When it comes to mixing and mastering music, our aesthetic sensibilities can be fickle. What sounds right to us one day might not seem right the next day, or when listened to at different volumes, or through different monitors, or in different listening environments. Our perception of sonic qualities, including instrumental balances and overall intensity, are influenced by a number of variables. As simple as it sounds, monitor volume is one of the main variables that we can control to help us achieve consistency in our mixes. One of our main goals while mixing and mastering is to achieve a certain amount of musical impact or feeling, which we call loudness.
While volume describes the actual sound pressure level (dB SPL) in a room, loudness can be thought of as how intense, dense, powerful or “loud” the sound feels to you. For example, a recording of an explosion played back at a low volume still sounds like a “loud” sound to our brain, while a recorded whisper played back at a high volume still feels like a soft sound. Both volume and loudness are important considerations and during mixing and mastering, where our job is to create the appropriate intensity, or loudness, of the music. Maintaining a consistent, or calibrated, listening volume helps us remain focused and consistent when making adjustments and decisions that affect the loudness of our project.
Is Monitor Volume Important While We Mix?
At different playback volume levels our brains perceive the loudness of different frequencies unevenly. For instance, when monitoring at low volumes, our ears and brain focus more on the midrange while the low and high frequencies are not perceived with the same emphasis. At higher listening volumes, the low and high frequencies appear more prominent while the midrange frequencies command less of our aural attention. Objectively, and in terms of physics, the actual frequency balance stays the same at all volume levels, but the human brain’s interpretation of the frequency balance changes.
In the 1930’s two acoustic researchers, named Harvey Fletcher and Wilden Munson, studied the phenomenon of human perception of frequencies vs. loudness. They developed a loudness contour, called the Fletcher-Munson Curve (fig. 1), which indicates the volume (dB SPL) levels across the frequency spectrum necessary for the listener to perceive a constant loudness level when presented with pure steady tones. This calculation was refined in 1956 and is now used as the ISO 226 standard for Equal Loudness Level Contours.
Given that we perceive the blend of frequencies differently at various volume levels, how loud should we monitor when we are mixing or mastering? From the Fletcher-Munson Curve graph (fig. 1), we see that a monitoring level of 85 dB SPL (in a large room) provides the flattest hearing curve. If you are working in a smaller space, like a typical bedroom studio, the flat response is probably closer to 79 dB SPL. Now that we are aware that our sensitivity to bass and treble frequencies becomes more flat as volume increases, we can infer that monitoring too softly will cause us to want to increase the bass and treble content of our mixes (because at low volumes the midrange is most prominent and bass and treble seem too low).
Our goal is to find a trustworthy, or calibrated, monitoring level which sounds well balanced in your listening environment. To calibrate your monitoring level, you will need either a software or hardware SPL meter. I use an iPhone app called SPLnFFT Noise Meter, but you could also use a dedicated SPL meter, like the Velleman analog meter or the Extech digital meter.
Start by bringing your monitor levels down for now–we will set monitor volume as the final stage of the overall adjustments. Set up a signal generator plugin in your DAW to output pink noise at -20 dBFS and verify that level on the master fader output meter. If you use a stereo signal, keep your output panning at 100% L/R, but if you use a mono signal generator, make sure to center the pan knob.
If you have a software control panel, like UAD’s Console application, you can double check that the level reaching the hardware is showing as -20 db. Once this is established, set the output gain of your audio interface all the way up at 0 dB.
We typically reference -20 dBFS for digital levels because that level matches the analog world’s headroom standards, where -20 dBFS is roughly equal to +4dBu or 0VU. [Note that -20dBFS = +4dBu is the SMPTE/AES standard, but Pro Tools and the BBC default to -18 dBFS = +4dBu, the EBU standard. Among mastering studios -14dBFS = +4dBu is also common.]For techies…
Grab your SPL meter or launch your SPL app and position it in your listening position, where your head would usually be, with the microphone pointing up. If the app or SPL meter gives you the option, select the C weighting scale and slow response time, as these are the most accurate settings for this type of measurement. Pan the pink noise to one speaker and slowly bring up your monitor volume until the SPL meter is registering 82 dB SPL. Pan the signal to the other speaker and verify that you have the same 82 dB reading (less than ± .5 dB between the left and right speakers is close enough). When you pan the pink noise back to the center you will notice the meter will now read 85dB, which is a 3 dB increase because both speakers are playing. You should also notice focused center image, which will confirm that your speakers are in polarity with each other. For mixing or mastering pop music, an 85 dB playback level may feel too loud, so it is common to readjust this level down to 79 dB. This level (79 or 85 dB) is now your calibrated monitor level. On monitor controllers with a digital readout it is easy to store and recall this precise setting, otherwise you may want to place a mark on your monitor controller to indicate this playback level.
With your monitor level set and both speakers matched, you can now mix, confident that you are working at an optimal and consistent level. Because of the Equal Loudness phenomenon, it is vital to keep your monitoring level consistent when mixing. Of course you will find it useful to occasionally monitor at quieter levels to verify that the important elements of the mix remain audible even at low levels. Many audio interfaces and monitor controllers have an adjustable DIM function, which allows you to press a button and drop the monitor volume by a preset amount. DIM is useful for checking balances at a lower listening level and also simply to allow you to have a quiet conversation while the music is playing. Switching off DIM will instantly set your monitors back to your calibrated monitoring level.
If you regularly mix or master on headphones, level calibration is not as straightforward. We can’t easily measure the SPL level of our headphones, so we must rely on our musical sensibilities. For headphone calibration, I suggest you listen to well-mastered reference songs at various levels until you feel that you have found a level that provides the proper feeling of impact, intensity and a full-range frequency response. Then, try to mark your headphone monitor level control at this calibrated level. As with monitor speakers, the more consistent you are with your listening level, the more consistent your mixes and masters will turn out.
To set your optimum monitor level for mixing, import some reference tracks that have been mastered and lower their playback level in your DAW by 6 to 8 dB and listen to it at your calibrated level. Then make small (2 or 3 dB) level adjustments to find your optimum calibrated listening level. By lowering the level of mastered tracks by 6 or 8 dB you can more fairly compare your unmastered mix to mastered reference songs.
To set your optimum mastering monitor level, import some reference tracks that have been mastered and listen to them at your calibrated level. Then make small (2 or 3 dB) level adjustments to find your optimum calibrated mastering level. Now you can confidently compare your masters to commercial reference masters for their loudness, impact and feeling without relying on level meters.
Calibrating your room for loudness is an essential step towards consistent mixing and mastering. Calibrating your monitors for a flat frequency response is equally important and tools like Sonarworks Reference 4 create the optimum EQ curve for your monitors in your listening environment. Calibrating your monitors for proper loudness and frequency response, puts you well on your way to creating professional sounding, competitive mixes and masters.