Sound Waves and the Acoustic Spectrum | Ultrasound Physics | Radiology Physics Course #1 — Transcript

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1. 0:00Hello and welcome to the ultrasound
2. 0:01physics module. My name is Dr. Michael
3. 0:03Now. Now in the next series of talks
4. 0:05we're going to be diving into the world
5. 0:07of ultrasound physics and before we get
6. 0:08started in today's talk, I want to make
7. 0:10sure we have a clear concept of what
8. 0:12exactly a sound wave is and what it
9. 0:15isn't and we'll start by having a look
10. 0:16at the definition. Now sound is
11. 0:18mechanical energy that propagates
12. 0:20through a continuous elastic medium by
13. 0:23the compression and rarefaction of the
14. 0:25units in that medium. So let's break
15. 0:27down this definition a little bit more.
16. 0:29It's mechanical energy. It's not
17. 0:31self-propagating energy like in our
18. 0:33electromagnetic radiation. It requires a
19. 0:35mechanical force. When we are creating
20. 0:37sound, we are mechanically moving our
21. 0:40voice box. When we create ultrasound
22. 0:42waves, we are moving an element within
23. 0:44that ultrasound. And that energy
24. 0:46propagates through a medium that needs
25. 0:48to be continuous. Sound needs a medium
26. 0:51to propagate. When we looked at
27. 0:52electromagnetic radiation, it could
28. 0:54travel through a vacuum because it's
29. 0:56self-propagating. Here sound is
30. 0:58dependent on a medium. And if that
31. 1:00medium's not continuous, there are
32. 1:02regions of vacuum within that plane, the
33. 1:05sound will not propagate. Secondly, the
34. 1:08medium needs to be elastic. Now, what
35. 1:10does that mean? Well, the molecules or
36. 1:12the units within that medium need to be
37. 1:14able to move, transfer energy, and
38. 1:17return back to where they started. If I
39. 1:20had my hand in some water and I push the
40. 1:22water, the molecules that I initially
41. 1:24pushed will then return back to where
42. 1:26they were and a wave will propagate
43. 1:28through that water. If I had a pile of
44. 1:31sand in front of me and I push the sand
45. 1:33like that, a wave wouldn't propagate
46. 1:35through that sand. The sand that I
47. 1:37pushed wouldn't return back to its
48. 1:39resting place and we wouldn't get that
49. 1:40transfer of energy. The sand has no
50. 1:42elasticity. And we'll look in later
51. 1:45talks at the elastic and inertial
52. 1:47properties of a medium and how that
53. 1:48affects the speed of the wave traveling
54. 1:50through that medium.
55. 1:52So when we look at a wave, we can see
56. 1:54its regions of compression and
57. 1:56rarefaction. Now these regions of
58. 1:58compression and rarefaction represent
59. 2:01localized pressure changes within that
60. 2:03medium, and we can plot those pressure
61. 2:05changes on a graph, and we get a sine
62. 2:07wave here. The regions of compression
63. 2:10have high localized pressures, and the
64. 2:12regions of rarefaction have low
65. 2:14localized pressures, and we can plot
66. 2:16these with the x-axis being the normal
67. 2:19pressure within the tissue. Now, in
68. 2:21practice, the amplitude of compression
69. 2:24is actually higher than the amplitude of
70. 2:26rarefaction, but for practical purposes,
71. 2:28we will represent a sound wave like this
72. 2:30within the talks, and you'll see that in
73. 2:32textbooks as well.
74. 2:33Now, the energy here is transferring
75. 2:35from left to right, but the molecules
76. 2:37are not moving all the way across. They
77. 2:40are staying in the same place
78. 2:41oscillating back and forth. When I'm
79. 2:43speaking to someone, the air molecules
80. 2:45leaving my mouth is not the same air
81. 2:47that is reaching someone's eardrum
82. 2:49there. The energy has passed through.
83. 2:51It's not the molecules going all the way
84. 2:53through. It's a transfer of energy.
