Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as temporal frequency for clarity, and is distinct from angular frequency. Frequency is measured in hertz (Hz) which is equal to one event per second. The period is the interval of time between events, so the period is the reciprocal of the frequency.
For cyclical phenomena such as oscillations, waves, or for examples of simple harmonic motion, the term frequency is defined as the number of cycles or vibrations per unit of time. The conventional symbol for frequency is f; the Greek letter ν (nu) is also used. The period T is the time taken to complete one cycle of an oscillation or rotation. The relation between the frequency and the period is given by the equation f = 1 T . \displaystyle f=\frac 1T.
The SI unit of frequency is the hertz (Hz), named after the German physicist Heinrich Hertz by the International Electrotechnical Commission in 1930. It was adopted by the CGPM (Conférence générale des poids et mesures) in 1960, officially replacing the previous name, cycle per second (cps). The SI unit for the period, as for all measurements of time, is the second. A traditional unit of frequency used with rotating mechanical devices, where it is termed rotational frequency, is revolution per minute, abbreviated r/min or rpm. 60 rpm is equivalent to one hertz.
As a matter of convenience, longer and slower waves, such as ocean surface waves, are more typically described by wave period rather than frequency. Short and fast waves, like audio and radio, are usually described by their frequency. Some commonly used conversions are listed below:
For periodic waves in nondispersive media (that is, media in which the wave speed is independent of frequency), frequency has an inverse relationship to the wavelength, λ (lambda). Even in dispersive media, the frequency f of a sinusoidal wave is equal to the phase velocity v of the wave divided by the wavelength λ of the wave: f = v λ . \displaystyle f=\frac v\lambda .
An old method of measuring the frequency of rotating or vibrating objects is to use a stroboscope. This is an intense repetitively flashing light (strobe light) whose frequency can be adjusted with a calibrated timing circuit. The strobe light is pointed at the rotating object and the frequency adjusted up and down. When the frequency of the strobe equals the frequency of the rotating or vibrating object, the object completes one cycle of oscillation and returns to its original position between the flashes of light, so when illuminated by the strobe the object appears stationary. Then the frequency can be read from the calibrated readout on the stroboscope. A downside of this method is that an object rotating at an integer multiple of the strobing frequency will also appear stationary.
Higher frequencies are usually measured with a frequency counter. This is an electronic instrument which measures the frequency of an applied repetitive electronic signal and displays the result in hertz on a digital display. It uses digital logic to count the number of cycles during a time interval established by a precision quartz time base. Cyclic processes that are not electrical, such as the rotation rate of a shaft, mechanical vibrations, or sound waves, can be converted to a repetitive electronic signal by transducers and the signal applied to a frequency counter. As of 2018, frequency counters can cover the range up to about 100 GHz. This represents the limit of direct counting methods; frequencies above this must be measured by indirect methods.
Above the range of frequency counters, frequencies of electromagnetic signals are often measured indirectly utilizing heterodyning (frequency conversion). A reference signal of a known frequency near the unknown frequency is mixed with the unknown frequency in a nonlinear mixing device such as a diode. This creates a heterodyne or "beat" signal at the difference between the two frequencies. If the two signals are close together in frequency the heterodyne is low enough to be measured by a frequency counter. This process only measures the difference between the unknown frequency and the reference frequency. To reach higher frequencies, several stages of heterodyning can be used. Current research is extending this method to infrared and light frequencies (optical heterodyne detection).
All of these waves, from the lowest-frequency radio waves to the highest-frequency gamma rays, are fundamentally the same, and they are all called electromagnetic radiation. They all travel through vacuum at the same speed (the speed of light), giving them wavelengths inversely proportional to their frequencies. c = f λ , \displaystyle \displaystyle c=f\lambda , where c is the speed of light (c in vacuum or less in other media), f is the frequency and λ is the wavelength.
Sound propagates as mechanical vibration waves of pressure and displacement, in air or other substances. In general, frequency components of a sound determine its "color", its timbre. When speaking about the frequency (in singular) of a sound, it means the property that most determines its pitch.
The frequencies an ear can hear are limited to a specific range of frequencies. The audible frequency range for humans is typically given as being between about 20 Hz and 20,000 Hz (20 kHz), though the high frequency limit usually reduces with age. Other species have different hearing ranges. For example, some dog breeds can perceive vibrations up to 60,000 Hz.
In many media, such as air, the speed of sound is approximately independent of frequency, so the wavelength of the sound waves (distance between repetitions) is approximately inversely proportional to frequency.
FDA has developed this guidance document to assist industry and FDA staff in identifying and appropriately addressing specific considerations related to the incorporation and integration of radio frequency (RF) wireless technology in medical devices. There has been rapid growth in medical devices that incorporate RF wireless technology due to the expansion of this technology. With the increasing use of RF wireless medical devices, continuing innovation and advancements in wireless technology, and an increasingly crowded RF environment, RF wireless technology considerations should be taken into account to help provide for the safe and effective use of these medical devices. This guidance highlights and discusses RF wireless technology considerations that can have an effect on the safe and effective use of medical devices. These considerations include the selection of wireless technology, quality of service, coexistence, security, and electromagnetic compatibility (EMC). Consideration of these areas can help provide reasonable assurance of safety and effectiveness for medical devices that incorporate RF wireless technology, and are supplementary to other device-specific guidances or guidelines.
Cell phones are currently used by 95% of American adults. The U.S. Food and Drug Administration (FDA) nominated radio frequency radiation (RFR) used by cell phones for an NTP study because of widespread public use of cell phones and limited knowledge about potential health effects from long-term exposure.
Q: Can NTP provide more information on how these studies were conducted? A: The studies were conducted in three phases. First, because radiofrequency radiation generates heat when absorbed by the body, NTP did pilot studies to determine exposure levels that did not exceed the ability of the animals to maintain normal body temperatures. Next, the scientists did short duration studies to determine exposure levels that did not affect the normal growth and development of rats and mice. And finally, they performed studies in which pregnant rats and their offspring, and young adult mice, were exposed to radiofrequency radiation for the better part of their natural lifetime, or approximately two years.
Although the general rule is that all consignments of material listed in Annex V Part B must be inspected on arrival in the EU, plant health checks can be carried out at a reduced frequency where this can be justified - Commission Implementing Regulation (EU) 2022/2389.
Any interceptions associated with a particular trade are assessed and allocated a value depending on the estimated mobility of the harmful organism concerned. The total of the allocated values for the particular trade becomes its risk index which is compared against a table of values differentiated according to the number of inspected consignments. The resulting figure is the recommended inspection frequency for the consignment. The final inspection level may be adjusted to ensure a minimum of 200 consignments per year are inspected.
A food frequency questionnaire (FFQ) consists of a finite list of foods and beverages with response categories to indicate usual frequency of consumption over the time period queried. To assess the total diet, the number of foods and beverages queried typically ranges from 80 to 120.
Usual portion size can be asked separately for each food and beverage. Alternatively, portion size can be combined with frequency information by asking respondents to translate usual consumption amount to number of specified units (e.g., How often do you eat a cup of rice?). Some questionnaires include portion size images in an attempt to enhance reporting [glossary term:] accuracy.
An FFQ is sometimes loosely referred to as a [glossary term:] diet history. However, the term diet history can refer generally to any instrument that asks about diet in the past (Learn More about Diet History). It also is used to refer specifically to a meal-based FFQ or the Burke diet history, which includes a food frequency cross-check component, and its various adaptations. 041b061a72