Portable sound-pressure meter:

Few audio enthusiasts possess, or have access to, equipment required for accurately measuring the performance of a loudspeaker or the acoustics of a given hall or room. The unit described in this article is an instrument that does not give the performance of a professional meter but, in conjunction with a test CD, makes possible fairly accurate sound level measurements that enable the frequency response of an acoustic system to be as certained. Moreover, its small size makes it a very handy unit to carry about.