question: Why do sirens and vehicles change pitch when they pass you?
comment: it's called the doppler effect. the sound waves get smushed as they approach you, and stretched as the source of the sound moves away from you. this changes the pitch, because pitch is based on the size and frequency of the sound waves. it works for light, too, but the object has to be moving pretty fast. that's how we know the universe is expanding.
passages: Section::::Application.
  
  Section::::Application.:Sirens.
  
  A siren on a passing emergency vehicle will start out higher than its stationary pitch, slide down as it passes, and continue lower than its stationary pitch as it recedes from the observer. Astronomer John Dobson explained the effect thus:
   In other words, if the siren approached the observer directly, the pitch would remain constant, at a higher than stationary pitch, until the vehicle hit him, and then immediately jump to a new lower pitch. Because the vehicle passes by the observer, the radial velocity does not remain constant, but instead varies as a function of the angle between his line of sight and the siren's velocity:
  
  where formula_31 is the angle between the object's forward velocity and the line of sight from the object to the observer.
  
  Section::::Application.:Astronomy.
   Doppler effect
  
  The Doppler effect (or the Doppler shift) is the change in frequency or wavelength of a wave in relation to an observer who is moving relative to the wave source. It is named after the Austrian physicist Christian Doppler, who described the phenomenon in 1842.
  
  A common example of Doppler shift is the change of pitch heard when a vehicle sounding a horn approaches and recedes from an observer. Compared to the emitted frequency, the received frequency is higher during the approach, identical at the instant of passing by, and lower during the recession.
   BULLET::::- All clear: A 30 second steady tone that marks testing of the sirens every first Saturday each month at noon and the end of an emergency for which the immediate danger signal was sounded.
   Section::::Civil defense sirens around the world.:Europe.:Finland.
  
  A general alarm consists of a repeating 1-minute sound that consists of tones ascending for 7 seconds and descending for 7 seconds. The end of danger is signaled by a 1-minute continuous tone. Warning sirens are tested on the first Monday of every month at noon. The testing alarm is shorter than the general alarm (only lasting for 7 or 14 seconds) and may be a flat tone.
  
  Section::::Civil defense sirens around the world.:Europe.:France.
   "Note: For an individual driver this effect is known as directivity, and is observable in both vertical and horizontal planes, and d is now the driver's diameter relative to the wavelength, whereas, the lobing pattern due to two or more drivers is primarily an effect in the vertical plane, as a result of the distance between the two drivers."
   Section::::Function of the Doppler Shift Compensation Mechanism.
   Section::::Trucks.:Emergency vehicles.
  
  Many fire trucks, ambulances, and other large emergency vehicles operate air horns as a means of warning vehicles to clear the right-of-way.
  
  There are also electronic horns for emergency vehicles, which produce a similar easily recognizable sound. These are typically integrated into the same system as the vehicle's electronic siren, and sound through the same speakers. In the last several decades, electronic sound systems with more widely varying frequencies have been chosen as common supplemental warning systems.
  
  Section::::Locomotives.
   The most common tone, 'alert', is widely used by municipalities to warn citizens of impending severe weather, particularly tornadoes. This practice is nearly universal in the Midwest and parts of the deep South, where intense and fast moving thunderstorms that can produce tornadoes occur frequently. The sound of 'alert' is a steady continuous note. In seaside towns, 'alert' may also be used to warn of a tsunami. Sirens that rotate will have a rising-and-falling tone as the direction of the horn changes. The 'attack' tone is the rising-falling sound of an air raid or nuclear attack, frequently heard in war movies. It was once reserved for imminent enemy attack, but is today sometimes used to warn of severe weather, tsunamis, or even fire calls, depending on local ordinance.
   Temporal Theory posits that the cause is from looking at the phase locking to tell what the pitch is. This theory has a hard time explaining diplacusis. There are some examples of pitch which don’t have an "edge" on the basilar membrane, which this would account for—i.e. white noise, clicks, etc. (need a reference)
   BULLET::::- H.O.R. – A New York company that started around 1930. They made vertical sirens 3, 5, and 7.5 HP. They also produced the Super Sirex, a large, double-ended siren usually available in single tone, dual tone, and rotating versions with a 25, 40, or 50 Hp. motor. Their dual-tone sirens had 7 and 10 ports, producing the infamous "Devil's interval" of a diminished fifth, or tritone. They went out of business by the 1980s or the 1990s.
   The Doppler effect (or Doppler shift), named after Austrian physicist Christian Doppler who proposed it in 1842, is the difference between the observed frequency and the emitted frequency of a wave for an observer moving relative to the source of the waves. It is commonly heard when a vehicle sounding a siren approaches, passes and recedes from an observer. The received frequency is higher (compared to the emitted frequency) during the approach, it is identical at the instant of passing by, and it is lower during the recession. This variation of frequency also depends on the direction the wave source is moving with respect to the observer; it is maximum when the source is moving directly toward or away from the observer and diminishes with increasing angle between the direction of motion and the direction of the waves, until when the source is moving at right angles to the observer, there is no shift.
   Section::::Applications.:DVL.
  
