Oxford Student Web Media TeamMon 25 October 2010

Audio Recording: Microphones and Digital Audio Recorders

1) Introduction

Audio is one of the most important aspects of any type of filmmaking, but is also one of the least valued by inexperienced filmmakers. How many times have you watched a low-budget short film where the director has clearly given a lot of thought to the story, framing, lighting and performance, only for wind noise or traffic to completely distract from the dialogue. An audience will often forgive a bad picture; sometimes you can even pass it off as an artistic decision. Not so with bad audio. Your audience will simply tune out.

Sound is as important as the picture. Treat them 50-50, at least. For interview work at the University, remember that if the picture turns out bad, you can always release your interview as an audio-only podcast. If the sound is bad, you may have to abandon it altogether.

There is one golden rule when it comes to sound. You can’t “fix it in post”. Get as good – clean, untouched - sound as you possibly can at the time. This means monitoring it constantly during the shoot. Understand that there is very little that can be done to fix bad sound after the fact. If the audio is unusable, consider re-recording as this will usually be the least time-consuming method.

This brief guide examines some of the equipment and practical techniques that will ensure that you get the best possible audio for your production. The one key piece of location equipment it doesn’t cover is mixers. Provided you know how to use it, you’ll get better sound more easily with a mixer, but good mixers are expensive and out-of-scope for most of the work we do at the University.

2) Microphones

Microphones contain a diaphragm that vibrates when sound waves cause air particles to move over it. This mechanical vibration is converted into electrical power by a transducer in the microphone, and the electrical signal travels down a cable and is processed and amplified by the mixer or recorder.

2a) Microphone Types

There are two main types of microphones used in location recording – the dynamic microphone and the condenser or electret microphone.

2a - i) Dynamic Microphones

In dynamic microphones the signal is generated when a small coil of wire attached to the diaphragm moves inside a magnetic field as the diaphragm vibrates with sound waves. Dynamic microphones don’t require batteries or phantom power (more on phantom power later), but only produce a tiny voltage and need to be very close to the sound source to pick up an adequate signal. This means you have to increase the gain.

Dynamic microphones are most commonly used as vocal mics for live concerts. During a public lecture Q&A session, the microphones passed around amongst audience members will be dynamic.

Shure SM58 dynamic microphone Sennheiser UHF radio microphone (and receiver)

2a - ii) Condenser Microphones

Condenser mics differ to dynamic mics by having a variable capacitor with two plates, one of which is fixed, the other is attached to the diaphragm. Sound waves cause the diaphragm to vibrate and the distance between the two plates changes. This change is read by a small amp in the microphone head.

Because the condenser microphone contains an amp, it must be powered by an external source such as a mixer or camera. This extra power means that the microphone can pick up a higher quality sound from much further away than a dynamic microphone, which makes condenser microphones ideally suited to any situation where the microphone must remain out-of-shot – i.e. most film and broadcast work.

Condenser microphones are extremely sensitive to sound and, unlike dynamic microphones, are not designed to be held in the hand. Even when using a boom pole, condenser microphones will pick up the slightest rustle of hand movement on the pole.

Unlike dynamic microphones which are very rugged, condenser microphones are particularly susceptible to damage through excessive vibration, dampness and extreme temperatures. They should be treated with care.

Rode NTG-3 condenser microphone Sennheiser MKH 416 condenser microphone

2b) Microphone Pickup Patterns (Polar Response)

In addition to variations in signal strength, different microphones are suited to different tasks by virtue of their pickup pattern i.e. how well they pick up, or reject, sound in certain areas around the microphone.

For example, when interviewing a person on the roadside, you want the boom microphone to pick up what the person is saying but reject as much as possible the traffic noise from behind the microphone and camera. However, when recording a round-table discussion in a small room, you’ll want to record all speakers equally.

The two most common kinds of polar response patterns in location recording are omnidirectional and cardioid.

2b – i) Omnidirectional

An omnidirectional (or nondirectional) microphone's response is generally considered to be a perfect sphere in three dimensions (although in the real world this is not quite the case).

