Everywhere in daily life, there are frequencies of sound and electromagnetic waves, constantly changing and creating the features of the visible and audible world familiar to everyone. Some aspects of frequency can only be perceived indirectly, yet people are conscious of them without even thinking about it: a favorite radio station, for instance, may have a frequency of 99.7 MHz, and fans of that station knows that every time they turn the FM dial to that position, the station's signal will be there. Of course, people cannot "hear" radio and television frequencies—part of the electromagnetic spectrum—but the evidence for them is everywhere. Similarly, people are not conscious, in any direct sense, of frequencies in sound and light—yet without differences in frequency, there could be no speech or music, nor would there be any variations of color.
HOW IT WORKS
Harmonic Motion and Energy
In order to understand frequency, it is first necessary to comprehend two related varieties of movement: oscillation and wave motion. Both are examples of a broader category, periodic motion: movement that is repeated at regular intervals called periods. Oscillation and wave motion are also examples of harmonic motion, or the repeated movement of a particle about a position of equilibrium, or balance.
KINETIC AND POTENTIAL ENERGY.
In harmonic motion, and in some types of periodic motion, there is a continual conversion of energy from one form to another. On the one hand is potential energy, or the energy of an object due to its position and, hence, its potential for movement. On the other hand, there is kinetic energy, the energy of movement itself.
Potential-kinetic conversions take place constantly in daily life: any time an object is at a distance from a position of stable equilibrium, and some force (for instance, gravity) is capable of moving it to that position, it possesses potential energy. Once it begins to move toward that equilibrium position, it loses potential energy and gains kinetic energy. Likewise, a wave at its crest has potential energy, and gains kinetic energy as it moves toward its trough. Similarly, an oscillating object that is as far as possible from the stable-equilibrium position has enormous potential energy, which dissipates as it begins to move toward stable equilibrium.
Though many examples of periodic and harmonic motion can be found in daily life, the terms themselves are certainly not part of everyday experience. On the other hand, everyone knows what "vibration" means: to move back and forth in place. Oscillation, discussed in more detail below, is simply a more scientific term for vibration; and while waves are not themselves merely vibrations, they involve—and may produce—vibrations. This, in fact, is how the human ear hears: by interpreting vibrations resulting from sound waves.
Indeed, the entire world is in a state of vibration, though people seldom perceive this movement—except, perhaps, in dramatic situations such as earthquakes, when the vibrations of plates beneath Earth's surface become too forceful to ignore. All matter vibrates at the molecular level, and every object possesses what is called a natural frequency, which depends on its size, shape, and composition. This explains how a singer can shatter a glass by hitting a certain note, which does not happen because the singer's voice has reached a particularly high pitch; rather, it is a matter of attaining the natural frequency of the glass. As a result, all the energy in the sound of the singer's voice is transferred to the glass, and it shatters.
Oscillation is a type of harmonic motion, typically periodic, in one or more dimensions. There are two basic types of oscillation: that of a swing or pendulum and that of a spring. In each case, an object is disturbed from a position of stable equilibrium, and, as a result, it continues to move back and forth around that stable equilibrium position. If a spring is pulled from stable equilibrium, it will generally oscillate along a straight path; a swing, on the other hand, will oscillate along an arc.
In oscillation, whether the oscillator be spring-like or swing-like, there is always a cycle in which the oscillating particle moves from a certain point in a certain direction, then reverses direction and returns to the original point. Usually a cycle is viewed as the movement from a position of stable equilibrium to one of maximum displacement, or the furthest possible point from stable equilibrium. Because stable equilibrium is directly in the middle of a cycle, there are two points of maximum displacement: on a swing, this occurs when the object is at its highest point on either side of the stable equilibrium position, and on a spring, maximum displacement occurs when the spring is either stretched or compressed as far as it will go.
