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Sound- type of wave and characteristics
Sound is a mechanical, longitudinal wave

Travels in a straight line

Carries energy, not matter

Needs a medium to travel through (cannot travel in a vacuum)
Sound waves are made up of...
...cycles of increasing and decreasing variables
- In phase means the peaks and troughs overlap eachother
-Out of phase means the peak of one wave sits on the trough of the other
When In Phase waves meet they create...
Constructive Interference
(they become one big wave)
When Out of Phase waves meet they create...
Destructive Interference
(they cancel eachother out)
Sound Frequency (f)
-Number of cycles per second
-Units: Hertz
-Determined by: the source
Sound frequencies- infrasound, audible sound, ultrasound
Infrasound: less than 20 Hz

Audible Sound: 20-20,000 Hz

Ultrasound: greater than 20,000 Hz (or 20kHz or 2MHz)
In Ultrasound, frequency is determined by...
the thickness of the piezoelectric element

Thicker element: lower frequency
Thinner element: higher frequency
Period (T)
-Period is the reciprocal of frequency
-Period is the time (T) it takes for one cycle to occur
-Units: s, microseconds
Propagaton Speed
It is the speed that sound moves through a medium

Only determined by two characteristics of the medium: stiffness and density

If you increase the stiffness, you increase the speed

If you increase the density, you decrease the speed

ONLY DETERMINED BY THE MEDIUM
What is the speed of sound through soft tissue?
1540 m/s or 1.54 mm/microsecond

Abbreviated by c
Propagation speed- lowest to highest- different material in the body
Air (lowest)
Fat
Water
Liver
Blood
Muscle
Bone (highest)
Wavelength
Wavelength is the distance over which one cycle occurs

Units: mm, m, km, etc (units of length)

Determined by source and medium
Wavelength equation
wavelength= propagation speed/ frequency
Amplitude
Amplitude describes the strength (magnitude) of the soundwave

Also known as pressure amplitude

Units: Pascals

Decreases with depth due to attenuation

Measured from baseline to peak (does not include both parts of the cycle)

Unrelated to frequency
Power
Also related to the strenth of the beam, power is the rate at which work is done

Units: Watts

Diminishes with depth
Intensity
Represents the concentration of energy in the beam

Intensity= Power/ area

Units= Watts/ cm2

Amplitude/ Power/ Intensity Units
Amplitude (pressure amplitude): pascals

Power: Watts

Intensity: W/ cm2
Amplitude/ Power/ Intensity relationships
Power is proportional to amplitude squared

Intensity is proportional to amplitude squared

Intensity if proportional to power
Impedance
-Acoustic resistance to sound in a medium

Impedance= density x prop. speed

Determined by the medium

Units: Rayls
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