There is an interesting phenomenon in life: when an ambulance is approaching, the sound is higher than the original; when the ambulance is leaving, the pitch of the sound is lower than the original. What you may not realize is that this phenomenon and the color ultrasound used in hospitals belong to the same principle, which is the "Doppler effect".
The Doppler effect is named in honor of the Austrian physicist and mathematician Christian Johann Doppler, who first proposed the theory in 1842. The main idea is that the wavelength of an object's radiation changes because of the relative motion of the source and the observer. In front of the moving source, the wave is compressed, the wavelength becomes shorter, and the frequency becomes higher (blue shift blue shift); behind the moving source, the opposite effect occurs. The wavelength becomes longer and the frequency becomes lower (red shift); the higher the speed of the source, the greater the effect. Depending on the degree of red (blue) shift, the speed of the source in the direction of observation can be calculated.
The displacement of a star's spectral lines indicates how fast the star is moving in the direction of observation, and the extent of the Doppler shift is generally small unless the speed of the source is very close to the speed of light. The D?bler effect is present in all fluctuating phenomena.
Basic introduction Chinese name ：Doppler effect Foreign name ：Doppler effect Alias ：Doppler's law Proposed by ：Christian John Doppler Proposed time ：1842 Set of disciplines ：Physics Scope of application of the field ：Physics, medicine, transportation Discoveries, principles, formulas, applicable, embodied, set of categories, medical set, transportation set, aviation set, related events, Discoveries, principles, formulas, applicable, embodiment, set of classifications, medical set, transportation set, aeronautical set, related events, Discoveries, principles, formulae, applicable, embodiment, set of classification, medical set, aviation set, relevant events. Discovery Although not as miraculous as the apple hitting Newton's head and inspiring the idea of gravity, the D?bler effect was also an accidental discovery made by an Austrian mathematician and physicist named D?bler in 1842. One day, he was passing through the railroad crossing, coincided with a train from his side, he found that the train from far and near when the whistle becomes loud, the pitch becomes sharp, and the train from near and far when the whistle becomes weak, the pitch becomes low. He was greatly interested in this physical phenomenon and studied it. It was found that this was due to the relative motion between the source and the observer, so that the frequency of the sound heard by the observer was different from the frequency of the source. This is the phenomenon of frequency shift. Because, when the sound source is in motion relative to the observer, the sound heard by the observer changes. When the source is away from the observer, the wavelength of the sound wave increases, the tone becomes low, when the source is close to the observer, the wavelength of the sound wave decreases, the tone becomes high. The change in pitch is related to the ratio of the relative velocity between the source and the observer to the speed of sound. The larger the ratio, the more significant the change, which is later called the "Doppler effect". The principle of the Dobler effect states that waves are received at a higher frequency when the source moves closer to the observer, and at a lower frequency when the source moves away from the observer. The same conclusion can be obtained when the observer moves. However, due to the lack of experimental equipment, Dobler did not experimentally verify at that time. A few years later, a group of trumpet players were asked to play on a flatbed truck, and then a trained musician was asked to use his ears to recognize the change in pitch in order to verify the effect. Assuming that the wavelength of the original wave source is λ, the wave speed is u, and the observer is moving with a speed of v (the following analysis does not apply to light waves): The Doebler Effect1 The frequency of the wave source observed when the observer approaches the source is (u+v)/λ, and vice versa. A frequently used example is the sound of a train whistle, when the train approaches the observer, and if the observer is far away from the wave source, its whistle will sound harsher than usual. You can hear the change in harshness as the train passes. The same is true for the sirens of police cars and the engines of race cars. If you think of sound waves as pulses emitted at regular intervals, imagine that if you emit a pulse with every step you take, then every pulse before you is closer to you than if you were standing still. And the sound source behind you is one step farther away than when you were standing still. Or, the frequency of the pulse before you is higher than usual, and