NOTE: This is an experimental resource. We hope to maintain the VLF audio stream 24 hours a day with infrequent interruptions for maintenance. However, we can't guarantee that it will be available all the time. Let us know if you encounter problems.

You can hear sferics, tweeks, whistlers and other VLF radio sounds at any time of the day, but the hours around dawn and dusk are generally best. Nighttime is also better than daytime. In Huntsville, AL, where our online receiver is located, dawn happens at about 1200 UT and dusk is ten hours later at 2200 UT. Please read the Science@NASA story "Earth Songs" to find out what these strange sounds represent.

Sferics: Sferics, short for "atmospherics", are impulsive signals emitted by lightning. They sound like twigs snapping or bacon frying. Sferics are caused by lightning strokes within a thousand kilometers or so of the receiver. The dynamic spectra of sferics are characterized by vertical lines indicating the simultaneous arrival of all audio frequencies. Click on one of these links to hear sferics in the audio format of your choice: [MP3] [RealPlayer G2] [NEXT/Sun] [Microsoft WAV]

Tweeks: Tweeks are sferics that travel considerable distances through the ionosphere -- a "dispersive medium" where low frequencies travel slower than high frequencies do. As a result of dispersion, tweeks don't sound like sferics. Instead of crackling like bacon they sound like a quick musical ricochet. The dynamic spectrum of a tweek shows a vertical line at the higher frequencies with a curved section (called the "hook") appearing at ~2 kilohertz. Click on one of these links to hear tweeks in the audio format of your choice: [MP3] [RealPlayer G2] [NEXT/Sun] [Microsoft WAV]

Whistlers: Whistlers are sferics that are dispersed even more than tweeks. The sound of a whistler is a musical descending tone that lasts for a second or more. Their dynamic spectra reveal a long sweeping arc that illustrates how the high frequencies arrive first, followed by lower ones. Click on one of these links to hear whistlers (interleaved with sferics) in the audio format of your choice: [MP3] [RealPlayer G2] [NEXT/Sun] [Microsoft WAV]

Editor's note: What is a dynamic spectrum? Dynamic spectra show the evolving intensity of a signal as function of time (horizontal axis) and frequency (vertical axis). The colorful ones on this page show VLF signals lasting about 5 seconds in the frequency range 100 to 2400 Hz. False colors denote intensity. Black and blue areas are frequency-time regions of relative quiet. Yellow, orange and red are louder. You can create your own dynamic spectra of voices, radio stations, etc., using Spectrogram Ver 5.1 by Richard Horne. It's free.