Reply to “Comment on ‘Acoustical observation of bubble oscillations induced by bubble popping’ ”

We reported on the sound pressure generated by aqueous foam bursts in our paper [Ding et al., Phys. Rev. E 75, 041601 (2007)]. Blanc et al., [Phys. Rev. E 91, 036401 (2015)] found that sound from one of three mechanisms of bubble burst (the prepopping) actually results from an acausal artifact of the signal processing performed by their acquisition system which lies outside of its prescribed working frequency range. We examined the same hardware used in our paper and found that the frequency range is not the cause of the artifact. The prepopping sound was a result from a built-in finite impulse response filter of analog-to-digital converters in the Brüel & Kjær data acquisition system.

Figure 1

The digitalized sine wave signals using the Brüel & Kjær Pulse data acquisition system. The sine waves were generated by a function generator with an amplitude of 100 mV and different frequencies: (a) 5 kHz, (b) 25 kHz, (c) 32 kHz, (d) 34 kHz, (e) calculated 25 kHz, and (f) calculated 32 kHz. The signal above 34 kHz is well cut off, which indicates the antialiasing function works fine. The spectral analysis for all digitalized signals proves the frequency component below 32 kHz is preserved.

Figure 2

A digitalized signal from a hammer impact sound. The inset shows a preringing signal similar to the bubble prepopping sound we falsely assigned in our previous paper.