## 1 Introduction  
Acoustic resonance testing (ART), also referred to as resonant or resonance inspection (RI), for example, is a nondestructive inspection method, see References [1; 2; 3; 4; 5; 6; 7]. It evaluates eigenfrequencies or other characteristic values derived from the measured natural vibration behavior of the parts to be inspected. ART is mainly used for 100% in-line inspection of serial parts that are manufactured in large quantities and with short cycle times. Some benefits to this method include the ability to test the entire component volume within seconds, moderate costs for test hardware, and good automation capability in contrast to other nondestructive testing (NDT) methods. Therefore, in contrast to image producing NDT methods like thermographic testing or scanning ultrasound methods, ART is a particularly effective and fast screening method. In addition, ART is an environmentally friendly procedure as no consumables are required. The method is especially suitable for metallic, for example, forged or cast, but also ceramic components. ART can be used to examine parts for various properties or quality deviations. Applications include testing for structural (surface as well as volume) defects like cracks or voids as well as analyses with regard to material mix-ups or microstructural irregularities. Multiple specific ART applications or research can be found in the literature, even from the last decade. This shows the method's wide application spectrum and its relevance. For example, there are works focusing on metallic automotive components and the detection of cracked parts with the help of intelligent classification methods [8; 9]. For defect detection in metallic serial components, ART can be combined with further measured component information and used within an allover good-bad distinction [10]. Other works use ART for the characterization of ceramic tiles according to (mechanical) properties and processing parameters [11] or deal with the evaluation of metallic samples from a selective laser melting process by analyzing their vibration behavior [12]. ART was also used to investigate cast iron test samples with the aim to collect information about microstructural variations and to identify, for example, anomalies in graphite shape. Identification of heat treatment or other parameters from sample processing could also be achieved [13]. There are also works regarding other material classes like eigenfrequency analysis for damage detection within structures made out of composite materials [14; 15]. Furthermore, ART was used for analyzing additive manufactured lattice structures [16] or for detection of glass bottles with defects [17]. There are also many works that deal with ART-like approaches for agricultural products such as fruits or eggs [18]. ART exploits the fact that the natural vibration behavior of a (free) body depends on its geometry, elastic constants, and density (since these are temperature-dependent, the natural vibration behavior is also a function of temperature). However, it is also influenced by various structural anomalies [19; 20]. Thus, the eigenfrequencies of a component reflect both the mechanical properties and macroscopic defects such as cracks. The aim of ART is therefore either to identify defect components using measured eigenfrequencies or other acoustic signal characteristics or to classify parts according to quality-related mechanical properties, respectively. The procedure of a series-integrated ART application starts with the defined and reproducible active mechanical excitation of the test object. Therefore, the test object is usually positioned and supported at only a few points such that it can vibrate as freely as possible. Despite this, other boundary conditions are possible for ART measurements or ART-like research investigations [12]. Excitation is accomplished either by a mechanical pulse of a small automatable hammer or by a continuous vibration signal, for example, via one of the component's supports, which is swept in frequency.