Complete silence is found only in laboratories called anechoic rooms. The wails and ceilings, made of blocks of special sound-sucking materials, are more than three feet thick, while floor coverings are six-foot layers of feathers or cotton wool. Silence here can be as painful to the ears as the din (continuous loud noise) of a steelworks or a rocket blast-off, yet scientists get used to this and stay in these silent rooms for hours at a time, using microphones and electronic equipment to test the various materials being developed to make the world a less noisy place.
    Architects have used scientific discoveries to solve noise problems in a number of ways. Walls are hollowed (having empty space inside) and then filled with sound-sucking materials similar to cotton wool. Extra-thick carpets cover the floors, and thick woolen curtains cover the windows. Air conditioning and heating channels are made less noisy by sound-sucking materials.
    Unfortunately, these techniques and others often work too well in some buildings. Noise-proof rooms become almost anechoic and people living in them are disturbed by the lack of sound. One way of handling this problem is to use what they call "sound perfume"—artificial (similarly produced, made by man) noise is piped to rooms through small loudspeakers.  Scientists use anechoic room for ______.