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

sumitagarwal

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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

Netflix-post-production.png

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.

K-System
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.

Bass
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!
 

DJNX

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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.
 

Curvature

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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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Dj7675

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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.
 
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