This pattern enables the microphone to pick up sound in equal strength all the way round its capsule. Most lapel/personal mics in broadcast are omnidirectional – useful because you don’t always have time to attach the mic exactly the way you’d like, and it may slip and point to one side during the recording.

It also means you can turn the mic head upside down to avoid nose snorts or overpronounced ‘P’s’.

2b - ii) Cardioid

Cardioid microphones are so labelled because their polar response is supposed to resemble the shape of a heart. Most boom/gun mics conform to a variation of the cardioid or hypercardioid polar pattern.

The cardioid pattern means that the mic hears more sound from its front side and ignores sound occurring behind it and somewhat from the sides, depending on the exact pattern.

The mic should be pointed directly at the sound source because this is the most responsive area of the mic (and also why it is referred to as agun).

A third type is the “figure of eight” or bi-directional microphone, which receives sound from both the front and back of the element.

The spectrum of microphone pattern possibilities is actually pretty limited. At one end are all omnidirectional mics, at the other end are all figure of eight mics. Everything else, including all of the various cardioid mics, fall somewhere between.

Hyper cardioid Super cardioid Shotgun Figure of Eight

2c) Frequency Response

The frequency response of a microphone relates to how well it reacts to the tonal range of the source sound – in most instances, the human voice. The human voice typically covers a range of frequencies between around 20 Hz to 20 kHz, with a peak in the 1-3 kHz region. Most of the energy is concentrated in the 125 Hz to 6kHz region

The figure below shows the frequency response of a Rode NTG-3 boom mic. This microphone has a excellent frequency response for speech and dialogue because the line is flat with a slight peak at the top end. This means that it is able to effectively pick up a wide tonal range but does slightly better for the higher frequencies you get from most people’s speech, making it good at capturing clean crisp dialogue.

We’ll come back to the frequency of the human voice when editing our audio in Audacity or Soundtrack Pro.

2d) Application-specific designs

2d – i) Boom Microphone (condenser, cardioid)

Ideal for documentary or film. Can be mounted on camera for one-man documentary shoots, or if you have an experienced second pair of hands, can be attached to a boom pole or pistol grip in order to get it closer to the sound source, thereby increasing the signal-to-noise ratio. Often used with a blimp and softie (“dead cat”) to reduce wind noise when shooting outdoors. Be aware of hand-movement on the boom pole – this microphone will pick up every creak of your knuckle.

Could also be mounted on a stand on the lectern to capture a lecture, but highly-directional so make sure your subject isn’t going to wander across the room mid-talk. Also be aware of trailing cables with this approach. Popular examples include the Sennheiser ME66, Sennheiser MKH 416 and the Rode NTG-3.

2d – ii) Lavalier Microphone (condenser, usually omnidirectional)

Made for hands-free operation and worn on the body. Can be connected directly to the recording device or, more commonly, to a wireless transmitter which is placed in the speaker’s pocket or clipped onto their belt. Can be hidden from view and is Ideal for any situation where you can’t get a boom mic. They are particularly well suited to recording a lecture by a single speaker, as typically, they will pick up very little background noise. Also well-suited to outdoor documentary news shooting, particularly for a one-man crew.

Using a lavalier microphone does have its disadvantages. The quality is lower than that of a boom mic and not ideally suited to drama (though perfectly acceptable for broadcast and documentary work). They are also prone to clothes rustle, “accidental” muting, and they’re not particularly easy to attach to some clothing (e.g. a formal dress). They are also susceptible to interference from other wireless microphones in the immediate vicinity, and a loose connection between the microphone and transmitter (a fairly common occurrence) will render the audio useless.

When relying on a lavalier microphone, try to get backup audio with a second device e.g. a portable audio recorder with built-in microphone.

2d – iii) Boundary microphone (condenser, omnidirectional or cardioid)

Used in videoconferencing and ideal for panel discussions and other situations where there is more than one speaker and you don’t have enough lapel microphones (or don’t have time to switch the microphone from speaker to speaker).

