Step 3: All Monitoring systems have volume controls, of course, and although that might appear to make a mockery of the concept of a calibrated monitor system, we really do need to be able to adjust the volume sometimes! Often we need to turn the volume up to identify unwanted low‑level noises, or to rock our clients. And sometimes we need to turn it down because we want to check how the mix works at low levels, or just to audition commercial reference tracks without going completely deaf.
So, what we need is a volume control that can be set to a precise calibrated position when we want to adopt our reference level. Some monitor controllers have a dedicated preset level setting for this purpose, most have a specific '0' mark on the volume control scale, and at worst you can add a chinagraph pencil mark to the control panel yourself! In a perfect world, the monitor volume control will also be calibrated in decibels so that you can adjust the replay volume in precise increments away from the reference position. This is a useful option as it allows you to assess the relative 'loudness' of auditioned material in a qualitative way. So you can assess how a compressor or limiter has made the track louder by noting how far you have to adjust the volume control to match perceived loudness with your reference level.
This SPL 2Control monitor controller has a clear '0' position on the volume control scale, which can be used as the reference position.
Most computer audio interfaces have some form of monitor volume control built in, and they can be used directly if required, but I recommend using an external analogue monitor controller whenever possible — even if it's just a simple passive controller like the SM Pro Nanopatch. There are several reasons for this, but the most important is that if the computer goes mad, or you accidentally create a howlround, you will generate a peak‑level signal. When working with an operating level of ‑20dBFS, that means the volume will be 20dB louder than usual and so you'll really want to be able to turn things down in a hurry! That's a lot easier to achieve if the monitor controller is (a) right in front of you, and (b) not reliant on the computer still recognising control inputs! A separate monitor control also makes it easier to optimise the gain structure through the monitoring chain between converter output and active speaker (or power‑amp) inputs.
When working with 20dB of headroom, it's almost essential to have a reliable means of turning down the volume, in the event of digital noise or howlrounds. If your DAW or interface crashes for whatever reason, then a software control isn't going to do that job. Something like the SM Pro Audio Nano Patch Plus makes a simple but cost‑effective passive monitor controller.
Step 4: An SPL meter will be required to measure the acoustic sound-pressure level produced by the monitors. The critical requirements are that it has a C‑weighting filter option (a flatter response than the A‑weighting curve usually employed for general measurements), and a slow averaging mode. I use a cheap analogue SPL meter for this purpose — the kind of thing you'll find on Amazon for about £15$30. There are many variations on the theme, but I recommend the analogue VU‑meter versions, as the average level of a noise signal is much easier to determine visually than digital displays, which may be constantly cycling around the digits! Despite the low cost, I've found this kind of SPL meter perfectly adequate for simple monitor alignment duties like this, although if you can check it's accuracy against a properly calibrated SPL meter, so much the better.
This style of budget SPL meter on the left is easy to use and perfectly adequate for project-studio monitor system alignment. The one on the right is much more expensive and versatile, and has guaranteed accuracy from the factory, but is overkill for simple monitor alignment duties.
An SPL meter app on a smartphone can also be used, of course, but do be aware that their accuracy can vary wildly depending on the quality of their design and whether they can be configured for the specific phone's mic sensitivity and frequency response. Again, if possible, compare the phone SPL meter reading with a properly calibrated SPL meter to check its alignment, and adjust the app calibration parameters as necessary.
Having said all that, the absolute precision of the SPL reading is not very critical for setting up the monitoring in an independent project or home studio. Matching levels consistently between different speakers, and for the system as a whole at different times (such as when the monitors are changed) is what matters for most people. The only time absolute SPL accuracy is critical is if trying to conform with a specific SPL standard used in other studios with which you want to share work.
Step 5: The next step is to decide what reference SPL level to use, and this, I am afraid, is where it gets a little vague! There are basically two options: one is to adopt an industry‑recommended reference SPL (as detailed below), and the other is to arrive at your own preferred reference SPL empirically (see side box). Both approaches are equally valid in a private project studio, but the former is desirable in commercial setups where compliance with other studios is important.
Historically, Dolby‑approved professional cinema mix rooms are aligned so that ‑20dBFS (RMS) pink‑noise produces 85dB SPL from each individual speaker. This level was chosen as it is right in the middle of the flat‑ish portion of the Fletcher‑Munson curves, and it produces an acceptable listening level for most people in large cinema‑like spaces. However, it's worth appreciating that this reference level requires speakers capable of delivering 105dB SPL individually to the listening position for a peak‑level digital signal — and few project‑studio monitors can achieve that, even if your ears could tolerate it in a smaller space.
This 85dB SPL Dolby standard has been widely adopted in many professional audio circles, and there are references to it from numerous audio standards bodies (for example, the SMPTE's RP200 and the ATSC's A/85). Confusingly, though, it was subsequently discovered that the way in which Dolby measured the pink noise was slightly inaccurate, and so the 'standard' reference level has been tweaked slightly with the revised figure now being 83dB SPL (as measured on a full‑bandwidth SPL meter with C‑weighting and slow averaging from a ‑20dBFS RMS pink noise source).
However, although this 83dB SPL reference level (with 103dB peaks) is perfectly acceptable when listening in a big space, like a cinema or a film dubbing theatre, or even a very large and well‑treated commercial studio control room, it will be completely overwhelming in a smaller space, because the listener is inevitably sitting much closer to both the speakers and the room boundaries. The very different nature of early reflections in these conditions makes the level seem, psychoacoustically, much higher than it would be in a larger room.