the frequency of the pulse after you is lower than usual. The reason: the sound source completes one full vibration, and sends out a wave of one wavelength, the frequency indicates the number of full vibrations completed in a unit of time, so the frequency of the wave source is equal to the number of full waves sent out by the source in a unit of time, and the pitch of the sound heard by the observer is determined by the frequency received by the observer, i.e., the number of full waves received in a unit of time. When there is relative motion between the wave source and the observer, the frequency received by the observer changes． In unit time, the number of complete waves received by the observer increases, i.e., the received frequency increases. By the same token, when the observer moves away from the source, the number of complete waves received by the observer per unit time decreases, i.e., the received frequency decreases. Formula Observer (Observer) and the source (Source) frequency relationship (this formula does not apply to light waves, light waves of the Doppler effect see below): for the observed frequency; for the emission of the source of the original frequency of emission in the medium;for the speed of the wave in the medium;
for the speed of the observer to move, if close to the source of the front of the operator symbols is the speed at which the wave is traveling in the medium;
is the speed at which the observer is moving, if it is close to the source then the forward operator sign is the - sign, if it is close to the observer then it is the + sign. Through this formula, we know the train approaching you when the reason for the change in pitch: the formula is the numerator of the sound propagation speed and the sum of the observer's speed (v + v 0), the denominator is the sound propagation speed and the difference between the train's speed (v-v s), and then and the original frequency of the sound source () multiplication operation. The frequency received by the observer becomes higher than the original frequency of the train whistle, so the train whistle is heard at a higher pitch. On the other hand, when the observer is far away from the train, the subtraction operation of the numerator becomes smaller, and the addition operation of the denominator becomes larger, and the calculated frequency becomes lower than the original frequency of the train whistle, so the pitch of the train whistle becomes lower. The Dobler effect applies not only to sound waves, but to all types of waves, including electromagnetic waves. Scientist Edwin Hubble used the Doppler effect to conclude that the universe was expanding. He found that light emitted by objects moving away from the Milky Way became less frequent, i.e., moved to the red end of the spectrum, called the redshift, and that the faster the objects left the Milky Way the greater the redshift, which indicated that the objects were moving away from the Milky Way. Conversely, if the objects are moving toward the Milky Way, the light is blueshifted. In mobile communication, when a mobile station moves towards the base station, the frequency becomes higher, and when it moves away from the base station, the frequency becomes lower, so we have to take the Doppler effect into full consideration in mobile communication. Of course, due to the limitations of our moving speed in daily life, it is not possible to bring very large frequency shift, but this undeniably will bring the impact on mobile communication, in order to avoid this effect causing problems in our communication, we have to take all kinds of considerations in technology. It also increases the complexity of mobile communication. In the case of monochrome, the color perceived by our eyes can be interpreted as the frequency of the vibration of the light waves, or as the number of times that the electromagnetic field alternates in one second. In the visible region, the lower the frequency, the more red it tends to be, while the higher the frequency, the more blue or violet it tends to be. For example, the bright red color produced by a helium-neon laser corresponds to a frequency of 4.74 x 10^14 Hz, while the violet color of a mercury lamp corresponds to a frequency of 7 x 10^14 Hz or more. The same principle applies to sound waves: the perception of the height of a sound corresponds to the frequency of the vibration of the pressure exerted by the sound on the tympanic membrane of the ear (high frequency sounds are sharp, low frequency sounds are muffled). If the source of the wave is stationary, the vibration of the wave received by the immobile receiver is the same as the rhythm of the wave emitted by the source: the emitted frequency is equal to the received frequency. If the wave source is moving relative to the receiver, e.g. away from each other, then the situation is different. The distance between the two peaks produced by the source is lengthened with respect to the receiver, and therefore