Resident Physics Lectures 02: Sound Properties and Parameters.

Resident Physics LecturesResident Physics Lectures

02:02:Sound Properties and Sound Properties and ParametersParameters

Sound Wave Definition?Sound Wave Definition?

• Sound is a WaveWave• WaveWave is a propagating (traveling)

variation in a “wave variablewave variable”

• “An elephant is big, gray, and looks like an elephant.”

http://www.tourism.state.ct.us/images/evphotos/dumbo.jpg

Sound Wave VariableSound Wave Variable

• Examples– pressure (force / area)– density (mass / volume)– temperature

• Also called acoustic variableacoustic variable

Sound is a propagating (moving) variation in a “wave variablewave variable”

Sound Wave VariationSound Wave Variation

• Freeze time• Measure some acoustic variable as

a function of position

Position

AcousticVariableValue

PressureDensityTemperature

MOREMORE• Make many measurements of an

acoustic variable an instant apart• Results would look the same but

appear to move in space

1

2

MOREMORE• Track acoustic

variable at one position over time

Sound WavesSound Waves• Waves transmit energy• Waves do not transmit matter• “Crowd wave” at sports event

– people’s elevation varies with time– variation in elevation moves around stadium

» people do not move around stadium

Transverse WavesTransverse Waves

• Particle moves perpendicular to wave travel

• Water ripple– surface height varies with time– peak height moves outward

» water does not move outward

Compression (Longitudinal) Waves

Compression (Longitudinal) Waves

• Particle motion parallel to direction of wave travel

1

2

1

2

Wave Travel

Motion ofIndividual Coil

Sound Waves are Compression Waves

Sound Waves are Compression Waves

• Regions of alternating low and high pressure move through air

• Particles oscillate back & forth parallel to direction of sound travel

• Particles do not move length of sound wave

Wave Travel Motion of IndividualAir Molecule

MediumMedium

• Material through which wave moves

• Medium not required for all wave types

– no medium required for electromagnetic waves» radio

» x-rays

» infrared

» ultraviolet

– medium is required for sound» sound does not travel through vacuum

Talk louder! I can’t hear

you.

Sound WavesSound Waves

• Information may be encoded in wave energy

– radio– TV– ultrasound– audible sound

Sound FrequencySound Frequency

• light frequency corresponds to color

• sound frequency corresponds to pitch

Sound FrequencySound Frequency# of complete variations (cycles) of an

acoustic variable per unit time

• Unitscycles per second

1 HzHz = 1 cycle per second

1 kHzkHz = 1000 cycles per second

1 MHzMHz = 1,000,000 cycles per second

• Human hearing range 20 - 20,000 Hz

Sound FrequencySound Frequency

• Ultrasound definition> 20,000 Hz

– not audible to humans» dog whistles are in this range

• Clinical ultrasound frequency range

1 - 10 MHz

1,000,000 - 10,000,000 Hz

PeriodPeriod

• time between a given point in one cycle & the same point in the next cycle

– time of single cycle

• Units– time per cycle (sometimes expressed

only as time; cycle implied)

period

Magnitude of acoustic

variable

time

PeriodPeriod

• as frequency increases, period decreases

• if frequency in Hz, period in seconds/cycle

1Period = ------------------- Frequency

PeriodPeriod

• if frequency in kHz, period in msec/cycle• if frequency in MHz, period in sec/cycle

1 kHz frequency ==> 1 msec period

1 MHz frequency ==> 1 sec period

Period = 1 / Frequency

Reciprocal UnitsReciprocal Units

Frequency Units

Period Units

Hz (cycles/sec) seconds/cycle

kHz (thousands of cycles/sec)

msec/cycle

MHz (millions of cycles/sec)

sec/cycle

Period / FrequencyPeriod / Frequency

If frequency = 2 MHz then sound period is 1/2 = 0.5 sec

If frequency = 10 kHz then sound period is 1/10 = 0.1 msec

If frequency = 50 Hz then sound period is 1/50 = 0.02 sec

If sound period = 0.2 sec then frequency = 1/0.2 = 5 MHz

If sound period = 0.4 msec then frequency = 1/0.4 = 2.5 kHz

If sound period = 0.1 sec then frequency = 1/0.1 = 10 Hz

Sound Period & Frequency are

determined only by the sound source. They are independent of medium.

Who am I?

