The natural phenomena of lightning generates electromagnetic waves over a broad spectrum of frequencies. A substantial component of the energy occurs in the spectrum that is normally considered part of the audio frequency range below 20 KHz. The spectrum covers the ELF (extremely low frequency) range from 30 Hz to 3 KHz and the VLF (very low frequency) range from 3 KHz to 30 KHz. However, since the energy is in the form of electromagnetic waves and not the movement of air, one cannot actually hear these phenomena by ear. A simple receiver has been devised which converts the ELF and VLF electromagnetic waves into sound. The receiver is in fact a very sophisticated audio amplifier, however instead of a microphone or other acoustic instrument at the input of the amplifier, a radio antenna is used. The ASV whistler receiver will enable continuous study of the ELF/VLF radio spectrum.
Lightning typically generates a short duration transient which is heard as a click in the receiver. This is what we normally describe as static and in the receiver the sounds are named “sferics” which is an abbreviation of atmospherics. Local lightning strikes within a distance of a thousand kilometres range or thereabouts generate this kind of sound in the receiver. The phenomenon however does not end there. Over medium distances of a few thousand kilometres the transient pulse is stretched in time by the characteristics of the radio wave interaction between the earth and the ionosphere. This causes the higher frequencies to arrive at a remote receiving location earlier than the low frequencies. The resulting sound of such distant lightning strikes sound like water droplets or short “plop” sounds with a duration of about 5 to 10 milliseconds. These are named “tweeks”. The sounds are very pleasing and may be considered musical in nature.
A lightning strike may also initiate a radio wave which propagates out along the earth’s magnetic field lines and travel many thousands or hundreds of thousands of kilometres out into space eventually returning back to the earth typically on the opposite side of the planet from where the lightning initially occurred. Over the long journey along the magnetic field line, the transient pulse is stretched even further, to the extent that an observer listening to the signal which returns to earth hears a tone with a descending pitch that is sustained over a period of one to several seconds, a truly astounding and surprisingly ethereal experience.
This image shows the three examples of lightning phenomena graphically in the frequency domain versus time. The top and bottom of each trace has been shown as a thin line to illustrate the reduced level of intensity of the received signal above 10 KHz and below 300 Hz. This is both a function of the actual spectral energy of the lightning static as well as the characteristics of the receiver. As shown, a sferic is observed as a vertical line. This indicates a wide spectrum of radio frequency energy occurring in a short period of time. For the tweek the line has a “hook” feature towards the end of the event at the lower frequencies. Note also how the spectral energy also abruptly cuts off just below 2 KHz. This is a function of the earth to ionosphere propagation characteristics. There are many and varied sounds also associated with the way the electromagnetic waves of lightning propagate. Recurring whistlers in a sequence create a sound described as a “chorus”. Whistlers can also occur in the reverse pattern where the pitch begins at a low frequency and sweep up to a high frequency.
The Whistler receiver audio is connected to a computer that streams the sound in real-time. A Stream Server using Icecast software can send the Whistler audio to multiple users of this website. The audio stream player also shows a Spectrogram of the sound. The spectrogram shows the Frequency and Intensity of the whistler audio. When a lightning strike thousands of kilometers away generates a tweek or whistler the visualized sound shows in the spectrogram.