Running ACF measurement for designing a concert hall

A concert hall, a listening room, car audio, and an outdoor space should be designed corresponding to music to be performed. It is found that some kinds of the optimal acoustic parameters are strongly affected by te (effective duration of the autocorrelation function) of the music. Running ACF analysis is performed to estimate te

Music and sound field

(1) The music suitable for the room is comfortable

Lecture room and a conference room should be designed for speech, and an opera house should be designed for vocal music. There cannot be a room suitable for all kinds of music.

For example...

  • A bathroom is suitable for singing, but not suitable for a quick conversation
  • The music of fast tempos, such as rock, is comfortable with little reverberation.
  • Orchestra music needs much reverberation.

    • (2) The stereo is more comfortable than monaural.
    Stereo system is preferred than monaural system, because it reproduces information about direction and other spatial impressions.

    (3)Players change their performance style according to the space and music type.
    They are going to compensate sound in the room with a performance style, and are going to give listeners comfortable sound.

    Optimal acoustical parameters from te (effective duration of the autocorrelation)

    1. The value of te is obtained at the ten percentile (-10 dB) delay of the envelope of the autocorrelation function. The –60 dB delay time of the autocorrelation envelope corresponds roughly to the "reverberation time" contained in the source signal itself, which is given by 6*te.

    2. Preferred reverberation time is estimated as 23*te, that is about four times the "reverberation time" contained in the source signal itself.

    3. Preferred delay time of the first reflection, D‚s1, is estimated from te and A-value (Total amplitude of the reflective sound), as the following equation.

    As for single reflection

    Individual difference in subjective preference

    As a musical preference may be different, there is individual difference also in a preference for the acoustic parameter. Individual difference is found mainly for the temporal factor such as the reverberation time (Tsub) and the delay time of the first reflection (Dt1). For the spatial factor such as IACC, there is not such a large individual difference. Most people prefer a sound field with low IACC (i.e. more diffused sound field). Individual difference in the listening level is caused by the difference in our hearing ability.

    When we are aged and our hearing ability decreases, we tend to like loud and slow tempo music.

    Summary

    1. Reverberation time of a room should be as about four times of the reverberation time of music itself. Reverberation time of the music signal is estimated as 6*te.

    2. Preferred delay time of the first reflection is almost the same value of te.

    3. The arrival direction of the initial reflective sound that minimizes IACC is around 55 degrees for 1kHz and 36 degrees for 2kHz.

    4. Optimal listening level is around 77-79 dBA for slow tempo music, and 79-90 dBA for fast tempo music.

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