Maybe one of the most intuitive and known technical characteristic of microphones is directivity.
Also referred to as directionality, polar pattern or pickup pattern, I mean intuitive in the sense that most of us have used a dynamic cardioid microphone (in speeches, playing in a band, singing in karaoke and so on) and we have intuitively pointed it at our mouth so as to make our voice to be heard. And that without any knowledge about audio engineering nor miking techniques.

Nevertheless, the directivity or directionality of a microphone is more complex than that and miking instruments can become a difficult or non intuitive matter. In the following of this post I will explain the concept of directivity and the most common kinds of microphone directivity that we can find.

DIRECTIVITY

Directivity is related to how a microphone picks up sound from a sound source depending upon their relative orientation. That means that depending on the specific microphone characteristics, the sound might be picked up with different intensity according to the direction where it comes from (except for a pure omnidirectional mic and taking into account that the noise source it is at the same distance).

That property allows discarding undesired sound sources or enhancing the recording of desired ones, with a simple microphone movement that changes its position.

KINDS OF DIRECTIVITY

HOW TO MEASURE THE DIRECTIVITY OF A MICROPHONE

The directivity of a microphone is represented by means of a diagram called polar pattern, polar plot or polar diagram. The values are determined in a special room called anechoic chamber where the dB level is recorded at each desired point (from 0 to 360 degrees). Read this other post about anechoic chambers.
A sound source is used to generate a specific frequency and the microphone is rotated around it (or the sound source around the microphone) so it is possible to get a measurement, for example, each 1, 3 or 5 degrees.
The maximum intensity value registered (on-axis at 0º) is used to set the reference value equal to 0 dB. The rest of the measurements are drawn in the polar pattern as negative values and represent the dB falloff respect the origin.
The sense of a negative value is not related as the absence of sound, but the microphone pick up falloff because of its directivity characteristics.
Once we have the 360º measurements taken, the plot is ready to show how the directivity of the measured microphone looks like.
Read this other post where it is shown a simple model for microphone directivity to process raw data obtained in anechoic chambers.

Sound loudness is measured in dB and that is a relative scale. When we talk about a 0 dB sound, we do not refer that there is absence of sound, but that our ears are not capable to hear sound pressures below it. Don’t confuse here the 0 dB scale of the polar pattern with the 0 dB from hearing. In a polar plot the 0 dB value is used as a reference scale, and the lose of intensity will referred to as negative values expressed in dB.
More about this topic in DPA Microphone University

DIRECTIVITY AND SENSITIVITY

It is important to clarify at this point that directivity sometimes is confused with sensitivity. Sensitivity is related to the performance of a microphone when is reached by sound pressure in giving electrical output (normally expressed in mV/Pa or dB V/Pa).

What do you think? Could we say that directivity is a kind of sensitivity when we move the sound source around the microphone?

OK, lets dive a little more into sensitivity and directivity from a less technical point of view:

Sensitivity is expressed as dB referred to 1V/Pa, and it is measured at a reference distance per frequency. When we determine a microphone’s sensitivity we can calculate the mV that the transducer gives when is exposed to the sound source.
For example, an AT 5040 presents a sensitivity (transfer factor) of 56.2 mV/Pa and an AT 4053 b presents a sensitivity (transfer factor) of 19.9 mV/Pa (see more in technical specs of microphones included in Arapolarmic).

The Directivity is a relative scale that is expressed as negative dB respect to the on-axis maximum intensity level picked up by the mic. It is not related to the distance to the sound source, but to the performance when it is rotated around it. So the relative proportion of dB falloff does not depend on the distance between the mic and the sound source.
Compare for example, in the figures below, the dB loose of a figure-8 microphone at 90º with a cardioid microphone at 90º.

From an electrical point of view, take into account that microphones are transducers that transform the sound pressure to electrical output. So if a sound source is rotated around the mic, the mV generated will vary, being maximum at the on-axis position and being less at other orientations. It is also easy to understand that if we place the microphone far away from the sound source, the mV generated are diminished too.

That is the reason why it is possible to scale the polar patterns in Arapolarmic as a visual reinforcement tool without disturbing the directionality properties of microphones and with no relation to its sensitivity. It is useful because it provides a visual reinforcement per frequency to check if it is possible to adjust a little bit the microphone pose. Read our F.A.Q. in resizing polar patterns in Arapolarmic.

INTRODUCING FREQUENCIES IN THE STORY

Frequently, in the theoretical representation and explanation of directivity, the polar patterns used to teach about it show shapes like those presented above in the post. So, we hear about cardioid polar patterns, supercardioid, hypercardioid and so on, and we get a single image to show that kind of directivity.
Nevertheless, it is important to highlight that a microphone has frequency range that ranges from 20 Hz to 20000 kHz. Taking as an example a cardioid directivity, that means that the plot representing the polar pattern will vary (more or less) for different frequencies.

Most of microphone manuals show polar curves for frequencies that ranges from 125 Hz to 16 kHz. It is easy to see 8 curves in a polar pattern for 125 Hz, 250 Hz, 500 Hz, 1000 Hz, 2000 Hz, 4000 Hz, 8000 Hz and 16000 Hz.

Look at the figure above where it is represented the directivity of a cardioid microphone, where we can see the 8 curves corresponding to each frequency. It is possible to see that the curves do not overlap exactly and show different shapes.

WHAT WE FIND IN REAL MICROPHONES

Despite the fact that we have described 6 types of directivity, it has been explained from a theoretical point of view. That means that when we work with real microphones, they do not fit exactly the models we have listed before.

For example a cardioid microphone might have different shapes than those presented in the above figures. Look at the following microphone polar patterns for different frequencies in the below supercardioid and cardioid microphone example images.

Arapolarmic is an audio tool software that overlaps the polar pattern of the microphones included in the miclibrary over the real microphone to show its directivity in real time. See more about Arapolarmic showing polar patterns.