Volume vs. Loudness
People often complain about the volume of TV commercials compared to the volume of the actual program. And theyre surprised when I tell them that theyre exactly the same! They both have a maximum volume (10dB below digital zero). Why then are commercials so much louder?
First, understand the difference between loudness and volume. The basic difference can be illustrated with the question... "if a tree falls in the forest and theres no one around, does it make a sound?"
The answer depends on whether youre talking about volume or loudness. If you define sound in physical terms youre describing volume. The tree falling disturbs the air and creates measurable sound waves that have a definite volume.
If you define sound in a physiological sense, youre talking about loudness. Loudness depends on the response in a living creature. For example, to a deaf person the tree wouldnt make a sound, even though they might feel the impact of the tree hitting the ground. In the same way, a sound outside the frequency range of human hearing (like a high-frequency dog or dolphin whistle) would have a definite volume, but would have zero loudness for any human.
Speaking Dolphin?
But you say, how does that explain commercials being louder than the programs they appear on? The actors in the program arent "speaking dolphin", and the music and sound effects are all in the audible range for humans. The answer is the average volume.
To use an illustration, if you drive 50 miles in one hour, one might assume that you drove 50 mph the entire way. But your speed probably varied from zero to well above 50 mph during the trip. Volume levels in a program are not always constant either. There are portions at higher levels and also at lower levels. The difference between the highest and lowest volume levels of sound effects is called the dynamic range.
Absolute Peak Volume
There is an absolute peak audio volume that television networks require program material and commercials not to exceed. I mentioned it earlier as 10 dB below digital zero. But how often is the volume at this level? In commercials its at or near this level much more than in movies and other television program material.
Compressors and Limiters
Commercials stay close to the maximum volume from beginning to end. This is done by using signal processing devices called compressors and limiters. These devices act like the cruise control on a car, which allow drivers to drive at the maximum speed limit almost all the time. Its not that movie and television program mixers (and broadcasters themselves) dont use these devices. They do. They just dont compress and limit the dynamic range as much as commercial mixers.
So by maintaining a higher average volume level, commercial mixers make the audio track louder. Now that you know why and how, maybe those loud commercials will seem less annoying...but probably not.