Choosing a microphone
suessian megaphone / Michael / CC BY-NC 2.0
This guide looks at some of the different types of microphone you may encounter, and gives an introduction to some of the features which may help you choose the right type for your needs, and the techniques you can use to get the best from them.
Types of Microphone
Microphones are normally categorised by two characteristics, the type of transducer (i.e. the way sound is converted into an electrical signal) and the direction from which they capture sound (the 'pickup pattern’).
Microphones most commonly use two types of transducer, dynamic and condenser. Dynamic microphones do not require power and are generally less expensive and more robust than condenser microphones. Condenser microphones need a power supply (e.g. a battery) and tend to produce a ‘warmer’ sound. Shown below are examples of dynamic and condenser microphones.
The most common pickup patterns of microphones are directional (“cardioid”) or omni-directional. In some microphones it is possible to select from a variety of pickup patterns.
Directional microphones are most sensitive to sound directly in front of them. This sensitivity drops off as the sound source moves off-axis until it has reached the rear of the microphone, where no sound is picked up. As a result it is good for reducing unwanted sounds in a recording but it is important that it be pointed directly at the desired sound source.
Omnidirectional microphones pick up sound equally well from all directions. As a result they are less good at isolating a single sound source but they require no special positioning.
Another microphone which is sometimes encountered is the shotgun or hypercardioid mic. This is similar to a cardioid but is even more directional. As such it gives even greater sound isolation but is correspondingly more sensitive to positioning.
Microphones record to a single channel of audio, also known as mono recording. There are, however, such things as stereo microphones. These are, in fact, two directional microphones which are mounted together in a single case and oriented in different directions. A stereo microphone will record to two channels of audio, left and right, giving a stereo recording.
However it is more common to record audio in mono and then to simulate stereo in the mixing process. This is done by 'panning' each mono recording, that is, to vary its loudness in the left and right tracks: the greater the difference in loudness between left and right, the more the recording will appear to come from the loud side; if left and right are equal, the recording will appear to come from straight ahead.
Microphones for Voice
The majority of educational audio and video productions will feature the spoken word in one form or another. Recording the human voice is one of the most demanding media production activities - its subtleties and nuances, and therefore ability to transmit information, can be enhanced by a good microphone used well, or damaged by poor equipment, configuration, or technique.
The best type of microphone for recording the voice is generally considered to be a condenser microphone, ideally one with a large diaphragm.
Single voices are normally recorded in mono. Multiple speakers can also be recorded in mono, but stereo will give a better sense of space and separation. This can be done either by recording with a stereo microphone or by using a number of normal microphones and then mixing the finished recording to assign apparent locations to each voice.
Microphones for Video
In video production, there are a few different ways to use microphones:
1. Camera-mounted
The microphone is attached to the camera.
2. Hand-held
For example, the interviewer points the microphone to the interviewee.
3. Lapel (or lavalier or clip-on)
Lapel is quite popular in video production. This has the advantage, not only of close proximity to the speaker, but also of being attached to the speaker and so moving around as the speaker moves around. Lapel microphones are often used with radio systems.
4. Shotgun microphones
Shotgun microphones can be mounted on the camera, a boom pole, or a stand. This is particularly useful in situations where the microphone cannot be placed very close to the speaker (often because the video maker doesn't want the microphone to be seen in shot); in such cases, the extra directionality goes some way towards compensating for the microphone's distance.
Many consumer video cameras come with poor-quality microphones or are not capable of using an external microphone, and in this case a separate portable audio recorder should be used. Recorded audio can then be combined and synched with video in post-production. A simple handclap made on camera once video and audio are being recorded will make synching of the audio and video a simple procedure.
Whatever microphone you use, you should ensure that it is compatible with the camera or separate audio recorder. This includes correct cable connector types, whether the microphone is mono or stereo and, if a condenser mic is used, how power is supplied to the mic. Professional and semi-professional video cameras and recorders may provide some or all of these options, but consumer cameras and even the best video-enabled DSLRs may not. However, there are DSLR adapters available to enable the microphone to connect to the DSLR cameras. Below is a photo of an audio adapter.
Beachtek DXA-SLR PRO - DSLR Audio Adapter
With regard to powering condenser microphones, professional and semi-professional video cameras and recorders usually have a feature called phantom power, which is the ability to supply power for a microphone down the same cable which is transmitting the microphone signal to the recorder. The power will not affect the audio signal and the microphone will not need its own power supply.
USB Microphones
USB microphones combine a microphone capsule with a built-in digital converter to allow direct connection to a computer without the need for a PC soundcard or, indeed, an audio input.
USB microphones are the simplest way to get a good quality microphone signal into a computer with the minimum of configuration. Some newer USB microphones have adapters to enable them to be compatible with mobile devices which offer no conventional microphone inputs (e.g. the iPad).
Yeti USB Microphone from Blue [Attrib: Mark Seymour]
Many USB microphones also provide a headphone output, allowing you to monitor sound during recording - something that would always be done in a professional voice-over session.
Samson Go Mic mini USB multi-pattern condenser microphone with headphone output. Windows/Mac
USB microphones come in a variety of physical forms: table-top 'stem' microphones, boundary microphones, stand-mounted mics of many shapes and sizes, or combined with headphones into USB headsets. Each of these can produce usable results depending on context, though be aware that there can still be a wide range of audio quality, as demonstrated by this audio comparison:
Choosing a Microphone
To get good sound from a microphone it is essential that you choose one which is suitable for your activity and environment, and which is compatible with your system. It is advisable to read the specifications before selecting. It can also be informative to look at equipment reviews on the Internet.
