Transcription
Hello and welcome to the ultrasound physics module. My name is Dr. Michael Nell.
Now, in the next series of talks, we're going to be diving into the world of ultrasound physics. And before we get started in today's talk, I want to make sure we have a clear concept of what exactly a sound wave is and what it isn't. And we'll start by having a look at a definition.
Now, sound is mechanical energy that propagates through a continuous elastic medium by the compression and rarefaction of the units in that medium. So let's break down this definition a little bit more. It's mechanical energy. It's not self-propagating energy like in our electromagnetic radiation. It requires a mechanical force. When we are creating sound, we are mechanically moving our voice box. When we create ultrasound waves, we are moving an element within that ultrasound, and that energy propagates through a medium that needs to be continuous. Sound needs a medium to propagate. When we look at electromagnetic radiation, it could travel through a vacuum because it's self-propagating. Here, sound is dependent on a medium, and if that medium is not continuous, there are regions of vacuum within that plane, the sound will not propagate.
Secondly, the medium needs to be elastic. Now, what does that mean? Well, the molecules or the units within that medium need to be able to move, transfer energy, and return back to where they started. If I had my hand in some water and I pushed the water, the molecules that I initially pushed will then return back to where they were, and a wave will propagate through that water. If I had a pile of sand in front of me and I pushed the sand like that, a wave wouldn't propagate through that sand. The sand that I pushed wouldn't return back to its resting place, and we wouldn't get that transfer of energy. The sand has no elasticity. And we'll look in later talks at the elastic and inertial properties of a medium and how that affects the speed of the wave traveling through that medium.
So when we look at a wave, we can see its regions of compression and rarefaction. Now, these regions of compression and rarefaction represent localized pressure changes within that medium, and we can plot those pressure changes on a graph, and we get a sine wave. Here, the regions of compression have high localized pressures, and the regions of rarefaction have low localized pressures. And we can plot these with the x-axis being the normal pressure within the tissue. Now, in practice, the amplitude of compression is actually higher than the amplitude of rarefaction, but for practical purposes, we will represent a sound wave like this within the talks, and you'll see that in textbooks as well.
Now, the energy here is transferring from left to right, but the molecules are not moving all the way across. They are staying in the same place, oscillating back and forth. When I'm speaking to someone, the air molecules leaving my mouth is not the same air that is reaching someone's eardrum. There, the energy has passed through. It's not the molecules going all the way through. It's a transfer of energy.
Now, when we look at these waves, as with electromagnetic radiation, as with any wave, we can define some properties of that wave. The first being wavelength, the distance between successive regions on a wave. So from one region of compression to the next region of compression, or one region of rarefaction to the next region of rarefaction, that distance there is the wavelength. The next thing we can look at is the frequency of the wave, how many cycles of that wave pass a particular point in a given period of time. And we measure frequency in Hertz. One Hertz is one wave passing a point in one second.
Now, as we looked at when we looked at electromagnetic radiation, we can calculate the speed of the wave by looking at the product of the frequency and the wavelength of that wave. Now, as you see throughout this ultrasound physics module, we are going to be looking at a lot of formulas, and without context to these formulas, we can go about misinterpreting these formulas. When we looked at electromagnetic radiation, electromagnetic radiation traveled at a constant speed. It traveled at the speed of light, no matter if it was X-rays or radio waves, the speed stayed constant. When we're looking at sound, this is not the case. The speed of a sound wave is purely determined by the medium through which it is traveling. We cannot control the speed of a sound wave, but it changes as it goes through different mediums. What we can control is the frequency of the wave. When we are speaking, we set the frequency with our vocal cords. When we are using an ultrasound transducer, we set the frequency of that wave.
Now, independent of that frequency, the sound wave will travel at a set speed depending on the material it is traveling through, and the wavelength will be the compensatory mechanism to link that frequency and that speed. Speed and frequency and sound are completely independent of one another, and don't worry, we are going to be going through this in some depth in the following talks.
Now, when we are looking at electromagnetic radiation and the acoustic spectrum, there are some subtle differences, and these subtle differences make marked differences when we're looking at how these waves interact with tissue. And from what I've seen with the types of questions that get asked in exams, they're trying to test, do you understand the unique properties of sound and how they differ from electromagnetic radiation? And a lot of people get tripped up by not understanding these differences.
So when we looked at the electromagnetic spectrum, we divided it up into multiple different sections, and we classify these sections by the wavelength of the wave. We could do that in electromagnetic radiation because speed stayed constant, so our wavelength acted as a proxy for frequency. And when we looked at the energy of a wave, the frequency determined the energy of that wave. Now, sound waves are a little bit different. We set the frequency of the wave, and the speed of that wave is dependent on the material it travels through. Now, depending on the material, the wavelength will change. We don't have a tight link between wavelength and frequency if the material is changing like we had in electromagnetic radiation. So depending on the frequency and depending on the medium through which it's traveling, our wavelength will change. We can't use wavelength to sub-categorize the acoustic spectrum, so we use frequency. That is what we set. That's the variable that we have control over.
Now, audible sound is between the region of 20 Hertz and 20 kilohertz, 20,000 Hertz. So 20 cycles of a wave passing a point in one second to 20,000 cycles of a wave passing a point in a second. Anything with a frequency lower than that is known as infrasound. We can't hear it. Sound frequencies higher than that is called ultrasound. Anything over 20,000 Hertz is known as ultrasound. We can't hear these frequencies. Now, diagnostic ultrasound is between 2 and 20 megahertz, 2 and 20 million Hertz, 2 and 20 million cycles going past a particular point in one second. We are dealing with really high frequency waves here, and that's an important point to remember.
Now, we can represent electromagnetic waves and sound waves graphically. We've seen this here. If you've done the X-ray module, an electromagnetic wave is a transverse wave, orthogonal waves that self-propagate through space. They can travel in a vacuum. They don't need a medium. The movement of the electric wave and the movement of the magnetic wave self-propagate one another. They have a constant velocity, no matter the frequency or the wavelength, the velocity of that wave is exactly the same, and the energy being transferred through time and space is the electromagnetic energy.
Now, when we look at a sound wave, it's known as a longitudinal wave. The movement of the units within that medium happen in the same direction as the movement of energy within that medium. These units oscillate in this direction, parallel to the movement of energy, unlike our electromagnetic wave where the oscillation of the energy happened in a perpendicular fashion to the movement of energy in that wave.
Now, as we've said, sound waves require a medium to travel, and that medium needs to be continuous and elastic. And depending on that medium, depending on various properties of that medium, the speed of that sound wave will change, and the wavelength will change accordingly. The frequency will not change. If we set a frequency of an ultrasound probe, the speed of that sound will change as it goes through various tissues, but the frequency will remain the same. Here, our frequency is constant. In electromagnetic radiation, our speed was constant. And the last is that energy transferring through is a mechanical energy. It requires a mechanical force to propagate that wave, to move energy through the medium.
Now, I've spent some time discussing this concept, and I really want you to have a good understanding of what acoustic waves are and what they aren't, because when we look at more complex topics in future talks, if you don't have this core basic fundamental knowledge, you're going to fall short when it comes to those more complicated calculations. So in our next talk, we're going to have a closer look at the wavelength, frequency, period, and speed of waves as they travel through a medium. So I'll see you all there. Goodbye, everybody.