Modify the gain after filtering to prevent TTS clipping in en-US.

#define FILTER_LOWSHELF_ATTENUATION -18.0f // in dB

#define FILTER_TRANSITION_FREQ 1100.0f     // in Hz

#define FILTER_SHELF_SLOPE 1.0f            // Q

#define FILTER_GAIN 6.0f // linear gain

#define FILTER_GAIN 5.5f // linear gain

// such a huge gain is justified by how much energy in the low frequencies is "wasted" at the output

// of the synthesis. The low shelving filter removes it, leaving room for amplification.