Christopher Blair on stage acoustics

At the time my PhD thesis was completed Christopher Blair, both an acoustician and conductor, gave his views on acoustic conditions for symphony orchestra on a blog posting on the Adaptistration web site. Several other interesting blog postings by Blair are also available on this web site. I was unaware of his posting on acoustics for orchestras when I completed my thesis, so it appears that Blair’s views on beneficial conditions for an orchestra are independent from the results in covered in my thesis.

In the blog posting referred to above Blair writes: “The art of designing good on-stage acoustics boils down to providing just enough early energy to help with coordination, but not so much as to mask audibility of the late-energy room response.” This is in very good agreement with the conclusions from my PhD thesis: the main purpose of early reflections back to the orchestra was found to an effective compensation for low direct sound level within the orchestra. Additionally, the most popular venues covered in our research project had a certain balance between early and late acoustic response on stage (C80), as well as a certain level of the late acoustic response in the main auditorium (Glate).

Blair has previously written about the importance of reflections from the main auditorium reaching the stage, in the Journal of the Conductors Guild, Vol. 19, No. 2, 1998. In this article (unfortunately not freely available) the relevance of perceptual masking effects for perceived conditions is described, along with a study of plan and section drawing for a set of existing symphony orchestra venues. For popular stages there are identified reflections from the back section of the auditorium that reach the stage with delay(s) within typically 100–300 ms. Unfortunately this article was not included in the literature review of my PhD thesis, but the conclusions in this article are also similar to my conclusions.

Improving the acoustics for orchestras under a low ceiling

One of the major findings from the research project documented on this website was that low ceiling can be very problematic for a symphony orchestra. Recently a paper by Higini Arau-Puchades (Arau Acustica) has been published in Acta Acustica united with Acustica; Increasing the Acoustic Volume of Performance Spaces without Altering the Internal Dimensions. The cases covered in this paper suggest that a low ceiling is the major reason for difficult acoustic conditions experienced by the orchestras playing in their specific venues. The paper also includes a design of a grid of vertically oriented solid panels suspended above the orchestra, called an acoustic labyrinth. The picture below is taken from the mentioned paper.

The apparent effect of this construction is to block the direct reflection from the ceiling particularly above 500 Hz. Blocking the direct path results in the sound travel a longer path from the orchestra via the ceiling back down towards the orchestra, resulting in lower level and larger delay on the ceiling reflection.The grid also contributes to scatter the ceiling reflection into different directions. The ceiling of the actual venues had curved ceiling that focused the reflected sound down towards the orchestra. Blocking the direct reflection by the grid will in such cases have a very significant effect on the level of reflection sound from the ceiling.

The orchestras reported on a significantly improved acoustic conditions with the grid installed. This is very encouraging since it suggests that there can be a cost-effective alternative to improve conditions for orchestras under a low ceiling. Raising the ceiling physically will in most cases be too expensive to be feasible.

There are only given a few suggestions in the paper to why the grid improves the perceived conditions. Apparently the experienced improvement can largely be explained by the findings from my PhD thesis; a high ceiling was here found beneficial for avoiding loud instruments becoming too loud and contributing to make the string section and the acoustic response from the main auditorium audible to the whole orchestra. This conclusion was based on comparing acoustic conditions with the orchestra present on stage, related to the psycho-acoustic effects masking, precence effect and the cocktail-party effect. The real stages studied in the PhD also supported this conclusion.

It will be interesting to hear more about the players’ experiences with Arau-Puchade’s grid in the future. To further understand the effect of the grid it would be very interesting to see measured acoustic responses in detail and results for Glate and C80, both on stage and in the main auditorium for these venues – not only Schroeder curves, T30, G and STearly on stage.

New material update

1) A new spreadsheet for calculating G has been added. See the spreadsheets page.

2) A journal paper is now due to be published in Building Acoustics. See the articles page.

3) The results from the project suggest that important features a stage enclosure is to provide effective compenstaion for the inherent skewed level balance between the different instrument groups of the orchestra. Such an effective compensation appears to be best accomplished by having reflections with small delay from specific directions – particularly from surfaces close to the orchestra at the sides. But maybe as important, or even more important: to allow or aid the players to hear the late reverberant response from the main auditorium. (The response from the main audiutorium will always be late due to the distances involved). This is likely to contribute to a sense of acoustic communication with the audience area, that appears to be an important aspect of the acoustic conditions for the player. Measuring Glate on stage and in the stalls area (in the main auditorium) appears to be a useful method to investigate the level of contribution of late reverberant sound from the stage enclosure and main auditorium on stage. A brief article has been added regarding details on measured  Glate in the stalls area (audience area) compared to on stage for eight different venues (the BSO venues).

Why G appears superior to ST for acoustic levels

For more details and background on why measures based on G appears to be both more reliable and subjectively relevant compared to the ST measures, I have written a 3 pages long article that include some results previously not presented. See the Articles page.

The message from the article in brief: feel free to continue measuring ST on stage, but please make sure that you also measure and report T, C80, Ge and Gl on stage as well as in the audience area for the venues you investigate!

Creating a fuller sound of the orchestra

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.

Details on the proposed architectural measures

There may be some confusion regarding how the proposed architectural measures should be obtained and what aspects of the stage enclosure the meaures can assess. I have therefore written a two-pages long article going into some more details on these measures. See the Articles page to read the article. Your opinions, questions or comments to this are most welcome.

Project results in brief

For a brief overview on stage acoustics and the major results from the research project, please see this two-pages long article, also available on the Articles page. (This article has been slightly revised 16 September 2011).