the time taken for the two upper peaks to reach the receiver is longer. Then the frequency decreases when it reaches the receiver, and the perceived color shifts toward red (the opposite is true if the wave source is closer to the receiver). To give the reader an idea of the magnitude of the effect, the Doppler shift is shown to give an approximation of the frequency received by a source that is moving away as its relative velocity changes. For example, in the red spectral line of the helium--neon laser mentioned above, the frequency received drops from 4.74 x 10^14 Hz to 2.37 x 10^14 Hz when the speed of the source is equal to one-half the speed of light, a value which is substantially downshifted into the infrared band. Embodiment of the Doppler effect of sound waves In our daily lives, we all have the experience that when a train with a whistle passes by an observer, he will notice that the pitch of the train's whistle changes from high to low. Why does this happen? This is because the pitch of the whistle is determined by the frequency of the vibrations of the sound waves. If the frequency is high, the pitch of the whistle sounds high; if the frequency is high, the tone sounds high; if the frequency is high, the tone sounds low. This phenomenon is known as the Dobler effect, which is named after the discoverer Christian Dobler, who was an Austrian physicist and mathematician. He first discovered this effect in 1842. In order to understand this phenomenon, it is necessary to examine the laws that govern the propagation of sound waves emitted by a whistle when a train is approaching at a constant speed . The result is that the wavelength of the sound wave is shortened, as if the wave were compressed. As a result, the number of waves propagating in a given interval of time increases, which is why the observer perceives the sound as becoming higher in pitch; conversely, as the train moves away, the wavelength of the sound wave becomes larger, as if the wave is being stretched. As a result, the sound sounds low . Quantitative analysis yields f1 = (u+v0)f /(u-vs), where vs is the velocity of the wave source with respect to the medium, v0 is the velocity of the observer with respect to the medium, f denotes the intrinsic frequency of the wave source, and u denotes the propagation speed of the wave in a stationary medium. When the observer moves toward the wave source, v0 takes a positive sign; when the observer moves away from the wave source (i.e., along the wave source), v0 takes a negative sign. When the wave source moves toward the observer, vs takes a positive sign in front; when the wave source moves away from the observer, vs takes a negative sign. From the above formula, it is easy to know that when the observer and the source are close to each other, f1>f; when the observer and the source are far away from each other, f1<f. Dobler effect3 Let the source of the sound S, the observer L, respectively, with a speed Vs, Vl in the static medium along the same straight line of the same direction of the movement of the sound source sends out the propagation speed of sound waves in the medium for V, and Vs is less than V, Vl is less than V. When the sound source does not move, the sound source emits sound waves with frequency f and wavelength X, and the frequency of the sound waves received by the observer is: f'=(V+Vl)V/[(V-Vs)X]=(V+Vl)f/(V-Vs) So it is obtained that (1) When the observer and the wave source do not move, Vs=0 and Vl=0, and from the above equation, f'=f (2) When the observer does not move and the sound source When the observer is close to the observer, the observer receives the frequency F = Vf / (V - Vs) Obviously at this time the frequency is greater than the original frequency From the above equation can be obtained from all the manifestations of the Dobler effect. The Dobler effect of light waves This effect also occurs with fluctuating light, and it is also known as the Dobler-Fischer effect. It is also known as the Dobre-Fischer effect, because the French physicist Fischer (1819-1896) independently explained the wavelength shift from stars in 1848, and pointed out a way to utilize this effect to measure the relative velocities of stars. Light waves differ from sound waves in that a change in the frequency of a light wave is perceived as a change in color. If the star is moving away from us, the spectral lines of light move in the direction of red light, known as redshift; if the star is moving towards us, the spectral lines of light move in the direction of violet light, known as blueshift. The Dobler effect of light (electromagnetic wave) formula is divided into the following three kinds: (1) longitudinal Dobler effect (i.e., the speed of the wave source and the line of the wave source and the receiver *** line): f'=f [(c+v)/(c-v)]^(1/2) Dobler effect 4 China Aerospace Sets of Technology where v is the relative speed of the wave source and the receiver. When the source is close to