  Acoustic holograms have promising applications in sound-field reconstruction,
particle manipulation, ultrasonic haptics and therapy. This paper reports on
the theoretical, numerical, and experimental investigation of multiplexed
acoustic holograms at both audio and ultrasonic frequencies via a rationally
designed transmission-type acoustic metamaterial. The proposed meta-hologram is
composed of two Fabry-Perot resonant channels per unit cell, which enables the
simultaneous modulation of the transmitted amplitude and phase at two desired
frequencies. In contrast to conventional acoustic metamaterial-based holograms,
the design strategy proposed here, provides a new degree of freedom (frequency)
that can actively tailor holograms that are otherwise completely passive and
hence significantly enhances the information encoded in acoustic metamaterials.
To demonstrate the multiplexed acoustic metamaterial, we first show the
projection of two different high-quality meta-holograms at 14 kHz and 17 kHz,
with the patterns of the letters, N and S. We then demonstrate two-channel
ultrasound focusing and annular beams generation for the incident ultrasonic
frequencies of 35 kHz and 42.5 kHz. These multiplexed acoustic meta-holograms
offer a technical advance to tackle the rising challenges in the fields of
acoustic metamaterials, architectural acoustics, and medical ultrasound.
