Directional microphones — myths and reality. Abalmazov E.I.

Directional microphones — myths and reality. Abalmazov E.I.

Directional microphones: myths and reality.

Abalmazov Eduard Ivanovich, Doctor of Technical Sciences, Professor

The article is reprinted from the journal «Security Systems» No. 4, 1996.

There are all sorts of rumors about the capabilities of directional microphones. Some sincerely believe in their long range, citing distances of 100, 200 and more meters, while others, on the contrary, believe that this is unjustified advertising, bordering on disinformation. Let's try to find out the real state of affairs using simple mathematical calculations.

  Instead of an introduction

When speaking about directional microphones, we mean, first of all, situations of acoustic control of sound sources in the open air, when the effects of the so-called reverberation of acoustic stoves can be neglected. For such situations, the decisive factor is the distance of the sound source from the directional microphone, which leads to a significant weakening of the level of the controlled sound field (In addition, at a large distance, the weakening of the sound becomes noticeable due to the destruction of the spatial coherence of the field, due to the presence of natural energy scatterers, for example, medium- and large-scale atmospheric turbulence, creating interference in the wind).

Thus, at a distance of 100 m, the sound pressure is weakened by at least 40 dB (compared to a distance of 1 m), and then the loudness of a normal conversation of 60 dB will be no more than 20 dB at the receiving point. Such pressure is significantly less than not only the level of real external acoustic interference, but also the threshold acoustic sensitivity of conventional microphones. Thus, unlike conventional microphones, directional microphones must have:

— high threshold acoustic sensitivity as a guarantee that the weakened sound signal will exceed the level of the receiver's own (mainly thermal) noise. Even in the absence of external acoustic interference, this is a necessary condition for monitoring sound at a significant distance from the source;

— high directivity as a guarantee that the weakened sound signal will exceed the level of residual external interference. High directivity is understood as the ability to suppress external acoustic interference from directions that do not coincide with the direction of the sound source. To fully comply with these requirements in practice (for one microphone) — is an extremely difficult task. It has become more realistic to solve specific problems, for example, to create a weakly directional microphone with high sensitivity or, conversely, to create a highly directional microphone with low sensitivity, which has led to a variety of types of directional microphones.

2. Types of directional microphones.

There are at least four types of directional microphones:

— parabolic;
— flat acoustic phased arrays;
— tubular, or «traveling» wave microphones;
— gradient.

A parabolic microphone is a parabolic sound reflector with a conventional (non-directional) microphone at its focus. The reflector is made of either optically opaque or transparent (e.g. acrylic plastic) material. 

Fig. 1 Parabolic microphone.

The outer diameter of the parabolic mirror can be from 200 to 500 mm. The operating principle of this microphone is explained in Fig. 1. Sound waves from the axial direction, reflecting from the parabolic mirror, are summed in phase at the focal point A. This results in amplification of the sound field. The larger the diameter of the mirror, the greater the amplification that the device can provide. If the direction of sound arrival is not axial, then the summation of the sound waves reflected from different parts of the parabolic mirror arriving at point A will give a smaller result, since not all the terms will be in phase. The attenuation is stronger, the greater the angle of arrival of the sound relative to the axis. This creates angular selectivity in reception. The parabolic microphone is a typical example of a highly sensitive, but weakly directional microphone.

Plane phased arrays implement the idea of ​​simultaneous reception of a sound field at discrete points of a plane perpendicular to the direction of the sound source (Fig. 2). At these points (A1, A2, A3…) either microphones are placed, the output signals of which are summed electrically, or, most often, open ends of sound guides, for example, tubes of a sufficiently small diameter, which provide in-phase summation of sound signals from a source in a certain acoustic summator.

Fig. 2. Flat phased array.  

A microphone is connected to the output of the adder. If the sound comes from the axial direction, then all signals propagating along the sound waves will be in phase, and summation in the acoustic adder will give the maximum result. If the direction to the sound source is not axial, but at some angle to the axis, then the signals from different points of the receiving plane will be different in phase and the result of their summation will be smaller. The greater the angle of arrival of the sound, the greater its attenuation. Usually the number of receiving points Ai in such arrays is several dozen. Structurally, flat phased arrays are built in either the front wall of the attaché case with subsequent camouflage, or in a vest-shirt, which is worn under a jacket or shirt. The necessary electronic units (amplifier, power elements, tape recorder) are located respectively either in the case or under the clothes. Thus, flat phased arrays with camouflage are visually more conspiratorial compared to a parabolic microphone.

Tubular microphones, or «traveling» wave microphones, unlike parabolic microphones and flat acoustic

grilles, receive sound not on a plane, but along a certain line that coincides with the direction of the sound source. The principle of their operation is explained in Fig. 3. 

Rice. 3 Tubular microphone.

The basis of the microphone is a sound guide in the form of a rigid hollow tube with a diameter of 10-30 mm with special slotted holes placed in rows along the entire length of the sound guide, with a circular geometry of the arrangement for each of the rows. Obviously, when receiving sound from the axial direction, there will be a summation in phase of the signals penetrating the sound guide through all the slotted holes, since the axial propagation speeds of sound outside the tube and inside it are the same. When the sound arrives at a certain angle to the microphone axis, this leads to a phase mismatch, since the speed of sound in the tube will be greater than the axial component of the speed of sound outside it, as a result of which the reception sensitivity decreases. Typically, the length of a tubular microphone is from 15-230 mm to 1 m. The greater its length, the more strongly interference from the side and rear directions is suppressed.

