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Topic: Very low frequency

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Ultra low frequency

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	Aurora Very Low frequency NMR spectrometer
		The Aurora, a low field NMR research spectrometer, is designed to work in the 0 kHz to 100 kHz frequency range.
		The Aurora system is especially useful in applications such as hyper-polarised gas NMR, where the low magnetic field strengths result in NMR frequencies outside the operating range of standard NMR spectrometers.
		The Aurora is very affordable which means it can be deployed for applications such as monitoring the production of hyper-polarised gas by NMR on a routine basis.
	www.magritek.com /aurora.html (256 words)

	Very Low-Frequency (VLF) Method
		Therefore, the high frequency of the VLF transmitters means that in more conductive environments, the exploration depth is quite shallow; for example, the depth of exploration might be 10 to 12 m in 25-Ωm material.
		VLF response is a maximum when the target strikes in the direction of the transmitter, falling off roughly as the cosine of the strike angle for other directions.
		Finally, the major disadvantage of the VLF method is that the high frequency results in a multitude of anomalies from unwanted sources such as swamp edges, creeks, and topographic highs.
	www.freewebtown.com /geophysics2/Vlf.html (0 words)

	Very Low Frequency
		VLF techniques utilizes a plane-wave radio signal (15-30 kHz) emanating from one of several worldwide radio transmitters used for submarine communications.
		The perturbed field may be phase shifted and oriented in a different direction than the primary field depending on the shape or geometry of the conductor, the orientation of the conductor, and the conductivity contrast with the surrounding material (e.g., the host rock).
		All VLF instruments measure two components of the magnetic field or equivalently the “tilt angle” and ellipticity of the field.
	nga.com /Geo_ser_VLF.htm (365 words)

	NationMaster - Encyclopedia: Subtractive synthesis (Site not responding. Last check: )
		Typically, the complexity of the source signal and the cut-off frequency and resonance of the filter are controlled in order to simulate the natural timbre of a given instrument.
		In acoustics and telecommunication, the harmonic of a wave is a component frequency of the signal that is an integral multiple of the fundamental frequency.
		Frequency modulation synthesis (or FM synthesis) is a form of audio synthesis where the timbre of a simple waveform is changed by frequency modulating it with a modulating frequency that is also in the audio range, resulting in a more complex waveform and a different-sounding tone.
	www.nationmaster.com /encyclopedia/Subtractive-synthesis (1565 words)

	Very Low Frequency TMD
		These advantages came to the fore when CSA Engineering was asked to design a very low frequency tuned mass damper for a large solar array that would power a spacecraft in orbit.
		At these very low frequencies, stiffness forces in the TMD must be very small and even slight friction can "lock up" the device and prevent it from damping.
		Because the very low frequency causes elastic (spring) forces to be small compared to weight forces, a special means was required in the ground versions for offloading the weight of the moving mass.
	www.csaengineering.com /actsys/lowfrequencydampers.shtml (0 words)

	Low Frequency Machinery Monitoring
		Low frequency readings are generally expressed in terms of velocity (inches per second), or displacement (mils peak to peak).
		Low frequency accelerometers are susceptible to high frequency overload and may contain low pass filters to attenuate high frequency signals.
		The low frequency noise of an accelerometer is proportional to the gain (amplification) of the circuit and inversely proportional to the charge sensitivity of the piezoelectric sensing element
	www.wilcoxon.com /technotes/lowfreq.html (5451 words)

	Frequency Summary
		This frequency is proportional to the square root of the acceleration due to gravity divided by the pendulum's length.
		Frequency is the measurement of the number of times that a repeated event occurs per unit of time.
		In Europe, the frequency of the alternating current in mains is 50 Hz (close to the tone G), however, in North America, the frequency of the alternating current is 60 Hz (close to the tone B flat — that is, a minor third above the European frequency).
	www.bookrags.com /Frequency (887 words)

	Spartanburg SC \| GoUpstate.com \| Spartanburg Herald-Journal (Site not responding. Last check: )
		VLF waves can penetrate water only to a depth of roughly 10 to 40 metres (30 to 130 feet), depending on the frequency and the salinity of the water.
		Frequency is increased by 25 Hz (for example) from the carrier to indicate a binary '1' and reduced by 25 Hz to indicate binary '0'.
		Very long wave transmitters on frequencies under 10 kHz are the Russian ZEVS on the frequency 82 cycles per second and the American transmitter Sanguine on the frequency 76 cycles per second.
	www.goupstate.com /apps/pbcs.dll/section?category=NEWS&template=wiki&text=Very_low_frequency (933 words)

	Reference 3A Loudspeakers
		Very stiff and light cones of Fibreglass and Kevlar in honeycomb construction are used to eliminate driver break-up modes.
		With a low mass voice coil and a copper Faraday-Ring in the motor it is very well behaved even at very loud volume levels.
		Frequency response: 36Hz - 20 kHz (+/-) 3dB.
	www.reference3a.com /grandveena.htm (541 words)

