For optimal transfer of power to the surrounding medium, a sound source should have a radius of 1/6 to 1/4 of the sound wavelength. Sound-waves propagate from the source as compressions and rarefactions of the fluid medium, which decay by spreading and viscous losses. Higher frequencies are more easily refracted and reflected by objects in the environment, causing degradation of signal structure. In open air or water, the sound spreads spherically and decays by the inverse square law. If the sound is restricted to two dimensions rather than three, it decays as the inverse of range, whereas waves within a rod decay largely due to viscous losses; such calls are usually rather simple pulses and rely on the initial time of arrival because of multiple pathlengths or different propagation velocities in the environment. Because of the relationship between calling success and reproductive success, singing insects are under selective pressure to optimize the range, and to maintain the specificity, of their calls. Smaller insects have less muscle power; because of their small sound sources, higher frequencies will be radiated more efficiently than lower frequencies, but in order to produce brief loud pulses from a long-duration muscle contraction they may use both a frequency multiplier mechanism and a mechanical power amplifier. Airborne insect sounds in the range from 1 to 5 kHz tend to have sustained puretone components and a specific pattern of pulses which propagate accurately. Where the song frequency is higher, the pulses tend to become briefer, with a rapid initial build-up that gives a reliable time of onset through obstructed transmission pathways. These scale effects may be related both to the sound-producing mechanism and the auditory system of the receiver. Tiny insects have the special acoustic problem of communicating with only a small amount of available power. Some, such as fruit flies, communicate at low frequencies, at close range, by generating air currents; these currents may also be used to waft specific pheromones. Other small insects, such as Hemiptera, beetles, etc., communicate using substrate vibration. This enables long-range communication, but signal structure degrades with distance from the source; vibration signals tend to be confined to certain types of linear substrate, such as vegetation.