Sound Production Theory

Lecturer - Dave Paterson

Sound Production theory is covered by four Learning outcomes, listed below:

  • LO1: Demonstrate an understanding of  common components  found in the audio recording and reproduction process.

  • LO2: Demonstrate an understanding of microphones and microphone techniques

  • LO3: Describe the principles of stereo audio imaging

  • LO4: Demonstrate an understanding of audio signal processing.

Each of these learning outcomes was assessed by means of an open book assessment.

LO1:

Inputs and Outputs:

  • Line Inputs: Analogue, Usually unbalanced 1/4inch jacks. Used for line level sources such as CD players and DI Boxes.

  • Tape inputs: Analogue. Used to connect tape machine (commonly now DAW outputs) to the mixing desk to allow us to monitor the signal being recorded.

  • Mic inputs: Analogue. Used to connect mic level signals, such as microphones and DI boxes. May have the facility for phantom power (+48v)

  • Digital: Used to connect CD players and DAT machines . Usually using SPDIF (opposite)

  • Optical: Used to connect devices such as ADAT.

  • Insert points: these are in the signal path, and are used to insert compression on a track, for example.

  • Group Outputs: Allows many tracks to be grouped, and sent to a multi-track recorder

  • 2 Track/Stereo Output: output from a mixer to a two-track recorder/DAW, allowing us to create a mixdown

  • Auxiliary Outputs/Sends: Allows the engineer to send a mix of different channels to an outboard unit or headphone amp, for example.

  • Direct Outs: These occur before the signal hits the Pre-amp, and sends each track  to another desk/recorder. Used extensively in Live, to allow a recording of the performance to be made.

  • Control Room Out: Allows us to monitor the mix in the control room.

  • Studio Out: Feeds the studio floor monitoring system.

Monitoring and Meters

We then moved on to have a look at typical characteristics of a studio monitor;

  • Wide, flat frequency response (not one frequency should be boosted or attenuated significantly)

  • Relatively flat off axis response (the HF should not change drastically as the engineer moves around the desk.)

  • Good response to transients and clarity (should be able to replicate the nuances of musical performance)

  • High output capability (allowing the engineer to hear quiet elements in the music)

There are commonly 2 types of monitor; the smaller near field Monitor and the larger, soffit mounted monitor. (see opposite). Engineers normally use soffit mounted monitors to mix the low and highs, then switch to near-field monitors to ensure the mid-range is clear, and the mix sounds good on more 'consumer' audio reproduction devices.

Meters provide a visual representation of how the audio passing through a desk is behaving, and ensuring the audio is travelling at optimum levels. There are generally 3 types of metering:

  • PPM: Peak Program Meter, 1-5ms response time, shows peaks very well.  0dBu = 4PPM. PPM are the meters I have mostly encountered live.

  • VU: Volume Unit. Response and Decay time average about 300Ms. Gives a more accurate depiction of overall levels. 

  • LUFS: Measures loudness, a new standard designed to prevent a second iteration of the loudness wars.

My PreSonus Eris 4.5 Near-Field Monitors.

The larger speakers to the rear are soffit mounted speakers in Eastlake 1 (Featuring Geoff Allan)

Spaced pair overheads being used in a recording session.

LO2

Microphones and Microphone Techniques:

 

LO2 began with us looking into different types of microphones:

  • Condensers (Capacitors): Sensitive and fragile microphones with a good frequency response. Commonly found as drum overheads or close miking string instruments. Require Phantom power. 

  • Dynamic: Cheap, rugged, simple. Lower output than a condenser, with a more limited frequency response.

  • Ribbon: Sensitive, fragile, not great with high SPL's. Tend to sound less bright than condensers - make good drum overheads for quieter genres.

Each different microphone has a specific polar pattern (the angles at which sound reaches the diaphragm and at which sound is rejected) and frequency response diagram. Find an example from an SM58 opposite.

Two microphone techniques I commonly employ live are:

  • Accent Miking: One element of a large ensemble is miked (a solo Vocalist in a choir, for example). This is a mono technique

  • Spaced Pair: Commonly employed for drum overheads, Two microphones are placed over the drum kit, equi-distant from the snare drum. This allows the engineer to give a depiction of how the drum kit sounds to the drummer/audience. (see opposite)

LO3 & LO4

Localising sound sources

The human brain uses 5 different methods of localising a sound source. Knowledge and exploitation of these concepts can help make mixes much bigger, more immersive and generally more enjoyable. The methods are:

  • Interaural Time Difference: the time it takes for a sound to travel from one ear to the other ear.

  • Interaural Level Difference: the drop in level from one ear to another ear.

  • Frequency difference: the loss of some frequency content due to the sound travelling around the head.

  • Comb filtering: when multiple signals reach the ear canal, through close reflections from the pinna, the ear is able to distinguish the directionality of the sound.

  • Phase Differences: the phase difference of the sound, created by the interaural time difference.

 

One technique commonly used to exploit these concepts is the HAAS effect, whereby one has the same sound both left and right, and delays one by roughly 50ms, creating the illusion the sound is coming from the non-delayed side.

Stereo and Pan Potted Mono

Pan pots on mixing desks essentially allow an engineer to place an instrument where he wants it to be in the stereo field. An example of pan potter mono is The Coostie Folks 'Shetland Square Park' (can be found here). In this track, the guitar is panned slightly left, along with the fiddles. To balance this, I placed the whistle on the right by the same amount that the fiddles are to the left, as well as placing the mandolin slightly to the right. I kept the bass, kick, snare and whistle harmonies centred.

One must be wary of panning in a live environment, as if something is panned too far left or right, it will affect the coverage, discriminating against some audience members.

Equalisation

An engineers best friend, that can also be their worst enemy. EQ is the act of boosting or attenuating frequencies, generally by up to 15dB. EQ can be used to eliminate unwanted frequencies (for example, using a High-Pass Filter on drum overheads to eliminate any unwanted low frequencies in that microphone.). They can also be used to boost desired frequencies (for example, increasing the body of the snare drum)

Compression

Compression reduces the dynamic range of audio signals, making the dynamic range more even. When used on a kick drum, it ensures the kick punches through the mix evenly at all times, even if, when it was recorded, the drummer was inconsistent with the level.

Reverb

Reverb are the complex reflections that result when a noise is produced in an enclosed space. Artificial reverb (Plugins or reverb units) are often applied to reduce the artificial sound of a track, especially if the audio has been recorded in a small, non-reverberant space. Through experience, I have discovered that instruments like the fiddle and tin whistle either need to be recorded in a large room (natural reverb) or have a generous amount of reverb applied, to ensure that the harmonic content is accurate.

Delay (AKA Echo)

Delay is a distinct repeat of the initial sound. Delay is a great way to add depth and character to a mix, but can quite easily ruin it if too much is applied. 

 

Phase, chorus and flange are all types of delay based effects.

Gates/Expanders

These are effects that attenuate the lowest level of signal, while leaving the louder portion of the signal untouched - the opposite of a compressor. Noise Gates are extremely useful on drum kits, as it can eliminate sound below the threshold, leaving only the signal from the element of the kit that the microphone is on. this can clean up the sound of the drum kit, but can also make a drum kit sound horribly artificial if over-used, as it will eliminate the resonant frequencies after the initial hit.

A 7-Band EQ in ProTools, with a high-pass filter applied.

A compressor in ProTools, being employed at the mastering stage.

© 2019 Will Derries.

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