Reflections Diffraction Diffusion Sound Observations Report AUD202 Audio and Acoustics Theory

The Doppler Effect Delay Perception SPL Meters Last Week >

Upcoming Events 18 Days - Sound Observations Report 28 Days - Exam

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 meters 7 meters 1 meter Comb Filtering

Remembering that odd harmonics cancel and even harmonics reinforce, what are the first 3 cancellations for the following fundamental frequencies: 100Hz 2500Hz 600Hz 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

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

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

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 Concepts Absorption Co-Efficients Next Week >