Wave motion is a type of harmonic motion that carries energy from one place to another without actually moving any matter. While oscillation involves the movement of "an object," whether it be a pendulum, a stretched rubber band, or some other type of matter, a wave may or may not involve matter. Example of a wave made out of matter—that is, a mechanical wave—is a wave on the ocean, or a sound wave, in which energy vibrates through a medium such as air. Even in the case of the mechanical wave, however, the matter does not experience any net displacement from its original position. (Water molecules do rotate as a result of wave motion, but they end up where they began.)
There are waves that do not follow regular, repeated patterns; however, within the context of frequency, our principal concern is with periodic waves, or waves that follow one another in regular succession. Examples of periodic waves include ocean waves, sound waves, and electromagnetic waves.
Periodic waves may be further divided into transverse and longitudinal waves. A transverse wave is the shape that most people imagine when they think of waves: a regular up-and-down pattern (called "sinusoidal" in mathematical terms) in which the vibration or motion is perpendicular to the direction the wave is moving.
A longitudinal wave is one in which the movement of vibration is in the same direction as the wave itself. Though these are a little harder to picture, longitudinal waves can be visualized as a series of concentric circles emanating from a single point. Sound waves are longitudinal: thus when someone speaks, waves of sound vibrations radiate out in all directions.
There are certain properties of waves, such as wavelength, or the distance between waves, that are not properties of oscillation. However, both types of motion can be described in terms of amplitude, period, and frequency. The first of these is not related to frequency in any mathematical sense; nonetheless, where sound waves are concerned, both amplitude and frequency play a significant role in what people hear.
Though waves and oscillators share the properties of amplitude, period, and frequency, the definitions of these differ slightly depending on whether one is discussing wave motion or oscillation. Amplitude, generally speaking, is the value of maximum displacement from an average value or position—or, in simpler terms, amplitude is "size." For an object experiencing oscillation, it is the value of the object's maximum displacement from a position of stable equilibrium during a single period. It is thus the "size" of the oscillation.
In the case of wave motion, amplitude is also the "size" of a wave, but the precise definition varies, depending on whether the wave in question is transverse or longitudinal. In the first instance, amplitude is the distance from either the crest or the trough to the average position between them. For a sound wave, which is longitudinal, amplitude is the maximum value of the pressure change between waves.
Period and Frequency
Unlike amplitude, period is directly related to frequency. For a transverse wave, a period is the amount of time required to complete one full cycle of the wave, from trough to crest and back to trough. In a longitudinal wave, a period is the interval between waves. With an oscillator, a period is the amount of time it takes to complete one cycle. The value of a period is usually expressed in seconds.
Frequency in oscillation is the number of cycles per second, and in wave motion, it is the number of waves that pass through a given point per second. These cycles per second are called Hertz (Hz) in honor of nineteenth-century German physicist Heinrich Rudolf Hertz (1857-1894), who greatly advanced understanding of electromagnetic wave behavior during his short career.
If something has a frequency of 100 Hz, this means that 100 waves are passing through a given point during the interval of one second, or that an oscillator is completing 100 cycles in a second. Higher frequencies are expressed in terms of kilohertz (kHz; 103 or 1,000 cycles per second); megahertz (MHz; 106 or 1 million cycles per second); and gigahertz (GHz; 109 or 1 billion cycles per second.).
A clear mathematical relationship exists between period, symbolized by T, and frequency (f ): each is the inverse of the other. Hence, and
If an object in harmonic motion has a frequency of 50 Hz, its period is 1/50 of a second (0.02 sec). Or, if it has a period of 1/20,000 of a second (0.00005 sec), that means it has a frequency of 20,000 Hz.
Grandfather Clocks and Metronomes
One of the best-known varieties of pendulum (plural, pendula) is a grandfather clock. Its invention was an indirect result of experiments with pendula by Galileo Galilei (1564-1642), work that influenced Dutch physicist and astronomer Christiaan Huygens (1629-1695) in the creation of the mechanical pendulum clock—or grandfather clock, as it is commonly known.
