Sound spectrograms of vibrations caused by friction closely resemble the spectrogram of the Slow Down. WDF spectrograms are visually distinctly different than FFT spectrograms. Starting in 1863, he began collecting the spectra of stars, accumulating some 4,000 stellar spectrograms. In addition, spectrograms display the amplitude of sound, a measure of sound intensity. When applied to an audio signal, spectrograms are sometimes called sonographs, voiceprints, or voicegrams. He became known as a meticulous observer, obtaining photographs and difficult spectrograms of faint galaxies. Good strings nearly sound like steel strings, and in spectrograms they show about 7-8 harmonics. The spectrograms of the whale voices showed peaks, valleys, and gaps. Rather than taking photographs, Bush collated write-ups, publicly available satellite imagery, spectrograms and maps. Watson found the CDs spectrograms and audio were consistent with being mastered from mp3 files. Her papers, including many spectrograms, are archived at the American Philosophical Society. Thousands of Earth resources photographs were taken, as were more than 40,000 spectrograms. Spectrograms may be used to visualise formants. Spectrograms of light may be created directly using an optical spectrometer over time.