One good way to choose a microphone is to try it yourself. If you can, call or visit a vendor and ask for a demonstration. If you have a laptop, camera or portable recorder which you intend to use, take it with you so that you can hear the mic in action with your kit.
Portable Recorders and Phones
Many portable digital audio recorders are available which record stereo or multi-channel audio to internal memory and/or memory cards, and feature built-in microphones (some also have external microphone inputs).
Zoom H4N recorder [Attrib David King]
Smartphones (e.g. the iPhone) also have good quality built-in microphones, dedicated audio recording apps, significant on-board memory and facilities to transfer or upload recordings wirelessly and/or via the internet.
Radio systems
When you are delivering training or lectures, you will often walk around the available space and using cabled microhphones becomes too restrictive. The cabling connecting the microphone to the recorder can be eliminated with the use of a wireless system. A wireless system consists of a transmitter and a receiver. The transmitter (attached to the microphone) is usually a small device which can be worn by a speaker or attached to the end of a microphone and transmits the audio to a receiver. The receiver receives the live audio feed from the transmitter and is normally connected to a recording device (which may be a video camera) and/or audio system such as the lecture speakers to ampify the speakers voice. There are also some systems, such as Bluetooth microphones, where the microphone and transmitter are combined into a single unit.
Care should be taken when choosing a radio system to follow current legislation regarding radio frequency bands, and that the system will not interfere with any in its proximity (e.g. local theatres and lecture theatres). If in doubt, contact an authorised vendor.
Microphone Inputs
Soundcards, computer audio interfaces, cameras and audio recorders all offer microphone inputs with a variety of different connectors and characteristics. While it may be tempting to think that they are all doing the same simple job, there is in fact a huge range in the quality and features of microphone input.
The mini-jack ‘Mic In’ on a generic PC soundcard or a cheap video camera can add hiss and interference, even if used with a great microphone, and have other sonically undesirable attributes. Some types of microphone (especially many types of condenser) are not compatible with these type of inputs.
At the other extreme, dedicated studio quality mic pre-amplifiers can cost extraordinary amounts of money, with ultra-linear, transparent response, or carefully tailored ‘vintage’ character, depending on the engineer’s preference. Good mic preamps will have a control for altering microphone gain before digital conversion (i.e. a physical gain knob).
Pay attention to providing your microphone with a suitable quality of amplification and conversion to get the best from it. However, while they will complement a good microphone, high quality pre-amps cannot make a silk purse from a sow’s ear; all elements of the chain need to be of similar standard. A simple way to avoid this concern is to use a USB microphone, which does its own amplification and conversion.
Making Good Recordings
What makes a good recording? The two main criteria are the best possible signal to noise ratio and a lack of distortion.
The signal to noise ratio is simply the difference in loudness between what you want to hear (a speaker's voice, a musical instrument, etc.) and what you don't want to hear (room noise, sounds from outside, hiss from the recording equipment). Distortion occurs when the sound source is too loud for the recording setup and is, as a result, not faithfully recorded. This results in a harsh sound which, in the case of the human voice, can make the speaker harder to understand.
The most important weapon in the sound recordist's arsenal is proximity. The closer your microphone is to the sound you want to record, the louder it will be relative to the sounds you don't want to hear, i.e. the signal to noise ratio will be better. When recording a voice the microphone should be placed as close to the speaker's mouth as reasonably practical. Note that if the microphone is too close you may get "popping" -- sharp explosive sounds whenever the speaker pronounces a 'p' or 'b'. One way to avoid this is to use a pop shield, usually a piece of metal mesh or screen placed between the speaker and the microphone.
Another easily controlled variable which can greatly improve the quality of a recording is the location chosen for that recording. The location should ideally be one with no extraneous noises (traffic, conversations, air conditioning) and also one that is acoustically suitable (no echos, not too reverberant a space).
Having found a good location and a good microphone position, the most important remaining task is to set the recording level correctly. This is the sensitivity of the equipment to the sound source. Essentially, you want to ensure that the sound source is as loud as possible without there being any danger of it distorting. This is best done with the aid of a recording meter, which will almost certainly be found on any piece of recording equipment: the meter can be monitored while the sensitivity of the recorder is adjusted. Since any sound source will vary in loudness it's a good idea not to adjust the sensitivity so that the sound is recorded at a level just below distortion. If the loudest sounds you encounter while setting the levels reach 80% or -6dB on your meter you will probably have the best setting. However it is a good idea to monitor the levels during the actual recording in case the sound gets substantially louder, in which case you should reduce the sensitivity more.
Stereo Recording
Recording in stereo allows you to give the recording a sense of space. For example when you are recording a group student activity you may want the recordng to sound like there is group in the session and recording in stereo enables you to hear sounds from various directions and distances invoking a sense of the group being together.
There are several different recording techniques used to give a better sense of space, and each has its relative merits, be they simplicity, flexibility, realism, or mono and surround compatibility. If you are not in a position to spend time manually configuring a stereo recording rig, then preconfigured stereo microphones offer ‘out of the box’ stereo recording capabilities, and are set up to give an optimised result for their chosen specifications and characteristics.
Sony ECM-MS907 stereo microphone
[Attrib Evan Amos]
Further reading
