DSLR Audio – Hear what the camera Hears
“It’s easy to fool the eye, but it’s a lot harder to fool the ear.”
Probably the most important and difficult process for still photographers converting to DSLR Video is Audio. While great pictures and story are two of the three key elements of video production, it is audio that engages the viewer, immersing them in your production. It is not only important, it is crucial to have a basic understanding of audio and audio capture.
Just the Facts:
Sound as we humans hear it as vibrations transmitted through the air (or water) and known as Sound Waves. Sound waves are measured in Frequency and Amplitude. In air, sound pressure can be measured (or recorded) using a microphone, and in water with a hydrophone with varying sound pressure levels.
Sound Frequency is measured in the distance between the sound waves. A low frequency sound wave is further apart and has a deeper sound than a high frequency sound wave which as closer together and is higher pitch.
Audio Frequencies are measured in Hertz starting out at 1Hz and going as high as 100kHz (100 Kilo Hertz = 1,000,000Hz).
The typical human speaking voice fits inside the audio frequency range with the male voice range of 85Hz to 180Hz and voice range of an adult female 165Hz to 255Hz.
The human hearing range is roughly 20Hz to 20kHz (20,000Hz) and there is a considerable hearing variation between individuals, especially with hearing at higher tones (above 11kHz). So if we can measure audio from 1Hz to 100kHz there are all sorts of audio frequencies we humans cannot hear. A dog whistle for instance is above the human hearing range at 23kHz to 54 kHz.
Note: The Nikon D4s (Vocal Range) records audio between 300Hz and 3kHz, which is a good range for most spoken voice recordings. The Nikon D4s (Wide Range) records audio between 200Hz and 20kHz, which falls short for music, nature and sound effect recording.
Sound Amplitude refers to the amount of energy present in the sound wave. A human ear perceives amplitude as volume. Louder sounds have more energy behind them than soft sounds for instance but could be the same frequency. We measure the amplitude of sound waves in Decibels (dB) and while there is a direct correlation between Amplitude and Sound Volume, human hearing can vary where some can hear different sound frequencies at different volume levels.
Decibels are a unit of measurement used for measuring the amplitude in a sound wave and is written as dB. We are really measuring the Sound Pressure Level (SPL) of a Sound Wave and how loud it is in relation to the humans hearing threshold of perceived silence or 0dB. The human ear can perceive a range from 0dB to 140dB. In the graphic on the right is an approximate conversation of a Sound Pressure Level (SPL) to Decibels (dB).
Signal Measurement and Recording
Important Note: The measuring of sound and the recording of sound is measured differently. On most digital audio equipment today we see a Digital Full Scale Meter (dBFS) more commonly referred to as a dB meter running from about -60dB (or just 60 on the scale) to 0dB. The threshold of recording and human hearing and silence on a Digital Full Scale Meter is -95dB. and running up to 0dB which would be the maximum level of recording without distortion. Here are some approximate common sounds and recorded levels, which will vary, dependent on distance to subject and equipment used.
0db = Door Slam
-20db = Dialog
-50dB = Footsteps
-95dB = System noise or Silence
A 6dB increase/decrease of amplitude would be double/half the volume as perceived by the ear.
An audio amplifier is an electronic amplifier that increases the level of low-power audio signals (signals composed primarily of frequencies between 20Hz-20000Hz) to a level suitable for driving typical audio playback.
Is a measure of the ability to increase the power or amplitude of a signal from the input device to the final recording device by adding energy from an amplifier. While most photographers will just boost the gain for their audio recording it comes at a price just like boosting your ISO on your camera. Increasing the gain will amplify the signal, but it will also increase audio noise (from equipment) as well as other sounds like camera handling and possibly unwanted background sound.
Sample Rate and Bit Depth
Since all audio like today’s digital cameras has to be converted from analogue to digital there are two measurements that define the speed and quality of the audio being captured and encoded into our videos.
Is simply the number of times a sound wave is sampled over a 1 second time period. Sample rates in audio are measured in Kilohertz (KHz) and most in camera audio today is measured at 48Khz or 48,000 times per second. Higher sample rates produce a higher quality audio file than lower sample rates.
Is the sampling of audio signals amplitude and is referred to as 16-bit or 24-bit. The higher the bit depth the less noise will be apparent in the audio file.
The Inverse Square Law that says; the volume of audio level is doubled when the distance between the microphone and audio source is cut in half.
It is always better to place your microphone as close to your audio source as possible rather than amplify your signal.
A look at in camera audio recording:
The problem with most in camera DSLR audio recording is two fold.
First is the microphone placement. While it is always preferable to place the microphone about 6-12 inches from your subject its hard for obvious reasons to get it that close to the subject. Also care has to be taken with the camera during recording to avoid unwanted camera handling noise.
The second stems from the built in amplifiers used. While they work well for voice under normal conditions they fall short when the audio signals have to be amplified with the in camera audio controls. Amplifying audio signals increases not only the level of your target audio but background sounds and possibly noise from the various equipment used in your recording process.