Burt Mustin

Propagation SpeedPropagation Speed

• Speed only a function of medium

• Speed virtually constant with respect to frequency over clincial range

WavelengthWavelength• distance in space over which single cycle

occurs OR

• distance between a given point in a cycle & corresponding point in next cycle

• imagine freezing time, measuring between corresponding points in space between adjacent cycles

Wavelength UnitsWavelength Units

• length per cycle– sometimes just length; cycle implied

• usually in millimeters or fractions of a millimeter for clinical ultrasound

Wavelength EquationWavelength Equation

Speed = Wavelength X Frequency [ c = X (dist./time) (dist./cycle) (cycles/time)

• As frequency increases, wavelength decreases

– because speed is constant

WavelengthWavelengthSpeed = Wavelength X Frequency

[ c = X (dist./time) (dist./cycle) (cycles/time)

mm/sec mm/cycle MHz

Calculate Wavelength for 5 MHz sound in soft tissue

Wavelength = 1.54 mm/sec / 5 MHz

Wavelength = 1.54 / 5 = 0.31 mm / cycle

5 MHz = 5,000,000 cycles / sec = 5 cycles / sec

Wavelength is a function of both the

sound source and the medium!

Who am I?

John Fiedler

Pulsed SoundPulsed Sound• For imaging ultrasound, sound is

– Not continuous

– Pulsed on & off

• OnOn Cycle (speak)– Transducer produces short duration sound

• OffOff Cycle (listen)– Transducer receives echoes

– Very long duration

ON OFF ON OFF

(not to scale)

Pulse CyclePulse Cycle

• Consists of– short sound transmission

– long silence period or dead time» echoes received during silence

• same transducer used for– transmitting sound– receiving echoes

sound silence sound

Pulsed Sound ExamplePulsed Sound Example

• ringing telephone– ringing tone switched

on & off

– Phone rings with a particular pitch

» sound frequency

sound silence sound

Parameters Parameters

• frequency

• period

• wavelength

• propagation speed

• pulse repetition frequency

• pulse repetition period

• pulse duration• duty factor• spatial pulse

length• cycles per pulse

Sound Pulse

Pulse Repetition FrequencyPulse Repetition Frequency

• # of sound pulses per unit time

• # of times ultrasound beam turned on & off per unit time

– independent of sound frequency

• determined by source

• clinical range (typical values)– 1 - 10 KHz

Pulse Repetition PeriodPulse Repetition Period

• time from beginning of one pulse until beginning of next

• time between corresponding points of adjacent pulses

Pulse Repetition Period

Pulse Repetition PeriodPulse Repetition Period

• Pulse repetition period is reciprocal of pulse repetition frequency

– as pulse repetition frequency increases, pulse repetition period decreases

• units– time per pulse cycle (sometimes simplified to just time)

• pulse repetition period & frequency determined by source

PRF = 1 / PRP

Higher FrequencySame PulseRepetition Frequency

Pulsed SoundPulsed Sound

• Pulse repetition frequency & period independent sound frequency & period

Same FrequencyHigher PulseRepetition Frequency

Pulse DurationPulse Duration• Length of time for each sound pulse• one pulse cyclepulse cycle =

– one sound pulse and one period of silence

• Pulse duration independent of duration of silence

Pulse Duration

Pulse DurationPulse Duration

• units– time per pulse (time/pulse)

• equationpulse duration = Period X # cycles per pulse

(time/pulse) (cycles/pulse) (time/cycle)

Pulse Duration Period


Longer Pulse Duration

Shorter Pulse Duration

Same frequency; pulse repetition frequency,period, & pulse repetition period


Pulse duration is a controlled by

the sound source, whatever

that means.

Duty FactorDuty Factor• Fraction of time sound generated• Determined by source• Units

– none (unitless)

• EquationsDuty Factor = Pulse Duration / Pulse Repetition Period

Duty Factor = Pulse Duration X Pulse Repetition Freq.

Pulse Duration

Pulse Repetition Period

Spatial Pulse LengthSpatial Pulse Length

• distance in space traveled by ultrasound during one pulse

HEYH.......E.......Y

Spatial Pulse Length


So, can you like show me an example?


• depends on source & medium

• as wavelength increases, spatial pulse length increases

Spat. Pulse Length = # cycles per pulse X wavelength

(dist. / pulse) (cycles / pulse) (dist. / cycle)


• as # cycles per pulse increases, spatial pulse length increases

• as frequency increases, wavelength decreases & spatial pulse length decreases

– speed stays constant


Wavelength = Speed / Frequency

Why is Spatial Pulse Length Important

Why is Spatial Pulse Length Important


Wavelength = Speed / Frequency

Spatial pulse length determines axial resolution

Acoustic ImpedanceAcoustic Impedance• Definition

Acoustic Impedance = Density X Prop. Speed

(rayls) (kg/m3) (m/sec)

• increases with higher– Density

– Stiffness

– propagation speed

• independent of frequency

Why is Acoustic Impedance Important?

Why is Acoustic Impedance Important?

• DefinitionAcoustic Impedance = Density X Prop. Speed

(rayls) (kg/m3) (m/sec)

• Differences in acoustic impedance determine fraction of intensity echoed at an interface

Resident Physics Lectures 02: Sound Properties and Parameters.

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