	How Elephants Communicate
		African savanna elephants produce a broad range of sounds from very low frequency rumbles to higher frequency trumpets, snorts, screams, barks, roars, cries and other idiosyncratic calls(3.) The most frequently used category of calls is the very low frequency rumble.
		The measured limit of high frequency hearing of air born sound in mammals varies from 12 kHz (elephants) to 114 kHz (little brown bat), and the low frequency hearing limit varies from less than 0.016 kHz (elephants) to 10.3 kHz (little brown bat), a range of more than nine octaves(26).
		In the lower frequencies there tends to be a higher level of background noise, and so animals that specialize in low frequency hearing must have a way of distinguishing signal from noise.
	www.elephantvoices.org /how_comm/acoustic.html (2643 words)

	Unbelievable! - Eliminate Pain and Inflammation, Increase Range of Motion with MendMeShop Ultrasound!
		While a low frequency means deeper penetration, using two low a frequency will mean that the waves are too wide to properly move the molecules.
		Typically, these types of injuries are very slow to heal and because people continue to try to use the injured body part, the injury is repeatedly stressed.
		A low cost, safe, easy to use unit allows professionals to provide their patients with units to take home and use for the duration of the healing process.
	www.sonicmend.com (1569 words)

	On-Line Glossary of Solar-Terrestrial Terms
		An extended region of the CORONA, exceptionally low in density and associated with unipolar photospheric regions.
		The maximum ORDINARY MODE radiowave frequency capable of reflec- tion from the SPORADIC E REGION of the IONOSPHERE.
		The maximum ORDINARY MODE radiowave frequency capable of reflec- tion from the F2 REGION of the IONOSPHERE.
	sec.noaa.gov /info/glossary.html (0 words)

	BWEA - Low frequency noise and wind turbines
		The frequency, or 'pitch', of a sound is measured in cycles per second (or 'Hertz' (Hz)) although most noise in the environment contains energy at many different frequencies combining together to give it its overall character.
		Above and below this, the ear becomes decreasingly sensitive and is very in-sensitive at very low frequencies, meaning that sound levels have to be very high for such sounds to be perceived.
		Wind turbines are not, therefore, a significant source of low frequency or infrasonic noise but, as with noise from any other sound source, the high frequency components are reduced when heard from a distance and overall levels are very low.
	www.bwea.com /ref/lowfrequencynoise.html (729 words)

	Electricity Today, June 1997 -- "Very-Low-Frequency" Cable Testing:
		VLF 0.1Hz cosine square wave testing causes existing large electrical trees and mechanical defects in the cable to grow and to cause a breakdown during the three-phase one-hour test duration.
		More than 7000 VLF tests and subsequent observation of the cables for three to four years have yielded statistics which prove that this method is very reliable, and that the tested cable operates problem-free in later years.
		The authors found that, owing to the low partial discharge inception voltage and relatively high channel growth speed of electrical trees, 0.1 Hz VLF with cosine square wave voltage is the most advanced on-site test method.
	www.electricity-today.com /et/june97/wittig.htm (1114 words)

	Very Low Frequency (VLF)
		VLF is useful for detecting lateral changes in subsurface electrical properties, and is used primarily for locating water bearing fractures in bedrock.
		VLF is often used in conjunction with EM, electrical and/or seismic methods for sighting high yield water wells and locating preferential contaminant pathways in bedrock, but VLF can be used as a stand alone method.
		Hager-Richter was hired to perform seismic refraction, EM-34 and VLF surveys to determine the depth of bedrock and detect a fracture zone that may have been acting as a preferential pathway for contaminant transport.
	www.hager-richter.com /vlf.htm (414 words)

	BBC NEWS \| Science/Nature \| Science in search of the low rumble
		The study of low frequency sound is giving science new perspectives on the natural world as well as helping to develop novel weapons technologies.
		Sound waves so low in frequency that the human ear cannot detect them may be behind ghostly sightings and haunted buildings.
		Dr Richard Wiseman, of Hertfordshire University, UK, believes it is the vibrations of low frequency acoustic waves, felt particularly in the stomach, which may cause the recipient to feel uplifted or create unease depending on the environment in which they are experienced.
	news.bbc.co.uk /2/hi/science/nature/2309505.stm (661 words)

	HF and Lower Frequency Radiation - Introduction
		The actual frequency used within the HF band for skywaves depends on the properties of the ionosphere, which are highly variable.
		The Navy's VLF systems serve as a back-up for global communication use during hostilities when nuclear explosions may disrupt higher frequencies or satellites are destroyed by enemy actions.
		VLF is also used for aircraft and vessel navigation beacons and for transmitting standard frequencies and time signals.
	www.weather.nps.navy.mil /~psguest/EMEO_online/module3/module_3_1.html (1661 words)

