The electromagnetic spectrum is the range of all possible electromagnetic radiation frequencies. Shortwave radio is radio transmission using shortwave radio frequencies. Aperture antennas are the main type of directional antennas used at microwave frequencies and above. Radiance is the integral of the spectral radiance over all frequencies or wavelengths. Some systems have multiple, distinct, resonant frequencies. Objects have a tendency to selectively absorb, reflect or transmit light of certain frequencies. They may control parameters such as amplitude (volume), filters (frequencies), or pitch. The oscillators are designed to have (when uncoupled) almost equal resonant frequencies. A spectrum of all the frequencies of absorption in a sample is recorded. Sounds consist of waves of air molecules that vibrate at different frequencies. Skip contributes to noise on CB frequencies. For audible frequencies above it is relatively constant. Hz, though less accurately at higher frequencies. This dielectric causes increasing losses at higher frequencies. Materials which do not allow the transmission of any light wave frequencies are called opaque.