WEEK 11 Revision class 1

Assignment Two – 20% Sound Measurements and Observations Due: Week 11 via electronic submission Weighting: 30% Learning Outcomes: a,c,e,f,g,h,k,l,m Description: Each student is to investigate and report on different acoustic spaces in terms of their characteristics and ambient noise levels.

Assignment Two – 30% The quietest space you can – measure the ambient noise levels over a 5 minute period and report on the sources of any noise observed (air conditioning, outside traffic etc). Give maximum and minimum noise levels recorded. What weighting did you use on the SPL meter? A safe observation point 2 metres away from a busy road – measure the ambient noise levels over a 5 minute period and report on the sources of any noise observed (large trucks, sirens etc). Give maximum and minimum noise levels recorded. What weighting did you use on the SPL meter?

Assignment Two – 30% Find a relatively constant sound source (eg. an air conditioning unit) and measure the Sound Pressure Level using A-weighting at 1 metre from it. Now switch to the C-weighted scale and repeat your measurement. What do you observe? Why is this the case? Now move to 4 metres away and repeat your measurements. Does this correlate with your expectations from what you learnt in class? Report on your findings.

SPL Meters The various options on an SPL meter are: A-weighted, C-weighted and fast or slow response A-weighted is closer to human perception of loudness (subjective) C-weighted is a relatively flat frequency response (more of an objective measurement)

Noise White noise is: Equal energy per frequency (or bandwidth) Pink noise: Equal energy per octave Pink noise is simply White noise with a ‘pinking filter’ added (-3dB/octave roll off) What is pink noise used for?

Simple Harmonic Motion A constant frequency without regard to amplitude Principle necessary for most acoustic musical instruments e.g. most strings and horns

Simple Harmonic Motion

Phase

Comb Filtering When a delayed version of a signal is added to itself, causing constructive and destructive interference.

Comb Filtering

Beat Frequencies Two sound waves of different but close frequency, the alternating constructive and destructive interference causes the sound to be alternatively soft and loud. E.g. 400Hz and 402Hz will cause a beat frequency of 2Hz

Envelope of Sound ADSR A = Attack D = Decay S = Sustain R = Release Envelopes describe the time vs amplitude of sounds

Envelope of sound

Harmonics The nodes of a vibrating string are harmonics

Harmonics Harmonic frequencies are equally spaced by the width of the fundamental frequency and can be found by repeatedly adding that frequency. For example, if the fundamental frequency is 25 Hz, the frequencies of the harmonics are: 50 Hz, 75 Hz, 100 Hz etc.

Overtones An overtone is any frequency higher than the fundamental frequency of a sound. The fundamental and the overtones together are called partials. Harmonics are partials whose frequencies are whole number multiples of the fundamental.

Classes of instruments Stringed Instruments Wind Instruments Percussion Instruments

Stringed instruments Includes all instruments whose standing wave constraint is that at each end of the medium there must be a node. The bridge lifts the strings so they can vibrate with the air as well as amplifying the strings The soundboard radiates with the strings as well as helping to amplify the sound

Percussion instruments Can be considered in 3 classes: Bars (xylophone, glockenspiel, triangle) Membranes (drums) Plates (cymbals)

Equal Loudness Curves An equal-loudness contour is a measure of sound pressure (dB SPL), over the frequency spectrum, for which a listener perceives a constant loudness when presented with pure steady tones.

Equal Loudness Curves The unit of measurement for loudness levels is the phon, and is arrived at by reference to equal-loudness contours

Psychoacoustics

Definition of Psychoacoustics Psychoacoustics is the study of the perception of sound - How we hear - How we can separate different sounds - Our psychological responses - The physiological impact of sound/music

Psychoacoustic Effects Beat frequencies: we can hear a single ‘beating frequency’ when there is in fact two closely spaced frequencies being played. Masking: Where a loud frequency will mask another softer frequency that is close in frequency. The principle behind ‘perceptual encoding’ and mp3s.

Psychoacoustic Effects The Haas Effect: The ear will lock onto the first arriving sound and ignore the direction of subsequent repeats and in fact fuse them with the original.

Perception of Direction One ear can’t discern the direction of a sound’s origin

Perception of Direction Using two ears to localise a sound source is based on three acoustic cues received by the ears: 1.Interaural intensity differences 2. Interaural arrival-time differences 3.The effects of the pinnae (outer ear)

Perception of Direction Interaural intensity differences for perceiving direction of high frequencies Interaural time differences for perceiving direction of low frequencies

Perception of Direction Interaural Intensity Difference (IID) – used for higher frequency sounds where the head ‘shadows’ the sound source and we hear a louder sound in one ear compared with the other due to the limited diffraction capabilities of the sound.

Perception of Direction Interaural Time Difference (ITD) – used for lower frequency sounds where there is significant diffraction around the head and the interaural amplitude differences are very low. The brain senses the ‘phase difference’ between the two ears to determine the direction of the sound source.

Intensity and delay cues allow us to perceive the direction of a sound’s origin… But not whether the sound originates from front, behind, or below.

Perception of front and back The ridges in the pinna introduce tiny time delays between the direct sound and the sound that’s reflected from the ridges. These delays tell us that a sound is coming from behind or above

Questions How do we localise mid to high pitch sounds? How do we localise low pitch sounds? How do we localise sound to be behind us? How do we localise sound on the vertically? What effect does the Pinna have on our perception of direction of sounds?

RT60 RT60 is the time for reverb to drop 60dB below the level of the direct sound

RT60

Reverb Parameters Direct sound is the original sound Early Reflections are the first reflections you hear after the direct signal.

Reverberation Parameters Pre-delay is the amount of time before the first reflection Early reflections are the first group of echoes (after the direct sound) Decay Time is the time it takes for the reverb to reduce by 60dB

Reverb Parameters Diffusion is the scattering of sound in all directions so discrete reflections are not heard Damping will typically ‘dampen’ the high frequency of the reverb Mix is the parameter for mixing between the ‘dry’ and ‘wet’ signal

Reverberation If a room isn’t designed and treated to properly absorb, diffuse and reflect sound issues such as comb filtering and phase cancellation can adversely affect frequency response Room Modes>

Absorption co-efficient An ‘Absorption co-efficient’ tells us how much sound a material will absorb at a particular frequency. It is a value of between 0 and 1 1 being completely absorbent 0 being completely reflective

Absorption co-efficients Floor materials125 Hz250 Hz500 Hz1 kHz2 kHz4 kHz carpet Concrete (unpainted, rough finish) Concrete (sealed or painted) Marble or glazed tile Vinyl tile or linoleum on concrete

Simple Waveforms Sine Square Triangle Sawtooth

Harmonic Content Sine wave: no harmonics Square wave: All odd harmonics at inverse amplitudes Triangle wave: All odd harmonics at inverse squared amplitudes Sawtooth wave: All harmonics (odd and even) at inverse amplitudes

Making a sawtooth from a sine wave A sawtooth wave's sound is harsh and clear and its spectrum contains both even and odd harmonics of the fundamental frequency

SPL Meters The various options on an SPL meter are: A-weighted, C-weighted and fast or slow response A-weighted is close to human perception of loudness

Decibel Revision Reference values for Power, Voltage and SPL are: 1. dB SPL for Sound Pressure Level - 20µPa 2. dB V for Voltage - 1 volt 3. dB U for Voltage volts 4. dB P for Power (Watts) watt