Waves, sound & music (in one-dimension, mainly)
30 Slides2.40 MB
Waves, sound & music (in one-dimension, mainly)
Making sounds Many sources of sound are very familiar: the human voice animal sounds straw oboe whistling tubes musical instruments (string, woodwind, brass, percussion, electronic) electric & combustion motors, rolling wheels & tyres, other noise What words are commonly used to describe these sounds?
A hearing test What is the lowest frequency that you can hear? the highest? At what frequency does the sound seem loudest?
Learning outcomes illustrate that all sounds are produced by vibrations distinguish between transverse and longitudinal waves describe the propagation of sound in terms of density waves relate pitch of a sound to wave frequency and volume of a sound to wave amplitude recall and use the relationship between wave speed, frequency and wavelength to make quantitative predictions use concepts of phase and phase difference to explain superposition of linear waves describe how a standing wave is created in musical instruments make links with music to motivate learning about waves interpret and use graphical representations of waves develop confidence in using signal generator & oscilloscope use units of frequency, Hz and kHz
Teaching challenges Students can have difficulty visualising how a wave moves through space i.e. that wave motion differs from the motions of particles in a medium through which a wave travels. Representing waves: – Sound waves are often displayed on an oscilloscope, which involves representing a longitudinal wave with a transverse wave. – Many students confuse displacement - distance graphs with displacement - time graphs. The fact that wave speed depends entirely on the medium and not on the sound source (i.e. frequency or amplitude) is not generally understood. Standing waves: Typically it takes many examples for students to grasp the concept of ‘phase’. They find it difficult to appreciate that standing waves are a superposition of two identical waves travelling in opposite directions.
Radiation ‘any process in which energy emitted by one body travels through a medium, or through space, ultimately to be absorbed by another body.’ Includes sound light (and the whole electromagnetic spectrum) nuclear radiation ( , positrons, neutrons)
A general model for radiation SOURCE MEDIUM DETECTOR journey: may involve transmission, reflection, refraction, partial absorption detector: absorption at the journey’s end SPT simulation
Vibrations make sounds Making a sound source visible vibrating tuning fork touched into a beaker of water, or against a suspended ping pong ball candle in front of a vibrating loudspeaker rice or polystyrene balls on a vibrating loudspeaker cone that faces upwards using a ‘wobbleboard’ You can also feel your larynx vibrate.
Two types of wave travel Particles in a medium vibrate about their mean positions, transferring energy but not matter. longitudinal wave – vibration along the direction of energy transfer transverse wave – vibration perpendicular to the direction of energy transfer
Longitudinal waves can be thought of as ‘density waves’ in a material medium (solid, liquid or gas). SPT simulation 2 AKA ‘pressure’ or ‘compression’ waves, because compressions alternate with rarefactions. slinky spring PP experiment Waves along a line of students
Detectors of sound ear – structure, range of hearing, locating a sound source by comparing arrival times microphone sound level meter
Visualising sounds Faroson software standard oscilloscope Soundcard Oszilloscope
Measuring sound Frequency (pitch) – vibrations or cycles per second (Hz, KHz) Amplitude – size of the vibration Loudness – perceived strength of a sound (frequency dependent) Intensity – energy carried by a sound (dB scale)
Wave changer An oscilloscope displays any longitudinal wave as a transverse wave. Why? oscilloscope display: voltage – time microphone diaphragm vibrates, producing an a.c. waveform So how do you explain this at an introductory level? use a ‘wave changer’
Representing waves graphically displacement – distance graph shows where the particles are at one instant of time. A snapshot. Gives wavelength, .
Representing waves graphically displacement – time graph shows what happens to a single particle in the medium over time. ‘Period’, T, is time for one complete cycle. Gives frequency, 1 f T
Wave speed distance wave travels in a second (m/s) wavelength (m) x number of waves each second (s-1) In symbols, v f To find the speed of sound, measure a distance and a time. speed of sound in a solid speed in a liquid speed in a gas The speed of sound in air depends on temperature & humidity. Medium, not source, generally determines wave speed (though both water & glass are ‘dispersive media’ – v depends on f).
Measure the speed of sound with a double beam oscilloscope or use echoes from an exterior school wall.
Diffraction, at edges & gaps Diffraction: spreading of wavefronts as they pass the edge of an object. only noticeable when the gap/obstacle size is hole similar to the wavelength Examples: Sound waves diffract through a doorway Radio waves diffract around hills & buildings pinhead Visible light with gap/obstacle mm ripple tank
Refraction As a wave travels from one medium to another, its speed changes at the boundary. The frequency of waves arriving and departing is the same. v f so wavelength changes at the boundary.
Wave interference Two waves can pass through a common point without affecting each other. Superposition principle: The displacement at any point will be the vector sum of the displacements caused by each wave.
Phase relationship Wave phase at a point describes the stage in a cycle of the vibrating particle. Where two waves cross, what happens depends on the phase of each wave, i.e. their phase relationship. in phase: constructive interference out of phase: destructive interference
Workshop session in the lab Experiments: signal generator and oscilloscope Waves with trolleys speed of sound – – – in air, using Audacity software in a metal bar, using Picoscope using the clap-echo method plus demonstration experiments Questions C21 Activity 6.8 Questions on v f C21 Activity 6.9 Do you understand wave motion?
Beats Two waves with slightly different frequencies. Their phase relationship constantly changes. This effect is used to tune musical instruments (reduce the beat frequency to zero).
Reflections at a boundary Transverse pulse in a string: When the pulse reaches a rigid support, the reflected pulse is upside down. When the pulse reaches an open end, the reflected pulse is upright.
Standing waves Watch two waves pass through each other. W.Fendt simulation: Standing Wave Phet simulation: Wave on a string Note that amplitudes vary along the wave path. Any point of maximum amplitude is called an antinode. Any point of zero amplitude is called a node. All musical instruments involve standing waves.
Musical standing waves In stringed instruments, there is a node at each end of a string. In wind instruments, there is an antinode at each end of an air column.
Web-based resources Institute of Sound and Vibration Research, University of Southampton Acoustic, Audio and Video Engineering, University of Salford Voicebox SEP booklet and UCL interactives
Applications Speech and hearing (speech therapy, Include information on careers voice coaching, hearing impairment, audiology) Sound transmission and production (mobile phone, radio, TV, film) Control of sound in the environment (acoustics, noise reduction, health and safety) Music (instruments, recording, performance) Microphones and loudspeakers Sonar (echo sounding e.g. mapping the sea bed) Ultrasound (medical (ultrasound images), non-destructive testing (flaw detection), cleaning jewellery), distance measurement) 29
Support, references www.talkphysics.org SPT 11-14 Light & Sound Ep1 Describing sound Ep2 Quantifying sound Ep3 Using sound David Sang (ed., 2011) Teaching secondary physics ASE / Hodder Phet simulations Sound and waves Use Google Earth for examples of refraction & diffraction