Why do some commercial DVDs/Blurays have poor/low sound?

[technicality1]

This is probably a common question, and I'm no techie with this kind of thing, but does anyone know why a lot (or some) films and TV shows on commercial DVDs and blurays have very poor audio when compared to broadcast versions?. I watched a bluray copy of a film recently and the audio was barely audible. I had to turn the sound right up to nearly 100% to be able to hear it within a good range. Is it to do with bitrate, compression, or is it a manufacturing defect?. Ever since DVD arrived on the scene I often heard people complaining about the low audio of purchased movies, and these were often people who worked in repairing TVs and VHS players. In the days of VHS I never heard anyone complain about the low sound levels on rental or sale tapes.

[DB83]

AC3 sound levels are often low. If the dvd has a LPCM track you could always try that.

[turk690]

16bit audio of the kind on DVDs and blu-ray discs has a dynamic range of 96dB. People responsible for editing and composing the various DTS and Dolby encoded 5.1ch soundtracks on them follow guides which suggest what types of audio, what levels they should have, and which channels they should be assigned to, to best manage what can be fit within 96dB, among other things. For example, a scene inside a typical room of two people normally conversing means dialogue, so this is routed to the center channel and usually has an average level of -20 to -10dB. Effects like explosions etc can appear on any channel and may well peak at 0dB. Due to these rules/guides, on a movie which only or mostly has dialogue therefore, the whole soundtrack may well rarely stray above -20dB. It would have been easy to normalize that such that you get relatively loud volumes on a mostly dialogue disc, but average levels between different discs will then jump up and down. A Dolby Digital stream can also have metadata embedded (if the editors created and embedded it), which can be used by the DVD or blu-ray player to restrict dynamic range if chosen by viewer.

[JVRaines]

It could be Dialog Normalization, an AC-3 feature that tells the player how much to reduce gain to put dialog at a level that Dolby thinks is proper. Not everyone agrees with Dolby, though, and some discs ignore dialnorm and set it to maximum. (I often do!) Broadcasters follow different standards for loudness and they will sometimes apply dynamic compression to keep dialog and explosions closer in volume.

By the way, quiet audio is not necessarily poor audio. Some listeners are actually paying full attention and want to hear a range of soft to loud sound.

[pandy]

It may be few reasons, mentioned Dolby policy where audio are targeted to -23dBFS level but it can be also improper downmixing.
Usually selecting proper audio mode on amplifier solve issues.

[acheter]

Effects like explosions etc can appear on any channel and may well peak at 0dB. Due to these rules/guides, on a movie which only or mostly has dialogue therefore, the whole soundtrack may well rarely stray above -20dB.

[lordsmurf]

DTS is worse than AC3 for low volumes.
It's not a "volume" issue, but "dynamic range".