In the twentieth century, architects in large cities designed structures in a way

that reduced noise and yet made living as comfortable as possible.  They used such

techniques as making walls hollow and filling this wall space with materials that

absorb noise.  Thick carpets and heavy curtains were used to cover floors and

windows.  Air conditioners and furnaces were designed to filter air through

soundproofing materials.  However, after much time and effort had been spent in

making buildings less noisy, it was discovered that people also reacted adversely

to the lack of sound.  Now architects are designing structures which reduce

undesirable noise but retain the kind of noise that people seem to need.

In line 2, the word “they” refers to