A full or warm sound of the orchestra is normally preferred. An experience of a full sound is normally associated with sufficient levels and acoustic response at low frequencies. An increase of reverberation time at low frequencies is often recommended for this purpose. By having bass-reflecting surfaces close to the orchestra at the sides and at the back, close to coherent reflections will be added to the source that will effectivily boost the sound source levels at low frequencies. The need for low frequency reverberant sound to produce a fuller orchestra sound may therefore be reduced with surfaces close to the orchestra. Such proximate bass-reflecting surfaces can therefore potentially contribute to both a full (warm) and distinct orchestra sound.
This apparent benefit of side and back reflecting surfaces was covered in my PhD thesis with emphasis on raising the levels of the double basses. But the percussion section may also benefit from being close to surfaces that reflect at low frequencies. The back wall has been associated with negative effects, like unnecessary raising the level of percussion. But such negative effects of percussion could to be relevant primarily at frequencies above 500 Hz. The back wall behind the percussion can be made reflective at low frequencies, while more absorbing or diffusing (sound scattering) at higher frequencies to avoid the negative effects. The brass instruments appears loud enough in the first place so there may be no need to raise the levels of brass at all with reflecting surfaces close to them. The results from the questionnaire studies suggest that the exception could be the horn section. Horn players have commented positively on having a reflecting surface behind them, that could be caused by the directivity of the horn.
Phase relations between sound waves are normally not included when sound levels are summed in computer simulating software for auditorium acoustics. By ignoring phase information the effect of the surfaces will be underestimated regarding total levels at low frequencies. For a source close to one reflecting surface, calculated acoustic gain G (Strength) from the simulation software can be typically 2–3 dB lower than the real value. If making design desicions based solely on the resulting G values from software the need for long reverberation time at low frequencies can be overestimated and resulting in too high levels in the bass for the orchestra and other users of the hall, like pop/rock bands
Our paper published in JASA (more or less Chapter 4 of my PhD thesis) demonstrates how to calculate the combined level based on the direct sound and a reflection with phase relation taken into account. A spreadsheet is also available for carrying out such calculations (comb filtering interference), see the Spreadsheets page. The total level presented in the spreadsheets will be relative to the direct sound level. If knowing the source-receiver distance, the total value of G can be found. If exporting wave files for auralisation from the computer simulation software, phase relations between the direct sound and early reflections can be included, but then there can often be a problem with obtaining correct values of G based on the exported (often normalised) wave files.