The frequency of a pendulum, a swing-like oscillator, is the number of "swings" per minute. Its frequency is proportional to the square root of the downward acceleration due to gravity (32 ft or 9.8 m/sec2) divided by the length of the pendulum. This means that by adjusting the length of the pendulum on the clock, one can change its frequency: if the pendulum length is shortened, the clock will run faster, and if it is lengthened, the clock will run more slowly.
Another variety of pendulum, this one dating to the early nineteenth century, is a metronome, an instrument that registers the tempo or speed of music. Consisting of a pendulum attached to a sliding weight, with a fixed weight attached to the bottom end of the pendulum, a metronome includes a number scale indicating the frequency—that is, the number of oscillations per minute. By moving the upper weight, one can speed up or slow down the beat.
As noted earlier, the volume of any sound is related to the amplitude of the sound waves. Frequency, on the other hand, determines the pitch or tone. Though there is no direct correlation between intensity and frequency, in order for a person to hear a very low-frequency sound, it must be above a certain decibel level.
The range of audibility for the human ear is from 20 Hz to 20,000 Hz. The optimal range for hearing, however, is between 3,000 and 4,000 Hz. This places the piano, whose 88 keys range from 27 Hz to 4,186 Hz, well within the range of human audibility. Many animals have a much wider range: bats, whales, and dolphins can hear sounds at a frequency up to 150,000 Hz. But humans have something that few animals can appreciate: music, a realm in which frequency changes are essential.
Each note has its own frequency: middle C, for instance, is 264 Hz. But in order to produce what people understand as music—that is, pleasing combinations of notes—it is necessary to employ principles of harmonics, which express the relationships between notes. These mathematical relations between musical notes are among the most intriguing aspects of the connection between art and science.
It is no wonder, perhaps, that the great Greek mathematician Pythagoras (c. 580-500 b.c.) believed that there was something spiritual or mystical in the connection between mathematics and music. Pythagoras had no concept of frequency, of course, but he did recognize that there were certain numerical relationships between the lengths of strings, and that the production of harmonious music depended on these ratios.
RATIOS OF FREQUENCY AND PLEASING TONES.
Middle C—located,, appropriately enough, in the middle of a piano keyboard—is the starting point of a basic musical scale. It is called the fundamental frequency, or the first harmonic. The second harmonic, one octave above middle C, has a frequency of 528 Hz, exactly twice that of the first harmonic; and the third harmonic (two octaves above middle C) has a frequency of 792 cycles, or three times that of middle C. So it goes, up the scale.
As it turns out, the groups of notes that people consider harmonious just happen to involve specific whole-number ratios. In one of those curious interrelations of music and math that would have delighted Pythagoras, the smaller the numbers involved in the ratios, the more pleasing the tone to the human psyche.
An example of a pleasing interval within an octave is a fifth, so named because it spans five notes that are a whole step apart. The C Major scale is easiest to comprehend in this regard, because it does not require reference to the "black keys," which are a half-step above or below the "white keys." Thus, the major fifth in the C-Major scale is C, D, E, F, G. It so happens that the ratio in frequency between middle C and G (396 Hz) is 2:3.
Less melodious, but still certainly tolerable, is an interval known as a third. Three steps up from middle C is E, with a frequency of 330 Hz, yielding a ratio involving higher numbers than that of a fifth—4:5. Again, the higher the numbers involved in the ratio, the less appealing the sound is to the human ear: the combination E-F, with a ratio of 15:16, sounds positively grating.
Everyone who has vision is aware of sunlight, but, in fact, the portion of the electromagnetic spectrum that people perceive is only a small part of it. The frequency range of visible light is from 4.3 · 1014 Hz to 7.5 · 1014 Hz—in other words, from 430 to 750 trillion Hertz. Two things should be obvious about these numbers: that both the range and the frequencies are extremely high. Yet, the values for visible light are small compared to the higher reaches of the spectrum, and the range is also comparatively small.
