Musical Instruments and Sound Simple Harmonic Motion Envelope of sound NIHL report AUD202 Audio and Acoustics Theory

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COMB FILTERING

Comb filtering is caused by a wave combining with a delayed version of itself

Comb Filtering A wave delayed by 180 degrees (half a wavelength) results in the fundamental frequency of cancellation. Frequencies delayed by 1.5x, 2.5x, 3.5x the wavelength will cancel, while whole number multiples (1, 2, 3 etc) will reinforce.

Comb Filtering A wave delayed by half a wavelength results in the fundamental frequency of cancellation

Single signal, no comb filter issues

1ms delayed version of the signal added

Comb Filtering Common causes of comb filtering: 1.Two microphones at different distances from the same source. 2.Reflective nearby surface causing a reflected sound to arrive at the microphone slightly after the direct sound. 3.Two identical sounds in a DAW with a 1 to 10ms delay

Comb Filtering Question If one mic is positioned 2 metres from a sound source and another mic is 3.5 metres from a sound source, what will be the fundamental frequency of cancellation if they are combined?

WHITE NOISE & PINK NOISE

Noise White noise: Equal energy per frequency Pink noise: Equal energy per octave Pink noise is simply White noise with a ‘pinking filter’ added (-3dB/octave roll off)

ENVELOPE OF SOUND

ADSR (Attack, Decay, Sustain, Release)

Attack initial run up of level Decay decay in the signal directly after the attack Sustain level during the main sequence Release how quickly the sound fades

Resonant Frequencies A resonant frequency is a natural frequency of vibration determined by the physical parameters of the vibrating object Resonant frequencies of a body are frequencies at which energy is most efficiently converted to physical displacement

Resonant Frequencies 1. It is easy to get an object to vibrate at its resonant frequencies, hard to get it to vibrate at other frequencies. 2. A vibrating object will pick out its resonant frequencies from a complex excitation and vibrate at those frequencies, essentially "filtering out" other frequencies present in the excitation 3. Most vibrating objects have multiple resonant frequencies.

STANDING WAVES IN MUSICAL INSTRUMENTS

Two sine waves travelling in opposite directions can result in a standing wave

Different instruments employ different methods to create pitches String instruments use strings Wind instruments use a vibrating column of air Percussion instruments use methods such as bars, membranes and plates

Classes of Instruments Stringed Instruments Wind Instruments Percussion Instruments

Standing Wave in Stringed Instruments Include: violin, cello, guitar, bass etc The standing wave constraint of string instruments is that at each end of the medium there must be a node. They can produce a fundamental and all odd and even harmonics

Stringed Instruments The bridge lifts the strings so they can vibrate with the air as well as amplifying the strings The soundboard radiates with the strings helping to amplify the sound

Wind Instruments Wind instruments include brass and woodwind families (categorised by the type of reed used) Brass: trumpet, trombone, french horn, tuba, didgeridoo. Uses Lip Reeds. Woodwind: flute, recorder, saxophone, oboe, clarinet, bassoon. Uses either Mechanical or Air Reeds.

Wind Instruments Lip Reed (brass) instruments vibrate under the influence of the air flow. Mechanical Reed (woodwind) instruments have a flexible reed or reeds at the mouthpiece, forming a pressure-controlled valve For Air Reed (woodwind) instruments such as the flute, the flow of air over the mouth of the instrument forms a flow-controlled valve.

Percussion Instruments Bars (xylophone, glockenspiel, triangle) Membranes (drums) Plates (cymbals)

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