Objective room acoustics is an essential part of the evaluation of the acoustics of performance venues. The subjective quality of concert hall acoustics is traditionally formed around various descriptors, such as loudness, reverberance, clarity, definition, and envelopment in the sound field [1]. Well known standardized objective parameters, such as G (strength), T30 (reverberation time), EDT (reverberance), C80 (clarity), and LEF (lateral energy fraction) have been developed to quantify different subjective aspects in the acoustics. However, the general objective measures rely heavily on the statistical properties of the sound fields, thus, they are not suitable for accurately describing the acoustic details in rooms. Instead, they aspire to compress the acoustic information in a small set on numeric values.

Spatiotemporal analysis of room acoustics requires measuring of spatial room impulse responses. Earlier methods have mainly concentrated on visualizing sound intensity vectors, where as a potential downside the contribution of e.g. certain early reflections is obscured. The forward-integrating approach aims to overcome this deficiency. Particularly when used with a large number of source positions on stage, the effect of the hall geometry will be emphasized for features that are relevant for most of the source positions.

The current development stage of the method employs the Spatial Decomposition Method (SDM) [2] in the directional analysis. By applying SDM to a spatial impulse response, we obtain directional information for each sample in a monaural impulse response. From the combination of the monaural pressure and spatial metadata, it is then possible to calculate directional energy histogram over different time windows. The histograms are stacked in the visualization so that each of the cumulative time frames indicate where the sound energy is arriving to the receiver position around that time.

Recording process

The instruments of a symphony orchestra were recorded one by one in an anechoic chamber. The musicians played their parts by watching a conductor in a monitor and by listening to a pianist playing the whole score. This way the musicians were able to adapt their playing style and tempo, and the synchronization between different players was possible. Since the size of a typical orchestra and the complexity of the music texture varies between periods, recordings of different music styles can provide more information about the acoustics in auralization. To have a more comprehensive selection of anechoic music excerpts, passages representing different styles were selected for the recordings.

Editing of recordings

To gather takes from all recorded instruments and to form an ensemble playing together, some editing was required. In the first editing stage, one complete take was joined from several clips, if necessary. A common task was to replace accidental wrong notes in otherwise good take. All editing was performed in sample-accurate manner, thus the length of resulting files were kept unchanged. Second editing stage was essential for correcting any timing inaccuracy between the instruments. First imported parts in each passage were edited by using the piano track as a timing reference. These parts included usually some string instrument parts and a wind instrument. Timing inaccuracies were corrected. After the first completed parts, the piano track was muted and the actual instrument recordings were used as timing reference from this point forward. The goal in editing was not to create an unnaturally accurate synchronization. Therefore slight timing discrepancies were left unchanged. However, all the corrections were attempted to accomplish in a delicate manner so that the edits would not be easily perceived even by listening individual tracks.

Tips and tricks:

• The recordings are all made with exactly the same gain settings. This means that in some parts the signal-to-noise ratio is not as good as it could be, since the gains were adjusted according to the loudest instruments. However, now all tracks contain the "natural" dynamics played by the musicians and if you just add all tracks together the balance should be correct. Naturally, some manual fine tuning might be needed.
• When adding all tracks together the background noise might be too disturbing. One solution is to first process individual tracks with a noise gate (i.e. remove the noise from parts where no signal is present) and then add all tracks together. By this way, the background noise should not anymore be a problem.