The present ASTM procedure for field measurement of the noise reduction between spaces is time consuming and costly because it requires numerous measurements in order to compute the noise isolation class (NIC), particularly when separate measurements must be made at multiple microphone positions to obtain adequate spatial averaging. The purpose of the present investigation is to explore the relationships between NIC values based on13‐octband data and more easily obtained ratings of isolation based on either octave band data or onA‐weighted orC‐weighted sound level data. A large number of curves of13‐octband noise reduction versns frequency were calculated from data on transmission loss of partitions, assumed room geometry, assumed values for the source sound power spectrum, and assumed values for the sound absorption in the source and receive rooms. For each of these13‐octband noise reduction curves, the following quantities were calculated and compared: NIC rating,Iarating (13‐octband data),Iarating (octave band data), theA‐weighted sound level difference, and the difference between theC‐weighted level in the source room and theA‐weighted level in the receive room.