What is Loudness Correction?
Human hearing is not as sensitive in the higher and lower frequency ranges as it is in the mid-range. What this means in practice is that when not listening to a recorded program – let it be music, movies or TV shows – at the volume the program was recorded at or intended to be played at, the treble and base seem to drop off quicker than the midrange as volume is decreased. This actually upsets the tonal characteristic of the program material – even though you may have a completely flat frequency response for your speakers.
Here you were thinking you had it all sorted once you applied EQ to your system, hey? Not so fast… Now you actually know why a flat frequency response for a system is only actually perceived flat if it is playing at the right volume… unless of course we apply loudness correction.
I won’t go into the biological reasons for this or the estimations of how human hearing responds to changes in sound pressure. If you want to know more about the technical ins and outs of Loudness Correction, do a search for any of the following terms on the Internet:
- Fletcher-Munson equal loudness contours
- Robinson-Dadson curves
- Normal Equal-Loudness Level Contours, ISO 226:1987
Loudness correction is not something new, it has been around in stereo systems for the last 30+ years. In fact, I have a loudness control on my 12-year-old stereo system in my car. However, auto-loudness correction is relatively new in the field of Home Cinema – first introduced by Denon and Marantz and other receiver manufacturers about 8 years ago through the inclusion of Audyssey’s Dynamic Equaliser, which is still the most sophisticated to date. Since then, THX, Dolby and more recently Yamaha have come up with their own version of it.
When it comes to movies, reference level for all channels – except the subwoofer channel – is calibrated by adjusting the playback system such that a pink noise signal recorded at -20dB relative to full scale (0dB) creates 85dB sound pressure level as measured with a C weighted SPL meter at the seating locations. Volume levels are adjusted for each channel individually until they reach 85dB.
The premise is valid: when listening to movies below the reference level (85dB), the tonal characteristic of the movie is changed. However, since loudness correction is not an exact science – human hearing is difficult to measure precisely – the different systems implement it slightly differently.
Yamaha YPAO Volume
I took some measurements with regards to Yamaha’s YPAO Volume to see how much loudness correction is applied to the high (above 6.5Kz) and low end (between 20Hz and 400Hz). Results are below:
- volume at -40dB : 4dB added both high and low end
- volume at -35dB: 3dB
- volume at -30dB: 2dB
- volume at -25dB: 1dB
- volume at -22dB: 0dB – originally measured this at -22dB, but actually this should be -20dB as I am looking at the graphs now.
Since I don’t have a unit that has YPAO volume – Yamaha sent me a unit (RX- A3060) to test for this – I wanted to know if I could re-create the effect using simply the base and treble controls on my unit. Below are the results:
As you can see from the graph, applying 4dB to both treble and bass at -40dB, the results very closely match the low end, but the high end is not as closely aligned between 3KHz and 8Khz. Let’s have a look at higher volume levels:
At -30dB, adding 2dB to both the high and low end, the effect is decreased, but also the errors. Since I actually listen between -30dB and -25dB normally, I have added -1.5dB to both bass and treble permanently on my older a3020 equivalent receiver.
How does it sound in practice? I have to be honest, Yamaha’s new RX-A3060 sounds noticeably clearer than my RX-A3020, even when matching the loudness curve using bass and treble controls as closely as possible and setting them up the same. (Please note that for the graphs above, the subwoofer EQ was turned off and the Yamaha was left to its own devices when it came to EQ.)
Now was it due to YPAO Volume, which works really well when listening to both movies and music at lower volumes, or was it the difference in DACs, the new 64bit YPAO or other component changes between the units? It is hard to know, but likely a combination of all the above. In the same room, with the same speakers, the difference in clarity was noticeable.