85. 2:55Now, when we look at these waves, as
86. 2:57with electromagnetic radiation, as with
87. 2:58any wave, we can define some properties
88. 3:01of that wave. The first being
89. 3:02wavelength. The distance between
90. 3:05successive regions on a wave. So, from
91. 3:07one region of compression to the next
92. 3:10region of compression, or one region of
93. 3:12rarefaction to the next region of
94. 3:13rarefaction, that distance there is the
95. 3:17wavelength.
96. 3:18The next thing we can look at is the
97. 3:19frequency of the wave. How many cycles
98. 3:22of that wave pass a particular point in
99. 3:25a given period of time, and we measure
100. 3:26frequency in hertz. 1 hertz is one wave
101. 3:30passing a point in 1 second.
102. 3:33Now, as we looked at when we looked at
103. 3:35electromagnetic radiation, we can
104. 3:37calculate the speed of the wave by
105. 3:39looking at the product of the frequency
106. 3:41and the wavelength of that wave.
107. 3:43Now, as you see throughout this
108. 3:45ultrasound physics module, we are going
109. 3:46to be looking at a lot of formulas. And
110. 3:48without context to to formulas, we can
111. 3:50go about misinterpreting these formulas.
112. 3:53When we looked at electromagnetic
113. 3:55radiation, electromagnetic radiation
114. 3:57traveled at a constant speed. It
115. 3:59traveled at the speed of light. No
116. 4:01matter if it was x-rays or radio waves,
117. 4:03the speed stayed constant. When we're
118. 4:05looking at sound, this is not the case.
119. 4:07The speed of a sound wave is purely
120. 4:09determined by the medium through which
121. 4:12it is traveling. We cannot control the
122. 4:14speed of a sound wave, but it changes as
123. 4:17it goes through different mediums. What
124. 4:19we can control is the frequency of the
125. 4:22wave.
126. 4:23When we are speaking, we set the
127. 4:24frequency with our vocal cords. When we
128. 4:26are using an ultrasound transducer, we
129. 4:29set the frequency of that wave.
130. 4:31Now, independent of that frequency, the
131. 4:33sound wave will travel at a set speed
132. 4:36depending on the material it is
133. 4:37traveling through. And the wavelength
134. 4:39will be the compensatory mechanism to
135. 4:42link that frequency and that speed.
136. 4:44Speed and frequency in sound are
137. 4:46completely independent of one another.
138. 4:48And don't worry, we're going to be going
139. 4:49through this in some depth in the
140. 4:51following talks.
141. 4:52Now, when we are looking at
142. 4:53electromagnetic radiation and the
143. 4:55acoustic spectrum, there are some subtle
144. 4:57differences. And these subtle
145. 4:59differences make marked differences when
146. 5:01we're looking at how these waves
147. 5:02interact with tissue. And from what I've
148. 5:04seen with the types of question that
149. 5:06gets asked in exams, they're trying to
150. 5:08test, do you understand the unique
151. 5:10properties of sound and how they differ
152. 5:12from electromagnetic radiation? And a
153. 5:14lot of people get tripped up by not
154. 5:16understanding these differences.
155. 5:18So, when we looked at the
156. 5:18electromagnetic spectrum, we divided it
157. 5:20up multiple different sections. And we
158. 5:23classify these sections by the
159. 5:25wavelength of the wave. We could do that
160. 5:28in electromagnetic radiation because
161. 5:30speed stayed constant. So, our
162. 5:32wavelength acted as a proxy for
163. 5:34frequency. And when we looked at the
164. 5:36energy of a wave, the frequency
165. 5:38determined the energy of that wave.
166. 5:41Now, sound waves are a little bit
167. 5:42different. We set the frequency of the
168. 5:45wave, and the speed of that wave is
169. 5:47dependent on the material it travels
170. 5:49through.
171. 5:50Now, depending on the material, the
172. 5:52wavelength will change. We don't have a
173. 5:54tight link between wavelength and
174. 5:56frequency if the material is changing,
175. 5:59like we had in electromagnetic
176. 6:00radiation.