  For underwater vehicles, the bottom tracking feature can be used as an important component in the navigation systems. In this case the velocity of the vehicle is combined with an initial position fix, compass or gyro heading, and data from the acceleration sensor. The sensor suite is combined (typically by use of a Kalman filter) to estimate the position of the vehicle. This may help to navigate submarines, autonomous, and remotely operated underwater vehicles.
  
  Section::::Applications.:Wave measurements.
   The all clear signal is used three times per year to denote a moment of silence (of one or two minutes): once on Israel's Holocaust Remembrance Day and twice on the Day of Remembrance.
  
  Section::::Civil defense sirens around the world.:Asia.:Singapore.
   Section::::Civil defense sirens around the world.:Europe.:Germany.
   Section::::Advantages of the Doppler Shift Compensation Mechanism in Biosonar Systems.
   Where "l" is the latitude and "L" is the longitude.
  
  Thus signatures from spots at higher latitudes will be restricted to spectral line centers, which will also occurring when the rotation axis is not perpendicular to the line of sight. If the spot is located at high latitude it is possible that it will always be seen, in which case the distortion in the line profile will move back and forth and only the amount of distortion will change.
   Doppler tracking
  
  Doppler tracking. The Doppler effect allows the measurement of the distance between a transmitter from space and a receiver on the ground by observing how the frequency received from the transmitter changes as the it approaches the transmitter, is overhead, and moves away.
  
  When approaching, the frequency of the transmission appears to be higher and as the transmitter moves away, the frequency appears to be lower. When overhead, the transmitted frequency and the received frequency are the same.
   Research has shown that sirens mounted behind the engine grill or under the wheel arches produces less unwanted noise inside the passenger cabin and to the side and rear of the vehicle while maintaining noise levels to give adequate warnings. The inclusion of broadband sound to sirens has the ability to increase localisation of sirens, as in a directional siren, as a spread of frequencies makes use of the three ways the brain detects a direction of a sound: Interaural level difference, interaural time difference and head-related transfer function.
   The standard deviation of the Doppler spectrum is as follow:
  
  where P(f) is the Doppler power spectrum and formula_14 is the centroid value of the Doppler frequency shift.
  
  Changing of flow velocity impacts value of the standard deviation. When the difference of flow velocity becomes larger, the Doppler frequency spectrum becomes wider. As a result, the standard deviation becomes larger.
  
  Section::::Fourier domain phase-resolved doppler OCT method.
   Section::::Feedback Control of Call Frequency.
   Today, signals are determined by state and local authorities and can vary from one region to another. The most common tones produced by sirens in the United States are "alert" (Steady) and "attack" (wail). Other tones include Westminster Chimes (commonly used for the testing of electronic sirens), Hi-Lo, Whoop, Pulse, Air Horn, and fast wail.
   BULLET::::- Pitch: The up/down rotation of a vessel about its transverse/Y (side-to-side or port-starboard) axis. An offset or deviation from normal on this axis is referred to as "trim" or "out of trim".
   BULLET::::- No all-clear signal is defined by either document.
   BULLET::::- The geometry of the towing line must have a well-defined value. Ideally, one would expect it to be exactly horizontal and straight. This is impossible in reality, because
  
  BULLET::::- the line falls into a catenary due to its weight;
  
  BULLET::::- the two fixed points of the line, being the bollard on shore and the ship's towing hook or cleat, may not have the same height above water.
  