22 Recording Studio Design traffic in the street of a small town. The low frequencies may in fact, sound very much louder. This fact affects the thinking on sound isolation, which can also be rated in dBA. There are measurements known as Noise Criteria (NC) and Noise Ratings (NR) which are close to the dBA scale. They allow for reduced isolation as the frequency lowers, in order to be more realistic in most cases of sound isolation, although they are perhaps more suited to industrial noise control than to recording studio and general music use. Where very high levels of low frequencies are present, such as in recording studios, 60 dB of isolation at 40 Hz may well be needed, along with 80 dB of isolation at 1 kHz, in order to render both frequencies below the nuisance threshold in adjacent buildings. An NR, NC or A-weighted specification for sound isolation would suggest that only 40 dB of isolation were necessary at 40 Hz, not 60 dB, so it would underestimate by 20 dB the realistic isolation needed. Not many industrial processes produce the low frequency SPLs of a rock band. If all of this sounds a little confusing, it is because it is. The non-flat fre- quency response of the ear and its non-uniform dynamic response (i.e. the frequency response changes with level) have blighted all attempts to develop a simple, easily understood system of subjective/objective noise level analysis. Although the A-weighting system is very flawed, it nevertheless has proved to be valuable beyond what could ever have been academically expected of it. However, all of these measurements require interpretation, and that is why noise control is a very important, independent branch of the acoustical sciences. Failing to understand all the implications of the A, C and unweighted scales