An elastic wave device includes a piezoelectric substrate made of LiNbO.sub.3, interdigital transducer electrodes on the piezoelectric substrate, and a first dielectric film provided on the piezoelectric substrate and the first dielectric film to cover the IDT electrodes and made of a silicon oxide. The IDT electrodes include a first metal film made of one metal selected from Pt, Cu, Mo, Au, W, and Ta. The Euler angles (ϕ, θ, ψ) of the piezoelectric substrate are (0±5°, −90°≤θ≤−70°, 0°±5°). The metal for the first metal film and the thickness hm/λ (%) match any of the combinations as follows: TABLE-US-00001 Metal for the first metal film Thickness hm/λ (%) of the first metal film Pt 6.5 ≤ hm/λ ≤ 25 Cu  13 ≤ hm/λ ≤ 25 Mo 15.5 ≤ hm/λ ≤ 25  Au 6.5 ≤ hm/λ ≤ 25 W 7.5 ≤ hm/λ ≤ 25 Ta .sup.  7 ≤ hm/λ ≤ 25.

An elastic wave device includes a piezoelectric substrate, an IDT electrode on the piezoelectric substrate, and a silicon oxide film arranged on the piezoelectric substrate to cover the IDT electrode. The IDT electrode includes first and second electrode layers laminated on each other, the first electrode layer is made of metal or an alloy with a density higher than a density of metal of the second electrode layer and a density of silicon oxide of the silicon oxide film, the piezoelectric substrate is made of LiNbO.sub.3 and θ is in a range of equal to or greater than about 8° and equal to or less than about 32° with Euler Angles (0°±5°, θ, 0°±10°) of the piezoelectric substrate, and the silicon oxide film contains hydrogen atoms, hydroxyl groups, or silanol groups.

An elastic wave device includes a piezoelectric substrate, an IDT electrode on the piezoelectric substrate, and a silicon oxide film arranged on the piezoelectric substrate to cover the IDT electrode. The IDT electrode includes first and second electrode layers laminated on each other, the first electrode layer is made of metal or an alloy with a density higher than a density of metal of the second electrode layer and a density of silicon oxide of the silicon oxide film, the piezoelectric substrate is made of LiNbO.sub.3 and θ is in a range of equal to or greater than about 8° and equal to or less than about 32° with Euler Angles (0°±5°, θ, 0°±10°) of the piezoelectric substrate, and the silicon oxide film contains hydrogen atoms, hydroxyl groups, or silanol groups.

An acoustic wave device includes first and second IDT electrodes electrically connected in series with each other by a common busbar common to the first and second IDT electrodes. In each of a first acoustic impedance layer and a second acoustic impedance layer, at least one of at least one high acoustic impedance layer and at least one low acoustic impedance layer is a conductive layer. At least a portion of the conductive layer in the first acoustic impedance layer and at least a portion of the conductive layer in the second acoustic impedance layer do not overlap with the common busbar when viewed in plan from a thickness direction of a piezoelectric layer. The conductive layer in the first acoustic impedance layer and the conductive layer in the second acoustic impedance layer are electrically insulated from each other.

An acoustic wave device includes a support substrate including silicon, a piezoelectric layer provided directly or indirectly on the support substrate, and an interdigital transducer (IDT) electrode provided on the piezoelectric layer. When a wavelength defined by an electrode finger pitch of the IDT electrode is λ, a thickness of the piezoelectric layer is about 1λ or less. V.sub.L, which is an acoustic velocity of a longitudinal wave component of a bulk wave propagating through the piezoelectric layer, satisfies Unequal Equation (2) below in relation to an acoustic velocity V.sub.Si-1 determined by Equation (1) below:
V.sub.Si-1=(V.sub.2).sup.1/2 (m/sec)  Equation (1),
V.sub.Si-1≤V.sub.L  Unequal Equation (2), V.sub.2 in Equation (1) is a solution of Equation (3), and
Ax.sup.3+Bx.sup.2+Cx+D=0  Equation (3).