Each of the colors has a frequency, and the value grows higher from red to orange, and so on through yellow, green, blue, indigo, and violet. Beyond violet is ultraviolet light, which human eyes cannot see. At an even higher frequency are x rays, which occupy a broad band extending almost to 1020 Hz—in other words, 1 followed by 20 zeroes. Higher still is the very broad range of gamma rays, reaching to frequencies as high as 1025. The latter value is equal to 10 trillion trillion.
Obviously, these ultra-ultra high-frequency waves must be very small, and they are: the higher gamma rays have a wavelength of around 10−15 meters (0.000000000000001 m). For frequencies lower than those of visible light, the wavelengths get larger, but for a wide range of the electromagnetic spectrum, the wavelengths are still much too small to be seen, even if they were visible. Such is the case with infrared light, or the relatively lower-frequency millimeter waves.
Only at the low end of the spectrum, with frequencies below about 1010 Hz—still an incredibly large number—do wavelengths become the size of everyday objects. The center of the microwave range within the spectrum, for instance, has a wavelength of about 3.28 ft (1 m). At this end of the spectrum—which includes television and radar (both examples of microwaves), short-wave radio, and long-wave radio—there are numerous segments devoted to various types of communication.
RADIO AND MICROWAVE FREQUENCIES.
The divisions of these sections of the electromagnetic spectrum are arbitrary and manmade, but in the United States—where they are administered by the Federal Communications Commission (FCC)—they have the force of law. When AM (amplitude modulation) radio first came into widespread use in the early 1920s—Congress assigned AM stations the frequency range that they now occupy: 535 kHz to 1.7 MHz.
A few decades after the establishment of the FCC in 1927, new forms of electronic communication came into being, and these too were assigned frequencies—sometimes in ways that were apparently haphazard. Today, television stations 2-6 are in the 54-88 MHz range, while stations 7-13 occupy the region from 174-220 MHz. In between is the 88 to 108 MHz band, assigned to FM radio. Likewise, short-wave radio (5.9 to 26.1 MHz) and citizens' band or CB radio (26.96 to 27.41 MHz) occupy positions between AM and FM.
In fact, there are a huge variety of frequency ranges accorded to all manner of other communication technologies. Garage-door openers and alarm systems have their place at around 40 MHz. Much, much higher than these—higher, in fact, than TV broadcasts—is the band allotted to deep-space radio communications: 2,290 to 2,300 MHz. Cell phones have their own realm, of course, as do cordless phones; but so too do radio controlled cars (75 MHz) and even baby monitors (49 MHz).
WHERE TO LEARN MORE
Beiser, Arthur. Physics, 5th ed. Reading, MA: Addison-Wesley, 1991.
Allocation of Radio Spectrum in the United States (Web site). <http://members.aol.com/jneuhaus/fccindex/spectrum.html> (April 25, 2001).
DiSpezio, Michael and Catherine Leary. Awesome Experiments in Light and Sound. New York: Sterling Juvenile, 2001.
Electromagnetic Spectrum (Web site). <http://www.jsc.mil/images/speccht.jpg> (April 25, 2001).
"How the Radio Spectrum Works." How Stuff Works (Web site). <http://www.howstuffworks.com/radio~spectrum.html> (April 25, 2001).
Internet Resources for Sound and Light (Web site). <http://electro.sau.edu/SLResources.html> (April 25, 2001).
"NIST Time and Frequency Division." NIST: National Institute of Standards and Technology (Web site). <http://www.boulder.nist.gov/timefreq/> (April 25, 2001).
Parker, Steve. Light and Sound. Austin, TX: Raintree Steck-Vaughn, 2000.
Physics Tutorial System: Sound Waves Modules (Web site). <http://csgrad.cs.vt.edu/~chin/chin_sound.html> (April 25, 2001).
