3. Microphone Characteristics
Knowing that every microphone has a ‘directional response’ is an important fact that every videographer should learn to ensure that they get the most effective and clean recording possible. It is essential that we utilise the design of our microphones to minimise the pickup of unwanted sound from sources other than our chosen subject, but at the same time not compromise the recording of the sounds we utilise the design of our microphones to minimise the pickup of unwanted sound from sources other than our chosen subject, but at the same time not compromise the recording of the sounds we’re interested in.
The directional response of a microphone refers to its sensitivity to sound in relation to its front and the angles of its central axis, and the resulting output level. This sensitivity is known as the ‘polar pattern’ or ‘polar response’. If you think of the polar pattern of any given microphone as a balloon around the microphone that expands in all directions as you increase the gain, or shrinks as you decrease the gain, then you will see how external noise or interference can creep in.
In addition, every microphone has a “frequency response”, which is sensitivity to sounds of different frequencies, usually illustrated by a
The better the mic, the less the graph will deviate from a flat line, and the further the flat area will extend at either end of the frequency spectrum. In this context, a mic with a “flat” frequency response is generally a good thing. You can easily change this flat- line response with EQ (using a mixer or at the post production stage), but it’s very hard to flatten out a mic with a very uneven response.
In an ideal world, the directional response would not interact with the frequency response, but in reality it does, to a greater or lesser degree, because the methods employed to achieve a directional response do not work as well at some frequencies as others. As such, even for the finest mics, that wonderful looking frequency response graph is only valid for recording directly “on-axis” (ie directly to the front of the mic, perpendicular to the diaphragm). Likewise, the specified polar pattern is typically only representative for a mid-range frequency (eg 1KHz). How the frequency response degrades as you move progressively off-axis, as it always does do, is partly an inherent consequence of the directional class of microphone (as listed overleaf), and partly the individual manufacturer’s design capabilities. This is rather like the off-axis degradation seen when viewing LCD screens, where the brightness/contrast/colour reproduction degrades as you move off-axis. Better quality microphone models are able to preserve their smooth frequency response over a wider angle when dealing with sound sources that are off-axis.
3.1 Omnidirectional
Omnidirectional microphones, as the name suggests, theoretically pick up sound equally in all directions, although in practice this is not quite the case. More often than not they are used in Lavalier (tie-clip or lapel) mic systems, or as hand-held reporters’ mics, where their relative immunity to handling noise is a benefit. The omnidirectional pattern for lavaliers has several benefits. They can be clipped upside down without the sound quality changing much, useful for speakers who breathe strongly through their nose, blowing down and causing “wind noise” on an upward facing mic.
They also do not suffer from the ‘proximity effect’ which is the disproportionate rise of the bass response of the microphone as the mic is positioned close to the sound source. However, due to the usual mounting position being close to the chest cavity, most small electret lavaliers reduce the bass response slightly to improve speech clarity, together with a slight top-end boost.
3.2 Bi-directional
Most ribbon microphones have this type of polar pattern, also known as a “Figure of Eight”, with equal lobes front and rear (which are out of phase with each other) and rejection of sound from either side. Bi-directional microphones are often used in radio studios where there are two people involved in an interview; one on either side of the microphone though this is not a sound recording method that would be used by a videographer very often.
3.3 Cardioid
Of all the microphone pick-up patterns available, Cardioid is by far the most commonly used and gets its name from its heart-shaped polar pattern. The cardioid microphone is most sensitive to sounds coming in on its primary axis (front), and it rejects sounds from the rear and sides. This is the most common type of microphone used in live sound reinforcement (PA systems, etc.) as its directional attributes can cut down on feedback. However, it suffers from the “bass proximity effect” and is more sensitive to breath popping and wind noise.
3.4 Supercardioid and Hypercardioid
Supercardioid and Hypercardioid microphones have a polar pattern part way between a cardioid and a “figure-of- eight” bi-directional mic. The Supercardioid microphone has a narrower frontal lobe than a cardioid. As a result, it is often used in situations where the subject is further away and there needs to be a greater rejection of noise from the sides. In contrast to the cardioid microphone it does pick up more sound from its rear pickup lobe and therefore care must be taken.
A Hypercardioid microphone is similar to a supercardioid microphone but with a slightly different balance between the front and rear lobes, as per the diagrams.
Supercardiod Microphone pattern
Hypercardiod Microphone pattern
3.5 Shotgun
Shotgun microphones are the most highly directional of all the microphones available to us, and they are used to pick up sounds normally from longer distances, particularly where it would be impossible to place any other microphone closer. But, as with all microphones, any sound source or reflected sound that falls between the microphone and the desired sound will also be picked up by the microphone.
Most shotgun microphones are distinguished by their long thin tube-like structure. Shotgun microphones do not amplify sound from a distance, but they are designed to be highly directional. This is achieved by placing the microphone capsule inside an “interference tube”, a tube with slots cut along the side. This design greatly reduces any off-axis sound signals by a process of wave cancellation between the sound waves from the sides that enter the end of the tube, and those that take a slightly shorter route and enter via the slots, but are out of phase. However, these increased directional characteristics come at a price. At lower frequencies, where the wavelengths are too long for the interference tube to have any effect, the mic gradually reverts to something between a cardioid and an omni. Consequently, make sure that your shotgun microphones are directed towards the high frequency sources that you wish to record, or else the frequency response will be very irregular and “muddy” (ie lacking high-frequency clarity). A misdirected microphone is referred to as being ‘off axis’ or the sound source as being “off mic”.
3.6 Parabolic
The parabolic microphone is a standard microphone with a built- on reflector that concentrates sound on to the capsule in similar way that a satellite dish reflects radio waves to its pick-up. The size of the reflector will affect the bass response, as the longer wavelengths produce the opposite of the normal “bass proximity effect” and reduce the bass response for anything less than a huge reflector. Parabolic microphones are fairly specialised in that they are often used in nature recording and are particularly suitable for picking up birdsong where the lack of bass response is not a concern.