IThe measurement of sound characterisitcs is inevitable in Sound Engineering. From the facts explained on the previous page it is obvious that not only pure physical measurements, but also measurements of perceipts are necessary. These types of measurements can not be performed using traditional measurement tools, but using special scientific methods including listening tests with subjects. These tests can be regarded as measurements is they are planned and conducted with the necessary knowledge and experience. Proper methods allow to test the significance and reproducability of the results, so that they are not less objective than traditional measurements although they are ofter refered to as subjective tests. Anyhow, it is of cruical importance that the test are carefully planned and that all influencing factors - including non-acoustical and cognitive factors - are properly considered.
In order to avoid the tests with subjects it is often intended to develop intrumental tools which reproduce specific aspects of perception. A rather abstract but first approach was the development of the A-weighted sound pressure level (dB(A)). Here the spectrum of the sound is weighted with a curve similar to the hearing threshold of human hearing. It was the intention that only those frequency components which can be heard should contribute to the overall sound pressure level.
Anyhow, it has to be considered that the frequency weighting of human audition changes with level. As a result, the A-weighting only corresponds to human frequency weighting if sounds are close to the hearing threshold. The follwing acoustical example includes three sounds with equal A-weighted level.