Studies indicating that BS.1770 is inaccurate at very low frequencies
Another weakness of BS.1770 is that, unlike the CBS loudness controller and meter as implemented in Orban products, the BS.1770 algorithm does not take into account the loudness contributed by the LFE channel, for good reason. Nacross and Lavoie17 tried to extend the BS.1770 algorithm to include the LFE channel by summing the K-weighted LFE channel’s power into the current BS.1770 algorithm, where the gain is weighted for the fact that LFE channel receives a 10dB gain boost on playback, per Dolby’s standards.
This modified BS.1770 algorithm failed to agree with the judgments of a subjective listening panel unless a 10dB attenuation “fudge factor” was applied to the LFE channel prior to its power summation with the other channels. Nacross and Lavoie concluded:
A problem exists, however, should ITU-R BS.1770 be modified to simply include an attenuated version of the LFE channel. Because the LFE channel receives a 10dB boost on playback, the low-frequencies on this channel would contribute differently to a loudness measure if they were moved to one of the other main channels, even though the perceived loudness would not appreciably change. This suggests that while LFE content does contribute to the perceived loudness, Equation (2)18 does not sufficiently predict how that content should be included.
An Australian study may shed light on the failure of BS.1770 when program material contains considerable energy at very low frequencies.19 The authors used octave-band noise in subjective listening tests with the goal of verifying the K-weighting curve used in BS.1770. The authors state:
Comparison of the test results with an image of the filter curve currently specified in ITU-R Recommendation BS.1770 shows good agreement at 250Hz and above 500Hz, reasonable agreement at 500Hz, but marked difference in the bottom two octaves.
The relatively good performance of the BS.1770 algorithm in ITU trials suggests that, in partial loudness terms, there was probably not much test content in the 125Hz band or below. While the existing BS.1770 filter curve is probably a good choice in applications where the program is dominated by speech, and it is certainly an improvement on the A and B curves in that application, it is likely to give significant errors in measuring the loudness of other programs with more partial loudness in the lower frequencies, such as movie soundtracks and popular music. It is, therefore, desirable to improve on this filter for more general measurement of program loudness.