Leq(m) Loudness - What Is It And When Is It Used?
Following our article, Loudness - Everything You Need To Know, we received a request to explain how Leq(m) fits into the Loudness scene. In this article loudness guru Mike Thornton explains what Leq(m) is and what it is used for.
Community member ‘socal CRM-114’ asked…
“I wonder if you could talk about what Leq is? I'm dimly aware it's used for regulating trailer loudness but I'm curious how it's calculated and if it has any other applications.”
Thank you for highlighting this, it is still a valid measurement in certain workflows.
What Is Leq(m)?
Leq(m) has some similarities to BS1770 loudness measurements like LUFS and LKFS, which we have covered in many articles, including our article Loudness - Everything You Need To Know in that it is a measurement of average loudness over time, measured through a weighting filter.
Let's take a look at this table which summarises the key differences and shows why they cannot be compared with BS1770 loudness measurements.
Filter Curves
The first difference between BS1770 and Leq(m) is the weighting curve or filter, through which the loudness is measured. BS1700 loudness is measured through a K-weighting filter as shown by the red curve in the diagram above, whereas Leq(m) is measured through a modified CCIR curve as shown by the pink curve.
Back in the days of analogue audio, I remember using the CCIR curve to measure the noise floor of electronic equipment including mixers and tape machines to check compliance with the IBA code of practice.
We understand that the shape of the filter was chosen because…
“the weighting curve is more sensitive to annoying frequencies like breaking glass, explosions, and other piercing sounds. It also allows for trailers and adverts to have loud explosions with enough quiet parts to balance it out so as not to annoy the audience.”
For completeness, the other two curves are ‘A’ weighting, which was originally based upon the 40-phon curve, but nowadays is mainly used in Sound Level Meters to predict potential hearing loss from environmental noise. Finally, the ‘C’ curve that was originally based upon the 100-phon curve and still finds some use in electro-acoustic system calibrations.
Short Term Measurement
With BS1770 loudness metering, there are 3 measurements, momentary (M), which is averaged over 400ms, Short Term (S) which is averaged over 3 seconds and Integrated (I), which is the average over the complete program.
With Leq(m), the averaging is taken over an 850ms window, but there is a second difference: the amount of overlapping of the averaging blocks or windows. In BS1770 loudness meters, the 3-second blocks overlap by 75%, effectively giving a sliding window, which is designed to give a smoother running average, whereas with Leq(m) there is no overlapping it uses fixed blocks or windows.
1KHz Tone Measurement
The reference to a 1K stereo tone is different. With BS1770 loudness meters, when fed with a -20dB 1K tone will read -20LUFS/LKFS, whereas with an Leq(m) loudness meter a -20dBFS tone will read 85 dB Leq(m). This also shows that Leq(m) is not an inverse scale with a maximum 0LUFS (full scale) but a measurement scale that starts with 0 as a minimum and goes up as loud as it can go.
For example, the threshold of hearing being considered to be 0dB SPL (usually measured through an A-weighted filter) going up to the threshold of pain being around 130dB SPL.
Surround Weighting
BS1770 including a +1.5dB weighting for the surround channels. This means that the sounds in the rear surround channels have a slightly higher influence on the single combined loudness measurement. This was following extensive listening tests undertaken during the development of BS1770, it was found that sounds arriving from behind a listener may be perceived to be louder than sounds arriving from in front of the listener. Following more listening tests the +1.5dB lift for the surround channels was set.
Note that unlike BS1770, which has a lift on the surround channels, the Leq(m) spec does the reverse, it doesn’t take the surround channels into consideration as much. It is our understanding that this difference is because BS1770 depends on point source speakers, whereas because Leq(m) is for theatrical workflows where there are multiple surround speakers, there is a need to weight the surround channels down.
What Is Leq(m) Used For?
Leq(m) is used to measure the loudness of theatrical cinema adverts and trailers, the stuff that happens before the main feature.
"The TASA Standard" is an audio standard for motion picture trailer volume. It was developed by the Trailer Audio Standards Association at the request of The National Association of Theatre Owners (NATO) to address audience complaints about excessively loud trailers.
