The SOS Audio Test Files

Standard test signals can be incredibly useful when configuring and aligning audio equipment — and SOS's Technical Editor Hugh Robjohns has created a set of properly calibrated test files just for you! Read on to find out how to use them and where to find them.

Audio test signals can be produced by proper audio test equipment, generated by audio editing software or DAW platforms, or found as pre‑made audio files online, from a wide range of sources. In this article I’m going to describe a selection of bespoke, specially created test signals and explain how to use them to align audio systems or check that things are working correctly.

First, though, a brief warning: if misused, test signals can potentially be dangerous to your ears, your loudspeakers, and even to equipment being tested (particularly sensitive equipment like mic preamps). So, always be cautious and careful when using these test signals, particularly in respect of monitor speaker or headphone volumes, and especially when using the full‑level and high‑frequency test signals. Use of these test signals is entirely at your own risk: neither Sound On Sound Ltd nor the author can accept any responsibility whatsoever for any harm to persons or equipment!

The majority of test signals described here are 24‑bit WAV files created using Adobe Audition, and they are available to download as three ZIP files from the online resource page at https://sosm.ag/sos-audio-test-files. There are three sub‑folders, each containing identical audio signals at one of three different sample rates (44.1, 48 and 96 kHz) to suit common DAW project configurations. A few test signal files are not available in all three formats, simply because some are not relevant or appropriate at certain sample rates.

This table lists the included audio test files, their purposes, levels, frequencies and formats. Download the ZIP files to get your hands on them!

Without further ado, let me take you through what these files are and what you can use them for.

Operating Level Sine Tones

Probably the most basic test signal, but also an incredibly useful one, is a 1kHz sine wave tone at the nominal ‘operating level’. A sine wave is a pure signal that contains only a single fundamental frequency, and a system’s operating level is the default recording level (0VU or +4dBu in analogue terms), which is well above the system noise floor yet still leaves adequate headroom to cope with transient peaks.

In the digital audio world, there are two standard operating levels. The most common in professional and project studios is ‑20dBFS (SMPTE RP155 standard), while European broadcasters use ‑18dBFS (EBU R49). The latter has been included here, as it often works better with semi‑pro analogue equipment than the SMPTE standard.

Sometimes, though, it’s more appropriate to work with an alternative (and usually higher) operating level, for example if mastering within a peak‑normalised environment such as for a CD. As it happens, the EBU also specify a CD Exchange tone level (EBU RP89) for broadcasters working with commercial CDs. This sets a reference level at ‑12dBFS, so I’ve included a test signal at that level in the folders for convenience. For any other personalised local reference level requirements, the amplitude of one of these standard test tones can be altered simply by introducing a known level change using a fader, plug‑in or native gain process.

System calibration using any form of audio metering is always performed using a sine‑wave test tone. Audio programme material (music or speech) and pink noise are simply not appropriate for this application because different metering systems, and even different meter designs of the same nominal type, inherently have substantially different dynamic responses and may be measuring different things (eg. quasi‑peak or RMS). Consequently, different meter types often register quite different levels, even on the same source material. In contrast, a sine‑wave tone provides a known, stable, consistent signal level that will always read accurately on all meter types.

When connecting multiple devices, it is important that the equipment input and output levels are adjusted to maintain a consistent headroom margin and low noise floor through all stages of the signal path. A standard operating level test tone allows this very easily, and the process starts with playing a sine tone at the appropriate operating level and sample rate from your DAW. In most cases, a ‑20dBFS tone is the easiest to work with, since virtually all digital meter systems have a suitable mark corresponding to ‑20dBFS.

Both mono and stereo operating level test tones are provided in the folder, and for general signal path checking the stereo file is often the easiest one to use. That’s because a mono channel’s pan‑pot attenuates the signal level reaching each side of the stereo output, with the amount of attenuation being defined by the DAW’s current ‘pan law’. In most cases the default pan law is ‑6dB (so that if summed back to mono the level would read the same as the original mono source file), but alternative pan‑law values including ‑4.5dB, ‑3dB, or even 0dB may be employed or available. There are good reasons why these different options exist, but one effect is that a ‑20dBFS reference tone played from a mono channel is likely to read lower on the left and right masters (eg. ‑26dBFS), and this can be confusing when you’re trying to calibrate your gear!

Obviously, it’s possible to compensate for the pan‑law attenuation by raising the source channel fader by 6dB (or whatever the pan‑law attenuation is), thus ensuring the stereo output meters read the required ‑20dBFS. The aim is simply to make sure the DAW outputs are at the appropriate operating level so that you can then check the signal level through external analogue equipment connected to the DAW, or even through internal plug‑ins. However, as I said previously, using a stereo file avoids the need for compensation and is thus considerably easier and quicker to use.

In the analogue world, the standard operating level is normally indicated by the zero mark on a VU meter. The SMPTE (RP155) standard equates a ‑20dBFS digital tone with an analogue signal voltage of +4dBu, but in most cases the actual analogue signal voltage is irrelevant – the important thing is that the outboard equipment registers the tone at 0VU on its own meters, and that is achieved by adjusting the device’s input level controls as necessary. Similarly, device output level controls (or interface input levels) can be adjusted so that the return signal into the DAW also registers ‑20dBFS on its input meters.

It’s worth noting that the SMPTE alignment means a full digital level (0dBFS) equates to a whopping +24dBu in the analogue world. That’s almost 35V peak to peak, and while professional equipment is designed to handle that kind of signal level much semi‑pro equipment doesn’t have sufficient headroom and might clip at much lower levels. For that reason it may be more appropriate to employ the EBU digital operating level of ‑18dBFS. This is associated with a lower analogue operating level of 0dBu (instead of +4dBu) and, as a result, the EBU alignment sets a peak level which is 6dB lower than the SMPTE requirement (0dBFS = +18dBu). Most semi‑pro equipment can accommodate that and will sound rather better. For added real‑world context, Mackie’s analogue consoles are aligned such that the meter zero equals 0dBu, so they’re easy to align to the EBU format. Similarly, RME’s interfaces all support the EBU alignment, but only the very latest high‑end models can meet the SMPTE spec.

Although a 1kHz tone is the international standard for level testing, other tone frequencies can be useful. For example, a 440Hz tone is handy for reference tuning, and a 3kHz tone is a better choice for detecting the clicks and glitches that occur if a digital system’s clocking isn’t configured correctly. I have included tones at both of these frequencies within the test file folders (both at ‑20dBFS).

Real‑world testing of music recordings has found reconstructed sample peaks reaching as much as +9dB or so above the highest registered sample peak.

Sample & True Peak Meter...