A sensor system that combines the sensing application of surface acoustic wave (SAW) sensor and sensor signal transfer though the enclosure wall via acoustic means. The sensor system includes SAW sensor placed inside the enclosure and at least one pair of bulk acoustic wave (BAW) transducers, one mounted inside and second outside the enclosure wall, allowing the interrogation of SAW sensor from outside the enclosure. The external BAW transducer converts interrogation electrical pulse into acoustic pulse which travels though the enclosure wall to the internal BAW transducer. The internal BAW transducer converts the interrogation electrical pulse to electrical pulse and transfers it to SAW sensor. The response of the SAW transducer containing series of electric pulses is converted to the series of acoustic pulses by internal BAW transducer which propagates though enclosure wall. The external BAW transducer converts the series of acoustic pulses into series of electrical pulses and is received by the interrogation circuit for processing.

A sensor system that combines the sensing application of surface acoustic wave (SAW) sensor and sensor signal transfer though the enclosure wall via acoustic means. The sensor system includes SAW sensor placed inside the enclosure and at least one pair of bulk acoustic wave (BAW) transducers, one mounted inside and second outside the enclosure wall, allowing the interrogation of SAW sensor from outside the enclosure. The external BAW transducer converts interrogation electrical pulse into acoustic pulse which travels though the enclosure wall to the internal BAW transducer. The internal BAW transducer converts the interrogation electrical pulse to electrical pulse and transfers it to SAW sensor. The response of the SAW transducer containing series of electric pulses is converted to the series of acoustic pulses by internal BAW transducer which propagates though enclosure wall. The external BAW transducer converts the series of acoustic pulses into series of electrical pulses and is received by the interrogation circuit for processing.

When a current flowing in a series circuit including an equivalent resistance, an equivalent inductor, and an equivalent capacitance in an electric equivalent circuit of a specific resonator in each filter is defined as an acoustic path current, under conditions that a phase of an acoustic path current of a first transmission filter at a side of a common terminal at a frequency within a first pass band is represented as θ1.sub.Tx1, a phase of an acoustic path current of the first transmission filter at the side of the common terminal at a frequency within a second pass band is represented as θ2.sub.Tx1, a phase of an acoustic path current of a second transmission filter at the side of the common terminal at a frequency within the first pass band is represented as θ1.sub.Tx2, and a phase of an acoustic path current of the second transmission filter at the side of the common terminal at a frequency within the second pass band is represented as θ2.sub.Tx2, a multiplexer satisfies a first condition: |(2.Math.θ1.sub.Tx1−θ2.sub.Tx1)−(2.Math.θ1.sub.Tx2−θ2.sub.Tx2)|=180°±90°, or a second condition: |(2.Math.θ2.sub.Tx1−θ1.sub.Tx1)−(2.Math.θ2.sub.Tx2−θ1.sub.Tx2)|=180°±90°.

When a current flowing in a series circuit including an equivalent resistance, an equivalent inductor, and an equivalent capacitance in an electric equivalent circuit of a specific resonator in each filter is defined as an acoustic path current, under conditions that a phase of an acoustic path current of a first transmission filter at a side of a common terminal at a frequency within a first pass band is represented as θ1.sub.Tx1, a phase of an acoustic path current of the first transmission filter at the side of the common terminal at a frequency within a second pass band is represented as θ2.sub.Tx1, a phase of an acoustic path current of a second transmission filter at the side of the common terminal at a frequency within the first pass band is represented as θ1.sub.Tx2, and a phase of an acoustic path current of the second transmission filter at the side of the common terminal at a frequency within the second pass band is represented as θ2.sub.Tx2, a multiplexer satisfies a first condition: |(2.Math.θ1.sub.Tx1−θ2.sub.Tx1)−(2.Math.θ1.sub.Tx2−θ2.sub.Tx2)|=180°±90°, or a second condition: |(2.Math.θ2.sub.Tx1−θ1.sub.Tx1)−(2.Math.θ2.sub.Tx2−θ1.sub.Tx2)|=180°±90°.

An acoustic wave device includes a support substrate, a piezoelectric layer, and an IDT electrode. The piezoelectric layer is over the support substrate. The IDT electrode is on the piezoelectric layer, and includes a plurality of electrode fingers. An intersecting width of the plurality of electrode fingers is equal to or smaller than about 5λ.