Vibration audio system, vibration audio output method, and vibration audio program

A vibration audio output method comprising:

• an envelope detection step of, by an envelope detection unit, detecting an envelope signal by obtaining an absolute value of amplitude of an audio signal outputted from a sound source and then integrating the absolute value;
• a frequency conversion step of, by a frequency conversion unit, generating an audio signal frequency-converted on the basis of resonant frequencies by multiplying the envelope signal by sine waves, the resonant frequencies being obtained from an impulse response of a low-frequency output speaker disposed in a vibration transmission member configured to allow a listener to perceive vibration of a low-frequency sound, the sine waves having the same frequencies as the resonant frequencies;
• an audio signal output step of, by the low-frequency output speaker, outputting the audio signal frequency-converted in the frequency conversion step;
• a distortion factor measurement step of, by a distortion factor measurement unit, obtaining a distortion component by removing signal components of the resonant frequencies from signal components of all frequencies of a low-frequency sound, the low-frequency sound being obtained by causing the low-frequency output speaker to output the sine waves having the same frequencies as the resonant frequencies while changing signal levels of the sine waves and then by collecting the sine waves, and measuring a distortion factor of the low-frequency output speaker by calculating a ratio of the signal components of the resonant frequencies to the distortion component in accordance with the changed signal level;
• and
• a dynamic range compression step of, by a dynamic range compression unit, reducing a signal level of the envelope signal for each of the resonant frequencies on the basis of the distortion factor measured in the distortion factor measurement step so that a signal level of a low-frequency sound outputted from the low-frequency output speaker becomes equal to or lower than an upper limit of a signal level that can be reproduced by the low-frequency output speaker, wherein
• in the frequency conversion step, the frequency conversion unit frequency-converts the envelope signal whose signal level has been reduced in the dynamic range compression step.