# Sound Effects on Standing Postural Strategies in the Elderly via Frequency Analysis Approach
## 1 Introduction  
The standing position is the fundamental posture for humans to perform daily activities and requires a certain level of postural control for balance maintenance. Aging degrades sensory functions or declines the ability of sensory integrations, which further induces losing balance and falling in the elderly. Postural control involves the visual, vestibular, and somatosensory systems [1], and there could be reweighting between each system depending on external stimulations [2]. Auditory noise sound has been documented to improve postural control [3, 4], and act as additional information to postural regulation [5]. In older adults, white noise sound reduces postural sway [6], and sound frequency has been documented to positively affect posture control [4]. Specifically, a high-frequency band, such as 1000 or 4000 Hz, could improve postural control and significantly decrease the swayarea during extreme loudness [4]. [PERSON] and [PERSON] [7] also reported that high frequency sounds affected postural regulation in the anterior-posterior direction. The intensity and frequency of sounds influencing postural regulation of stability is associated with the relationship between the vestibular system and organs of [PERSON] in the inner ear [8]. Vestibular modulation by hearing aids has been documented to improve postural control in individuals with hearing loss [9]. However, changes in postural regulations by vestibular interference induced by sound effects are not evident in the elderly. For revealing postural regulations, one approach is to evaluate the spectral analysis of body sway, which has been employed in young, older adults, and individuals with neurological impairments [10; 11; 12; 13; 14]. Frequency domain analysis provides corresponding postural strategies in controlling body sway. Center of pressure (COP) excursions are transformed into the power spectrum and divided into low-, medium-, and high-frequency bands [10; 11; 13]. Low-frequency is associated with visual regulation, medium-frequency with vestibular and somatosensory regulation, and high-frequency with somatosensory regulation [12; 13]. Redistribution of the magnitudes between low- and medium-frequency bands in the COP medial-lateral direction was observed in the absence of vision [12; 13]. In addition, older adults also showed an increase in high-frequency band in COP anterior-posterior and medial-lateral directions [14]. The outcomes of these studies confirmed that the spectral analysis approach distinguished strategies of postural control. Hence, the spectral analysis approach might be useful to identify the effects of sound on changes in postural regulations. Although older adults have shown less efficient postural control [15], previous studies of postural control generally focused on body sway in quiet standing [16; 17], and instant limb movements, such as pushing tasks [18; 19] or gait initiation [20; 21]. These instant limb movements have been considered as a dual-task perturbation to standing balance and significantly affected changes in COP displacements [18; 19; 21]. However, when performing daily activities, most hand tasks are continuous movements and require visual attention. This kind of voluntary movement as dual-tasking to postural control might superimpose the effects of perturbations and further affect postural regulations, particularly sensory degenerations in the elderly. However, the reweighting postural regulations might not be revealed by general COP displacement evaluations, thus an alternative approach is necessary, such as spectral analysis. Therefore, the objective of the current study was to investigate the effects of sound on postural strategy during a dual-task of continuous hand movement through spectral analysis of COP displacement. The experimental paradigm involved silence and sounds of 50 dB with three different sound frequencies, while performing a hand task using the Purdue Pegboard test.