ReflectionsDiffraction

DiffusionSound Observations Report

AUD202Audio and Acoustics Theory

The Doppler EffectDelay Perception

SPL Meters

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Topics So Far

Sound Properties

Wavelength

Frequency

Amplitude

Period

Comb Filtering

Speed of Sound

Wave Interaction

Phase Concepts

Standing Waves

The Inverse Square Law

Harmonics and Overtones

The Decibel

White and Pink Noise

Envelope of Sound

Anatomy of the Ear

Hearing Protection

OH&S principles

SPL Meters

NIHL

Psychoacoustics

Waveforms

Masking

Reflection

Diffraction

Diffusion

Haas Effect

Doppler Effect

Delay Perception

Beat Frequencies

Loudness Curves

Ranges of Hearing

Delay Perception

up to 10ms – comb filtering effects from wave interaction

10-40ms – a ‘thickening’ effect

40ms and over – a distinct repeat or echo of the sound

COMB FILTERING

When a delayed version of a sound interacts with the original sound causing constructive

and destructive interference

Comb Filtering

Write down three situations where this could occur

If two identical signals are shifted against each other in time some frequencies will be cancelled and some will be reinforced

Odd harmonics of the fundamental frequency of cancellation will cancel

Even harmonics of the fundamental frequency of cancellation will reinforce

Comb Filtering

How to Calculate the fundamental frequency of cancellation of a comb filter:

Find the longest wavelength that can occur between two points (a 2m room will have a 4m wavelength), from that find the frequency of the full wavelength. This is your fundamental frequency of cancellation.

Comb Filtering

Given the following room length axis measurements, calculate the fundamental frequency of cancellation

5 meters7 meters1 meter

Comb Filtering

Remembering that odd harmonics cancel and even harmonics reinforce,

what are the first 3 cancellations for the following fundamental frequencies:

100Hz2500Hz600Hz

Comb Filtering

REFLECTION

Sound acoustics is the field primarily concerned with the reflections of sound.

Reflected sound reveals the size and materials within a space. Changing the size or materials will alter the quality and amount of reflections.

Reflection

Reflection

Direct sound is the original sound

Early Reflections are the first

reflections you hear after the direct

signal.

Direct Sound vs. Reflected Sound

The diffuse field is where the reflected sound is higher in level than the direct sound. The near field is where the direct sound is the louder.

Reflection

Reflections

Sound bounces off reflective surfaces in the same behaviour as light – and follows the law "angle of incidence equals angle of reflection"

Reflections

Reflected waves can interfere with incident waves, producing constructive and destructive interference which can result in standing waves

Standing Waves

What can be done to combat standing waves?

Reflections

Reflection from a HARD boundary

Reflections

Reflection from a HARD boundary

DIFFRACTION

Diffraction

Diffraction is the bending of waves around small obstacles and the spreading out of waves beyond small openings.

Diffraction

Diffraction

When sound passes through a hole in a barrier, the hole becomes a new ‘point source’ and the sound will spread out from that point.

DIFFUSION

Diffusion

In acoustics, diffusion is the scattering of sound waves to reflect them in different directions. Or breaking the soundwaves into smaller pieces.

Diffusion is commonly used to reduce specular reflections and echo. Also to direct sound energy to specific areas.

Diffusers

The purpose of a diffuser is to break up sound waves and disperse them in various directions.

There are two main types of diffuser, the barrel diffuser and the quadratic diffuser.

Adding sound diffusion to your acoustic room treatment can improve the speech and instrument intelligibility and improve the overall listening environment within the room

Barrel Diffusers

Quadratic Diffusers

Quadratic Diffusers

SPL Meters

SPL Meters

A weighting is based on the human perception of loudness (e.g. the equal loudness curves) and is

used for most situations.

SPL Meters

C weighting provides a flatter response and is useful for peak sound analysis

Reverberation Parameters and ConceptsAbsorption Co-Efficients

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