The directional characteristic describes the angular dependence of the strength of received or transmitted waves , mostly related to the sensitivity or the intensity in a main direction. If the directional characteristic is uneven over the solid angle , which is also referred to as not isotropic , then there is a directional effect . The directivity is described quantitatively by the directivity factor .
The term is common in various areas such as antenna technology , both for transmitting and receiving antennas, for light sources such as light-emitting diodes and in acoustics for microphones and loudspeakers .
The directional characteristic is usually shown in polar coordinates in a "polar diagram". Its radial scaling is either linear, expressed in relative units of the drawn power , or logarithmic , for example expressed in decibels .
Areas of application
In microphone technology , the directional characteristic describes the dependence of the sensitivity of a microphone, i.e. the output voltage in relation to the sound pressure, on the angle of sound incidence . The directional characteristic is basically frequency-dependent. Often only a horizontal section is given. The directional character depends on the design of the microphone capsule and on external form elements (e.g. shotgun microphone ). The strength of the directional effect is described with the degree of bundling or the bundling factor . Prototypical polar patterns are named after their appearance in the polar diagram:
- Sphere (omnidirectional = non-directional)
- Eight ( figure-of-eight characteristic = dipole , opposite polarity front and rear)
- Club (club characteristic, shotgun)
Mixed forms are:
- Kidney (mixture of sphere and figure eight)
- Wide cardioid (wide cardioid characteristic)
- Supercardioid (supercardioid characteristic)
- Hypercardioid (hypercardioid characteristic)
The above representations are logarithmic down to −30 dB, the following references also contain linear representations (of the voltage values):
The directional characteristic of microphones is measured in anechoic rooms in the direct field , even if the lobe microphone directional characteristic , as with a shotgun microphone, is only used in the room sound field (diffuse field). Every directional microphone loses its directional effect in the diffuse field. That means: The effectiveness of the sound-bundling directional characteristics, as shown by the microphone manufacturers, only applies within the reverberation radius ; outside of this, the directional effect decreases with increasing distance from the sound source.
The directivity depends exclusively on the acoustic design of the capsule, not on the underlying transducer principle.
A pure pressure microphone (also: pressure receiver) has no directional effect, i.e. a spherical directional characteristic (omnidirectional). A pressure gradient microphone (also: pressure gradient receiver) in its pure form (e.g. ribbon microphone ) provides a figure eight as directional characteristic (the direction-dependent force on the membrane is cosine; the cosine in the polar diagram has the shape of an eight). By special construction of the capsule or by electrical mixing of the signals from two closely spaced capsules, directional characteristics between sphere and figure eight are obtained, which follow this mathematical description:
As standardized intermediate forms between the two basic forms
- Omnidirectional pattern (A = 1, B = 0)
- Figure eight characteristic (A = 0, B = 1)
- "Wide kidney" (A = 0.63, B = 0.37)
- "Kidney" (A = B = 0.5)
- "Supercardioid" (A = 0.366, B = 0.634)
- "Hypercardioid" (A = 0.25, B = 0.75)
The directional characteristic "lobe" is obtained through the principle of the interference tube ( shotgun microphone ; sound direction-dependent cancellation through acoustically effective transit time elements). Towards the front, the club only picks up higher frequencies in a very narrow area. This directional characteristic is strongly frequency dependent. The recording sensitivity is lower and diffuse towards the rear, i.e. H. "Indeterminable".
Due to the complex conditions of the sound field, the real directional character in practice differs individually from these theoretical patterns. Strong deviations in the pattern can be observed when the wavelength of the signal frequency is in the range of the capsule diameter. Therefore, the smaller the membrane diameter, the less these pattern distortions are.
The greatest deviations are to be expected with pressure gradient microphones , the directional character of which has been modified from a pure figure of eight to the cardioid with acoustic time-of-flight elements or a double-diaphragm design.
In the case of pressure microphones , for example, the pressure accumulation effect as well as sound shadowing by the microphone body lead to a directional effect at high frequencies.
The production of the directional characteristic “wide cardioid” as a “ Straus package ” from the use of two small microphones very close together is also known. Of Volker Straus a microphone KM is 83 (ball), and a microphone KM 84 (kidney) of Georg Neumann used simultaneously, which are connected in series at a microphone input.
The membrane tuning , which is often referred to as microphone tuning, is also of great importance in generating the directional characteristic .
The radiation characteristics of loudspeakers are important for the sound reinforcement of rooms, train stations and events.
While woofers (closed boxes and so-called subwoofers with bass reflex tube) - with a few exceptions - have an omnidirectional characteristic, midrange and tweeters behave very complex and differently:
- Dome tweeters have a wide radiation over a wide frequency range and are therefore well suited for living spaces.
- Tweeters with funnels have a low radiation angle and are used - if they are raised - to cover large areas.
- Midrange speakers or broadband speakers with a conical membrane have highly frequency-dependent radiation characteristics.
- Vertical stacking enables horizontally wide and vertically limited radiation angles to be achieved ("fans").
The directional characteristic of antennas is also referred to as antenna diagram or directional diagram and determines (regardless of whether it is transmitted or received) their angle-dependent gain and the front-to-back ratio (VRV).
As described above for microphones, there are omnidirectional, cardioid or figure-of-eight characteristics as well as (with strongly bundling antennas) lobe characteristics with several or a few so-called "side lobes ". The directional characteristic of antennas is shown in a directional diagram or antenna diagram horizontally and vertically in polar coordinates . It indicates the angle-dependent antenna gain relative to the maximum gain.
The directional characteristic is also important in order to avoid undesired received signals (e.g. avoidance of “ghost images” in analog TV reception).
- Radiotelephone base stations
- Radio transmitter (VHF and television on television towers and converters)
Horizontal arrays can also be used to reduce the horizontal beam angle, e.g. B. in ship radar antennas. One or two-dimensional antenna arrays can be fed with different phase positions of the signals for the individual antennas to electronically influence the directional characteristic (radar systems without moving antenna, phased array antenna ).
Large antenna arrays are used in radio astronomy to examine the sky or even neighboring planetary surfaces by evaluating the phase position of the individual signals with extremely high angular resolution.
The directional characteristic of a light source (light source or lamp), i.e. its angle-dependent light intensity , is used to assess the illumination of surfaces or the glare effect.
In the case of directed light sources (e.g. light-emitting diodes or reflector lamps , cold-light mirror lamps ), the radiation angle describes the angle that is enclosed by the lateral points with half the maximum light intensity. It can be different for lights in different spatial directions.
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- All logarithmic directional characteristics (dB) of ideal microphones , sengpielaudio.com (PDF; 191 kB)
- Microphone polar patterns - from pure pressure receiver to pure pressure gradient receiver , sengpielaudio.com (PDF; 77 kB)
- Difference between hypercardioid and supercardioid , sengpielaudio.com (PDF; 118 kB)
- Microphone directional characteristic wide cardioid etc. , sengpielaudio.com (PDF; 91 kB)