the observer, v is positive, called "purple shift" or "blue shift"; otherwise v is negative, called "red shift". (2) Transverse Doppler effect (i.e., the velocity of the source is perpendicular to the line between the source and the receiver): f'=f (1-β^2)^(1/2) where β=v/c (3) Generalized Doppler effect (the general case of the Doppler effect): f'=f [(1-β^2)^(1/2)]/(1-βcosθ) where β=v/c, and θ is the angle of the receiver's line with the source to the direction of velocity. . The longitudinal and transverse Doebler effects are special cases when θ is taken to be 0 or π/2, respectively. Classification of sets Medical sets The D?bler effect of acoustic waves can also be used for medical diagnosis, which is what we usually call color ultrasound. Color ultrasound is simply a high-definition black-and-white ultrasound plus color Doppler, first of all, ultrasound frequency shift diagnostic method, that is, D ultrasound, this method applies the principle of the Doppler effect, when there is a relative motion between the source of the sound and the receiver (i.e., the probe and the reflector), the frequency of the echo changes, such frequency changes are called frequency shift, D ultrasound, including pulsed Doppler, continuous Doppler, and color Doppler blood flow images. Color Doppler ultrasound is generally used in the autocorrelation technique for Doppler signal processing, the autocorrelation technique obtained by the blood flow signal after color coding in real time superimposed on the two-dimensional image, that is, the formation of color Doppler ultrasound blood flow images. As a result, color Doppler ultrasound (i.e., color ultrasound) has the advantages of two-dimensional ultrasound structural images, but also provides a wealth of information on hemodynamics, and its practical application has been widely valued and welcomed, and it is known as "non-traumatic angiography" in clinical practice. In order to examine the movement of the heart and blood vessels and to understand the speed of blood flow, ultrasound can be emitted. Since the blood in the blood vessels is a flowing object, the Doppler effect occurs between the ultrasound source and the blood moving relative to each other. As the blood vessel moves toward the ultrasound source, the wavelength of the reflected wave is compressed and thus the frequency increases. As the blood vessel moves away from the source, the wavelength of the reflected wave lengthens and thus decreases in frequency per unit of time inward. The amount of increase or decrease in the frequency of the reflected wave is directly proportional to the speed of blood flow, and thus the flow rate of blood can be determined based on the frequency shift of the ultrasound wave. We know the speed of intravascular blood flow and blood flow rate, which has a certain value for the diagnosis of cardiovascular diseases, in particular, it provides valuable diagnostic information on the oxygen supply in the circulatory process, the ability of atresia, the presence of turbulence, vascular atherosclerosis and so on. Ultrasound Dobler method of diagnosis of the heart process is this: ultrasound oscillator produces a high-frequency equal-amplitude ultrasound signals, excitation of the transmitter transducer probe, generating continuous ultrasound waves, to the human body cardiovascular organs to transmit, it produces the Dobler effect, when the ultrasound beam encounters the movement of the organs and blood vessels, the reflected signal for the transducer to accept, according to the frequency difference between the reflected wave and the transmitter blood flow velocity, according to the reflected wave and the transmitter frequency difference to seek, the blood flow rate. The direction of blood flow is determined by whether the reflected wave increases or decreases in frequency. To make it easier to align the probe with the blood vessel being measured, a plate-shaped, double-stacked probe is often used. Color Doppler Ultrasound Supplement: The Doppler effect can also be explained by the attenuation theory of wave propagation in a medium. Waves traveling in a medium exhibit dispersion, with high frequencies moving to lower frequencies as the distance increases. The development of ultrasound in the medical field is the direction of color ultrasound, let's talk about the characteristics of color ultrasound: Doppler effect5 Its main advantages are: ① can quickly visualize the two-dimensional distribution of blood flow. ② can show the direction of blood flow. ③It is helpful to identify arteries and veins. ④It is good for recognizing vascular lesions and non-vascular lesions. ⑤Facilitates the understanding of the nature of blood flow. ⑥ It can facilitate the understanding of the temporal phase and velocity of blood flow. ⑦ It can reliably detect shunts and refluxes. (8) It can quantitatively analyze the origin, width, length, and area of blood flow bundles. But the color ultrasound using the relevant technology