The Recording Environment part 1

The Recording Environment

stuart.jones3@southwales.ac.uk

www.stuartjones.org

Agenda

Sound Waves

Room Acoustics

Sound Wave Reflections

Absorption Coefficients

The Sabine Equation

Reverb Calculation Example

Estimating the Reverberation Time

Reverb Calculation Example 2

Correcting the Reverberation Time

Absorbers

Reverberation

Sound Waves

Sound Propagation

The Recording Environment

What is Sound?

Sound is produced when an object (the source) vibrates and causes the air around it to move.

The Recording Environment

Sound Propagation

• Sound travels in air as a lengthwise or longitudinal wave. This sort of density wave is the way in

which sound is transmitted through air, gases and liquids.

• In solids we also find transverse waves.

The Recording Environment

Sound Propagation

Longitudinal Wave

The Recording Environment

Snell's Illustration of Sound Waves

http://www.youtube.com/watch?v=OQPI5Ng-3vI&list=PL689066FDE3D631CE

Disturbance takes place perpendicular to the direction which it is moving

Sound Propagation

Transverse Wave

The Recording Environment

Sound Propagation

Transverse Wave Machine

The Recording Environment http://www.youtube.com/watch?v=tihcRFWeZlQ&list=PL689066FDE3D631CE

Clip taken from - Sound Waves and their Sources (1933)

The Recording Environment

The Recording Environment

Sound Waves

Sound Wave Properties

The Recording Environment

Sound Waves: Fundamental Mathematical Relationships

The Recording Environment

Time

Am

plit

ude

0

+

-

t

The rate at which the source oscillates is the frequency of the sound wave it produces, and is

quoted in hertz (Hz) or cycles per second (cps). 1000 hertz is termed 1 kilohertz (1kHz)

The Recording Environment

In air, the speed of sound is approximately 340 meters per second

The frequency and wavelength of a sound wave are related very simply if the speed of the wave

(usually denoted by the letter c) is known.

c = fλ

(speed of the wave = frequency x wavelength)

or

λ = c/f

(wavelength = speed of the wave x frequency)

(λ = The Greek letter lambda is often used to represent wavelength)

The Recording Environment

Relationship between elapsed time and traversed distance for the propagation of a sound wave in air

time (ms)

2 4 6 8 10 12 14 16 18 20 22 24 ms

0.7 1.4 2.0 2.7 3.4 4.1 4.8 5.4 6.1 6.8 7.5 8.2 m

distance (m)

The Recording Environment

Room Acoustics

The Recording Environment

Influence of Acoustical Space on the Sound Event

The Recording Environment

In enclosed performance spaces, a new phenomenon appears: reverberation, which is caused by sounds

being repeatedly reflected from all surfaces and objects in the room.

The Recording Environment

Room influences may be described in two ways:

1. Objectively through measurement of the sound events and their variation with time (room

acoustics.

2. Subjectively through verbal description of the audible experience (aural acoustics)

Both methods are necessary depending on the question at hand; either the objective or subjective

one may assume the greater importance.

The Recording Environment

The Recording Environment

Fundamentals of Aural Acoustics

The Recording Environment

Fundamentals of Aural Acoustics

The Recording Environment

1. The Listenability of a Room - Generally describes its suitability for certain sound events.e.g

Good listenability of a room for speech means that we hear speech well at every seat in the

house without the need for reenforcement.

2. The Transparency of a Room - The ability to differentiate between simultaneously played

instruments or instrument groups in spite of superimposed room reverberation. Transparency

is a basic requirement when we hear complex musical structures.

Fundamentals of Room Acoustics

The Recording Environment

Sound propagation in an enclosed room (the rays show the propagation direction and intensity)

The Recording Environment

Direct Sound

First Reflections

Reverberation

Sound Level

Time

Direct Sound and diffuse sound (reverb build-up, early reflections and decay) in an enclosed space

The Recording Environment

The term RT60 refers to the time it takes the reverb to decay by 60dB. RT is measured at the point at

which the reverb decays to -60dB of its peak level.

RT60

The Recording Environment

The Recording Environment

• Direct sound arrival is followed by reflections from room surfaces.

• Overlapping reflections are heard as reverberation.

• Direct-to-Reverberant ratio gives cues to size of room, type of room surfaces, and distance from source.

The Recording Environment

Significance of Room Tone for Microphone Placement and the Listening Experience

The Recording Environment

• The microphone generally picks up both direct and diffuse sound.

• While the direct sound is influenced little by the nature of the room, the diffuse room

tone transmits information about the room size and the nature of the wall treatment.

• The acoustical attributes of the room tone provide information about the cultural and

social environment into which a musical performance has been placed.

Significance of Room Tone for Microphone Placement and the Listening Experience

The Recording Environment

• Thus church music requires the acoustics of a large church for which it generally is

written; symphonic music is written for concert halls, chamber music for the small,

private room in a castle or home.

• Folk music needs the intimate atmosphere of a pub.

• In pop music and other similar musical forms, we see the creation, through the use of

artificial reverberation of new acoustical surroundings which really do not exist in real

life.

Significance of Room Tone for Microphone Placement and the Listening Experience

The Recording Environment

Sound Wave Reflections

The Recording Environment

Sound Wave Reflections

Creation of Sound Reflections

The Recording Environment

Sound Wave Reflections

Creation of Sound Reflections

The Recording Environment

1. Sound reflections within a room occur when sound reaches a boundary surface

without too much absorption.

2. Dimensions and nature of the surface determine how the diffused sound is

scattered.

Sound Wave Reflections

The Haas Effect

The Recording Environment

The Haas effect can be summarised as follows:

• The ear will attend to the direction of the sound that arrives

first and will not attend to the reflections providing they arrive

within 30 ms of the first sound.

• The reflections arriving before 30ms are fused into the

perception of the first arrival. However, if they arrive after

30ms they will be perceived as echoes.

Angus, J & Howard, D (2009) Acoustics and Psychoacoustics. UK, Elsevier.

Comb Filtering

The Recording Environment

direct path

Comb Filtering

Diagram showing an example of a comb

filter created by the combining of two signals

with the same amplitude, but with a time

delay between them of just 1 ms.

A.C.M.E Mixing Desk

The Recording Environment

speaker

Mic

Reflected Sound

Sound Source

Direct Sound

Surface

Resulting Frequency Response

dB

The Recording Environment

An affected waveform shows lots of sharp peaks and troughs that look not unlike the teeth of a comb

Ceiling reflections cause acoustic

interference at the listeners position

Splayed Ceiling Design

The Recording Environment

Splayed ceiling reduces

unwanted reflections

The Recording Environment

Standing Waves

Max Min Max Min Max Min Max.

Pressure

Bo

un

dary

Maximum

Minimum

Pre

ssure

Reflection

The formation of a standing wave by reflection at a boundary

Room Resonances, Natural Frequencies & Modes

The Recording Environment

Image showing room resonances, natural frequencies & modes

Bo

un

dary

Bo

un

dary

F

2F

3F

11.3 Feet

100Hz

200Hz

300Hz

Image showing room resonances, natural frequencies & modes

100Hz

The Recording Environment

The same effect will

occur at frequencies that

are multiples of 100Hz

(200Hz, 300Hz, etc)

Node

Anti Node

The Recording Environment

The Recording Environment

stuart.jones3@southwales.ac.u

k

www.stuartjones.org