A quantitative model has been developed to study the relationship between the signal to noise ratio (SNR) and the magnetic spacing. The effects of the reading and writing processes, the linear density, and the noise mix (the ratio of the media noise power and the electronic noise power) on the SNR are studied, as a function of the magnetic spacing. The model demonstrates that magnetic spacing reduction effectively improves SNR and that the writing process contributes more to the SNR at low linear density, while the reading process contributes more at higher linear density.