Directional/cardioid vs. omnidirectional microphones

Microphones can have different directional characteristics. Omnis that pick up the sound all around them, and cardioids that mainly pick up the sound directly in front of them. Other directional patterns are: bi-directional/figure 8, super cardioid, hyper cardioid, and wide cardioid.

Generally a cardioid sounds appealing, since in a musical setting you only want the sound source and rarely want to record or amplify the surroundings. But directionality has a price and that sometimes is not worth paying.

Directional microphones need to have a much softer diaphragm than an omni. This softness results in handling, pop & wind noise which puts a limit to how close you can get to a vocalist, even when using pop-filters.

A directional also suffers from proximity effect, which means that the closer you get to the sound source the louder the low frequencies get.


The proximity effect exhibited by the cardioid microphone 4011A
A cardioid microphone can be adjusted to be linear at the distance it is normally used. A vocal microphone for live use, for example, is adjusted to be linear at a distance of approximately 1 – 2 cm and at longer distances the low frequencies drop dramatically. A studio microphone is typically adjusted to work at a longer distance. In the example of the DPA 4011A, when used closer than 30 cm it gives a low frequency boost, and when used further than 30 cm it gives a low frequency roll-off. That means that from any other distance than 30 cm equalization is needed unless the proximity effect is desired.

Additionally the off-axis sound of a cardioid is less linear than that of an omni. It is very hard to reduce the level of sound taken in from the sides without some coloration, and some directional microphones have a notably poor off-axis response. This means that sound entering the microphone from the sides and the rear are more or less strongly colored – the industry names this "the curtain effect". This effect can be seen on the microphones polar pattern as ‘spikes’.
On the other hand, miking live with high level monitors can make the omni mic feed back, which makes the cardioids more suitable for this application, although the use of in-ear monitors reduces that problem.

If you choose an omnidirectional microphone, channel separation may be less precise than with a directional microphone, because the omni will pick up sound from all directions. Therefore, if channel separation is preferred, the ratio between direct and indirect sound can become more unfavourable with an omni. The omni, however, can be moved closer to the source, without the penalty of proximity effect. As a general rule it can be said that if we place a cardioid at a distance of 17 cm to the source, then an omni placed at 10 cm gives the same ratio of direct and indirect sound as the cardioid.

Relative distance to sound source for equal balance between direct and indirect sound
Multi-pattern microphones with both omni, bi-directional and cardioid characteristics will always compromise the sound quality. It may be very convenient to have a 3-in-1 solution, but the drawback is reduced performance in each mode.  Due to the need of a pressure gradient design, a multi pattern microphone in omni mode has many of the weaknesses of the cardioid, such as popping, handling & wind noise and a less linear off-axis sound. In fact a multi pattern microphone in the same mode can have different characteristics depending on the frequency.

Large vs. small diaphragms

Before choosing between a large and a small diaphragm microphone it is important to know the difference in features between them, and microphone behavior can not be compared with that of a loudspeaker when considering size.

A large diaphragm microphone is not better at reproducing low-frequencies, but it may be less precise in reproducing high frequencies, which may make it sound as if it has more low end.

Both diaphragm sizes have their respective advantages and disadvantages. This is illustrated in the table to the left.

A small diaphragm has a higher self noise due to the fact that the small diaphragm is less compliant and therefore more sensitive to the bombardment of air molecules that causes some of the self noise of a microphone. And since the large diaphragm is softer than the small, it is easier to move and therefore more sensitive – even at very low levels. This means that the small diaphragm, because it’s stiff, can handle a higher sound pressure without clipping or distortion, but is less sensitive and needs more amplification, which also adds a little noise.

When reproducing very high frequencies, large diaphragm microphones have a more limited range than the small diaphragms. This is caused by three factors:

1. A large diaphragm tends to break up and will no longer act as a true piston. This phenomenon is also recognized in loudspeaker technology and is the reason why loudspeakers are manufactured with different sizes of diaphragms to handle different frequencies.
2. The weight of the diaphragm will attenuate the displacement of the diaphragm for higher frequencies.
3. The diffractions around the edges of the microphone capsule will limit the microphone's capability to handle very high frequencies.

ConclusionBoth diaphragm sizes have their respective advantages and disadvantages. This is illustrated in this table, which compares the specifications of DPA's small, medium and large diaphragm microphones:

How to read microphone specifications

When you read microphone specifications in order to compare different microphones, it is extremely important that you understand how to interpret them. In most cases the specifications can be measured or calculated in many different ways. This article is designed to help evaluate specifications in a meaningful way.

While microphone specifications provide an indication of a microphone's electro-acoustic performance, they will not give you the total appreciation of how it will sound – just as it is with cars. Knowing that it is a 3.0 turbo-engine with 4WD gives you an idea of a pretty good driving experience, but for the exact feeling, you need to drive the car yourself.
Frequency range/frequency response
Frequency range tells you the range of the frequencies (for example 20 Hz to 20 kHz)  that the microphone can pick up and reproduce, but not how well the different frequencies are reproduced. To see that you need the frequency response:

Ex. DPA 4006A Omnidirectional Microphone, P48
Here you see how linear the response is or if the microphone has any ‘spikes’. But pay attention to the scale on the left. The number of dB each step represents can vary a lot.

The frequency response normally refers to the on-axis response, which means from a sound source right in front of the microphone. The diffuse field response curve will illustrate how the microphone will respond in a highly reverberant sound field.

The off-axis response is also important to examine. A microphone always takes in sound from the sides too, the question is just how much and how good it sounds. In particular, directional microphones can, in their attempt to suppress sound from the sides, get an uneven off-axis response:

On-and off-axis responses of 4011A/4012/4011C cardioid microphones measured at 30 cm
Finally a polar plot can show the 360° response of selected frequencies. The response curves should be smooth and symmetric to show an uncolored sound. Extreme peaks and valleys are unwanted and the response curves should not cross each other. From the polar diagram you can also see how omnidirectional microphones usually become more directional at higher frequencies.



4006A Omnidirectional Microphone
Equivalent noise level /self noise
The equivalent noise level indicates the sound pressure level that will create the same voltage that the self-noise from the microphone will produce. A low noise level is especially desirable when working with low sound pressure levels so the sound will not drown in noise from the microphone itself. The self-noise also dictates the lower limitation in the microphone's dynamic range.

There are two typical standards:

1. The dB(A) scale will weight the SPL according to the ear's sensitivity, especially filtering out low frequency noise. Good results (very low noise) in this scale are usually below 15 dB(A).
2. The ITU-R BS.468-4 scale uses a different weighting, so in this scale, good results are below 25 dB.

Sensitivity, sound pressure level (SPL) handling & total harmonic distortion (THD)
Sensitivity tells you how well the microphone can convert the acoustic sound into electricity and according to the IEC 60268-4 norm, the sensitivity is measured in mV per Pascal (air pressure) at 1 kHz. The higher the sensitivity the better, because it reduces the need for amplification and therefore reduces the amplification noise.

SPL handling tells you how much sound pressure in dB the microphone can handle before it either clips (the diaphragm hits the backplate or the amplifier overloads) or reaches a certain level of distortion (THD or total harmonic distortion). Typically either 0,5% or 1%. The higher level of sound pressure before clip or distortion the better.

Example: DPA 4004 Hi-SPL Omnidirectional Microphone, 130 V:
Maximum sound pressure level: 168 dB SPL peak
Total harmonic distortion:142 dB SPL peak (<0.5 % THD), 148 dB SPL peak (<1% THD)