A few days ago, I had a twitter chat with my dear friend, ソノベ-san about an interesting auditory phenomenon called the missing 6 dB effect. Basically, it is a perceived loudness level deviation occurring in the lower frequency region, when loudspeakers and headphones are directly compared to each other.
At the threshold of audibility, this phenomenon is considered fully debunked by Killion (1978): there is no difference in the minimum audible pressure at the eardrum, once a proper procedure that eliminates the physiological variables has been followed.
Above the threshold of audibility, the effect is also considered resolved by Rudmose (1982): A pre-listener training has been found to be essential for eliminating the deviation. Thus, once a listener adapts to a linear distortion identification process, s/he can overcome the illusion. This also implies non-trained regular listeners are still sensing the loss of bass when listening to headphones.
Many researches including Fastl (1986), Theile (1986), Keidser (2000) Brixen (2001), Martens (2009), and Simon (2009) re-confirm the persistence of the issue. Brixen comes up with a deviation of 5-9 dB, while Keidser's is 8 dB around 500 Hz. In two studies from 2009, a whole-body vibration related value has been found: a 4 dB increase in vibration caused ~1.5 decrease in the frequency equalization below 100 Hz.
In 2011, Völk and Fastl find that the deviation of sound pressure in the ear canal can be effectively eliminated by utilizing a non-individualized binaural synthesis with headphones. With the introduction of a natural group delay in the sub-bass frequency range, there is the missing 6 dB effect no more. In other words, only when the association principles of headphone acoustics & speaker acoustics match, a direct comparison can be realized.
Yet, the studies from 2009 and 2011 contradict each other; they both utilize a non-individual binaural synthesis. However, since the former studies deal with automobile acoustics, of which vibration is much more prevalent in-situ, the latter one, which deals with reference, anechoic, and diffuse field conditions, shall be considered more commonly applicable in practice.
D.R. Begault, "Chellenges to the Successful Implementation of 3-D Sound," Journal of Acoustic Society of America, vol. 39. No. 11 (November 1991).
E.B. Brixen, "Report on Listening Level in Headphones," Document KKDK-068-01-ebb-1 for the Danish Radio, Copenhagen, Denmark (2001).
G. Keidser, R. Katsch, H. Dillon, and F. Grant, "Relative loudness perception of low and high frequency sounds in the open and occluded ear," Journal of Acoustic Society of America, Vol.107, No.6 (February 2000).
G. Simon, S.E. Olive, and T. Welti, “The Effect of Whole-body Vibration on Preferred Bass Equalization in Automotive Audio Systems,” presented at the 127th Audio Eng. Soc. Convention, preprint 7956 (October 2009).
G. Theile, "On the Standardization of the Frequency Response of High-Quality Studio Headphones," Journal of the Audio Engineering Society, Vol.34 No.12 (December 1986).
H. Fastl, "Gibt es den Frequenzgang von Kopfhorern?," in NTG-Fachberichte 7 (VDE-Verlag, Berlin, 1986), pp. 274-281.
M.C. Killon, "Revised estimate of minimum audible pressure:Where is the "missing 6 dB?," Journal of Acoustic Society of America, Vol.63, No.5 (May 1978).
F. Völk, and H. Fastl, "Locating the Missing 6 dB by Loudness Calibration of Binaural Synthesis," presented at the 131st Audio Eng. Soc. Convention, preprint 8488 (October 2011).
W. Martens, W. Woszczyk, H. Sakanashi, and S.E. Olive, “Whole-Body Vibration Associated with Low-Frequency Audio Reproduction Influences Preferred Vibration,” presented at the AES 36th International Conference, Dearborn, Michigan (June 2-4, 2009).
W. Rudmose, "The case of missing 6 dB," Journal of Acoustic Society of America, Vol.71, No.3 (March 1981).