Music on shellac records has a unique frequency range that is quite different from modern music. Because of this equipment that is adequate for pop music, for example, does not suit tango music in the same way. The common two or three field equalisation of common equipment is almost useless for tango. The bass is too low for tango, the midrange too high and the treble above the frequency range of tango. In order to equalise tango music properly you need more accurate equipment, where you can adjust more accurate frequency bands.
Here is an example of a graphical equaliser adjusted for the sound of a typical early thirties tango tune. The bass has been incrementally enhanced up to 5 dB, from 200 Hz down to 50 Hz. After that the low-bass has been rapidly shelved. The mids have been enhanced gradually by 3 dB from 1 kHz up to 4 kHz. After that the upper levels are rapidly shelved to cut hiss.
This image shows clearly the frequency ranges one has to operate with when adjusting tango music at a milonga. Sometimes, when that has not been done in production, one needs to shelf the highs and lows to reduce hiss and hum. Note that this occurs at a higher range when working with forties music. In the example above we start shelving at 4 kHz. For Pugielse’s music from 1944 we would need to take care not to start until about 7 kHz as the frequency range has become extended by this time.
A parametric equaliser enables one to define specific ranges to enhance or reduce specifically. Some of these have a graphical interface showing how the shape of the music changes. Above is an example of one of these, with similar equalisation as for the graphical example above. The first parameter is a low shelf, reducing the bass from 50 Hz downwards. Parameter 2 enhances the bass by about 6 dB at 100 Hz. Parameter 5 raises the mids by about 4 dB at 2000 Hz and parameter 6 is a high shelf, cutting off the treble from around 5000 Hz. As we see once set up it is easier to fine-tune thing with the parametric equaliser than with the graphical one. It can therefore be easier to use in the field, when one may need to adjust the equalisation individually for each tune.