the stereo microphone used as a long, gives almost mono signal. Taking into account the principle of “an echo is wider then sound source” (in fact reflections come to the listener in the audience from all sides), it makes sense to use the AB stereo system with a large base for ‘air’, having a bad location, but good for highlighting echo. You have to choose the distance between the microphones in a stereo system, try to avoid the obvious incompatibility. Typically, this distance is chosen is more than one and a half meters. However, loss of some bass notes in any arrangement is inevitable on the signals with the emergence of standing waves (for example, organ). For reducing the loss, you can use (“Three AB” system) installing Mono in the center, or stereo, which in this case gives mono too, but, of course, more mild.

AB-signal compatibility distant pair is easy to check with the ruler panel phase switch. Placing these signals of panoramic control at one point, you can check how antiphase switch of one channel is changing signal tone. Typically, room with good diffusive field and the distance between microphones that are longer than half meters the difference between the in-phase and antiphase combining is almost invisible, as in the diffuse sound field consists of a plurality of signals with a steady allocation of phase between 0 and 180 degrees.

In the large halls you must take into account the distance to the sound source, not to create a signal delay, leading to its repetition. This distance is especially crucial when recording instruments with percussive sound character (piano, percussion), or with fine notes of music. Based on the values commonly referred to the 50ms, after which the signal delay is perceived as an echo, it can be assumed that the distance between the near and distant microphones should not be more than 17 meters.

Deutsche Grammophone has been used 4D digital recording system for several years to avoid such a “fragmentation” of sound by introducing a time delay in the signals of all the microphones, “equating” them to the most far located. In digital consoles, this procedure is very simple. As a result, a significantly increased sound clarity.

In the small halls it is difficult to get a sufficient number of echoes – the sound always has large proportion of the direct signal. Furthermore, the time delay of the reflected signals is often small.

There is a way to install the microphones, which increases first rate and another: put the microphone, which is turned to the back of the performers at some distance from the back wall. If you are unable to install a microphone in the hall (the presence of the public, insufficient length of cables) you can try to install microphones in the same way (with the back side to the performers) at the edges of the scene, sending them to the reflecting wall.

You can also apply the latency of closely placed microphones, observing, however, considering that their presence in the direct sound signal can lead to a repetition of the signal with less time than in the natural run of the waves in a large room. Using this method, you need to use a microphone with a cardioid polar pattern, otherwise you can get a direct signal, distorted by the back side of the microphone frequency response.

When placing “air ” microphones you should consider a low sound pressure level at the place of installation, otherwise, you can get a lot of electro-acoustic noise due to interference on the long lines.

Almost every microphone is suitable for being used as “distant”. The main requirement to them is high sensitivity, since they operate in weak sound fields. Polar pattern of microphones plays a lesser role. However, it is better to use an omnidirectional microphone, because the transmission ratio of sound field is higher. The best recommendation in this case – Microphones DPA 4003, which have a sensitivity 40 mV/Pa. As for the frequency response, fading of high frequency, which is common with distances greater than 15 m, the microphone does not need to transfer of the highest frequencies. Moreover, if you have to use the ” air mic” near the source, it is good decision to fade high frequency (for example, on the remote), so you can simulate its remoteness. Microphones with correction for diffuse field (for example, a Neumann 130), or certain types of PZM-microphones should be located far from the artist, otherwise you may get a nasty hard sound unnaturally underlining sizzling. If this effect does not occur, the PZM-microphones as the distant sound great, artificially extending the space of the room. However, we must bear in mind that some of their models have bad sensitivity.

When recording in small rooms “Air Microphone” should be treated with caution, not focusing its sound in the mix. Better decision is to do is not very spectacular, but “clean” recording than trying to add “air” to receive a distorted frequency response due to the audible resonance of the hall and addition of signals from nearby microphones when they receive direct sound.

General Microphone

Setting of general microphone should provide a harmonious blend of music and acoustic balance. This means that, firstly, the total sound microphone must reflect all instruments equally, providing preservation of musical balance, which the hall really has. Secondly, in signal of general microphone ensemble instruments should be present in roughly equal measure both direct and reflected sounds. That is why the microphone is always installed in an area of buzz radius, at an equal distance from all the instruments. This is essentially the same one-point microphone, which can be the one for recording of balanced ensemble.

The requirement of equal distance to all the musicians of the ensemble insists on having a microphone placed quite high above the stage. Otherwise, instrument plan, located in the back of the stage, would be too remote, and the orchestra will be extended unnecessarily deep. For the same reasons it is useful to arrange an ensemble or orchestra in a semicircle to the edges, too, so the corners would not sound distant. To some extent the excessive remoteness of woodwind can be compensated by directing sensitive axis of general microphone on them, not on the string instruments, as is commonly done in order to get more vivid tone of violins.