Usually connected directly to a recorder but can sometimes be connected to a wireless transmitter.

2d – iv) On-board microphones (condenser, cardioid)

Prosumer cameras such as the Sony Z7, and field audio recorders such as the Marantz PMD661 actually have very decent condenser microphones built in. The challenge when using on-board microphones is getting them close enough to the sound source to achieve a high enough signal-to-noise ratio.

On-board microphones are ideally suited to small, silent rooms in one-on-one recording scenarios. They’re not very well suited to recording in noisy

environments (such as outdoors) and are next to useless when recording lectures in large halls, as they will pick up too much echo and background noise.

3) Audio recorders

On a film set, it is common practice to record audio separate to video. This allows the sound recordist to be free to move wherever he/she wants in order to capture the best possible sound, without being tied to the camera operator. Modern digital processes can achieve reliable and perfect synchronization, but for one-man documentary or lecture filming, it is more practical to record audio directly to the camera to avoid sync issues. By the end of a long take, such as a 1 hour lecture, audio recorded separately will typically be several seconds out of sync from the picture. Of course, it is still always a good idea to use an audio recorder to get backup audio for any filming, and they can be very useful in situations where you cannot record audio to the camera e.g. when filming using a cheaper camera with no external microphone input.

When thinking about creating a podcast, although video is in vogue, in many situations audio will be enough. It takes up less hard drive space than video and is easier to edit using free software such as Audacity (http://audacity.sourceforge.net). You can more easily remove and/or reorder audio to best tell your story than with video.

Both the Marantz PMD661 and the Zoom H4n can record audio at a very high quality, using either the in-built microphones, or by plugging in up to two microphones via the XLR inputs.

Marantz PMD661 Zoom H4n

Useful tip - it is always a good idea to put batteries in the audio recorder even if you plan to plug it into the mains. Unlike tape, where a sudden power failure at the end of the recording would mean you lose only the last few moments, digital recorders must finish writing the complete file or you will lose the entire recording. Always remember to press stop and wait for the file to finish writing. Don’t just pull the plug! Batteries will give you a lifeline if you make this mistake.

4) Terminology and Useful Info

Mono vs StereoIn location sound recording, sound is usually recorded in either mono or multiple mono, rather than stereo. If you are mixing sound into a camera using two microphones on one sound source, it is best to mix one mic into the left channel and the other into the right channel, as a dual-mono mix. This gives the sound editor full control over which mic to use and allows him to mix the feed from both mics into stereo if he chooses. When using the inbuilt microphone on a digital audio recorder, recording in mono means you will use only half the storage space than if you recorded in stereo, as the channel is simply duplicated.

Phantom PowerPhantom power is the 48V current that is supplied to a condenser microphone to enable it to work. The term Phantom Power was chosen because the power for the microphone runs down the same wires as the audio, but is engineered in such a way so as to not interfere with the audio. This Phantom Power is supplied by the battery in the camera or audio recorder. Sometimes you need to manually enable Phantom Power on the recording device. The more phantom mics you use, the more battery power you use.

Mic and Line LevelsIt is very important when mixing sound into a camera to set the input level to the right mode. The electrical current generated by a microphone is very tiny – it can be measured in millivolts. The current produced at line level (which is typically delivered by mixers or CD players) is 3 – 4,000 times greater than of microphones and can be up to 2 volts. Therefore, if you were to send a line- level signal out of your mixer into a camera expecting mic-level, it would result in a massive distortion, and not do the camera much good either! Likewise, if you were to plug a mic straight into a camera expecting line-level, you would barely hear any level from the mic.

5) Anti-panic checklist

1. No audio at camera: Check camera powerCheck camera input settingsCheck cablesCheck Phantom supplyCheck mic power/mute switch and batteries

2) No audio in headphones: Headphones plugged in right?Headphone volume turned up?Swap headphones

3) Distorted audio: Check camera inputs (mic/line) are correctCheck all batteriesSwap mic and cables