Hornmanufaktur has been designing rather unconventional speakers for years, ignoring prevalent standards and doubtful contemporary tendencies, focusing on the human ear instead.
The diagram shows lines of equal volume. The reference values are the volumes (dB) of a 1000 Hz tone, with the lowest perceivable pressure of 20 µPa. Relating to each of these values, test persons had to settle the actual volume levels which were perceived as equally loud. A linear frequency responce may be useful in studio monitors. For the actual listening at home, however, other parameters than linearity are decisive as the diagram shows: Speed, accuracy and fidelity are necessary to ensure natural reproduction.