At any given time, the TASA Standard identifies a specific Leqm number as a recommended upper volume limit for trailers in order to bring the audio level of trailers closer to the features they precede. The current upper volume limit is 85 dBLeq(m) as quoted in the What is TASA? page on the TASA website.
The Screen Advertising World Association (SAWA) has standardised Cinema commercial production formats such as Sound in Cinema & the Digital Medium of Cinema. Since 1 January 2000, many screen advertising companies adopted the SAWA maximum of 82 dBLeq(m) as laid out in the most recent document on the SAWA website Summary of Standards for Digital Screen Advertising – 3rd Edition dated 2019.
How Can I Measure Leq(m)?
Originally the Loudness meter of choice was the Dolby Media Meter. However, since Dolby discontinued their Media Meter in 2018, there are two plugin products that can read Leqm…
Nugen Audio VisLM 2
The Nugen Audio VisLM 2 is my preferred loudness meter, it does everything I need it to do, I like the way it displays the data and that I can colour it and resize the window to suit his screen real-estate.
The Rewritable Memory Edit Mode is absolutely amazing. It enables me to make small corrections to a mix and not have to re-analyze the whole programme. I also love the way I can resize the window and all the additional options they have added to the plug-in.
For people working in film production and especially trailers, Nugen Audio has included the Leq measurement for both the TASA and SAWA delivery specs as you can see from the image above.
VisLM-H2 is designed to promote an efficient workflow with the ability to drill down to any level of complexity the context demands. With VisLM-H2, your loudness workflow can be integrated into the production environment. With a fully resizable interface and many additional visualisations, it's even simpler to identify exactly the information you need and choose the configuration that matches your workflow precisely.
Check out why Mike makes Nugen Audio’s VisLM2 one of the plug-ins he loves for post-loudness - So It Has To Be Nugen Audio VisLM 2.
VisLM 2 also supports the new Netflix delivery spec using Dolby Dialog Intelligence. Check out our video from IBC 2018 when we got a sneak peek.
VisLM from Nugen Audio supports AAX, VST, VST3, AU and AudioSuite in both 64-bit and 32-bit versions for macOS and Windows. Note that RTAS is also available as 32-bit only. VisLM is also available as a standalone application for macOS and Windows.
Waves WLM Plus
The Waves WLM Plus is a popular loudness meter that covers many bases although it doesn’t have any histogram display. The upside of this is that the plug-in’s GUI takes up less screen real estate.
The Waves WLM Plus Loudness meter adds a True Peak Limiter and a Trim option, to their WLM loudness meter. They both provide loudness measurement, metering, correction and adjustment tools for broadcast, movie trailers, games, packaged media and more. Fully compliant with all current ITU, EBU and ATSC specifications, the WLM Plus offers Momentary, Short Term, Long Term and True Peak readouts, as well as a warning and logging system that keeps track of your levels, alerting you when you’ve exceeded them - or fallen short.
The Waves WLM Plus supports Leq(m) measurement and comes with a TASA Movie Trailer preset, but not a SAWA Avert prest, so I made a custom preset by simply adjusting the Target level down from 85.0 to 82.0 and saved it as a new preset.
WLM Plus also features Gain and Trim controls for correction of loudness levels and a True Peak Limiter, which can save you needing to utilise additional equipment or software.
Check out Mike’s review on Waves WLM Plus Loudness Meter Plug-in and his tutorial on How To Use Waves WLM Plus In An Audio Post Production Workflow.
The WLM Plus supports macOS 10.11.6 - 10.14 and Windows 7 with SP1 64 bit, Windows 8.1 64 bit, Windows 10 64 bit with AAX Native 64-bit, Audiosuite, Audio Units (Mac only), VST, VST3 plug-in formats.
In Conclusion
There you have it. Leq-m is an averaging loudness measurement system, using 850ms windows with a CCIR weighting filter to measure the loudness of cinema adverts and trailers. Note that there is no loudness spec for the main feature film. Once you have a calibrated room to 85dB c weighted and you set the Dolby RMU Dolby 7, its completely up to you.
If you want to learn more about loudness and cinema sound then check out these articles…