Investigating the relationship between pressure force and acoustic waveform in footstep sounds Francesco Grani, Stefania Serafin, Amalia de G¨ otzen, Dan Overholt Aalborg University Copenhagen Email: (fg, sts, ago, dano)@create.aau.dk Spencer Topel Dartmouth College Email: topel@dartmouth.edu Abstract—In this paper we present an inquiry into of the relationships between audio waveforms and ground reaction force in recorded footstep sounds. In an anechoic room, we recorded several footstep sounds produced while walking on creaking wood and gravel. The recordings were performed by using a pair of sandals embedded with six pressure sensors each. Investigations of the relationships between recorded force and footstep sounds is presented, together with several possible applications of the system. Index Terms—footstep sounds, ground reaction force, acoustic waveform. I. I NTRODUCTION Footstep signals present applications in different domains, ranging from entertainment to medicine and rehabilitation. In the context of audio-visual production, Chion writes of foot- step sounds as being rich in what he refers to as materializing sound indices – those features that can lend concreteness and materiality to what is on-screen, or contrarily, make it seem abstracted and unreal [1]. The recognition of footstep sounds has been applied to smart home environments, to track people presence using a semi-automatic capture system [9]. In the field of medicine, footstep sounds have facilitated the detection of different gait deficiencies and the consequent rehabilitation process [11], [2], [12]. In previous work, we used an envelope follower to extract the amplitude envelope from recorded footstep sounds. Such envelope was used as the overall amplitude value for syn- thetically generated footstep sounds created using physically informed sound synthesis models [10], [6]. While this technique was free of significant delay and afforded natural interaction, it did not allow a high degree of control of the synthesized sounds. This implied that sounds were controlled only by varying their amplitude envelope, without possibilities to interact directly with the physical parameters. To our knowledge, the relationship between ground reaction force and amplitude envelope in footstep sounds has not been investigated yet. In [3] a mapping of force to sound level was chosen, in order to perform the translation of gait to sound patterns, where forces were measured using a force plate. A similar assumption was made in [10]. In this previous paper the estimated amplitude envelope represented the ground reaction force corresponding footsteps. We posit that access to this information would improve the design of simpler footstep Fig. 1. The recording setup used for the studies presented in this paper. A person is walking on different surfaces in an anechoic room, and the produced sound is recorded together with sensors data captured from insole pressure sensors. tracking systems, which do not require a high number of pressure sensors. II. TECHNICAL SETUP Figure 1 shows the technical setup used in this study. In a fully isolated anechoic chamber, we used a microphone (Neumann U87) to record the sound produced by a person walking on a solid wooden surface and an aggregate surface, in this case a box full of peoples as shown in Figure 1. Inside the shoes, an insole with several pressure sensors captured six independent channels of sensor data for each foot. We interpreted the sensor data as Ground Reaction Force (GRF). The GRF can is described as the force exerted by the ground on a body in contact with it [5]. Through an analysis of the GRF [4], we chose pressure sensors that transduce varying contact forces between the ground and different feet DSP2013 978-1-4673-5807-1/13/$31.00 ©2013 IEEE