## 1 Introduction
Hence, these technologies have considerable potential to provide sensing information at home or in more controlled environments, such as residential homes and care facilities. Two examples of available ambient-assisted living sensor technologies involve the deployment of cameras and infrared sensors. The former requires a hard-wired power source, a high-bandwidth network connection, and a computer for the processing. Also, optical monitoring raises privacy issues. Furthermore, it is challenging to monitor the whole house with cameras [24]. Typically, infrared sensing generates data that is difficult to interpret, resulting in a higher occurrence of false alarms [25]. Remote sensing of vibrations and sounds, especially if processed appropriately, is unlikely to cause the same privacy issues as collecting images or videos. Typical hardware includes wireless sensor networks coupled with data analysis systems and distributed motion, pressure, vibration, and sound sensors [26]. An additional advantage of vibration and sound sensing systems is that they are cheaper than those incorporating more expensive modalities such as infrared and microwave [27]. However, sensing either vibrations or sounds alone can lead to inaccuracy because of the wide variety of vibrations and sounds around a domestic home throughout the day [28; 29; 30]. Since important events such as falls are associated with a particular combination of vibrations and sounds, identifying such combinations using a combined vibroacoustic approach would aid in the accurate identification and discrimination of these critical events from other events. Remote vibroacoustic sensors offer a way to address most of the issues associated with other remote sensing technologies, namely, discomfort (with wearable devices) and ethical challenges such as privacy, autonomy, dignity, safety, and trust [31]. However, it remains critical to ensure that ambient sensing technologies are acceptable to older adults, deliver the intended benefits, and are applicable to any environmental setting. In the context of working towards a future where the elderly, carers, and key professionals (such as organisations supporting carers) are more informed about and encouraged to use digital monitoring technologies, this review considers the following two research questions:  
RQ1: What are the currently available non-intrusive technologies and their acceptability when utilised in monitoring older adults' activities and health conditions in the United Kingdom? The primary aim of this first research question is to identify what non-intrusive technologies are currently available to monitor older adults' activities and health conditions in the United Kingdom. A secondary aim is to identify the extent to which these technologies are considered acceptable and useful by older adults or/and their carers and the extent of any increased safety and security felt by the users or/and their carers. RQ2: To what extent are remotely monitored sounds and vibrations used simultaneously in assisted living or smart homes? The primary aim of this research question is to assess the extent to which non-intrusive monitoring devices use sound and vibration technologies and their associated advantages and disadvantages. A secondary aim is to identify the extent of research both into the acceptability and into the simultaneous use of sounds and vibrations in remote monitoring of assisted living or smart homes.