The modular Cointrax models, the original metal cased models and the Millennium, have tones and modulated audio. The Millennium II and e-Class have a single tone. The SST has the choice of 9 tones, 2 tones or single tone. The 9 tone audio gives a different tone for each meter segment, the two tone audio gives a low tone on iron and high tone on everything else and the single tone mode gives the same tone for everything. The SST also has saturated audio instead of modulated, so deep targets sound pretty much the same as top of the ground targets until it reaches the edge of it's depth capability. and then it gives a weaker audio response.