If you wish to have a long career as an audio professional or musician it is important to practice safe sound:
• Monitor loudly for brief periods only.
• If you are in a situation where you are exposed to loud sounds, wear earplugs.
Protect your hearing. Loud noise exposure eventually destroys the sensitive hair cells
in the cochlea – modern medicine cannot yet make them grow back!
WEIGHTING
SPL measurements are usually weighted, meaning that they are biased, to better reflect how our ears perceive amplitude at different frequencies:
• dBA weighting severely reduces the measurement tool’s sensitivity to the low and high frequency extremes (much like we hear at lower SPLs), and is related to the 40 phon scale described earlier.
• dBC weighting is flatter, only slightly reducing the measurement tool’s sensitivity to lows and highs (much like we hear at high SPLs), and is related to the 100 phon scale described earlier.
recorded or played back. If you plug a mic in and there’s no sound, or the sound is distorted, a thorough understanding of signal flow makes it possible to quickly troubleshoot and fix the problem – and makes you a better and more employable audio professional.
Signal flow is case-specific, and differs depending upon the type of facility and specific gear in use – so it’s impossible to describe every scenario in this book. However, typical hardware mixing console and computer/digital audio workstation recording chains are shown in Figure 1.18.
MICROPHONES AND MIC LEVELS
Microphones change acoustical sound waves into electricity – specifically, variations in voltage. This voltage is very small, measured in millivolts. This type of signal level is known as mic level.
A mic level signal needs amplifying before it can go through the main mixing console circuits, or the analog to digital converters in a DAW system. A pre-amplifier, pre-amp, or
mic pre is the first circuit in the signal flow of a mixer that the mic is plugged into. A pre-amp
can also be a separate device, external to the mixer or audio interface. The pre-amp is a critical circuit – it needs to amplify the tiny voltage coming from a microphone, without increasing noise, undesirable distortions, or negatively affecting the frequency response or sound of the microphone.
Fi g u r e 1.18 Left: The signal flow of a typical hardware recording chain, including a mixing console. Right: An example computer/DAW recording chain.
MICS, SVNTHS, DI's ETC.
BE
LA
MONITOR SPEAKERS HEADPHONE MIXESMic/Line Control Room
Inputs Outs
MIXER
Aux Sends Insert Point Sends Insert Point ReturnsAux Sends FX or Line Inputs Direct/
Group/ Tape Main/Mix
Buss Outs Inputs Outs
J
C O M PRESSO RS/ GATES EFFECTS PR O C E SSO R S MULTI-TRACK 2-TRACK RECORDER RECORDER MICS, SYNTHS, DI's ETC. MIC PREAMPS Inputs AUDIO INTERFACE MONITOR SPEAKERSBE
_ Jf
- n
HEADPHONE MIXES B 3 D- 3 o n o 2 3 3 S3LINE LEVELS
A pre-amp applies gain (amplification) to the mic level signal to raise it to line level, which is the level the rest of the mixing console (or audio interface) operates at. There are two standards for line level signals:
• “+4 dBu” or professional level represents the audio waveform as voltages of up to about ±1.7 V.
• “−10 dBV” or consumer level represents the audio waveform as voltages of up to about ±0.4 V.
Consumer level “−10” interconnects are commonly found on domestic devices such as CD, DVD, Blu-ray players, and gaming systems.
• If you connect a −10 dBV device to a +4 dBu input you will see low levels (about 12 dB lower than they should be) and have to apply a lot of gain.
• If you connect at +4 dBu output to a −10 dBV input you will probably overload that input unless there is an input gain stage you can turn down on the receiving device. Connections between studio hardware are usually “+4” professional line level.
• For a “direct to stereo” recording, the main outputs of the mixing console are con- nected to a stereo recording device – a CD recorder, portable recording device, or an audio interface attached to a computer-based recording system.
• For multi-track recording, the subgroups, multi-track busses, or direct outs from the console can be used to send individual microphones to individual tracks in a DAW, hardware multi-track recorder, or an analog tape machine – yes, analog tape is still used! The playback outputs from a multi-track recording device will return into individual inputs on a mixing console. If mixing is taking place “in the box” (inside the DAW system), only a stereo left/right (or a set of surround outputs if mixing in surround) may come out of the DAW and be routed to the monitor speakers.
LOUDSPEAKER LEVEL
If passive monitor speakers are being used, the line level output of the mixer or DAW will need amplifying to loudspeaker level – which is the many volts required to move a loudspeaker cone (± several volts to ±50 V or more). The line outputs are connected to a power amplifier, which will handle that task. This amplification is built into powered or active monitors – hence the input is line level, and the speaker requires an AC electrical power connection for its ampli- fiers and crossover circuitry.
INSTRUMENT LEVEL
Electric guitars and basses feature instrument level connections. In terms of signal level, they are not dissimilar to mic level – so plugging a guitar or bass directly into a mic input will usually result in decent levels. However the impedances (the electrical load each device presents to the other) of the two devices are not designed to work together – and the sound will be muddy
1.15 GAIN STRUCTURE
Levels, levels, levels! A beautifully clean recording (as opposed to one that is full of hiss, noise, or distortion) is created in part by setting appropriate levels as signals leave one device and enter the next:
• If too low a signal enters a device, that device will add a greater relative percentage of its own hiss and noise to the signal. That extra noise will end up being turned up to compensate for the low input level somewhere later in the signal flow.
• If too high a level enters a device, then subtle or not so subtle distortion will result. Both these types of noise and distortion are undesirable.
With the exception of a pad control (which attenuates a microphone’s output by a fixed amount) mics do not feature a gain control on their output. They are connected to a pre-amp, and the gain control on the pre-amp is used to set the amount of amplification that the mic signal receives in order to bring it up to an appropriate line level.
On a digital dBFS scale, the golden rule is “do not clip.” To avoid potentially nasty distor- tion the “clip” or “over” indicators must not light. There are two schools of thought on digital recording levels:
• The older approach was to record with the levels as high as possible, without going “over,” while making sure enough headroom was left to avoid clipping unexpected louder moments.
• Given the huge dynamic range offered by modern 24 and 32 bit recording systems, it is much less necessary to “max the meters.” Peaking a little lower, maybe around −12 dB or so, leaving plenty of headroom means that that the digital convertors are not con- stantly running at or close to their upper limit. Many engineers claim they sound better that way.