It should be noted that a general microphone should also provide the lowest stereo deformation of front side. The fundamental mistake in this case – a “hole” in the middle when installing a stereo pair too close to the musicians.

It necessary to apply the best models that a sound engineer has as a general microphone.

Traditionally, the combined stereo microphone system is used.

Only a few firms in the world produces such microphones. First of all, it is necessary to name a company Neumann – one of the leading manufacturers of microphones for classical recording. Neumann produces stereo microphones SM 69, the RSM 191 with electronic adjustment of the angle between the orientation axes, and two models of the microphone “Dummy Head”: KU81i and KU100.

SM 69 has a bright, rich sound, the most spectacular in the near field. When you install it far enough away from the sound source microphone becomes more sluggish, “quilt” sound. The microphone has a smooth changeover of characteristics from the ‘circle’ to ‘eight’. The most commonly used polar pattern is Hypercardioid or Cardioid. Once while recording organ in a large church, the author used the wide cardioid (“half circle”).

Lovely “Dummy Head” KU81i has all the advantages and disadvantages of this system of microphones: the soft, noble sound, but rather a rough localization and a tendency of the “hole” appearance in the middle when placing the microphone close to the performers.

Widely publicized microphone SASS (Crown company), consisting of two PZM-microphones separated by a soft barrier (like “artificial head”) conveys spaciousness and localization of sound sources very well, but has a typical PZM-microphones of this firm hardness and sharpness of the sound. This makes it suitable for use only as a general, and not for all builds. Due low sensitivity (near 6mV/Pa), this mic is dangerous to use as general. The company has also released a Crown SASS-microphone DPA heads.

The high rating among similar products has a stereo microphone Beyerdynamic MC 742.

A Sony stereo microphone ECM-MS 5 and ECM-999R have pretty sharp, aggressive sound, which, however, allows you to achieve good transparency, producing a record in the hall, sitting away from the stage. These microphones are designed for MS system, they have hypercardioid directional characteristic and electronic adjustment of the opening angle.

The company Schoeps produces stereo sparks MSTC 6 cardioid capsule for MK 4 system ORTF, MS-microphone, consisting of ‘eight’ MK8 and head with a ‘circular’ or cardioid, and Spark UMSC for mounting a variety of heads, working in pair for XY and MS systems.

The company Shure produces a stereo microphone MS-VP 88 (cardioid and figure eight) having output immutable XY.

Excellent products are microphones by Calrec Soundfield Microphone (SMS series). It is four capsuled stereo microphones with an electronic sensor that generates output signals with simulated XY of all corners (0… 360 °) and any directional characteristics. One of the models can even simulate the microphone turn left and right and tilt up and down. The microphone has an excellent transparency and clearly conveys the details, but can only be used as a general in a hall with good diffuse field. In the near field, he is too harsh tone.

Microphones set Systems AB and “Three AB” can be used as general. Traditional AB System in this case must be used with great caution – it can give a very approximate localization of sound sources, especially at a large distance between microphones. Furthermore, different frequency-domain signal may have their phase ratio with the stereo, which creates discomfort to the listener, a different localization at different notes when the instrument on the passage “moving”. The most dangerous distance between microphones in this case is from 30 cm to one and a half meters.

This same distance is bad because area basic tones of musical instruments frequencies are falling in antiphase. It is clear that the implementation of AB quality system requires no frequency and phase differences. The best recommendation in case of the AB system is that can be set with two specially selected microphones DPA 4003. In addition to the beautiful tone and a neutral frequency response, these microphones have a high sensitivity 50 mV/Pa. That allows you to use line inputs on remote on low noise and low distortion.

Undeservedly rarely used system of “three AB” allows setting the microphones with a wide base, and it combines with the bulk transfer of diffuse-field. It should be used when recording large ensembles – choirs and orchestras as the sound system fill the base very well. Not to make it narrow, it is necessary to be careful when mixing medium microphone. However, stereo may be used as secondary microphone.

Good results can provide a stereo pair of cardioid microphones of ORTF system – it is located at a distance of 17 cm, two cardioid microphones directed at an angle of 110 ° to each other. When using a stereo pair close to the sound source, you may experience a “hole” in the middle of the base.

A combination of microphones, which is called the “Three XY’ are used successfully. It is ORTF system, but with the addition of a third microphone. When used as “near” this combination of microphones gives you more freedom to control the stereo image. The same combination can be placed inside the ensemble, while the central microphone may be directed in a direction opposite the other two. Once here was a very good experience of brass quintet recording with a microphone “three XY”, when the musicians were placed around it.

