Click here to purchase the entire book in PDF format. Calibration The calibration of your monitoring system is possibly one of the most significant factors that will determine the quality of your mixes. As a simple example, if you have frequency-independent level differences between your two-channel monitors, then your centre position is different from the rest of the world's. You will compensate for your problem, and consequently create a problem for everyone else resulting in complaints that your lead vocals aren't centered. Unfortunately, it is impossible to create the perfect monitor, so you have to realize the limitations of your system and learn to work within those constraints. Essentially, the better you know the behaviour of your monitoring system, the more you can trust it, and therefore the more you can be trusted by the rest of us. There is a document available from the ITU that outlines a recommended procedure for doing listening tests on small-scale impairments in audio systems [Union, 1997]. Essentially, this is a description of how to do the listening test itself, and how to interpret the results. However, there is a section in there that describes the minimum requirements for the reproduction system. These requirements can easily be seen as a minimum requirement for a reference monitoring system, and so I'll list them here to give you an idea of what you should have in front of you at a recording or mixing session. Note that these are not standards for recording studios, I'm just suggesting that their a good set of recommendations that can give you an idea of a ``good'' playback system. Note that all of the specifications listed here are measured in a free field, 1 m from the acoustic centre of the loudspeaker. Frequency Response The on-axis frequency response of the loudspeaker should be measured in one-third octave bands using pink noise as a source signal. The response should not be outside the range of $\pm2$ dB within the frequency range of 40 Hz to 16 kHz. The frequency response measured at $10^\circ$ off-axis should not differ from the on-axis response by more than 3 dB. The frequency response measured at $30^\circ$ off-axis should not differ from the on-axis response by more than 4 dB [Union, 1997]. All main loudspeakers should be matched in on-axis frequency response within 1 dB in the frequency range of 250 Hz to 2 kHz [Union, 1997]. Directivity Index In the frequency range of 500 Hz to 10 kHz, the directivity index, C, of the loudspeakers should be within the limit 6 dB $\leqslant$ C $\leqslant$ 12 dB and ``should increase smoothly with frequency'' [Union, 1997]. Non-linear Distortion If you send a sinusoidal waveform to the loudspeaker that produces a 90 dBspl at the measurement position, the maximum limit for harmonic distortion components between 40 Hz and 250 Hz is 3% (-30 dB) and for components between 250 Hz and 16 kHz is 1% (-40 dB) [Union, 1997]. Transient Fidelity If you send a sine wave at any frequency to your loudspeaker and then stop the sine, it should not take more than 5 time periods of the frequency for the output to decay to 1/$e$ (approximately 0.37 or -8.69 dB) of the original level [Union, 1997]. Time Delay The delay difference between any two loudspeakers should not exceed 100 $\mu$s. (Note that this does not include propagation delay differences at the listening position.) [Union, 1997] Dynamic Range You should be able to play a continuous signal with a level of at least 108 dBspl for 10 minutes without damaging the loudspeaker and without overloading protection circuits [Union, 1997]. The equivalent acoustic noise produced by the loudspeaker should not exceed 10 dBspl, A-weighted [Union, 1997].