There are quite a few guides, tutorials, videos, and other resources on how to capture usable boom audio. Most of these have a few usable sections with a bunch of fluff or useless information accompanying them. This is my attempt to compile a “quick guide” to capturing usable boom audio, assuming you don’t have a competent audio engineer in your employ (which I highly recommend). Audio is half of your deliverable product, as a cinematographer, so you should make sure that you capture the highest quality audio possible.
Proximity, Reflections, and the Inverse Square Law
Much like light, sound decreases exponentially as you move away from the source of the sound. One of the first things which you will realize when starting to capture audio is that it sounds increasingly awful as you move further away – which can be attributed to two things:
As you move further away, relative volume decreases. The background sounds (“noise floor”, for anyone trying to be technical), consisting of both the present background noises and any noises introduced in the recording process, increase when you increase the sensitivity of your recording equipment. As the source sound becomes quieter, bringing sound up in post production will also increase that noise. (If anyone says “just record some room tone”, ignore them; it won’t help you if your source recording is terrible, since that’s only a solution if you can gate and isolate the primary sounds in post.) This is one of the compelling reasons why boom operators are utilized; they allow a recording to be made from closer to the source.
Reflections. Light bounces off of everything, as we are taught, and we have to be careful to deal with those reflections, in terms of color, brightness, et cetera. Sound does the same thing. It bounces off of surfaces, but because sound travels slower than light, we notice when it takes a few milliseconds longer for sound to arrive at our recording device. Even more insidious, those reflections (which take on different properties and sounds based on the materials on which they are reflecting) can sound louder than the original sound – which will make your recording sound terrible. The closer you are in proximity to the source, the softer the reflections will be, in comparison to the original sound.
Not all microphones, or even microphone types are created equal. They all have different “polar patterns“ which describe the areas of sensitivity which the microphones use to pick up sound. When attempting to capture dialog, for example, an omnidirectional microphone would be a poor choice, as it picks up sound equally from all directions, taking away the ability to create greater isolation for the primary sound source. The most popular boom microphone type is the “shotgun” microphone, which has a very directed polar pattern, allowing specific isolation of the sound in question.
Levels, Peaking, and Limiting
Getting the recording level just right is one of the more tricky parts of recording external boom audio. If the audio is too high, “peaking” will occur. Peaking is the phenomenon which can be heard when the top part of a sinusoidal wave (which naturally recorded sounds have) becomes squared when the top of the wave is clipped by hitting the top limit for recording signal. If the audio is too low, the signal-to-noise ratio will be too low, and bringing the signal up to a usable level will bring the sound floor up to an obscenely loud level – making gating nearly impossible to perform.
To properly deal with this, you need to adjust the signal level so that the loudest sound comes in under 0 dB, which is where most recorders “peak”. If certain sounds surpass 0 dB, some audio recorders have the ability to apply a “limiter” effect, which will push the sound level back down to a usable value as it is recorded. It’s not a desired effect, but it can save you from clipping.
As you’re adjusting audio levels, you’ll see a constantly fluctuating level of audio when no primary sound source is active. This is the “sound floor”, and should be as far away from the bulk of the primary sound levels as possible.
Portable Recording Devices
I recommend avoiding the Tascam DR-40 unit unless you’re positive that you are using a balanced microphone. It tends to have a weird firmware issue which produces a strange clicking sound (almost impossible to remove) every quarter of a second when presented with an unbalanced microphone. If you’re worried about this, go for the Zoom H4n. It’s a bit more expensive, but it seems to handle less expensive microphones in a more able fashion, as well as having signal limiting and a host of other interesting features.
Compression and Formats
Most digital recorder units will record, at a minimum MP3 and WAV formats. WAV is an uncompressed audio format, which means that it takes up more storage space than a compressed format, but retains all of the information captured (at the resolution captured). MP3 is a compressed format, using a psycho-acoustic model, which means that it drops pieces of information which it figures we aren’t going to be able to hear. MP3 files take up a smaller amount of space compared to the same resolution WAV files – but that comes at the cost of throwing some of the information out. MP3 @ 192kbps or above tends to have enough information for most uses.
There’s no “correct” equipment which you need to purchase to be able to capture boom audio properly. There are, however, a few piece of equipment you’re probably going to need.
- Extendable Boom Pole. It’s tempting to go with a converted painters’ pole, but trust me – it pays to go with a decent boom pole. This is mainly due to the additional noises which can be generated by swinging around a makeshift boom pole. I personally recommend the On Stage MBP7000 boom pole for budget operators. It doesn’t have an internal mic cable, but it works very well.
- Microphone Cable (XLR). This is an easy place to skimp for some people, but you don’t want to pick up outside noise, so make sure you go with a shielded XLR cable which is a few feet longer than the maximum size of your boom pole, fully extended.
- Clip / Shockmount / Zeppelin / Windscreen. You need something to isolate the microphone from the wind, vibration, and other distorting effects of the environment, which would distract from otherwise relatively clean audio. Shockmounts can be had for relatively little, as can windscreens. A zeppelin can cost a bundle, unless you make a DIY one – but they produce very clean-sounding results.
- Wire clips / wraps. Either some electrical tape (a cinematographer’s best friend, after gaffe tape), or some inexpensive bobble hair ties (available at most dollar stores) will allow you to keep the XLR cable near the pole – otherwise you may find it dropping into frame at the most inopportune times.
- Microphone. There is a great deal of conjecture over the “best” budget microphone to use. The most important things to consider are the polar pickup pattern, the signal-to-noise ratio, and the frequency response of any microphone you’re testing.
- Portable digital recorder. Covered in the portable recorder section.
- Headphones. A set of headphones, preferably full cup earphones, are essential to monitoring the sound for disturbances and/or interruptions. My favorite pair is the Sennheiser HD-280 PRO, as they’re relatively inexpensive (under 100 USD), and produce a fairly accurate reproduction of live audio.
- Storage media. Make sure you don’t buy off-brand media cards. Try to stick with SanDisk and Lexar media, if you can. The trick is that these companies generally tend to QA their products a bit more rigorously than most off-brand manufacturers. This can make the difference between usable audio and a very upset director.
Technique and Directionality
Rather than iterate all of the techniques involved in actually operating a boom mic rig, and keeping in mind that pictures are worth a thousand words…
Audio is half of your deliverables – so make sure your boom operator knows how to deliver the best possible audio to your audio engineer for post production. Good luck!