25 Jun 2016

Microphones for Video: What you need to know

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Replay: A microphone is a simple component that transforms acoustical energy into electrical energy. However, there is a lot to gain if you select the right microphone for the job. Eddy B. Brixen of DPA Microphones explains.

This article references DPA microphones, which is not surprising because the author works for them. It is not, however, a sponsored article. We're printing it because it contains extremely useful iformation. Other microphones from other manufacturers are available!

You may have asked yourself why there are so many different microphones on the market. While it is nice to have a lot of choices, the downside is that too many options can lead to making a less than perfect choice. So what should you look for when selecting a microphone? This article aims to answer that question by looking at microphone quality and why it is important, what specs to take note of, how to power your mic, and what you can achieve with a wireless solution.

Why audio is important in video production

Audio and video should go hand in hand. Considering perceived quality of a given production, it does not help much to have great images if the sound is bad. The quality of the sound affects the perception of the images. Getting the right perspective and the right balance is important in both video and audio production.

As you will know from working with images, if the quality is low, there are limitations to the extent of possible enhancement. If the audio is noisy, distorted or at low resolution, there are limits to what can be fixed in the mix.

Craig Carter with DPA 4017B in Windshield.jpg

Specs that count

As with other equipment, the specifications of a microphone provide essential information. Serious microphone manufacturers will refer to an international standard (IEC) that states the conditions for the measurements in order to provide comparable data. Here are some of the important ones:

Sensitivity: This tells how much electrical output a microphone produces when it is positioned at a place where the sound pressure level is 94 dB re 20 µPa (94 dB equals 1 Pa). Low sensitivity (meaning only little voltage is produced) will be in the range of 1-5 mV, for instance like handheld vocal microphones. If the sound source is at a distance from the microphone, the microphone's sensitivity should be relatively high to produce a sufficient signal level: The sound source in the range of 10-30 mV. An example is the DPA d:dicate™ 4017B, a shotgun microphone with a sensitivity of 19 mV/Pa.

Selfnoise:  Every microphone produces noise. The noise or the noise floor determines how much the microphone can be amplified. Normally the 'built in' noise is compared to that of an external acoustical sound source that would produce a voltage on the output equivalent to the level, which now stems from the microphone itself.

So the maximum acceptable selfnoise-level is determined by the level of the sound source. If the sound source is loud, it does not matter that the selfnoise is above say 25 dB(A). However, if the sound source is weak, the selfnoise should preferably be below 25 dB(A).

In most specifications you will only find one measure, dB(A), A-weighted RMS-level, which shows the selfnoise in ideal settings. Serious manufacturers also present a peak measure, which is considered to provide better information, especially on condenser microphones, since it shows the maximum level of selfnoise (a higher selfnoise figure).

Directivity pattern: Directivity expresses how much the microphone picks up in the on-axis direction compared to all other directions in a three-dimensional space.

The directivity is expressed by the width of the pickup angle. However, we use descriptions as 'omni', 'cardioid', or 'figure eight', referring to the pattern of the polar plot. For microphones with high directivity, terms like 'shotgun' are used partly because of the look of the microphone, and partly because of the more narrow directivity. If you wish to minimize off-axis noise, the microphone must exhibit high directivity.

Directional microphones normally exhibit proximity effect. This means a rise of low frequencies (more bass) when the microphone gets closer to the sound source. If the microphone is designed to be used very close to the sound source (like a handheld microphone or a headset microphone) then the microphone is equalised to have a flat response in the nearfield. This equalizing also means however, that low frequencies of distant sound sources will be reduced. So if you produce stand-up/vox pop in a crowded and noisy place, be sure to use a directional handheld microphone in order to reduce background noise, especially if it has extensive low frequency content.  Omnidirectional microphones do not exhibit this effect.

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