Acoustically treated, doorless telephone booths can increase the effectiveness of telephone communication in noisy environments even though practical size and open area requirements limit the amount of noise reduction which can be achieved. The principal factors determining the noise reduction of an acoustically treated booth (where the noise reduction is considered as the decibel difference in octave band noise levels measured outside and inside the booth) have been investigated by use of full size and13model booths.The principal parameters affecting the reduction of an acoustically treated telephone booth were found to be (1) the sound absorption coefficients of the wall surfaces as function of frequency; (2) the ratio of open area to the wall area of the phone booth; (3) the transmission loss of the walls of the phone booth; and (4) the size and shape of the booth.The investigation resulted in a new triangular design of booth which is fully as effective acoustically as the rectangular booths, yet has added practical advantages.