Why does the loudness FIR filter of the DSP in the Eversolo DMP-A8 look so strange?
It appears that the Eversolo loudness FIR filter mimics the equal-loudness contour of the human hearing system. However, this is not something that should be directly “corrected.” What actually requires correction are the
differences between loudness contour curves—for example, when a music recording is mixed at 80 dB SPL and later listened to at home at 60 dB SPL. In this case, the difference between the 80-phon and 60-phon equal-loudness contours defines the appropriate shape of a usable FIR filter (Toole et al., 2026).
According to the ISO 226:2003 equal-loudness contours, curves from 20 to 80 phons are very similar in shape above approximately 500 Hz, differing mainly by a constant offset in phon level. This implies that, above 500 Hz, a change of 10 phons corresponds to roughly a 10 dB change in sound pressure level. Consequently, no correction is required at frequencies above 500 Hz to preserve tonal balance (Toole et al., 2026).
In contrast, below 500 Hz the equal-loudness contours converge in an SPL-versus-frequency plot. As a result, when listening at home at a lower SPL than the level used during mixing in a control room, the bass frequencies below 500 Hz must be boosted to maintain perceived loudness balance (Toole et al., 2026).
Therefore, it would be highly desirable to have access to the FIR parameters of the Eversolo DMP-A8 loudness function, such that its effect automatically decreases as playback volume increases, transitioning smoothly from low to higher listening levels.
Furthermore, as suggested by Toole et al. (2026), easily adjustable bass, treble, and tilt controls—implemented via virtual knobs or sliders—would be highly recommended.
Any thoughts on this?
Reference
Toole, F. E., Olive, S., & Welti, T. (2026).
Sound Reproduction: The Acoustics and Psychoacoustics of Loudspeakers, Rooms and Headphones (4th ed.). Routledge, Taylor & Francis Group, New York and London.
