# Surround by Sound: A Review of Spatial Audio Recording and Reproduction
## 3 Soundfield Recording and Reproduction
### Soundfield Reproduction
#### 3.3.1 Reproduction Methods  
Given multi-channel (or multi-microphone) recordings of an acoustic scene, the _channel-based reproduction_ plays these recordings with certain down/up mixing to replicate the scene. The _object-based reproduction_, on the other hand, is based on the location information of loudspeakers and the objects(or virtual source) in order to determine which loudspeakers are used and their driving signals for playing back the object's audio. The object-based reproduction formed on the panning law can render the audio objects in real time. Two well-known object-based pieces of audio equipment are stereo systems that consist of two channels, left and right, and surround sound systems that consist of multiple channels surrounding the listener. The stereo can provide good imaging (the perceived spatial location of the sound source) in the front quadrant while the surround sound can offer imaging around the listener. Based on the amplitude panning principle [94,95], the audio object can be positioned in an arbitrary 2D or 3D setup using the vector base amplitude panning (VBAP) or distance-based amplitude panning (DBAP). The VBAP is based on a triplet-wise panning law, where three loudspeakers are arranged in a triangle layout to generate sound imaging [96]. The DBAP only takes the positions of the virtual source and loudspeakers into account for reproduction and thus is well suited to irregular loudspeaker layouts [97]. In object-based reproduction, the listener normally is required to be at the centre of the speaker array (i.e., the sweep spot position) where the generated audio effect is best; DBAP, however, is a noticeable exception to this restriction. The first known demonstration of reproducing a soundfield within a given region of space was conducted by [PERSON] at the Illinois Institute of Technology in 1967, where loudspeakers were distributed on the surface enclosing the selected region and the listeners can move freely within the region [100]. Later, the well-known Ambisonics was designed based on [PERSON]'s principle for more advanced spatial soundfield reproduction over a large region of space [101,102]. The system is based on the zero and first order spherical harmonic decomposition of the original soundfield into four channels, i.e., Equation (1), and from a linear combination of these four channels to derive the loudspeaker driving signals. This low-order system is optimum at low frequencies but less accurate at high frequencies and when the listener is away from the center point. Higher Order Ambisonics (HOA) based on the higher order decomposition of a soundfield, such as cylindrical two-dimensional (2D or horizontal plane) harmonic [87,103,104] or spherical three-dimensional (3D or full sphere) harmonic [105,106,107] decomposition, was developed especially for high reproduction frequencies \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Method** & **Typical Systems** & \multicolumn{2}{c}{**Characteristics**} \\ \hline Stereo/Surround & Dolby Stereo, Dolby 5.1/7.1 & \(\blacksquare\) & Channel-based Object-based reproduction \\  & Dolby Atmos, NHK 22.2 & \(\blacksquare\) & Amplitude/phase encoding \\  & Auro 3D & & \\ \hline VBAP, DBAP & Software Demo [98] & \(\blacksquare\) & Object-based reproduction \\  & & & Amplitude encoding \\ \hline Ambisonics (B-format) & Youtube 360\({}^{\circ}\) & \(\blacksquare\) & Model-based reproduction \\ HOA & VR Audio Kit [99] & \(\blacksquare\) & Amplitude/phase encoding \\ \hline WFS & IOSONO & \(\blacksquare\) & Model-based reproduction \\  & & & Amplitude/phase encoding \\ \hline Inverse Filtering & – & \(\blacksquare\) & Channel-based reproduction \\  & & & Amplitude/phase encoding \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of soundfield reproduction methods and commercial systems. and large reproduction regions. Based on the same principle, soundfield reproduction using the plane wave decomposition approach was recently proposed [107; 108; 109].