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THX reference level, K system, loudness curves, and safe listening


Sep 23, 2022
So I've been rabbit-holing on loudness based on conversations in my thread here https://www.audiosciencereview.com/...ions-toe-in-enclosure-volume-etc.37741/page-6 and thought I'd share some findings/thoughts.

Program content volume levels
Maybe most interesting is this guide on Netflix loudness: https://www.pro-tools-expert.com/pr...oudness-and-dynamic-range-developments-update


Figure 3. Typical dynamic range and dialogue levels of a variety of content. Black lines indicate average dialogue level; red and yellow are used for louder/softer sounds.

Although reference pink noise is -20 dB, only documentary content tends to place dialogue there. Which kind of makes sense. Concert videos unsurprisingly clock the highest average levels and, in my opinion, represent a sensible target for determining safe program volume, at -10 dB for program content. At 105 dB peak reference level that gives us a program content level of an ear-scorching 95 dB!

"Safe" Reference level and duration
Safe listening volume is frequently pegged at 85 dB for 8 hours, with safe durations halving for each +3 dB above that. If we target "loud" content (Live Concert video) and a typical runtime of two hours, that gives us 91 dB for program content. So that's -4 dB from THX reference, or 101 dB peak per full-range channel instead of 105 dB.

Interestingly the K-System sets reference mixing level a bit below THX and Dolby standards as well, which makes complete sense as mixers are expected to spend long sessions working on audio and a higher listening volume would literally be hazardous to both their health and their career. While THX sets 85 dB as the per-channel reference for pink noise, K-System sets the target at 83 dB (or 86 dB for pink noise from two channels in a stereo mix). THX and Dolby theaters are looking to make big impacts on audiences perhaps once per week without regard to safety, whereas for mixing it's 5 days per week and it's their own people.

Of course, coming down 4 dB from the 105 dB peak target affects perceived loudness, so we need to adjust our loudness curve to compensate (most modern receivers do this already, if you want them to). As seen here, at 85 dB at 1000 Hz the equal loudness for 20 Hz is +36.34 dB at 121.34 dB (this is why reference peak for full-range channels is 105 dB while for LFE it is 115 dB). If we drop down -4 dB for safety to 81 dB at 1000 Hz, then our equal loudness for 20 Hz is +38.45 dB at 119.46 dB! And that's just for normal program content, not peaks! So the differential there is +1.88 dB for 20 Hz versus 1000 Hz when dropping from 85 dB down to 81 dB.

So, if my assumptions are reasonable, that means for 2-hour safe viewing at home we should be targeting 101 dB peak for the full-range channels, and 112.88 dB peak for LFE. In actuality the difference is maybe a little less since at higher volumes the loudness curve flattens a bit, and peaks are of course louder.

Home theater and limitations
Net-net what does this mean for home theater builds? Well, it seems clear to me, which I hadn't learned before meeting the helpful people at ASR, that by far the limiting factor in home theater is bass reproduction. You simply don't need gigantic speakers to reach 101 dB peak in mid and high frequencies. And, anyways, high bass output is pretty benign to ear health. The real struggle is in reaching that 112+ dB in the bass, which is why you see so many experts (Grimani, et al) pushing for 4 subwoofers in almost any installation. This thankfully creates the flexibility of using relatively small "satellite" style LCR and surround speakers with easier positioning and even shallow installation into walls, so long as you reserve the necessary space for serious, good-quality subs that can integrate with those speakers at a frequency below the localization threshold.

If budget is constrained, consider spending more on subs and less on speakers!

Maybe even lower?
The above assumptions are built around 2 hours of "loud" program content. With today's 3-hour action epics, Netflix binge-watching sessions (reality TV typically has limited dynamic range, is mixed loud, and is often watched for hours on end), long concert videos, and most of all *videogames* (very high average loudness and often very long listening sessions) all of the above targets would be moved down even more. But the bass targets less-so ;)

P.S. After reading through a bunch of decibel charts I am definitely going to be more conscientious about wearing hearing protection on NYC subways!


Sep 15, 2022
If the goal of this research is to avoid hearing loss, then I would worry more about dosage.

So basically you have to avoid loudness or wear hearing protection when unavoidable, so you can listen to your speakers/headphone at your preferred volume, when you actually want to enjoy music/movies.

If your workplace is a loud environment, or if your day was particularly loud, then a 2 hour movie at reference levels might not be the best idea.

By the way, a lot of mixing engineers develop a method where mixing is done just sufficiently above noise floor, and only go to 85dB to check certain things (mostly related to bass). You cannot just turn the knob up and down though; a specific gain structure is needed for that.


Addicted to Fun and Learning
May 20, 2022
By the way, a lot of mixing engineers develop a method where mixing is done just sufficiently above noise floor, and only go to 85dB to check certain things (mostly related to bass). You cannot just turn the knob up and down though; a specific gain structure is needed for that.
I don't follow. Gain structure?

Edit: Do you mean loudness compensation?
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Major Contributor
Forum Donor
Jan 12, 2019
You simply don't need gigantic speakers to reach 101 dB peak in mid and high frequencies. And, anyways, high bass output is pretty benign to ear health.
Producing clean 101dB at a typical seating distance of 10-15ft is certainly a problem in my opinion for many speakers. Many speakers simply can't do it. Either distortion, compression, or both become a limiting factor. When you want to be able to produce that SPL at that distance you are back to needing speakers that can hit 110-115dB peaks at 3ft without falling apart with compression, distortion etc... Anecdotally, this follows my experience. Many speakers have sounded quite good at low/medium volumes. But you have to select speakers with this kind of output in mind if you want to hit these levels and still sound good.


Major Contributor
Dec 7, 2019
use the first , return of the jedi 1983 , theatrical sound mix , Dolby Stereo 4.2.4 , THX/TAP , Lucasfilm Ltd THX Sound System

few basic controls on the THX to align levels electronically so the amps and then mics and SPL dB for final listening checks , not many own an actual cinema THX , this being 2nd version made by component engineering 1987 THX3417 ( female ) they made the unit box , THX made the crossover cards that are only leased out to THX cinemas and be returned when no longer THX , i own three and all have ad the THX crossover cards , first one being a , dbx 900 THX , last one i think made by franch manufacture THX D1138 is the ( male ) THX patent ran out and component engineering have some newer THX crossover cards
all the THX crossover cards are year dated on the card itself and handwritten serial number for each THX cinema
the THX badge had gone though few changes the one pictured is the 2nd , made of metal


3rd THX badge design around early 1990's

THX D11378 ( male )with my late cat Sooty sleeping on it , it came with plastic chrome badge design more monitor buttons to checking processor/amplifiers an improved monitor speaker , its all DSP type with standard 2-way crossover for the stage channels LCR it has support for daughter board ( additional card ) its also not easy to set up as needs windows XP/2000 program which i have , but once its installed it should work fine and i never installed it only tested it which for , fish and chips was cheap considering its DSP its a bit heavy


pictured is dbx 900 THX , i don't have this model its also bit complicated how where its connected but doesn't have monitor speaker just the THX crossover cards in dbx 900 frame box

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