## 1 Introduction  
An acoustic wave is the propagation of strain in a condensed matter, and its propagation speed is determined by the group velocity of low-energy phonons; that is, the speed of sound. Time-resolved measurements of acoustic waves based on short-pulsed lasers, a time-resolved measurement of acoustic waves, often called \"picosecond acoustics\" or \"picosecond ultrasonics\", have been employed to investigate thickness information [1; 2; 3; 4; 5], elastic properties [6; 7; 8; 9; 10], phonon properties [11; 12; 13; 14; 15], and interfacial quality [16; 17; 18; 19]. For example, a time delay between acoustic echoes can be used to accurately determine the thickness of a metal layer on top of an insulating substrate [1; 2; 3; 4; 5]. In addition, the interfacial quality between two layers has been investigated from the perspectives of the amplitude and attenuation of acoustic waves [16; 17; 18; 19]. To obtain a strong acoustic signal, a large acoustic mismatch is required. However, many condensed solid phases have a similar acoustic impedance, and acoustic oscillations are often suppressed. Inserting a mechanically soft layer with a small acoustic impedance between two layers can enhance acoustic oscillations, but a soft layer with a thickness of a few nanometers is prone to fabrication damage. A thin, soft layer at the interface affects not only acoustic propagation but also heat transport. A relatively well-studied system is the strongly coupled interface between two hard materials, such as metals and oxides; thermal conductances at interfaces have been explained by phonon scattering at interfaces and the phonon spectrum of constituent materials [20; 21; 22; 23]. A weakly coupled interface has been achieved by inserting a thin polymer or two-dimensional material at the interface [19; 24]. It has been reported that the thermal conductance of the weakly coupled interface strongly depends on the stiffness of interfacial bonds [24; 25]. However, the intrinsic nature of interfacial bonds may depend on the preparation method for the insertion layer [26] and the deposition process for the transducer layer. In this work, we use the Co\({}_{50}\)B\({}_{50}\) (atomic %) alloy as a transducer to detect the propagation of acoustic waves and thermal energy across an interface. Boron (B) is a glass-forming element, and alloying it with a ferromagnetic metal, such as Co, Fe, and Ni, results in an amorphous structure. The amorphous ferromagnet shows improved magnetic properties, such as higher magnetic susceptibility, smaller coercivity, and a smaller eddy current [27]. In addition, amorphous metals can have improved mechanical properties, such as high tensile yield strengths, high elastic strain limits, and a smooth interface with an underlying oxide [28; 29]. In this work, we demonstrate that the Co\({}_{50}\)B\({}_{50}\) transducer can drastically enhance acoustic waves compared to the single element of Co. The strong acoustic oscillation results from a soft intermediate layer between Co\({}_{50}\)B\({}_{50}\) and the substrate. By analyzing the frequency and damping of acoustic oscillations using acoustic modeling, we determine the thickness of the intermediate layer to be >2 \(\pm\) 1 A. We expect that the soft intermediate layer is preserved by low kinetic damage during the Co\({}_{50}\)B\({}_{50}\) deposition. We also investigate the heat transport at the interface by measuring the temperature response of the Co\({}_{50}\)B\({}_{50}\) transducer. Owing to its magnetic property, we were able to collect the temperature response of the transducer without contamination by any signal from the substrate. By analyzing the temperature response using thermal modeling, we determine the interfacial thermal conductance of 35 \(\pm\) 5 MW m\({}^{-2}\) K\({}^{-1}\), which is one order of magnitude smaller than that of the strongly coupled interface. This low thermal conductance supports the existence of the thin, soft layer at the interface.