Acoustics Acoustics is the science concerned with the production, control, transmission, reception and effects of sound.

A Definition of Sound ‘The sensation produced through the ear, what is or may be heard; vibrations causing this sensation; musical ~ (produced by continuous & regular vibrations, opp. to noise)…’ Oxford English Dictionary

Musical Sound Source - creates sound Medium - transfers sound Receptor - receives sound

Source Excitation Mechanism - energy supply Vibrating Element - tone characteristics Resonator - amplifies / modifies sound

ClassExampleExcitation Mech Vibrating Element Resonator Stringedviolin, guitar bow, pluckstringsound box/board Woodwindclarinet, saxblowing, reed air columnpipe & bell Brasstrumpetblowing, lips air columnpipe & bell Percussiondrum, piano stick, hammer membrane, string cavity/solid material/tu be Vocalsingerlarynx muscle air column/tiss ues tissues & mouth Electronicsynth, sampler electric current oscillator, waveform fx, amp., speaker

Medium Medium Proper - sound propagation (e.g. air, water, wood, steel) Boundaries - reflection, absorption, reverberation (e.g. walls, ceiling)

Receptor Eardrum - conversion into mechanical oscillations Inner Ear - primary frequency sorting, conversion into nerve impulses Nervous System - processing, display identification, storage and transfer to other brain centres

Ear Anatomy

Mechanism of a Wave Vibrations must be converted into pressure oscillations before they can be heard Sound is transmitted through compressed air

Mechanism of a Wave The molecules that make up air can be compressed together Sound pressure waves are longitudinal waves of compressed air

pistonair

compressed air direction of piston

air that is now compressed air that used to be compressed direction of disturbance

It’s not wind! It’s a compression easing along. Like a line of dominoes falling down. None of the dominoes move very much. They just hit the one in front.

A Musical Sound Wave is a Periodic Disturbance A tuning fork is attached to the end of the piston. This creates a periodic (regular) disturbance.

Just as compressed molecules are crowded and so hit those around them more than normal, rarefied molecules have lots of space and so don’t hit the molecules around them as much as air at normal pressure. Tuning Fork Pushes & Pulls Piston

Air molecules tend to move into a low pressure region, creating a new low pressure region further down the tube. Tuning Fork Pushes & Pulls Piston

Periodic pushing & pulling creates regular alternate waves of high and low pressure. Tuning Fork Pushes & Pulls Piston

pressure cm Compression = peak Rarefaction = trough Time Domain Representation

Distance Can Be Seen As Time Here Tube = 100cm Say sound travels 100cm/sec A compression will reach the end of the tube after 1 second It will get halfway along the tube in half a second, etc.

100cm pressure seconds It is possible to an measure waves in terms of time as opposed to distance.

Amplitude amplitude seconds pressure The greater the pressure variations above and below atmospheric pressure, the greater the amplitude of the wave.

Wavelength cm pressure The distance travelled by one cycle of a waveform.

Period seconds pressure   The length of time it takes for one cycle of a waveform to pass through a given point.

Frequency The number of cycles that move through a given point per second f = 1 / 

Frequency Cont... If  = ½ s Then f = 1/0.5s = 2Hz

Frequency Cont... If  = 80m s (1ms = 1/1000 th of a sec) Then f = 1/0.08s = 12.5Hz

Frequency Cont... If it takes 50ms for a wave to repeat (  = 50ms or 1/20 th of a second) Then f = 1/50ms = 1/0.05secs = 20Hz The higher the fundamental waveform frequency the higher the pitch

Phase A sine wave is 90 º out of phase with a cosine wave sine wave cosine wave