Flashcards in Module 1 - Waves, sound and matter Deck (76)

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1

## Define wave speed

### Speed (c) = distance (x) over time (t). Wave speed can be expressed in several different metric units of measurement that denote distance and times. For example km/h and m/s. Wave speed is independent of wavelength (λ) and frequency (f). It is a scalar quantity.

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## Define wave velocity

### Velocity describes the speed AND direction of a wave. It is a vector quantity where as speed is a scalar quantity.

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## Define wavelength

### It is the physical length of a single cycle. (λ) is measured as a disturbance in space and is the shortest repeat distance and is measured in units of length. Wavelength can be calculated using the formula c = f λ where λ = c/f. If frequency is in Hz and speed is in m/s wavelength will be in meters (m).

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## Define period

### It is the time between one cycle and the next. (T) is a measure of disturbance over time and is measured in units of time (s). It can be calculated using the formula T =1/f.

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## Define wave frequency

### (f) is measured in hertz (Hz); this used to be called cycles per second (cps). Frequency is the inverse of the period and can be calculated by, manipulating the equation T =1/f to f = 1/T

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## Define wave amplitude

### (A) is the maximum variation of pressure from its mean value in the tissues. It has the same units as pressure (Pa or N/m^2). It determines the amount of energy in the US wave.The term amplitude may only be used to refer to a sinusoidal wave. Some people refer to it as magnitude which is far more general. Amplitude refers to the change in pressure that occurs when particles in the wave become rarefied and compressed.

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## Define displacement amplitude

### The maximum excursion of an atom from its average position and is usually measured in nanometers (nm = mx10^-9)

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## Outline pressure variation

### (p) is measured in the units Pa or Nm^2. po is used to denote ambient pressure. If the pressure is greater than the ambient pressure we say the fluid is compressed (at the crests). If the pressure is less than the ambient pressure we say the fluid is rarefied (at the troughs).

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## Describe pressure variation on an atomic level in a wave

### p < po means rarefied. p > po means compressed. So if we compress a region of fluid the atoms within the fluid will become closer together. The interatomic forces will then push these atoms apart. As the atoms expand outward they transfer the compression to their neighbouring atoms and then the original atoms are at a slightly lower pressure. This is how a pressure waves moves at the wave speed. A sound wave in a fluid is a pressure variation with areas of compression and rarefaction.

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## Define volume in terms of US waves

### (loudness) is not used in ultrasound. It is determined by the energy of a sound wave. It is related to both pressure amplitude and frequency but as it is zero for sound frequencies used in ultrasound it is not used.

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## Define wave intensity in terms of US waves

### (I) The energy carried by a wave over a specific area in a specific time. In ultrasound this is usually expressed as mW/cm2, where 1 mW = 10-3 watt. It is a more precise measurement of the energy of a wave than loudness.

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## Define Power in terms of US waves

### The energy content of the beam per second and is closely related to the intensity. The unit of power is the Watt (W). 1 Watt = 1 joule per second.

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## What is the equation used to calculate wave speed, wavelength and frequency

### c = f λ

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## What is the frequency range for Ultrasound waves

### Approximately 1-15 MHz

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## What are the two different sinusoidal expressions for a wave?

### One as a function of disturbance (p) over time (s) where disturbance is the y axis and time is the x axis and period is represented crest to crest. The other is as a function of disturbance (p) over distance (x). Where disturbance is the y axis, distance is the x axis and wavelength is represented crest to crest.

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## Describe a transverse wave

### When particles that move do so perpendicular to the direction of wave travel.

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## Describe a longitudinal wave

### When particles that move do so parallel to the direction of wave travel. When the particles associated with the material that support the wave vibrate in a direction that is at all times parallel, or antiparallel, to the wave direction.

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## What is wave interference?

### Interference occurs when two waves of the same frequency are present in the same region of a material and they interact with each other. The combination of these two waves needs to be calculated to determine the resultant motion of pressure and particle movement. This is done by adding the two wave fronts together.

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## Describe constructive interference

### Constructive interference occurs when the crests of two waves occur at the same point. If we had two waves of amplitude A, due to constructive interference the amplitude is now 2A. As power and intensity are directly proportional to amplitude they are now 2^2 = 4.

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## Describe destructive interference

### Destructive interference occurs when a crest and a trough of each wave occur at the same point. In this case the resultant amplitude would be 0A, as would the power and intensity.

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## Recite the powers of ten and abbreviations in the SI measurement system.

###
G, giga = 109 = 1,000,000,000 (a billon)

M, mega = 106 = 1,000,000 (a million)

k, kilo = 103 = 1,000 (a thousand)

h, hecta = 102 = 100 (a hundred)

d, deci = 10-1 = 0.1 = 1/10 (a tenth)

(And not to be confused with deca = 10; ten)

c, centi = 10-2 = 0.01 = 1/100 (a hundredth)

m, milli = 10-3 = 0.001 = 1/1000 (a thousandth)

μ, micro = 10-6 = 0.000001 = 1/1000000 (a millionth)

n, nano = 10-9 = 0.000000001 = 1/1000000000 (a billionth)

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## What, in words, is represented by the symbol A

### Amplitude. Units are Pa or N/m^2

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## What, in words, is represented by the symbol λ

### Wavelength measured in units of length (mm, cm, m)

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## What, in words, is represented by the symbol f

### Frequency Measured in Hertz (Hz)

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## What, in words, is represented by the symbol T

### Period Measured in seconds (s)

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## What, in words, is represented by the symbol p

### Pressure, disturbance or shape away from the average. Pa or N/m^2

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## What, in words, is represented by the symbol x

### Distance measured in units of length (mm, cm, m)

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## What, in words, is represented by the symbol s

### Time measured in seconds (s)

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## What is the difference between loudness, intensity and power of a wave?

###
Loudness - determined by the energy in the sound wave. Not used for ultrasound waves as they have no sound and are zero.

Intensity – Determined by the energy of a sound wave. Used for ultrasound as it is more precise and removes the element of human perception that loudness has. It is the energy carried by the wave covering some specified area in a specified time . Expressed as mW/cm2

Power (P) - the energy content of the beam per second. Unit is the watt. Watt = 1 joule per second, or 1 J/s

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