An anechoic chamber (an-echoic or non-echoing) is a room designed to stop reflections of either sound or electromagnetic waves. They are also insulated from exterior sources of noise. The combination of both aspects means they simulate a quiet open-space of infinite dimension, which is useful when exterior influences would otherwise give false results. The wavelength of audible sound in air falls in the same range as that of commonly used radio waves, and their propagation patterns bear many similarities. This is why both types look similar.
Anechoic chambers range from small compartments to ones as large as aircraft hangars. The size of the chamber depends on the size of the objects to be tested and the frequency range of the signals used, although scale models can sometimes be used by testing at shorter wavelengths. The average Goldfinch is 12/13 cm long with a wingspan of 21/25 cm and a weight of 14 to 19 grams. The sexes are broadly similar, with a red face, black and white head, warm brown upperparts, white underparts with buff flanks and breast patches, and black and yellow wings. On closer inspection male Goldfinches can often be distinguished by a larger, darker red mask that extends just behind the eye. In females, the red face does not reach the eye. The ivory-coloured bill is long and pointed, and the tail is forked. Goldfinches in breeding condition have a white bill, with a greyish or blackish mark at the tip for the rest of the year. Juveniles have a plain head and a greyer back but are unmistakable due to the yellow wing stripe. Birds in central Asia (caniceps group) have a plain grey head behind the red face, lacking the black and white head pattern of European and western Asian birds.