For an object oscillation, amplitude is the value of the object's maximum displacement from a position of stable equilibrium during a single period. In a transverse wave, amplitude is the distance from either the crest or the trough to the average position between them. For a sound wave, the best-known example of a longitudinal wave, amplitude is the maximum value of the pressure change betweenwaves.
In oscillation, a cycle occurs when the oscillating particle moves from a certain point in a certain direction, then switches direction and moves back to the original point. Typically, this is from the position of stable equilibrium to maximum displacement and back again to the stable equilibrium position.
For a particle experiencing oscillation, frequency is the number of cycles that take place during one second. In wave motion, frequency is the number of waves passing through a given point during the interval of one second. In eithercase, frequency is measured in Hertz. Period (T ) is the mathematical inverse offrequency (f ) hence f =1/T.
The repeated movement of a particle about a position of equilibrium, or balance.
A unit for measuring frequency, named after nineteenth-century German physicist Heinrich Rudolf Hertz (1857-1894). Higher frequencies are expressed in terms of kilohertz (kHz; 103 or 1,000 cycles per second); megahertz (MHz; 106 or 1 million cycles per second);and gigahertz (GHz; 109 or 1 billion cycles per second.)
The energy that an object possesses due to its motion, as with a sled when sliding down a hill. This is contrasted with potential energy.
A wave in which the movement of vibration is in the same direction as the wave itself. This is contrasted to a transverse wave.
For an object in oscillation, maximum displacement is the farthest point from stable equilibrium.
A type of harmonic motion, typically periodic, in one or more dimensions.
In oscillation, a period is the amount of time required for one cycle. For a transverse wave, a period is the amount of time required to complete one full cycle of the wave, from trough to crest and back to trough. In a longitudinal wave, a period is the interval between waves. Frequency is the mathematical inverse of period (T ):hence, T =1/f.
Motion that is repeated at regular intervals. These intervals are known as periods.
The energy that an object possesses due to its position, as, for instance, with a sled at the top of a hill. This is contrasted with kinetic energy.
A position in which, if an object were disturbed, it would tend to return to its original position. For an object in oscillation, stable equilibrium is in the middle of a cycle, between two points of maximum displacement.
A wave in which the vibration or motion is perpendicular to the direction in which the wave is moving. This is contrasted to a longitudinal wave.
A type of harmonic motion that carries energy from one place to another without actually moving anymatter.
Any process that is repetitive or periodic has an associated frequency. The frequency is the number of repetitions, or cycles, during a given time interval. For example, a child who skips rope so the rope makes one complete circuit every two seconds has a frequency of 30 cycles per minute. The inverse of the frequency is called the period of the process. In this example, the frequency is one minute.
In physics, frequency (f) is often represented as f = 1/T, where T is the period of time for a particular event and the frequency f is the reciprocal of time.
In the international system (SI) of units, frequency is measured in hertz (Hz), which was named after German physicist Heinrich Rudolph Hertz (1857-1894). For example, 1 Hz indicates that something is repeated once every second, while 5 Hz shows that another body has a frequency that is five times every second, or five times faster. Other common ways to measure frequency is cycles per second, revolutions per minute, radians per second, degrees per second, and beats per minute.
Pendulums, as in a grandfather clock, also have a frequency of a certain number of swings per minute. A complete oscillation for a pendulum requires the pendulum bob to start and finish at the same location. Counting the number of these oscillations during one minute will determine the frequency of the pendulum (in units of oscillations/minute). This frequency is proportional to the square root of the acceleration due to gravity divided by the pendulum’s length. If either of these is changed, the frequency of the pendulum will change accordingly. This is why one adjusts the length of the pendulum on a grandfather clock to change the frequency, which changes the period, which allows the clock to run faster or slower.
Vibrating strings also have an associated frequency. Pianos, guitars, violins, harps, and any other stringed instrument requires a particular range of vibrational frequencies to generate musical notes. By changing the frequency, generally by changing the length of the string, the musician changes the pitch of the note heard.
