Chapter 19 Objective measurement and subjective evaluations Description of objective tests. Discussion of relevance. Assessment of results. Subjective views of close-field monitoring. Analysis of the NS10M. Inter- modulation distortion. The history of audio has had a frustrating tendency to produce `undefinables' in terms of quality differences. The hi-fi press has invented a whole vocabulary of adjectives to describe sound qualities, but without objective verification there is no way to know if the terms mean the same thing to different people, or not. Spaciousness is one obvious example. In the stereo reproduction of music, spaciousness is a highly valued attribute, yet no unit of measurement exists for its quantification. Transparency is another example: we think that we know when we hear it, but can we prove it? 19.1 Objective testing In 1998, Dr Keith Holland began a series of loudspeaker tests, carried out at the Institute of Sound and Vibration Research (ISVR), for the magazine Studio Sound. His format can be followed here in order to present a series of objective measurements in a logical form. In a summary article, after about 20 tests had been performed on small loudspeakers, all intended for close- field or mid-field studio monitoring purposes, it was pointed out that no two responses of any aspect of the performance of any two of the loudspeakers was the same. People are often heard to ask why so many loudspeakers sound so different when they all measure the same. Well they may seem to measure the same when reading the manufacturers' sanitised literature, but as Dr Hol- land pointed out, they do not sound the same simply because they do not measure the same. Figures 19.1­19.4, 19.6, 19.9 and 19.10 show the anechoic responses of three different loudspeakers. In each case (except 19.3(c)) the (a) plot is that of a JBL LSR32, the (b) plot is that of a Westlake Audio BBSM-5 and the (c) plot is a Tannoy System 600A. The pedigree of the manufacturers is beyond reproach, yet they have chosen three radically different physical layouts. The JBL is a conventional vertical 3-way design, the Tannoy is a 2-way concentric