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The directivity of the microphones

La directivité des microphones

This article brings together the main information on the directivity of the microphones.

 

A microphone has two main characteristics : its technology and its directivity. The knowledge of these two parameters will allow you to correctly choose your microphone and the best use of them.

We will be talking about in this part of the three polar patterns the most used. The other will be addressed in a future article.

 

Introduction

Microphones are transducers. They transform a physical quantity into another. Namely, an acoustic pressure variation ΔP (unit: Pascal) change in voltage ΔU (unit: Volt)

 

Definition of the directivity

The directivity defines the area around the membrane within which the microphone is most sensitive. Outside of this area, the micro drop in performance. On the one hand, the rendering will be qualitatively less good (for example, a voice détimbrée) ; on the other hand, it will also be less “strong” : the acoustic pressure variation is picked will be very low. Logical, since the signal will be in a non-sensitive area.

 

Polar patterns commonly used

Omni-directional

The microphone picks up sound in an optimal way (frequency/intensity), irrespective of their direction. The capture is done on 360°. This directivity is mainly on the telephones (the position “free hand” without a kit is the great example), cameras and DSLRS (digital cameras) “entry” and “middle” of the range (it is not uncommon to hear a door slam behind him, even his own breath). The omni’s are used mainly for the atmosphere because they capture up to 360°. They therefore add to the safety deposit box in a scene. Nevertheless, they become difficult to use if your shooting is polluted sounds (boiler room, busy street, noisy neighbors, etc).

Particular case of this directionality

A microphone is subjected to the proximity effect which causes a sharp increase in the ” low ” when it is closest to the source. This is very annoying in multiple cases such as those of the female voice or the male voice, serious nature. A micro omni is not subject to the proximity effect. As close to the source, it attenuates ambient noise.

As seen on the polar diagram, the quality of the recording of the source does not change according to the directions.

Our ears are omni-directional. Therefore, it would not be logical to least hear a sound because it comes too “right” or ” from behind “. Nature makes things well…

Cardioid

The micro is experiencing a high attenuation at the rear. It is less sensitive to sounds coming from “behind his back” compared to those that come to him ” face “. This is represented by a box in the shape of a heart reversed.

As shown in tapered form toward the front, there is a mitigation as to the sides (-3dB at 65°, and -6dB at 90°).

As you can see also on the polar diagram, the attenuation of the cardioid pattern on the sides makes it difficult to use in the story. The reason is the détimbrage or sub-mixing a sound that can occur if one does not pay attention to its direction.

On the other hand, on a plateau where everything is controlled, cardioid is widely used (cinema, TV). It allows you to isolate a source compared to the ambient sounds and limit the impact of the reverb, giving priority to the direct field. It is the micro of the poor. If your budget is reduced, it is necessary for you to fold on this directionality, as you will have the dialogue correct. You will also have the option to capture ambient sounds bit dirty (compared to omni).

N. B. There are also microphones polar pattern hypercardioid. To the front, they have the same directivity as the cardio, but they also have a catchment area to the rear which allows you to have a semblance of atmosphere. In the case, of course, where there is no noise pollution. The disadvantage is that one can not choose accurately the mix between what we receive at the front (one voice) and the surround sound. As the two recordings are mixed together, it will be difficult to isolate in post-production the taking of his “before” in cases where what we receive in “the back” is no longer satisfactory after the euphoria of the shoot last…

There is also supercardioïdes who are the intermediary between the cardio and the hyper. The sound to the rear will be more moderate than with a hyper, but the disadvantage of the latter remains (the problem of the mix between voice and ambient sounds). The advantage of this directivity in a place controlled is obvious. It’s time to add the safe and a natural feel to the sound. Thing that we can do, however, post-prod with greater freedom…

Two-way

The catchment area optimum is in the form of 8. The microphone picks up perfectly on the front and the rear instead of the sides.

The key of this directionality is that, in spite of its capture bi-Directional, there is only ONE track in the output. In fact, the stereo is obtained by TWO membranes. On this microphone, there is only one, the recording is made in mono. Nevertheless, the track can be doubled.

The use of such a microphone is double-edged. At first sight, it is ideal to record the dialogue. People are placed in vis-à-vis, and discuss each in front of the microphone.A concern arises then. Since there is only one track output, you can not post-prod to make the levels or mix of voice because they are “mixed” in a single file. Also, in the capture bi-directional, it is necessary to take the following precautions :

  • Estimate the volume of the voices of the people who share (to roughly the levels) ;
  • In general, there is always one that speaks a little bit less strong. It is sufficient in this case to place the mic more towards it as towards his (its) interlocutor(s) ;
  • If several people love to shout, or that the debate is lively, the place then in the middle.

However, be careful not to place the microphone too close to a person compared to another in order to avoid the proximity effect ! It would be unthinkable to add bass to a female voice or crystalline, on the contrary, to make a voice outrageously serious to a man. Except if you do the dubbing of Babylon A. D

 

Thibault Taccone

Category: Uncategorized
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