According to guidelines for the evaluation of church bell chimes, the timbre of a well-sounding bell is characterized by a soft, full, bright and clear tone with a salient fundamental pitch in a low-noise sound spectrum, where a luminous, clearly pronounced striking tone can be heard with as little percussive noise as possible. In a pilot experiment, 11 bell experts and 26 laypersons evaluated the ringing of 40 loudness-matched bell recordings in terms of tonal quality, pleasantness, salience of the fundamental pitch, clarity and softness of the attack. Via signal analysis, the bell sounds were analyzed in regards to 127 audio features. Correlation and regression analyses were used to determine relationships between the bells' timbre ratings and the calculated audio features. Timbral changes resulting from an optimization of the bell clapper turned out to be particularly noticeable in respect to timbral brightness, the pronouncedness of the tonal components in the spectrum (harmonic energy) and the noise components (spectral flux above 1000 Hz). Specifically, by modifying the clapper, subjective pleasantness of the bells' timbre can be improved. With increasing clapper weight and decreasing ringing angle, the fundamental pitch of bells becomes more pronounced, with stronger harmonic components and less background noise.

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