We’ve updated our set of free audio test files!
Back in SOS May 2023, we provided a comprehensive set of audio test files at three different sample rates to suit different DAW projects (44.1kHz, 48kHz and 96kHz). I also offered detailed explanations of their use in testing and aligning a variety of audio systems, which you can read at sosm.ag/sos-audio-test-files-article. Now, I’m describing some additional files that I created recently to test for ‘absolute polarity’, and we’ve added these to the original test file set, to keep everything in one place. The new files are as follows:
- 28: LF Asymmetrical Test signal (80Hz @ ‑20dBTP, stereo, 44/48/96 kHz)
- 29: LMF Asymmetrical Test signal (250Hz @ ‑20dBTP, stereo, 44/48/96 kHz)
- 30: MF Asymmetrical Test signal (1kHz @ ‑20dBTP, stereo, 44/48/96 kHz)
- 31: HMF Asymmetrical Test signal (3kHz @ ‑20dBTP, stereo, 44/48/96 kHz)
- 32: HF Asymmetrical Test signal (7kHz @ ‑20dBTP, stereo, 44/48/96 kHz)
Polarity Overview
Most analogue and digital audio equipment maintains the polarity of audio signals passing through, unless specifically configured otherwise (a ‘phase’ button, for example, will invert the signal polarity and some processing deliberately makes use of inverted signals). When the polarity of the equipment’s output matches its input, the device is said to maintain ‘absolute polarity’.
There’s one very common case where absolute polarity is often not maintained, though: it’s not unusual on budget mixers to find that the aux outputs have inverted polarity, due to intentional (cost‑saving) shortcuts in the electronic design. Usually, this doesn’t matter and it may not even be noticed. But if the Aux outputs are routing a signal to an outboard effects processor, for example, and that effect’s output is mixed back in with the original signal inside the mixer, then the polarity inversion might cause significant problems. So, having a means of checking the arrangement can be very helpful!
Asymmetrical Signals
To check for absolute polarity, we obviously need a special signal with its own unambiguous polarity, and several different types have been used in different applications. Unfortunately, our ears are pretty poor at detecting signal polarity, and so polarity test signals are usually intended to be viewed as waveforms, to confirm absolute or inverted polarity. Typically, the output from the device being tested is either recorded in a DAW and the signal viewed on its waveform display, or it could be viewed directly in real time on some form of oscilloscope display (real or virtual).
One common test signal is a half‑way rectified or asymmetrically‑clipped sine wave. This basically leaves the positive peaks and removes all (or most of) the negative peaks. A very commonly used alternative is a low‑frequency sawtooth wave, typically ramping upwards slowly before resetting and starting again. Not surprisingly, neither of these test signals is particularly nice to listen too! So I’ve come up with a much more tuneful alternative, but one that’s just as easy to use and delivers completely unambiguous results.
Harmonics & Phase
The symmetry of an audio test tone is largely a function of the phase relationships between its fundamental pitch and its harmonics. For the asymmetrical test signals provided here, instead of starting with a sine wave, I created a cosine wave — this is a sine wave, but shifted by 90 degrees. I then added its first four harmonics, each 6dB lower in level than the fundamental.
As you can see in the waveform display provided here, the result is distinctively asymmetrical. But it sounds quite reedy and bassoon‑like, rather than heavily distorted or clicky, as in the two common alternatives mentioned above.
The waveform resulting from passing the test tones through equipment that retains absolute polarity.
If this asymmetrical test signal is routed through equipment being tested, the output signal can be viewed as a waveform. If the equipment maintains absolute polarity (ie. there is no polarity inversion), then the display will resemble a bumpy plain with regular high mountain peaks. Where signal polarity is being inverted, it will display as a high mountain plateau with regular deep valleys, as shown in the lower image.
The waveform that results when equipment inverts the polarity.
Rather than just leave you with a single file at a single frequency, I’ve created a set of five asymmetrical test tones at five different frequencies, but all with True Peak levels of ‑20dBTP. Please note, though, that the signal level shown on standard digital sample‑peak meters will typically be a little lower, especially for the low‑frequency files. In general, they will register around ‑21dBFS, with the error being due to the extremely asymmetrical nature of the waveform and the relationship between its fundamental frequency and the sample rate.
Applications
For general equipment testing applications (mixers, interfaces, preamps, processors etc.), I’d recommend using either the 250Hz or 1kHz asymmetrical tones (files 29 or 30), simply because these tend to show up well on DAWs without too much display zooming.
When checking individual loudspeaker driver polarity in multi‑way speakers don’t assume all drivers should all have the same polarity!
The alternative frequency files are included because it’s often helpful to be able to check the polarity of each cabinet or driver in multi‑way loudspeaker systems. However, I must give an important note of caution: when checking individual loudspeaker driver polarity in multi‑way speakers don’t assume all drivers should all have the same polarity! Loudspeaker designers intentionally flip the polarity of one or more drivers in many loudspeaker designs in order to achieve the desired frequency response(s) and directionality through the crossover region(s)! This is often intentional, perfectly acceptable, and not necessarily a fault!
The 80Hz tone (file 28) is intended for testing subwoofer polarity, while the 80 or 250 Hz tones (files 28 and 29) can be used on bass cabinets and bass drivers (depending on their low‑frequency extension). The 3kHz and 7kHz tones (files 31 and 32) are useful for testing tweeters, and either the 1kHz or 3kHz tones (files 30 and 31) can be used for midrange units (depending on their crossover frequency and bandwidth). As always, beware sending high‑frequency tones at high level into speaker tweeters!
Downloading The Test Files
As mentioned at the top of this article, these new asymmetrical test files have been added to the ZIP files associated with the original SOS Audio Test Files article. They’re free to download, and can be found at sosm.ag/sos-audio-test-files. The new files are numbered 28‑32, and all five are included in all three sample rate sets.