Near Microphone

While setting a microphone to record sound near the artist every sound engineer must remember that all acoustic instruments have historically been designed to sound in large halls and have “reserved” quality. This means is the sound of the violin, which the audience hears, sitting in parterre, is completely different from the one which hears a violinist himself whose violin sounds right under his ear. If you set a microphone right there, the sound will be too harsh, rude, not in the flight, with poor transitions from note to note. These defects disappear as early as a few meters from the instrument. That is why you should use a short-range microphone carefully, not to spoil the timbre of the instrument.

Location of the near the microphone depends on the instrument itself (its direction, the radiation power, design features). Also the microphone installation specifications affect the quality of the acoustics of the hall.

Volatility of acoustic parameters of rooms and equipment of musicians each time makes a sound engineer, starting to record even in a familiar room, to perform the installation of microphones, especially near ones, to the ear. Various types of notes for memory, made on the last record, are good only to keep the overall plan of record. This helps you quickly restore the main features of sound, especially after a long break in the record. It is impossible to achieve a truly great result with the old balance and mixing. But we should not deny the benefit of fixing the position of the microphones and faders. To do this, use different techniques: drawing up detailed maps showing the microphone location and height, fill the faders and other controls on the remote, record the parameters used in signal processing devices (type and reverb modes, etc.). Some sound engineers even photograph remote and studio. These recommendations, of course, does not refer to the consoles that have a memory retaining state regulators.

It is good to know that when you are mixing a multi-track recording and the source signals are identical, the repeatable of mixing sound result will be very high.

There are some recommendations of aural installation.

The simplest of them – resetting microphones by sound engineer with the subsequent control of the result in the hardware by himself. The process is quite time consuming, especially if the distance to the tone room is big. In addition, the continuous “running” of a sound engineer from hardware to match the room can irritate the artist. In this case use the services of an assistant, interchanging the microphone by the instructions of the hardware engineer. It is only necessary to provide a direct link connection with headphones and mic to avoid using speaker system.

The following method is described by A. Nisbett. Install two microphones in places where, according to the engineer should be good sound. The signals of these microphones are compared, and less good sounding microphone is transferred to a new location. If at this point the microphone will sound better than the first, the latter is transferred to the third point. So you can very quickly and objectively find the optimal position of the microphone setup.

There is also a way to install more than required number of microphones in order to calmly choose the best. So, you can immediately install a few general microphones. It is possible to apply different systems to change the angles for combined microphone base for the system AB, find the optimum height and distance to the artist. In addition, a variability of microphones somehow enhances the prestige engineer in the eyes of the artist

Finally, there is elegant way to find the optimal point using headphones – the sound engineer sets the microphone, wearing well-insulated headphones. They are receiving signal from rearranged microphone (in the same way as is done with overdubbing) and soundman immediately evaluates the result of new setting. With some experience, you will be able to use this method quite effectively.

In the case than there are several near-field microphones, each of them must make sound primarily from the source for which it is intended. The difference between the levels of the wanted and unwanted signals must be at least 6 dB, to be able to operate independently and the signals enough to avoid acoustic opposition.

Typically, writing chamber music, stereo or two closely spaced microphones on the AB system are used as near ones. The orchestral recordings almost always use Mono. However, there are sound engineers, applying stereo microphones recording orchestra. At the same time, they claim that the Mono sound creates the “effect of the pipe.” in stereo image. Obviously, it is reasonable, especially if we consider the stereo image as the sum of the near microphones signals. Then, indeed, you may have a so-called “multipoint mono”, which is widely used on the stage.

In addition, signals of different microphones should form the apparent source of the sound as accurately as possible in one point of the base, otherwise it will be localized in several directions at different frequencies. It is not usually recommended to use stereo pair microphones (the AB system) as near ones.

Microphone signals of different areas should have more or less the same frequency response. This must be achieved by setting the proportional distance between the sound source and near, general and distant microphones. Otherwise, while mixing a set with very close near microphone and the distant general, each frequency components will be different. This applies mainly to the high-frequency components, which have only near microphone signal.

This defect is particularly notable in the echoing halls with good reflective surfaces and a long reverberation (such as churches). In this case, the sound wave is reflected many times, forming a stable diffuse field without high frequency. In large halls with absorbing walls large reverberation time is formed by the time of the sound wave path between the reflective surfaces. But the amount of signal reflection itself is small, and the frequency response of reverb is more steady. So that means that the more resounding room is, the smaller the difference in distances from the sound source to the near and distant microphones should be. Because it should not cause cleavage of the signal frequency.

In addition, signals of different microphones should form the apparent source of the sound at one point of base, otherwise it will be localized in several directions at different frequencies.

Giving the exact recipe on the share in the total balance of the microphone signal for each space zone, of course, is impossible. But for the first estimations, when setting up the remote before the upcoming broadcast without rehearsal and in other extreme situations, it can be assumed that the signal level of near and distant microphones should be the same.