# Sound Synthesis of Objects Swinging through Air Using Physical Models +
## 2 Background and Related Work  
Sound synthesis techniques can be split into two broad approaches, signal-based and physical models [2]. Signal-based models aim to replicate the sound properties; matching frequency components, replicating the time envelope or similar. Physical models aim to replicate the processes behind the natural sound creation by mathematical models. The advantage of a signal-based model is that it is relatively computationally inexpensive to replicate the spectrum of a sound using established techniques such as additive synthesis or noise shaping. A drawback of this approach is that it is rarely possible to relate changes in signal properties to the physical processes creating the sound. For example, an increase in speed of a sword not only changes the fundamental tone frequency, but also the gain. Therefore, changing one signal-based property could lose realism in another. Physical models aim to replicate the physics behind the sound generation process. Sounds generated by these models have the advantage of possessing greater authenticity in the generated sounds, especially in relation to parameter adjustments. A potential drawback is that the computational cost required to produce sounds is often high, and the physical models typically cannot adapt quickly to parameter adjustments, making real-time operation challenging and often not possible. In the middle of these traditional techniques lay physically-inspired models. These hybrid approaches replicate the signal produced, but add characteristics of the physics that are behind the sound creation. For a simple sword model, this might be noise shaping with a bandpass filter with centre frequency proportional to the speed of the swing. A variety of examples of physically-inspired models was given in [3]; the model for whistling wires being exactly the bandpass filter mentioned. Four different sword models were evaluated in [4]. Here, the application was for interactive gaming, and the evaluation was focused on perception and preference rather than accuracy of sound. The user was able to interact with the sound effect through the use of a Wii Controller. One model was a band-filtered noise signal with the centre frequency proportional to the acceleration of the controller. A physically-inspired model replicated the dominant frequency modes extracted from a recording of a bamboo stick swung through the air. The amplitude of the modes was mapped to the real-time acceleration data. The other synthesis methods in [4] both mapped acceleration data from the Wii Controller to different parameters; one using the data to threshold between two audio samples, the other a granular synthesis method mapping acceleration to the playback speed of grains. Tests revealed that the granular synthesis was the preferred method for expression and perception. One possible reason that the physical model was less popular could be the lack of correlation between speed and frequency pitch, which the band-filtered noise had. This may also be present in the granular model. A signal-based approach to a variety of environmental sound effects, including sword whoosh, waves and wind sounds, was undertaken in [2]. Analysis and synthesis occur in the frequency domain using a sub-band method to produce narrow band coloured noise. In [5], a rapier sword sound was replicated, but this focused on the impact rather than the suoosh when swung through the air. A physical model of sword sounds was explored in [6]. Here, offline sound textures were generated based on the physical dimensions of the sword. The sound textures were then played back with speed proportional to the movement. The sound textures were generated using computational fluid dynamics software (CFD), solving the Navier-Stokes equations and used [PERSON]'s acoustic analogy [7] extended by [PERSON]'s method [8]. In this model [6], the sword was split into a number of compact sound sources (discussed in Section 3.2), spaced along the length of the sword.