Sound waves How they are generated and travel Physics of Sound Part 1 Sound waves How they are generated and travel

Sound Waves Generation and Propagation Sound wave = changes in pressure caused by vibrating object Compression = High pressure Rarefaction = Low pressure Sound needs a medium to “vibrate” Usually air, but could be anything Speed of sound depends upon the medium Air = 1130 ft/sec Water = 5000 ft/sec Steel = 13000 ft/sec

Measuring sound waves Sound waves are longitudinal waves Vibrating object compresses the air around it. Pushes air away leaving an area of low pressure Vibrating object then compresses more air to create a “chain”

Measuring methods Cycle A single push and pull of the vibrating object One are of compression followed by one area of rarefaction An initial increase in atmospheric pressure from the norm, followed by a drop below the norm and then a return to normal Mathematically displayed by a sine curve Pressure on Y axis Time on X axis

Measuring methods Period (T) and Frequency (f) Period - The time it takes to create one cycle Frequency - The number of cycles in one second Measured in Hertz (Hz) or cycles per second

Measuring methods Example It takes ¼ sec to create one cycle. What is the sound wave’s frequency?

Measuring methods Frequency will determine pitch High frequency = high pitch Low frequency = low pitch Octave – a doubling of halving of the frequency

Measuring methods Human hearing range Low range between 15 to 30 Hz With enough power lower than 15 Hz can be felt, buy not heard as “sound” High range varies with age and gender Women - up to 20 kHz Men – between 15 to 18 kHz High frequency range will lower with exposure to high levels of sound and age

Tuning Traditional orchestra would tune First Chair Violin A first. Remaining instruments would tune relative to that A above middle C was tuned to about 420 Hz As halls grew larger it was found to be desirable to tune sharper 1939 A was established to be 440 Hz Corresponds to the 49th key on a full size piano Tuning is not a science. The relative frequency difference is what is important

Measuring methods Wavelength The distance from one area of compression to the next or one area of rarefaction to the next l=wave length V = velocity of sound in medium usually 1130 ft/sec f = frequency

Measuring methods Amplitude How high the pressure goes above and below normal atmospheric pressure Corresponds to how loud the sound is “loudness” is relative to frequency and dependant on the listener.

Timber and Harmonics Harmonics – multiples of a base frequency Timber – the characteristics of a particular sound or instrument Different harmonics combined in different levels

Basic Acoustics Inverse square law Reinforcement/cancellation Physics of Sound Part 2 Basic Acoustics Inverse square law Reinforcement/cancellation

Interference Phase measurement of where the amplitude of a wave is relative to another wave A cycle can start at any point in a waveform Two waves with the same frequency can start at different times Measured as an angle in degrees Related to the sine wave representation of the wave

Interference Constructive of destructive interference Waveforms will add by summing their signed amplitude at each instant in time

Beats Happens when two slightly different frequencies interfere Often used in tuning

Standing waves When sound waves bounce off of obstructions, they can interfere with themselves Tends to reinforce some frequencies and attenuate others Prevented by using Non- Parallel walls, ceilings Convex surfaces Multi-level ceiling sections

Reverberance (Reverb) Consisting of multiple, blended sound images caused by reflections from walls, ceilings and other structures which do not absorb sound NOT echo Echo consists of individual, non-blended sound images Reverb time is related to The time it takes for a sound to reduce to an inaudible level Loudness of sound relative to background noise Ratio of loudness of reverberant to direct sound Short reverb time (less than 1.5 sec) is better for speech or drama Long reverb time (more than 1.5 sec.) is better for music

Absorption Controlling reflections can reduce or increase reverb time Air tends to absorb frequencies above 2K Hz Sight line obstructions Frequencies above 10 kHz tend to not bend around corners well or other obstructions l=1.3 inches for 10 kHz tone Frequencies below 1kHz do very well l=5.65 feet for 200 Hz tone Specialists are often hired to “tune” a space acoustically

Acoustic attributes Defined by Leo Beranek after a 6 year study of 54 concert halls Used to define acoustic properties in terms that other trained professionals can understand

Acoustic attributes Intimacy – Indicates the size of a room How it sounds to the listener, not actual size Determined by the initial-time-delay-gap (ITDG) Interval between the sound that arrives directly at the ear and the first reflection Usually considered to be the most important attribute

Acoustic attributes Liveness Warmth Related to Reverberance Room size is related More reflections is live. Less reflections is dry or dead Warmth More low frequency sound relative to mid frequency Too much low frequency sound is said to be “Boomy”

Acoustic attributes Loudness of direct sound Definition or Clarity Inverse square law Loudness of sound will decrease by one quarter every time the distance from the source is doubled Definition or Clarity Good definition when sound is clear. Related to intimacy, liveness, loudness of direct and reverberant sound

Acoustic attributes Brilliance Diffusion A hall that has liveness, clarity and intimacy Diffusion Relates to the orientation of reverberant sound Where is the reflected sound coming from It is preferable to have reverb sound coming from all directions

Intensity Like pitch, loudness is a sensation in the consciousness of a listener To produce a sound twice as loud requires 10 times the power Inverse square law Sound level is reduced by a factor of the square of the distance away from the source If you move double the distance from the source, the sound intensity will by one quarter

Intensity Intensity is a measurable quantity SPL – Sound Pressure Level dB – deciBel A system of measuring a ratio between two powers dB (spl)=20log(P1/p0) p0 is a reference level, usually taken to be the minimum intensity audible to an average person – “Threshold of Hearing” 3 dB change is hardly noticed as a change 6 dB changes is heard as twice as loud

Thunderclap, Air Raid Siren 1 Meter 120 dB Jet takeoff (200 ft) 110 dB dB SPL Sound 150 dB Jet engine at 1m 140 dB Rock and Roll stack at 1m 130 dB Thunderclap, Air Raid Siren 1 Meter 120 dB Jet takeoff (200 ft) 110 dB Rock Concert 100 dB Train passing up close 90 dB Heavy traffic 80 dB Hair Dryer 70 dB City street 60 dB Noisy bar or restaurant 50 dB Open plan office environment 40 dB Normal conversation level 30 dB Library, Soft Whisper (5 Meter) 20 dB Quiet domestic environment 10 dB Broadcasting Studio, Rustling Leaves 0 dB Threshold of hearing in young adult

Sound Envelope Listener does not hear individual cycles of sound waves Attack – Time it takes for sound to rise from nothing to its greatest intensity. Usually short. Decay – Time it takes for a sound to fall from its attack level to its sustaining level. Decay time is usually short Sustain – The time during which the initial vibrating source continues to supply energy to the sound. Usually perceived as the duration and intensity of the sound Release – Time it takes for the sound to drop from its sustain level to inaudibility after vibrating object stops supplying energy

For Next Class Read Basic Electricity, Kai’s Sound Handbook P. 23-27, Leonard