is a pulsed wave, the speed of the detector is too high, the color of the color flow will be wrong, in the quantitative analysis is significantly inferior to the spectral Doppler, nowadays color Doppler ultrasound instrument with spectral Doppler's function, that is, for the color 鈹 Dual-function ultrasound. Color Doppler ultrasound flow map (CDF), also known as color Doppler ultrasound imaging (CDI), which obtains the source of echo information and spectral Doppler consistent with the distribution and direction of blood flow in two-dimensional display, different speeds with different colors to distinguish. Dual-function Doppler ultrasound systems, i.e., B-mode ultrasound images that show the location of blood vessels. Doppler measures blood flow, and this combination of B-mode and Doppler systems allows for more precise localization of any given vessel. In a spectral Doppler display, the direction of blood flow is distinguished by the zero baseline. Above the zero baseline indicates flow toward the probe, and below the zero baseline indicates flow away from the probe. In CDI, the direction of blood flow is color-coded, with a red or yellow *** spectrum indicating flow toward the probe (hot) and a blue or blue-green *** spectrum indicating flow away from the probe (cold). 2. Vascular distribution CDI shows blood flow within the lumen of a blood vessel and is therefore a flow channel type of display; it does not show the vessel wall or periphery. 3. Identify the types of blood vessels in cancerous nodules With CDI, the blood vessels of hepatocellular carcinoma nodules can be categorized. Distinguish them as perinodal circumflex vessels, curved vessels to the inner edge of the nodule. The blood vessels in the nodule, the blood vessels inside the nodule and the blood vessels out of the nodule can be classified. Color ultrasound clinical application (a) vascular disease using 10MHz high-frequency probe can be found in the blood vessels less than 1mm calcification points, for carotid artery sclerosis occlusive disease has a better diagnostic value, but also the use of blood flow probing local amplification to determine the degree of lumen stenosis, embolus whether there is a possibility of dislodgement, whether or not to produce ulcers, to prevent cerebral embolism occurs. Color ultrasound for all kinds of arteriovenous fistula can be said to be the best diagnostic method, when the probe to the colorful mosaic of ring color spectrum can be diagnosed. For carotid artery aneurysm, abdominal main vein aneurysm, vascular occlusive vasculitis, chronic venous disease of the lower extremities (including lower extremity varicose veins, the original occurrence of deep venous valve insufficiency of the lower extremities, lower extremity deep venous reflux disorders, thrombophlebitis and venous thrombosis) using color ultrasound of high-definition, local magnification and blood flow spectroscopy can be made a more correct diagnosis. (B) abdominal organs mainly used in the liver and kidney, but for the identification of benign and malignant lesions in the abdominal cavity, gallbladder cancer and large polyps, chronic inflammation, the difference between the common bile duct, hepatic artery, and other diseases have a certain auxiliary diagnostic value. For cirrhosis, color ultrasound can make a better judgment from the speed and slowness of flow rate in the liver, the size and direction of vascular lumen and the establishment of collateral circulation. For nodular sclerosis and diffuse hepatocellular carcinoma, which are difficult to be distinguished by black and white ultrasound, high-frequency exploration and blood flow spectrum exploration can be utilized to make differential diagnosis. For the differentiation of benign and malignant space-occupying lesions in the liver, cysts and various kinds of arteriovenous tumors, it has a better diagnostic value, and primary liver cancer and secondary liver cancer can be differentiated by internal blood supply. Color ultrasound used in the kidney is mainly used for renal vascular lesions, such as the aforementioned renal arteriovenous fistula, when the clinical manifestations of intervals, painless hematuria can not find the cause of the strong indications. For one of the common causes of secondary hypertension - renal artery stenosis, ultrasound can basically be a clear diagnosis, when exploring the stenosis of blood flow rate of more than 150cm/s, the diagnostic accuracy of 98.6%, while the sensitivity is 100%. On the other hand, it is also the differential diagnosis of renal cancer, renal pelvis metastatic cancer and benign tumors. (III) Small organs Among the small organs, color ultrasound has obvious diagnostic accuracy compared with black and white ultrasound mainly for thyroid, breast and eye, in a way, the 10MHz probe without color flow has been much clearer than the ordinary black and white ultrasound with 5MHz, the probe is much clearer, and the diagnosis and differential