177. 6:01So, depending on the frequency and
178. 6:03depending on the medium through which
179. 6:05it's traveling, our wavelength will
180. 6:07change. We can't use wavelength to
181. 6:09subcategorize the acoustic spectrum.
182. 6:12So, we use frequency. That is what we
183. 6:14said. That's the variable that we have
184. 6:16control over. Now, audible sound is
185. 6:18between the region of 20 hertz and 20
186. 6:21kilohertz, 20,000 hertz. So, 20 cycles
187. 6:25of a wave passing a point in 1 second to
188. 6:2820,000 cycles of a wave passing a point
189. 6:31in a second. Anything with a frequency
190. 6:33lower than that is known as infrasound.
191. 6:35We can't hear the sound.
192. 6:38Frequencies higher than that is called
193. 6:40ultrasound. Anything over 20,000 hertz
194. 6:43is known as ultrasound. We can't hear
195. 6:45these frequencies. Now, diagnostic
196. 6:48ultrasound is between 2 and 20
197. 6:50megahertz, 2 and 20 million hertz, 2 and
198. 6:5420 million cycles going past a
199. 6:56particular point in 1 second. We are
200. 6:59dealing with really high frequency waves
201. 7:01here, and that's an important point to
202. 7:02remember.
203. 7:03Now, we can represent electromagnetic
204. 7:05waves and sound waves graphically. We've
205. 7:07seen this here if you've done the x-ray
206. 7:09module. An electromagnetic wave is a
207. 7:11transverse wave, orthogonal waves that
208. 7:14self-propagate through space.
209. 7:16They can travel in a vacuum. They don't
210. 7:18need a medium. The movement of the
211. 7:20electric wave and the movement of the
212. 7:22magnetic wave self-propagate one
213. 7:24another. They have a constant velocity,
214. 7:27no matter the frequency or the
215. 7:28wavelength, the velocity of that wave is
216. 7:30exactly the same, and the energy being
217. 7:33transferred through time and space is
218. 7:34the electromagnetic energy. Now, when we
219. 7:37look at a sound wave, it's known as a
220. 7:38longitudinal wave. The movement of the
221. 7:41units within that medium happen in the
222. 7:43same direction as the movement of energy
223. 7:45within that medium. These units
224. 7:48oscillate in this direction, parallel to
225. 7:51the movement of energy, unlike our
226. 7:53electromagnetic wave where the
227. 7:54oscillation of the energy happen in a
228. 7:57perpendicular fashion to the movement of
229. 7:59energy in that wave.
230. 8:01Now, as we've said, sound waves require
231. 8:03a medium to travel in that medium needs
232. 8:05to be continuous and elastic. And
233. 8:07depending on that medium, depending on
234. 8:09various properties of that medium, the
235. 8:11speed of that sound wave will change and
236. 8:14the wavelength will change accordingly.
237. 8:16The frequency will not change. If we set
238. 8:19a frequency of an ultrasound probe, the
239. 8:21speed of that sound will change as it
240. 8:22goes through various tissues, but the
241. 8:24frequency will remain the same. Here,
242. 8:26our frequency is constant. In
243. 8:28electromagnetic radiation, our speed was
244. 8:30constant. And the last is that energy
245. 8:32transferring through is a mechanical
246. 8:34energy. It requires a mechanical force
247. 8:37to propagate that wave, to move energy
248. 8:40through the medium. Now, I've spent some
249. 8:42time discussing this concept and I
250. 8:43really want you to have a good
251. 8:45understanding of what acoustic waves are
252. 8:47and what they aren't. Cuz when we look
253. 8:49at more complex topics in future talks,
254. 8:51if you don't have this core basic
255. 8:53fundamental knowledge, you're going to
256. 8:54fall short when it comes to those more
257. 8:56complicated calculations. So, in our
258. 8:58next talk, we're going to have a closer
259. 8:59look at the wavelength, frequency,
260. 9:01period, and speed of waves as they
261. 9:04travel through a medium. So, I'll see
262. 9:05you all there. Goodbye, everybody.