¶ Whistle
Whistles (a.k.a. Radio Frequency Beat Notes) usually appear in time-frequency plots with a characteristic 'W' or 'V' shape. Whistles are caused by radio frequency signals (i.e. signals at MHz frequencies) that beat with the LIGO Voltage Controlled Oscillators (VCOs). When the VCO and radio frequency signals are close enough in frequency, the frequency of the resulting beat note, f_beat = |f_VCO - f_RF|, will enter into the LIGO frequency band. Because the VCOs usually have rapidly changing frequencies, f_beat can change quickly, sometimes thousands of Hz in just a second or two.
Whistles occur at both H1 and L1 and contaminate searches for binary black hole mergers. For more information, see http://arxiv.org/abs/1508.07316
¶ Examples
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Audio of Whistle (with associated spectrogram) Make sure to wear good headphones!