In any type of wave, the frequency of the wave is the number of wave crests (or troughs) passing a fixed measuring position in a given time interval; and, is also equal to the wave’s speed divided by the wavelength. As a wave passes by a fixed measurement point, a specific number of wave crests (or troughs) pass a fixed point in a given amount of time. In the case of waves, the frequency is also equal to the speed of the wave divided by the wavelength of the wave.
Light also exhibits the characteristics of waves; so, it too has a frequency. By changing this frequency, one also changes the associated color of the light wave. A lower amplitude wave has a larger frequency than a wave with larger amplitude. The opposite is also true; a higher amplitude wave has a small frequency that a small amplitude wave. When described like this, frequency is described with respect to wavelength (?) and speed (the magnitude of velocity (v)), or f = v/?. If the speed of light (c) is involved, v is replaced with c (which is a constant equal to about 186,000 miles per second [299,300 kilometers per second], that is, when in the vacuum of space or a vacuum generated artificially on Earth).
Any process that repeats on a regular basis has an associated frequency. The frequency is the number of repetitions, or cycles, that occur during a given time interval. The inverse of the frequency is called the period of the process.
Suppose you stand on a beach and watch the waves come in. You will notice that the waves arrive in a regular pattern, perhaps one every second. The frequency of that wave motion, then, is one wave per second. The period for the wave motion is the inverse of the frequency, or one second per wave.
All forms of wave motion have some frequency associated with them. That frequency is defined as the number of wave crests (or troughs) that pass a given point per second. Light waves, for example, have a frequency of about 4 × 1014 to 7 × 1014 cycles per second. By comparison, the frequency of X rays is about 1018 cycles per second and that of radio waves is about 100 to 1,000 cycles per second.
What makes a note from a musical instrument sound rich and pleasing to the ear? The answer is harmonics.
If you pluck a guitar string, the string vibrates in a very complex way. If you could actually watch that vibration in slow motion, you would see the whole string vibrating at once with a frequency known as the fundamental frequency. At the same time, however, the string would be vibrating in halves (the first overtone), in thirds (the second overtone), and in even smaller segments. The collection of overtones is known as the harmonics of the sound produced by the vibrating string.
The harmonics produced by a vibrating string depend on factors such as the place the string is plucked and how strongly it is pulled. The many different sounds produced from a single guitar string depend on the variety of harmonics that a player can produce from that string.
The frequency of some processes depends on other factors. For example, the frequency with which a string vibrates depends on factors such as the type of string used and its length. One way to change the frequency of a vibrating string is to change its length. The frequency of a vibrating string determines the pitch of the sound it produces. Thus, when a violinist plays on her instrument, she places her finger on different parts of the string in order to produce sounds of different pitches, or different notes.
Technical definition of the range of sounds audible to humans.
Humans can detect sound waves with frequencies that vary from approximately 20 to 20,000 Hz. Probably of greatest interest to psychologists are the frequencies around 500-2,000 Hz, the range in which sounds important to speech typically occur. Humans are most responsive to sounds between 1,000 and 5,000 Hz, and are not likely to hear very low or very high frequencies unless they are fairly intense. For example, the average person is approximately 100 times more sensitive to a sound at 3,000 Hz than to one at 100 Hz. People can best differentiate between two similar pitches when they are between 1,000 and 5,000 Hz.
The relationship between frequency and pitch is predictable but not always simple. That is, as frequency increases, pitch becomes higher. At the same time, if the frequency is doubled, the resulting sound does not have a pitch twice as high. In fact, if one listens to a sound at a given frequency, then a second sound at twice the frequency, the pitch would have increased by one octave in pitch. Each doubling of frequencies involves a one-octave change, for example, the Middle C note on a piano has a frequency of 261.2; the C note one octave higher is 522.4, a change of 261.2 Hz. The next C note on the piano has a frequency of 1046.4 Hz, or a change of 523.2 Hz.