High-order gradient microphones There are practically no open offers on the market. The exception is the first-order gradient microphone.

Unlike phased receiving acoustic arrays that use the operation of adding acoustic signals, gradient microphones are based on the operation of subtraction in the direction of signal arrival. This puts them a priori at a disadvantage in terms of threshold sensitivity, since each subtraction weakens the signal, but statistically sums up the internal interference. At the same time, the subtraction operation itself allows for the construction of small-sized directional systems. The simplest gradient directional microphone is a microphone that implements a first-order gradient (Fig. 4).

Fig. 4 The simplest gradient microphone.

It consists of two fairly miniature and closely spaced highly sensitive microphones M1 and M2, the output signals of which are electrically (or acoustically) subtracted from each other, realizing in finite differences the first derivative of the sound field along the microphone axis and forming a diagram of the form cos Q, where Q is the angle of arrival of sound. This ensures a relative weakening of acoustic fields from the lateral directions (O ≈ 90°). High-order gradient microphones are systems that realize spatial derivatives of the 2nd, 3rd and higher orders.

3. How to compare and evaluate directional microphones? The main user characteristic of directional microphones is their operating range in specific conditions. For open space and isotropic and independent in angular directions external acoustic interference, the operating range R is related to:

a) the spectral signal-to-interference ratio q at the output of the directional microphone,

b) the spectral speech level Вр;

c) the spectral level of external acoustic interference Вш by a ratio of the form:

q=Bp-Bш-20 lg R+G-Bп (1)

where

G is the so-called directional coefficient of the microphone (dB),

Вп is the threshold acoustic sensitivity of the microphone (dB).

The directional coefficient G included in formula (1) characterizes the degree of relative suppression of external acoustic interference: the higher it is, the stronger this suppression. Theoretically, it is associated with the normalized directional pattern of the microphone F (Q,j ) a ratio of the form:

  where

Q — is the angle of arrival of the sound wave relative to the microphone axis;

j — is the angle of arrival of the sound wave in polar coordinates of the plane,

perpendicular to the axis. For example, for a tubular microphone, when

where l is the wavelength of sound. and L is the length of the tube, we have (for L ? l . ):

G = 4 L/l . (4)

Similarly, an approximate formula is derived for the directivity of parabolic microphones and phased flat arrays:

G = 4p (S/l 2) (5)

where S is the area of ​​the entrance aperture; l .- wavelength of sound. For n-order gradient microphones

with optimal signal processing

G=n (n+1) (6)

where n is the order of the gradient. With known values ​​of GFormula (1) is sufficient to obtain absolute estimates of the expected spectral signal-to-noise ratio if the conditions are known. But in many cases, knowledge of these conditions is inaccurate. Therefore, it is more justified to use relative rather than absolute estimates of the range, as they do not require precise knowledge of the conditions, since the comparison occurs when they are equal. Accepting this ideology, let us compare the capabilities of directional microphones with the capabilities of human hearing not equipped with special devices. Formally, a relationship similar to (1) can be written for it. As a result of the comparison, we obtain:  

R=R0 x 10 0.05 (G-G0) – 0.005 D Bп (7)

  Here R0 is the range of sound audibility by the organ of hearing;

R— the operating range of a directional microphone with the same control quality.

Go — the directional action coefficient of the human hearing organ (binoural listening mode).

D Bп — the difference in threshold sensitivity of the directional microphone and the hearing organ. Fig. 5 shows a graph of the dependence of the relative operating range R/Ro of a directional microphone as a function of its directional action coefficient G for the case when D Bп = О(the option is technically feasible). The coefficient Go of the directional action of the human hearing organ is taken to be 6 dB.

The graph shows that at G = 15 dB (this value of G approximately corresponds to the data for most fairly good microphones of the phased array and parabolic type) a directional microphone will allow the implementation of a control range approximately 3 times greater than the distance Ro, at which the sound is perceived by a person without special devices. The comparison is carried out under the same conditions for the same sound source. In practice, this result means the following: if we are talking about acoustic monitoring of conversations in the city, on the street, when R0 = 2 — 4 m, then directional microphones will allow recording conversations at distances of 6-12 m. In suburban conditions, with a lower level of interference, when the value of Rocan reach 10 m and more, the control range using technical means can be more than 30 m.

Fig. 5. The range of a directional microphone R compared
with the range R» of sound audibility by an unequipped hearing organ.

These are the assessments of the situations of using directional microphones in open space conditions. But it is possible to use directional microphones in closed rooms, for which it is necessary to take into account reverberation, that is, reflections of sound signals from the walls of the room and interior objects.

Formally, in these conditions, the relation (7) remains valid if instead of G we use the reduced coefficient of directional action G0:

G0=(G+R)/(1+R) (8)

 where R is some parameter that takes into account the surface area of ​​the volume (the so-called acoustic ratio).

4. Thinking about the future

Speaking about the future of this special industry, we can highlight at least three areas of possible improvement of directional microphones. On the one hand, we should expect (by analogy with adaptive time filtering) the emergence of devices capable of adaptive spatio-temporal filtering of acoustic interference. The objective basis for such devices are achievements in the field of digital multichannel data processing. The second possibility of improving directional microphones is associated with progress in the field of highly sensitive acoustic sensors, which in principle makes it possible to create microphones with a threshold sensitivity of minus 10 — minus 15 dB and an extreme control range in the absence of external noise. And, finally, we cannot exclude the emergence of fundamentally new directional microphones using nonlinear and parametric effects to implement large-sized organoleptic covert antennas and capable of providing a c.n.d. of 20-25 dB or more.