Harry Kaufmann of Beachtek, (www.beachtek.com) says, “Since the in camera preamplifiers are relatively noisy at high gain (higher amplification), the key is to reducing the gain as much as possible (On a Nikon D810 camera sensitivity levels go from 0 to 20). It is quite simple – the lower the gain, the lower the noise. It is also very important to record at the proper levels. Too high and you will cause clipping and distortion, too low you will be close to the noise floor and pick up excessive hiss.”
External Recording Devices:
An active audio adapter is basically a relativity “high quality” amplifier used to take the signal from a Microphone, amplify the signal and pass it to the cameras preamp and on to in camera audio storage. The object is to let the audio adapter do the heavy lifting in amplifying the audio from the microphone passing a clean signal to the camera.
So when capturing in camera audio how high can you turn up the gain before the audio signal starts to degrade?
It will vary from camera model to camera model and with the microphones and connectors used. But according to Harry, a camera sensitivity level set to about “5” is the sweet spot (on the Nikon D810).
If you want to amplify your incoming audio above this consider using Beachtek’s DXA-SLR-ULTRA’s high quality audio pre amplifier. Set the camera’s sensitivity level according to the Ultra’s instructions and then adjust the gain on the adapter to give you a reading up to about 0dB and -12dB on the Ultra’s VU Meter. This is where an Audio Adapter provides the ability to record very clean audio for voice directly to the camera.
A word about Cables
It is important to use the proper cables for your audio recording (normally supplied with your devices). You should also check and replace faulty or damaged recording cables, as they can introduce noise and cause other recording issues.
Hand held audio recorders have a long history with DSLR Video used either to get better quality audio recording by replacing the in camera audio, or during a multi camera shoot where the audio recorder is the main source of audio replacing the audio tracks of all the other cameras. (Red Giant Software’s Plural Eye’s makes the task of replacing audio tracks a snap).
Gino Sigismondi of Shure Microphones says “There’s a 6 dB drop in level every time you double the distance microphone from audio source. So if you move the microphone from 6 inches away to 12 inches away, you’ve lost 6 dB on signal level.” So whatever and however you’re shooting, keep the microphone close to the sound source. Remember the inverse square law?
Microphone Types (Dynamic or Condenser Microphones)
Dynamic microphones don’t require an additional source of power but they can be somewhat limited in their sensitivity, so it’s especially important for the sound source to be very close to the microphone.
Condenser microphones require an additional source of power that can be supplied by a battery or the next device in the audio chain. They can be highly sensitive so they are better suited for capturing DSLR Audio
Shaped or Flat Frequency Response?
Making the right choice here depends on what you’re recording. The human voice versus a musical performance or outdoor sounds for example.
A shaped response is probably one of the more common dynamic microphones. Usually a shaped response is aimed at capturing the frequencies of the human voice and will give you everything you need to capture those frequencies.
A flat response means that the microphone is equally responsive at all frequencies. It will give you the most accurate sound and a wider frequency response. You’d probably choose this frequency response if you’re recording a musical performance or nature sounds.
(Nikon D4s (Vocal Range) is a shaped response and the Nikon D4s (Wide Range) would be an example of a Flat Response.)
Microphone Capture Patterns (Polar Patterns)
Microphones can capture audio in all directions or be more directional rejecting sounds from the sides or behind it.
Omnidirectional or Unidirectional?
A microphone’s polar pattern describes how it responds to sound coming at it from different directions.
Omnidirectional microphones are sensitive to sounds coming at it from any direction. The benefit is that you don’t have to worry about aiming the microphone. The downside is that it doesn’t reject sound coming from sources behind you for instance.
Unidirectional microphones can be especially useful in video since they can be aimed at a sound source (with precise degrees of coverage) and away from undesired sound sources.
There are three types of unidirectional microphones:
Cardioid, Super cardioid and Hyper cardioid. Also a shotgun microphone is an extreme version of a unidirectional microphone with a very narrow pickup pattern.
Microphone Types and Usage
A lavaliere microphone is placed about six to eight inches below the speaker’s chin. The most common polar pattern for a lavaliere microphone is omnidirectional though there are unidirectional types may be used to control excessive ambient noise. The Sennheiser G3 Series is a very popular versatile lavaliere wireless microphone.
The most common kind of microphone is the handheld type, they can be wired or wireless. Omnidirectional are the most common of handheld microphones and it should be positioned about 6”-12” from the audio source.
This microphone is named for the long, slotted interference tube in front of the microphone cartridge. This tube helps reject sounds coming from more than about 30 degrees off to the sides. They are not like telephoto lenses allowing you to focus sound on distant subjects, as they will pickup sounds from behind the subject.
Shotgun microphones can be positioned slightly above, below, or to the side of the sound source, so that the microphone doesn’t appear in the camera frame. Avoid aiming shotgun microphones at a hard surface, such as a tile floor, brick wall, or hard ceiling. These surfaces reflect sound waves, and may reflect background noise into the microphone or cause the sound to be slightly hollow.