	Programmed Test Sources, Inc. - Fast Switching Low Noise Frequency Synthesizers
		Frequency synthesizers are basically variable radio-frequency generators which are very accurately and quickly settable and possess high stability.
		This output frequency is as accurate and as stable as a built-in frequency standard, usually a crystal oscillator, or as accurate and stable as an external precision standard which may be connected to the synthesizer in lieu of its own standard.
		The theoretical maximum output frequency obtainable is one-half the clock frequency, although practical filtering considerations limit the output frequency to less than 45% of the clock.
	www.programmedtest.com /technology.html (706 words)

	Csound Magazine
		A high pass filter with a very low cut off frequency (~20 Hz) can be used to remove DC from a signal.
		Frequencies within a frequency band of the center frequency are allowed to pass.
		Frequencies within the specified bandwidth are blocked while higher and lower frequencies are allowed to pass.
	www.csounds.com /ezine/spring2000/beginners/index.html (777 words)

	Noise & Hearing Protection
		Frequency is measured in cycles per second, or Hertz (Hz).
		When hearing impairment begins, the high frequencies are usually lost first, which is why people with hearing loss often have difficulty hearing the high pitched voices of women and children.
		The better earplugs and muffs are approximately equal in sound reductions, although earplugs are better for low frequency noise and earmuffs for high frequency noise.
	www.entnet.org /healthinfo/hearing/noise_hearing.cfm (2100 words)

	1996 WhaleNet Archive: Case Study:US Navy Low Frequency Active Sonar (fwd) (Site not responding. Last check: )
		Another option that should be seriously considered is the use of a number of very intense mobile sound sources.
		A deaf whale or dolphin is unlikely to survive for long or to reproduce effectively.
		A frequency of a few hundred hertz might be optimal.
	whale.wheelock.edu /archives/whalenet96/0290.html (431 words)

	Very low frequency (VLF)
		VLF suffers from the same difficulties associated with ELF: small bandwidths are available; and antennas are large and inefficient, leading to low radiated powers.
		However, VLF has similar advantages to ELF and has low attenuation through the atmosphere and through seawater and is therefore used for some submarine communications and maritime radio navigation systems.
		Hughes, Csaba Ferencz and A. Gwal, "Very Low Frequency Vlf Phenomena".
	www.argospress.com /Resources/CommunicationsSystems/verlowfreque.htm (213 words)

	FAQ: Is Low Frequency Noise a Problem for Wind Turbines?
		1) Because of the low rotational rates of the turbine blades, the peak acoustic energy radiated by large wind turbines is in the infrasonic range with a peak in the 8-12 Hz range.
		Turbines that have their rotors upstream of the tower, except in very rare circumstances, do not generate impulses since there is nothing blocking the flow upwind of the rotor.
		The low-frequency noise generated from an upwind turbine is primarily the result of the interaction of the aerodynamic lift on the blades and the atmospheric turbulence in the wind.
	www.awea.org /faq/noise-lf.html (491 words)

	VLF - Very Low Frequency AC Hipots - High Voltage, Inc.
		High Voltage offers a full line of VLF hipots from 25kVac to 200kVac with models capable of testing up to 50uF of load, cables from 1500 feet to 90 miles in length and the largest of generators and motors.
		A VLF hipot is a portable testing solution for XLPE, PE and EPR cables enabling utilities and industry to economically field test cables and other high capacitance electrical apparatus.
		It is used in conjunction with a Very Low Frequency (VLF) AC Power Supply to measure the loss angle or Tan Delta of the insulation.
	www.hvinc.com /vlf4060prd.htm (391 words)

	Radio frequency spectrum
		Above 300 GHz, the absorption of electromagnetic radiation by Earth's atmosphere is so great that the atmosphere is effectively opaque to higher frequencies of electromagnetic radiation, until the atmosphere becomes transparent again in the so-called infrared and optical window frequency ranges.
		The ELF, SLF, ULF, and VLF bands overlap the AF (audio frequency) spectrum, which is approximately 20–20,000 Hz.
		Frequencies from 216 to 450 MHz were sometimes called P-band: Previous, since early British Radar used this band but later switched to higher frequencies.
	faculty.cs.tamu.edu /lively/GE_rf/radio_frequency_spectrum.htm (390 words)

	NGDC/STP - On-Line Glossary of Solar-Terrestrial Terms
		An extended region of the CORONA, exceptionally low in density and associated with unipolar photospheric regions.
		The appearance of a light bridge is frequently a sign of impending region division or dissolution.
		That portion of the radio frequency spectrum from 30 to 300 kHz.
	www.ngdc.noaa.gov /stp/GLOSSARY/glossary.html (2480 words)

	Yamaha Corporation of America - Multi-way Loudspeakers? (Site not responding. Last check: )
		In order to create a given sound level at a low frequency, the driver must move a much greater volume of air than is required to create the same sound level one or two octaves higher.
		Because low frequencies require a larger volume of air to be moved, they are typically reproduced by a large diameter cone.
		Since the wavelength of low frequency sounds is very large, the dispersion of sound remains sufficiently wide even though the diameter of the driver is large.
	www.yamaha.com /yamahavgn/CDA/ContentDetail/WrappedTextDetail/0,,CNTID%25253D48556%252526CTID%25253D227500,00.html (1684 words)

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