When listening to a mix, it is generally frequencies between around 40Hz and 12-14kHz that the ear is most sensitive to and a distinct lack of any one frequency in between these figures can result in a mix that may lack 'fullness'. Generally, a pleasant mix will contain a good balance of frequencies from across the spectrum. Each part of the frequency spectrum does however contribute its own unique characteristic. For instance, if there is too much low frequency content below around 300Hz, the mix can sound 'boomy' or 'muddy'. Too little presence from these frequencies however and the mix can sound 'thin' and lacking 'power' or 'energy'. Too much energy in the low mids (around 300Hz – 2kHz) can cause a really 'middy' sound which lacks drive and excitement. The mid ranges are very important as it is within these ranges that your vocal and lead guitar will be most prominent so it is essential that you get a good balance. Finally, the towards the top of the listening spectrum, frequencies here are generally adding sparkle and a sense of space to your overall mix. Lack of energy from as low as 2kHz can cause the mix to sound dull whilst to much top and your mix will sound overly bright. Some instruments use conflicting frequencies and so in order to balance up, you may need to make specific cuts and boosts to piece the sounds together. This commonly occurs with - kick drum and bass line, lead guitar and lead vocal, you should also be wary of piano/ keyboard sounds as they have a wide range and can conflict with bass-lines, vocals and other details. Hope this is useful! Cheeky Physics x