Shotgun microphones are more sensitive to wind noise than standard microphones, so try to avoid moving the microphones rapidly and use a foam windscreen if possible.
These external camera-mounted microphones can run on AA batteries and have become increasingly popular with videographers. Since they mount directly on top of the camera, handling and motor noise is practically eliminated, and the lobar pattern of the condenser shotgun microphone rejects off-axis sound.
Shotgun microphones do a great job of picking up frequencies in the vocal range, but they may not be the best choice for recording musical instruments. Use the windscreen to keep wind and drafts of air out of your audio and use the microphone’s low-cut switch to filter out undesirable low frequency sounds.
Mono or Stereo Recording
While most DSLR cameras now record in Stereo most microphones produced for external recording are Mono especially when looking at a Shotgun type of microphone. One of the big advantages for using stereo microphones is that you get a richer sounding playback.
So where to start:
This will depend on what you want to record and how your videos audio will be played back and of course how fussy you are about playback.
Voice or Music/Nature
Voice: The camera’s on board audio recording will work adequately when the camera/microphone are placed near the subject so the camera’s audio gain (on most cameras) can be kept down to as close to “Camera Microphone Sensitivity Level” of “5” as possible.
If this isn’t possible then consider using an external microphone and audio adapter like the Beachtek’s DXA-SLR-ULTRA to increase the gain cleanly.
This system will work well for all situations but when trying to record the expanded range of Music/Nature the cameras on board recording capabilities may comprise some audio frequencies. This will vary from camera to camera brand.
Music/Nature: In a shoot where audio is very important you may need to capture an expanded audio range beyond that of what your camera is capable of, it is here a hand held audio recorder becomes essential. While I will still use a DSLR with an external microphone and a Beachtek’s DXA-SLR-ULTRA to do my base recording, I will use a Zoom H5 (or Zoom H6) Handy recorder and a microphone to match the recording situation. And then use Red Giant Software’s Plural Eye’s software to replace the cameras the cameras audio tracks.
For Interviews it is hard to beat a Lavaliere, Omnidirectional or a Shot Gun style microphone and remember to place it as close to the audio source as possible and keep it just out of camera sight. Setting your Nikon camera to “Vocal Range” will give you the best audio range for the spoken word.
For music/nature look to getting an external stereo microphone and remember to place it as close as possible to your source. Setting Nikon camera to “Wide Range” will give you a wider audio range capture.
To record wildlife or sounds at a sports event where you cannot get your microphone close to the action there are highly directional recording microphones called parabolic microphones.
On camera recording get as close as you can to your subject and remember it does a pretty good job generally but it captures all kinds of sounds, from camera handling noise to background chatter.
Using an audio adapter
You will probably see the advantage in having an audio adapter right after you purchase your first microphone. The reason you bought a microphone was to get better audio and an audio adapter will give you better audio control and quality when recording into your camera.
When it comes to using an audio adapter I always look to Beachtek and Harry Kaufmann for advice. He would tell you different cameras and Beachtek Audio adapters work slightly differently so make sure you read the instructions specific to your camera and audio adapter.
Using an audio recorder
It is very important to match the sample rate of your audio recorder to the recording sample rate of your camera. This is measured in Kilohertz (kHz) and most Nikon cameras built in the last couple of years record at 48kHz per second and 16bit. If your camera records at a different sample rate your audio will drift over time and the pictures and audio will not match. If you are replacing your camera’s audio with audio from a audio recorder in your final production you could record at the same sample rate of your camera but choose the higher bit depths available on your recorder.
Before you record:
Hear what your camera or device hears by using headphones. Use over the ear “cup” type headphones rather than ear buds for this as it’s not only more convenient it will allow you to concentrate on hearing the output of your device.
If you were looking for a pair of headphone spend $80-$120 on a pair of flat response closed ear (cup) type headphones. Sony’s MDR-7506 headphones seem to be the most widely used by sound recordists.
There are a number of issues you should be looking for after you have your equipment set up.
#1 – Your microphone isn’t close enough to your subject. You will want to be lazy and just amplify the signal from a poorly placed microphone. But this will only create other issues for you like poor audio quality or background sounds becoming more dominant. Spend the time and place your microphone carefully.
#2 – There are ambient sounds or noises that interfere with your audio. A fan, other voices or traffic noise for example will have to be turned off, included in your video pictures or you will have to change location.
#3 – You record your audio either at too low of a level or too high of a level and it can’t be fixed in post. Do a quick audio test; watch your audio recording levels while recording. After finishing your recording check it before moving on to the next scene. Don’t assume you have an audio track until you have confirmed it, as many things can go wrong during recording from audio cables coming unplugged to batteries dying.
In short recording great audio is an essential part of any video, spend some time learning and practice good audio recording techniques. You will be thankful you did when you come to the postproduction stages of your video. – Nick Didlick