4.2 Details of recording approach

Plans for audio-visual recording and data collection were made based on the observations of the model and the architecture of the broch, a consideration of the acoustics inside the broch and the practicalities of transporting equipment to the island of Mousa. Important areas of the broch were identified and recording positions were mapped to the entrance, courtyard, cells, staircase, galleries and rooftop.

Sound sources (voice, drums, whistle, iron bars and wooden clacks) were recorded in the selected spaces using a Neumann's monophonic microphone. To convey an impression of how sound travels within and around the broch, recording locations were varied, for example from courtyard to cell C and from cell C to courtyard. In selected locations (entrance, cell A, cell B, cell C, rooftop, courtyard, staircase), 10 hits or whistle blows were created using iron bars, whistle and drum, and were recorded (48k 24 bit mono) (See section 7). From each wave file, 3 of 10 (real time) consecutive hits/impulses per sample were selected and edited for each location. This approach allows users to select and compare similar or contrasting sounds from all areas of the broch and to use these sounds as a basis for further analysis. From the collection of samples, single-hit examples of drum, whistle and iron bars were selected, edited and analysed using Steinberg Wavelab 4 software and the wave graphic and frequency graphs for these single hits were generated.

To convey an impression of the variety and character of sounds in and around the broch (an impression of the human voice inside the broch, ambient sound inside the broch and environmental sounds outside the broch), five soundscapes were created. Field recordings were made inside and outside the broch using a five microphone surround sound approach. This recording approach was unsuccessful because of interference from the wind blowing through the broch's entrance. To replace the surround sound recordings, audio material recorded by stereo field and shotgun microphone was used and sound recordings were later edited.

To explore further the sound of the human voice in different parts of the broch, the content of a guided tour was scripted by the author and was performed by a volunteer, tour guide Sarah MacBurnie. Recordings of Sarah's voice were made in three locations. Binaural microphones were worn by Sarah and by the author (following 5-6 feet (1.5-1.8m) away) and a stereo recording was made by Justin Vitello (10 feet (3m) away). Again, intermittent air currents coming through the entrance interfered with the surround sound recording process and despite attempts to alleviate this problem (creating substitute wind socks, adapting existing wind sock and changing recording positions) no solution was found and the approach to recording was adjusted. The results of the binaural three position approach and the five mic surround field recording approach were therefore disappointing, with poor-quality low recording levels and background noise interference. The surround sound approach was abandoned and replaced with a stereo image.

A collection of reverberation times for all the spaces was created and analysed (see 5.1 and Table 4). Reverberation times were obtained by recording clack sounds (48 K. 24 bit), created by clapping together two pieces of wood (2in x 2in x 2in x 2ft). Clacks were recorded in selected positions inside Mousa broch in one session on 20 June 2009 in dry weather (at sea level, average temperature 21°C). Ideally, these impulses would have been created with a starting pistol but logistical difficulties prevented this approach. A series of 10 clacks was recorded for each space, with one clack from each series chosen for analysis. Analysis of 21 audio files was made using Arta software, which analyses the wave file, measures the acoustical energy and plots this to a time axis. By measuring the time it takes for the energy to decrease by 60 db, an approximate reverberation time for each space was calculated.

Video footage and a series of photographs collected inside the broch on the day of the summer solstice were edited and are presented as a partial documentary record of the action of the sun around the walls of Mousa broch during the daylight period of the summer solstice. Further footage filmed inside and outside the broch show the broch's location in the landscape and highlights points of interest.


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