22 Jan WHAT IS LOUDNESS?
Loudness perception is surprisingly complicated. The physical sound pressure level, which is usually specified in dB, tells us very little about how loud a sound will be perceived by a listener. For example, a low-frequency tone might appear much softer than a high-frequency tone, even if it has a higher sound pressure level. This frequency-dependency of loudness perception is described by the equal loudness contours.
As if this wasn’t confusing enough, the frequency dependency of loudness perception itself varies with sound pressure level. Moreover, loudness perception is a highly subjective affair that depends on factors such as age and possible hearing damages. In short, loudness cannot be easily measured. Florian Camerer from the European Broadcasting Union argues that, “there’s never gonna be such a thing as a perfect loudness meter.”
Current measurement devices and the Loudness War
Perhaps because perceived loudness is so hard to measure, it has become common practice to normalize audio according to peak signal levels. This method, extensively discussed in the professional audio world, is driven by two somewhat incompatible concerns:
On the one hand, it is necessary to prevent clipping in digital audio. For this purpose, most broadcasting stations use QPPM (Quasi Peak Programme Meters) to measure quasi peak levels that, as we will see later, can actually lie below the ‘true peaks’. These quasi peaks are then normalized to -9 dBFS.
At the same time, peak normalization has been used as a method for maximizing the loudness of an audio program, despite the fact that peaks do in no way determine how loud a signal is perceived. The actual loudness of a peak-normalized program is therefore entirely unpredictable, which can cause annoying inconsistencies at transition points.
Loudness normalization with EBU R128
So what are the practical implications of all this stuff for audio production? Well, the EBU standard provides us with the means to normalize an audio production to an actually perceived loudness and punishes the tendency towards overcompression that stemmed from the practice of peak level normalization. In the words of Florian Camerer:
The fight for ‘Who is the loudest’ disappears, mixes can be more dynamic, there are fewer dynamic compression artefacts, such as ‘pumping’, and thus there is an overall increase of audio quality!
The EBU standard proposes -23 LUFS, a meaningful target loudness that all audio content should be normalized to. However, Auphonic still uses a higher loudness target as default (EBU R128, ATSC A/85 and other common targets are selectable).