what is the simplest method of calculating the speed of sound in air
- bang a drum while standing at a measured distance from the wall of a large building
- time the time it takes to hear the echo right when the drum was banged
how would you calculate the speed of sound with that method
- the distance the sound traveled would be double the distance the person is standing from the wall
- if the distance is originally x, then the distance you use in the calculation is 2x
- then you use speed = distance / time as v = 2x / t
how do sonar and radar work in ships to detect things
- they send out pulses of radio and sound waves
- the time taken for the waves to be reflected back to them as well as the direction they came from is noted
- this allows them to decipher whether there are objects around them and how far away they are
what is ultrasound
a sound wave with a frequency of 20,000Hz or more
what does A-scans stand for what are the y used for
- amplitude scans
- they are used to determine the depth of boundaries between tissue or bone and tissue
before pulses of ultrasound are are emitted by a transducer directly into the body at the region to be investigated , why is coupling gel smeared onto the body at the point of entry
so that very little ultrasound is reflected from the skin
how would an image be displayed when the ultrasound has been shot into the body by the transducer
- the ultrasound would be reflected back and received by the transducer at boundaries between different media
- this could be at the inner abdomen wall or the front and back of an organ
- the time between the reflections and the entry of the pulse are measured using the time base of the CRO
- then the depth of the boundaries are calculated using the pulse-echo formula
what does the fraction of sound that is reflected back depend on
the acoustic impedance of the tissue on each side of the interface
what does the acoustic impedance depend on
the density of the medium
what would be an example of a set up with a high acoustic impedance and low one
- a tissue-bone boundary would have a high acoustic impedance
- whereas a tissue-muscle boundary would have a low one
what does this mean about the fraction of sound that would be reflected back the transducer when met with these boundaries
a bigger reflection of sound occurs at the tissue-bone boundary than at the tissue-muscle
why would the amplitude of the reflected sound waves be reduced compared to when they were emitted into the body
- they would be reduced due to attenuation
- which is the energy absorbed or scattered within the body
how is the attenuation accounted for
the reflected pulses are amplified by a factor depending on the distance travelled
what are B-scans and what are they used for
- they are brightness scans
- they detect the position of the reflecting boundary
- they also give a display of the brightness of the reflection which represents the fraction of energy reflected
between ultrasound and x ray images, which ones have the lower resolution
ultrasound images
why do ultrasound images have a lower resolution
- because resolution depends on the wavelength
- the smaller the wavelength the higher the resolution and the sharper the image
- as x rays have shorter wavelengths than sound, they make clearer images
despite that, why are ultrasound waves still used in body and foetal scanning rather than x rays
- x ray radiation is ionising so it can kill or damage cells
- a foetus is very vulnerable so it is safer alternative
how could the resolution of ultrasound images be improved regardless
by increasing the frequency of the wave
what is a disadvantage that comes with increasing the frequency of the wave
- the shorter waves are absorbed more readily
- meaning the useful range is reduced
what is a compromise that is made when it comes to favoring between clearer images or being able to see more
- high frequency (3MHz) are used for more detailed images of regions close to the skin
- whereas lower frequencies are sued to examine deeper organs
