Acoustic microscopy acoustic microscope (CSAM), and C-mode scanning acoustic microscope (C-SAM). More recently acoustic microscopes based around picosecond ultrasonics systems have demonstrated acoustic imaging in cells using sub-optical wavelengths working with ultrasonic frequencies into the multi-GHz. Since the vast majority of acoustic microscopes in use today are C-SAM type instruments, this discussion will be limited to these instruments. Ultrasound is broadly defined as any sound having a frequency above 20 kHz, which is approximately the highest frequency that can be detected by the human ear. However, the acoustic microscopes emit ultrasound ranging from 5 MHz to beyond 400 MHz so that micrometre
Aspidistra (transmitter) seven bomb-free zones in central and southern Germany. All these announcements were false. German radio stations tried announcing "The enemy is broadcasting counterfeit instructions on our frequencies. Do not be misled by them. Here is an official announcement of the Reich authority." However, "Aspidistra" broadcasts included similar announcements, leaving the listeners confused. Although mainly intended for the military and propaganda transmissions described above, "Aspidistra" was also used during the war for some BBC European Service broadcasts to Europe on 804 kHz. After 1945, "Aspidistra" continued to be used by the BBC. Frequencies used included 1122 kHz (1945–1950), 1340 kHz (1950–1962),