When an individual hears a complex sound consisting of many different wavelengths, such as a human voice, music, and most sounds in nature, the ear separates the sound into its different frequencies. This separation begins in the inner ear, specifically the basilar membrane within the cochlea. The basilar membrane is a strip of tissue that is wide at one end and narrow at the other. When the ear responds to a low frequency sound, the entire length of the basilar membrane vibrates; for a high frequency sound, the movement of the membrane is more restricted to locations nearer the narrow end. Thus, a person can hear the different frequencies (and their associated pitches) as separate sounds.
The ability to hear declines with age, although the loss is greatest for high frequency sounds. At age 70, for example, sensitivity to sounds at 1,000 Hz is maintained, whereas sensitivity to sounds at 8,000 Hz is markedly diminished. As many as 75 percent of people over 70 years of age have experienced some deterioration in their hearing .
Any process that is repetitive or periodic has an associated frequency. The frequency is the number of repetitions, or cycles, during a given time interval . The inverse of the frequency is called the period of the process.
Pendulums, as in a grandfather clock, also have a frequency of a certain number of swings per minute. A complete oscillation for a pendulum requires the pendulum bob to start and finish at the same location. Counting the number of these oscillations during one minute will determine the frequency of the pendulum (in units of oscillations/minute). This frequency is proportional to the square root of the acceleration due to gravity divided by the pendulum's length. If either of these are changed, the frequency of the pendulum will change accordingly. This is why you adjust the length of the pendulum on your grandfather clock to change the frequency, which changes the period, which allows the clock to run faster or slower.
Vibrating strings also have an associated frequency. Pianos, guitars, violins, harps, and any other stringed instrument requires a particular range of vibrational frequencies to generate musical notes. By changing the frequency, generally by changing the length of the string, you change the pitch of the note you hear.
In any type of wave, the frequency of the wave is the number of wave crests (or troughs) passing a fixed measuring position in a given time interval; and, is also equal to the wave's speed divided by the wavelength. As a wave passes by a fixed measurement point, a specific number of wave crests (or troughs) pass a fixed point in a given amount of time. In the case of waves, the frequency is also equal to the speed of the wave divided by the wavelength of the wave.
Light also exhibits the characteristics of waves; so, it too has a frequency. By changing this frequency, you also change the associated color of the light wave.
fre·quen·cy / ˈfrēkwənsē/ • n. (pl. -cies) 1. the rate at which something occurs or is repeated over a particular period of time or in a given sample: shops have closed with increasing frequency during the period. ∎ the fact of being frequent or happening often. ∎ Statistics the ratio of the number of actual to possible occurrences of an event. ∎ Statistics the (relative) number of times something occurs in a given sample. 2. the rate at which a vibration occurs that constitutes a wave, either in a material (as in sound waves), or in an electromagnetic field (as in radio waves and light), usually measured per second. (Symbol: f or ν) ∎ the particular waveband at which a radio station or other system broadcasts or transmits signals.
Frequency ★★½ 2000 (PG-13)
In 1999, New York cop John (Caviezel) finds he can communicate with his dead father (Quaid) in 1969 through dad's old ham radio. Since his father was a fireman who died in a warehouse fire almost exactly 30 years ago, John decides to tell him how not to get killed. This leads to changes in the present (messing with the past always does), including the murder of John's mother by a serial killer. Father and son must solve and prevent the killings in the past and the present using their respective skills and, amazingly, John's knowledge of the 1969 World Series. It's all very convoluted and ridiculous (especially the ending), but the underlying sentiment, and the fine work of Caviezel and Quaid make it easy to suspend disbelief and go along for the ride. 117m/C VHS, DVD . Dennis Quaid, James (Jim) Caviezel, Elizabeth Mitchell, Andre Braugher, Shawn Doyle, Noah Emmerich, Jordan Bridges, Melissa Errico, Daniel Henson; D: Gregory Hoblit; W: Toby Emmerich; C: Alar Kivilo; M: Michael Kamen.