The methods of classifying adventitious sounds have changed repeatedly over the years, resulting in continuing confusion in the literature and in the clinical setting. The familiar terms used by Laënnec were replaced by terms that later physicians thought were more aesthetic or clearer. For example, the term rhonchus was substituted for rale because the latter was associated with the “death rattle” sound. Later, attempts were made to classify adventitious sounds on the basis of their acoustic qualities or their similarities to musical tones.

In 1977, the American Thoracic Society adopted a system of classifying adventitious sounds on the basis of acoustical qualities, timing, and frequency waveforms. This system divides adventitious sounds into two classifications: crackles and wheezes. These two classifications are further subdivided into two categories: discontinuous and continuous.