# Locating Image Sources from Multiple Spatial Room Impulse Responses
## 4 Discussion  
The proposed method has proved to be a powerful tool for visualizing image sources. In the variable acoustics room Arri, it was able to find prominent image sources up to third order. The method was not able to find all the image sources. This means that they are not in sufficient temporal isolation in a minimum of three measurements. However, there are at least two ways of improving the measurement data. First, the search result is easily improved by measuring more receiver positions. More positions also allow us to raise the minimum number of ER objects per image source, effectively making the method more resistant to detection noise. Second, the orientation of the loudspeaker could be optimized to find a desired reflections. By pointing the loudspeaker to a certain direction excites that direction with the shortest possible peak, which could be detected more easily. In other words, the directivity of both the source and the receiver enables us to find higher order reflections [21]. Figure 6: Colors used for each channel in the loudspeaker orchestra. The circles are the main loudspeakers and the triangle are the auxiliary loudspeakers that were used in some channels to approximate the directivity of certain instruments better. The loudspeaker orientation was found to affect the receiver fit accuracy. Most apparent in the concert hall datasets, the fit was found to be the most accurate when the receiver was situated in front of the sound source. In contrast, the found receiver position started to deviate from the measured positions when placing the receiver on the side or behind the loudspeaker. Apparently, the applied directional analysis quality seems to degrade when the direct sound is lacking high frequencies. This can however be circumvented by rotating the loudspeakers to always face the measuring microphone array. Combining multiple measurements also allows us to analyze complex spaces, such as a concert hall. For instance, the method found reflections from the lower part of the balcony front, that is, the softit reflections in Concertegebouw. In addition, image sources are found consistently for several sound sources and clearly separate from the wall reflection, implying that the reflection is not negligible. Another concert hall, Berlin Philharmonic serves as an example of a space without any parallel walls. Also here, the method was able to pinpoint many of the wall and ceiling reflections. The case of the Philharmonic also points out the first limitation of the presented method--we had to lower the ER object limit in order to detect the reflections from small surfaces. The reason is that different surfaces are visible to different receiver positions, thus \"visible\" image sources are different to different receiver positions [22]. Nonetheless, again this limitation can be circumvented by measuring more receiver positions. One limiting factor in the method is the geometry alignment algorithm. Although the applied fitting procedure appears to be accurate even in big spaces, the algorithm is susceptible to deviations in the receiver orientation. It is able to correct orientation errors of several degrees, but an error as big as 45\({}^{\circ}\) would be too much to recover from. Furthermore, the algorithm requires reference position for all sound sources in order to determine measurement system delay correction. The first limitation can be circumvented by paying at least some attention to the orientation during the measurement, or by manually entering a rough initial orientation estimate. However, both limitations could be solved by applying another fitting method either as a preliminary step or as a replacement for the current geometry calibrator. The image source alignment algorithm manages to gather first order image sources as expected. However, for convex or concave surfaces spread instead of converge. This is because Equation (8) assumes a reflecting plane. This last stage could be extended by a content-dependent translation model, which is left for future research.