Subscripts
APG	Amplified piezoelectric generator
BB	Buckled beam
BR	Bistable resonator
GB	Guide beam
EH	Energy harvester
HA	Hydraulic amplifier
HC	Hydraulic cylinder
HFT	High frequency transducer
HV	Hydraulic valve
LFA	Low frequency absorber
PFF	Piezoelectric force factor
Roman letters
a [m]	HV bending lever arm
$a_h$ [-]	Hydraulic amplification level
$Cf$ [Pa·s2·m−6]	Pressure loss coefficient
$C{f}_0$ [Pa·s2·m−6]	Opened HV pressure loss coefficient
$C{f}_c$ [Pa·s2·m−6]	Closed HV pressure loss coefficient
$C_p$ [F]	APG internal capacity
D [m]	Diameter of a random tube
$D_h$ [m]	Hydraulic diameter of the HV
$D_{hc}$ [m]	Hydraulic cylinder internal diameter
$D_r$ [m]	HV rigid sheath diameter
$D_t$ [m]	Kapton tube initial diameter
$E_d$ [J]	Global dissipated energy
$E_e$ [J]	Harvested electric energy
$E_{hyd}$ [J]	Available hydraulic energy from earplug
$F_{br}$ [N]	BR counter reaction force
$f_0$ [Hz]	BR natural resonance frequency
$f_d$ [-]	Dry friction loss coefficient
K [N/m]	APG stiffness
$K_{bb}$ [N/m]	8 BB’s stiffness
$K_{eq}$ [N/m]	Equivalent stiffness seen from the APG
$K_{gb}$ [N/m]	GB stiffness
$K_{HV}$ [N/m/rad]	HV stiffness
$K_s$ [N/m]	Spring stiffness
$K_{\phi }$ [N/m/rad]	Single hinge stiffness
$k^2$ [-]	APG electromechanical coupling coefficient
$k_{sys}^2$ [-]	Global electromechanical coupling coefficient
L [m]	Distance between 2 BR adjacent hinges on $\overrightarrow{x}$ axis
$l_0$ [m]	Distance between 2 BR adjacent hinges
m [kg]	BR dynamic mass
$p_c$ [Pa]	Comfort pressure
$p_{hc}$ [Pa]	Hydraulic cylinder internal pressure
$P_e$ [W]	Harvested electric power
q [m3/s]	Flow rate
$q_{ear}$ [m3/s]	Flow rate of the fluid exiting the earplug
Q [-]	BR quality factor
$r_{Cf}$ [-]	Hydraulic restriction coefficient
$R_l$ [Ω]	Load resistance
$Re$ [−]	Reynolds number
$S_{hc}$ [Pa]	Hydraulic cylinder internal section
$x_m$ [m]	BR mass position
$x_{hc}$ [m]	HC piston position
$x_m$ [m]	BR mass position
$x_0$ [m]	BR buckling height
$U_p$ [V]	APG tension
Greek letters
$\alpha$ [N/V]	Piezoelectric force coefficient
$\Delta p_{ear}$ [Pa]	Earplug pressure variation
$\Delta V_{ear}$ [m3]	Earplug volume variation
$ϵ$ [-]	BR buckling level
${\eta }_{br}$ [-]	BR energy conversion efficiency
${\eta }_g$ [-]	EH energy conversion efficiency
$\theta$ [rad]	HV bending angle
${\mu }_{air}$ [N·s/m]	Air dynamic viscosity
${\mu }_w$ [N·s/m]	Water dynamic viscosity
${\mu }_{hc}$ [N·s/m]	HC viscous loss coefficient
$\rho$ [kg/m3]	Water density
$\phi$ [rad]	BR hinge rotational angle
${\omega }_0$ [rad/s]	BR natural angular frequency
Indexes
${\square }_a$	Active hydraulic branch
${\square }_b$	Bottom side hydraulic branch ( $x<0$ )
${\square }_c$	Closed HV state
${\square }_i$	Inactive hydraulic branch ( $x>0$ )
${\square }_o$	Opened HV state
${\square }_t$	Top side hydraulic branch
${\square }_{max}$	Upper value
${\square }_{min}$	Lowest value