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See the chart below. If you tune your AM radio to 530 KHz at the bottom end of the AM dial, and then if you could somehow keep tuning lower....and....lower you would eventually reach the VLF band. If you could keep tuning down even lower you would reach the bottom of the electromagnetic spectrum where radio waves literally become millions of miles long, an example of which is the earth's magnetic field.
The charts shows radio band usage below 300 MHz. The FM broadcast band is between 88 to 108 MHz. The AM broadcast band is much lower and is found between 0.5 MHz to 1.7 MHz.
The frequencies below 0.5 MHz have many uses. There are marine and air navigation beacons, point-to-point communications, low frequency (hobby) experimenters (160 KHz - 190 KHz), LORAN Navigation (100 KHz). If you are curious as to where the 'Radio-Controlled Clocks and Watches' get their perfect time from, the answer is from the time-signal station WWVB on 60 KHz. WWVB covers all of North America and is located in Colorado. Other communication and navigation signals can be found below 60 KHz down to about 14 KHz. The OMEGA NAVIGATION SYSTEM used to operate between 10.2 KHz to 14 KHz. The invention of the GPS Navigation System made the Omega Navigation System an obsolete antique so all 8 transmitters left the air long ago.
Below 9 KHz there are many natural radio emissions and strange elecrical sounds to be heard. 9 KHz and below is reserved for radio astronomy, scientific, and geophysical research uses.
Tweeks are found between 1600 Hz to 5000 Hz. Whistlers are found between 1000 Hz up to a maximum of around 29000 Hz. (29 KHz).
The following is a looped audio recording (MP3) of a 1-second long whistler and static noise. This is NOT representative of the best. This example shows how a weak diffuse whistler sounds through the noise. Sometimes whistlers are very strong and last several seconds. At rare times when conditions are right, there are "whistler storms" where the whistlers overlap eachother continually. There are times when whistlers can echo or bounce for several seconds between the two North-South points on the earth, resulting in some very strange sounds.
[ Click to Listen to a looping MP3 file of a very weak whistler ].
Whistlers come and go seasonally, and are rare, heard mostly at night, depending on season. If in North America then FALL AND WINTER are the best times to listen, though whistlers can happen anytime day or night. Whistlers are caused by lightning flashes more than 2000 miles away, in the opposite hemisphere. The whistlers travel along the earth's magnetic field lines. This means you need to aim your ferrite antenna so it can detect NORTH - SOUTH. This is done by pointing the ENDS of the ferrite rod EAST - WEST. (imagine the rod as a compass needle that points East - West, instead of North - South).
Whistlers are usually weak-signals, are rare, yet the ferrite rod WILL detect them, BUT you will need to listen in a hum-free environment far away from power lines. A beach, public park or the middle of a lake is a good place to get away from power line hum. Take a friend along and be safe! Remember, you won't hear whistlers unless your antenna can sense waves from the north-south. If you live in the NORTHERN HEMISPHERE keep an eye on weather directly South of the Equator at the same latitude. Example: If you live 30 degrees NORTH of the Equator you will want to watch the area 30 degrees SOUTH of the Equator for thunderstorms that may launch whistlers along the earth's magnetic field lines. Whistlers can not be heard during local storms, as whistlers are the result of wave-spreading. Whistlers start out as wideband static sounds from a lightning flash. Whistlers travel far out into space along the earth's magnetic field ( up to several earth radii), and then back to earth again.
The higher frequencies arrive first, followed by the lower frequencies. The result is a down-ward whistle sound. When you hear a whistler, you are hearing a signal that has traveled a very great distance - out from the earth into space - and back again!
Go get your tape recorder and a good ferrite rod, find a quiet location and be ready to tape-record some very strange sounds!
Like shortwave radio, VLF can be reflected from the upper atmosphere. But what happens when the waves become too big to fit? Does the upper atmosphere expand to allow the waves room pass thru? No! There is a point in frequency where radio waves get too big to fit between the 35 to 60 miles space between the earth and the upper atmosphere. This point is 1600 Hz (click to listen). These radio waves are so low in frequency that you can actually hear them with your ears, when they are transformed into sound waves by a speaker. Radio waves below 1600 Hz can not travel too far beyond 60 miles.
Waves below 1600 Hz can travel by conduction through the earth itself. The earth's surface and underground area is very large and the waves below 1600 Hz have plenty of room to travel underground. Frequencies below 1000 Hz are used for earthquake research and other scientific studies. Lightning static drops out very quickly below 1600 Hz.
Voice and musical sounds of 20 Hz through 20 KHz, can be transformed into VLF radio waves when detected by a microphone. A tape recorder works by recording the VLF waves onto a magnetic cassette tape. During playback, a sensor detects the VLF radio waves from the tape as it is pulled past the sensor. An amplifier boosts the VLF radio waves and sends them to the speaker. The speaker vibrates and produces the sounds that you hear!
AM and FM radio broadcasts are actually carrying voice and music frequencies - 20 Hz to 20 KHz, this is VLF being carried through the air on much higher-frequency radio wave, but it is not VLF until it flows as electrical current to your radio's speakers.
Q: When should I listen to VLF to hear the strange sounds?
The very best time to listen to VLF is during magnetic storms and solar storms. See the space weather display below from NOAA.
When the dials move more toward the red, it means that geomagnetic storms are occuring.
If you have satellite TV and notice your picture going fuzzy several times per minute this is the result of solar storms. It is a good time to listen for strange VLF sounds during solar storms!
A: Any time is a good time, but tweeks are heard only after dark, until dawn. Night is usually better than day since the D-layer fades out after sundown.