diagnosis of thyroid lesions are mainly made based on the internal blood supply of thyroid, among which, the image of hyperthyroidism is the most typical, with specificity, and is a "sea of fire" sign. "sea of fire sign". Simple goiter, on the other hand, shows no significant changes compared to normal thyroid blood flow. Subacute thyroiditis, Hashimoto's thyroiditis between the two, can be used to distinguish between the nodules and the surrounding blood flow can be well differentiated nodular goiter, thyroid adenoma and thyroid cancer, so it is recommended that the diagnosis of the thyroid gland is not too clear, the patient has a certain financial ability to do ultrasound to further clarify the diagnosis. Breast ultrasound is mainly used for breast fibroma and breast cancer differential diagnosis, while the eye mainly has better diagnostic value for vascular lesions of the eye. (D) Prostate and seminal vesicles Because rectal exploration is the best method to diagnose prostate at present, it is specially proposed here. In this method, the prostate gland is divided into the migratory zone, the central zone, the peripheral zone, and a part of the fibromuscular stroma of the prostate. The migratory zone includes both sides of the periurethral sphincter and the abdomen, which is the origin of 100% of BPH, while the migratory zone accounts for only 5% of the size of the prostate in normal people. The central area is around the *** tube, the tip wall points to the seminal caruncle, and the peripheral area includes the posterior part of the prostate and the tip of both sides, which is the origin of 70-80% of cancers, while the perimembranous membrane of the tip is bookish or even disappears, forming an anatomically weak area, which is the common metastatic channel of cancers, and is the key area of the biopsy of the prostate gland. Rectal examination has good diagnostic value for various prostate and seminal vesicle diseases, and when combined with prostate biopsy, the diagnosis is basically clear, while the incidence of prostate diseases, especially prostate cancer, is on the rise in China, and the incidence of prostate cancer in Europe and the United States is even ranked behind lung cancer as the second highest incidence of cancer, and abdominal examination of the prostate is basically unable to make a diagnosis, so it is recommended that the clinic use more rectal Therefore, it is recommended to utilize rectal ultrasound to diagnose prostate diseases more often in clinical practice, so that rectal exploration can be used to diagnose prostate diseases without abdominal exploration. (E) Obstetrics and gynecology The main advantage of color ultrasound for obstetrics and gynecology lies in the identification of benign and malignant tumors and umbilical cord disease, fetal preterm heart disease and placental function assessment, for trophoblastic diseases have a better auxiliary diagnostic value, infertility, pelvic varicose veins through the observation of blood flow spectra, but also can be made in black and white ultrasound difficult to diagnose. The use of *** probe has certain advantages over abdominal exploration, its superiority is mainly reflected in ① uterine artery, ovarian blood sensitivity, high display rate. ②Shorten the examination time and obtain accurate Doppler spectrum. ③No need to fill the bladder. ④It is not disturbed by obesity, abdominal scars, or bowel insufflation. ⑤ With the activity of the tip of the probe to find the pelvic organ tenderness site to determine whether there is adhesion in the pelvis. Traffic set Traffic police to the traveling vehicle transmit frequency known ultrasound wave at the same time to measure the frequency of the reflected wave, according to how much the frequency of the reflected wave changes will be able to know the speed of the vehicle. A monitor equipped with a Dobler speed camera, sometimes mounted just above the road, photographs the license plate number of the vehicle while measuring the speed and prints the measured speed automatically. Aviation set March 8, 2014 Malaysia Airlines MH370 lost, 17 days later, Malaysia's Prime Minister Najib 24 evening temporarily held a press conference to announce: "According to the latest data, flight MH370 ended in the southern Indian Ocean." McLaughlin, vice president of Inmarsat, who was involved in the investigation of the lost flight, explained that they used the theory of the Doebler effect, combined with other reference factors, to give the final direction of MH370 based on a large amount of data analysis. Related Events On March 24, 2014, at 10:00, Malaysian Prime Minister Najib held an emergency press conference in which he said that according to new data analysis, Flight MH370 crashed in the southern Indian Ocean. Inmarsat 24 explained that they used the theory of the Dobler effect to analyze the signals from Malaysia Airlines Flight MH370 and concluded that the plane fell into the southern Indian Ocean.