## 1 Introduction  
Contemporary technologies are broadly employing composite plate materials developed by using various innovative materials (nanostructures, mica structures, etc.) [1]. Ultrasonic methods enable research of key properties of materials which cannot be assessed by merely using the traditional methods. The guided ultrasonic waves are extensively used for non-destructive testing and evaluation (NDT) of sheet type materials [2; 3]. They are commonly excited when the vibrations of an acoustic transducer are transferred to the investigated item by a direct contact via a narrow gap of liquid. However, liquids are impossible to use in a wide range of cases as the item under investigation may be contaminated, damaged, or otherwise negatively affected. This is relevant to the research of paper, cardboard, membranes and composite structures which are used across a wide variety of fields including aerospace industry [4; 5; 6; 7; 8; 9]. In this case the guided waves in sheet type objects are excited and picked-up via air gap using contactless air-coupled ultrasonic transducers. Usually for this purpose the lowest anti-symmetrical A\({}_{0}\) mode is exploited [10; 11; 12]. This mode is dispersive, e.g., its phase velocity is frequency dependant and is reducing with a reducing frequency [12; 13; 14]. In order to achieve efficient excitation and reception of such mode the air-coupled ultrasonic transducers are deflected according to the [PERSON]'s law. This mode is commonly applied for detecting of defects in composite materials as the leakage is extremely prominent at the defective site [14; 15; 16; 17; 18]. Attenuation of acoustic waves in air and in the investigated structure increases with the growth of the frequency; thus, in real life the frequency of ultrasonic waves is selected to be as low as possible. Usually, it does not exceed 1 MHz. The losses of ultrasonic non-destructive investigations may be reduced by generating the guided waves while using the lower frequencies (\(<\)100 kHz) [19; 20]. With decrease of a frequency the sub-sonic or slow anti-symmetric Lamb A\({}_{0}\) mode may be excited in which the phase velocity becomes lower than the velocity of sound in air. In this case, the slow guided waves propagating in the structure under investigation do not excite the leaky waves and may travel longer distances without losses [21; 22]. However, application of conventional methods for excitation of the slow guided waves based on exploitation of air-coupled transducers deflected according to the [PERSON]'s law is not efficient. Due to those reasons at low frequencies for excitation of the guided waves via air gap it is sensible to employ ultrasonic low frequency arrays [23; 24; 25]. Propagation of the slow A\({}_{0}\) mode within thin films is accompanied by evanescent waves in surrounding air whose amplitude decreases with the increase of the distance from the investigated films. This phenomenon can be employed for a non-contact reception of the A\({}_{0}\) mode applying specially designed air-coupled ultrasonic arrays. At the moment there are not many publications in which excitation and reception of a slow A\({}_{0}\) mode is discussed. Various techniques used for such purposes were proposed and investigated by us [26; 27]. For this purpose ultrasonic air-coupled arrays with high efficiency piezoelectric elements made of PMN-32%PT crystals were developed [26]. The elastic properties and/or thickness of thin plates and films can be estimated from the measured velocity of ultrasonic guided waves propagating in them. However, in the most of the published research the information about an internal structure of the object under investigation is obtained from ultrasonic signals which propagate some distance from the ultrasonic transmitter to the ultrasonic receiver. This means that in such cases only average values of the ultrasound velocity and, consequently, the elastic properties of the object can be determined and any information about spatial variations of those properties is lost.