Gunshots, noise off – and music

When acosusticians investigate the acoustic response of rooms they normally excite the room in such a way that they are able to study the room’s acoustic response to a perfectly impulsive sound.  This response is called the room’s echogram or reflectogram. Based on this echogram a set of acoustic measures can be calculated, for instance the reverberation time. Calculating the reverberation time is based on observing the long decay after a long-lasting sound source is switced off. In the past balloons were popped off, gunshots were fired off and/or loudspeakers switched off to obtain the relevant responses. With modern measuring techniques the same responses are obtained by use of loudspeaker and taylor made excitation signals like sine sweep. Most of the time the loudspeaker as well as the microphone are omnidirectional since this makes it easier to obtain reliable results among different investigations and between acousticians.

On the other hand music rarely contains perfectly impulsive sounds or long-lasting constant sounds being switched off (the ‘noise off’ method). Apart from at very high frequencies musical instruments excite the room over a certain time, leading to reflections from the room fusing together. Addtionally, the instruments are highly directional and we hear sound in stereo, not mono. Perceptual effects like masking and the cocktail party effect appear relevant for a listener inside a reflective room. Accummulated early reflections as the music is running can contribute to mask the direct sound and make it difficult to separate different sound sources spatially. This can as proposed by David Griesinger make the sound appear more muddy and the listener will not feel engaged in the sound. Since localisation is affected by the shape of our head and outer ear, the extent to which a sound appears defined or fully meddled can only be judged by an individual listening inside the actual room.

Based on this we may claim that measurability and reliability has been given priority compared to validity, when assessing room acoustic spaces. Focusing mainly on measured characteristics may have limited the scope and approaches when searching for relations between architectural design and experienced conditions. My impressions based on the results from the PhD work is that acoustic measures are relevant to see if the proposed design is ‘in the ballpark’, to ensure that the most catastrophic results can be avoided – formally or just to ensure the end users are happy with the acoustics.

One paradox within room acoustics may be that even if absolute acoustic levels are widely used within other fields of acoustics, like formal limits for sound pressure levels from traffic noise, it is not (yet) an established single measure that is always included when assessing listener and performer acoustics. The acoustic measure Strength (G) represents the absolute acoustic gain by room, but is not always measured/reported. For stage conditions, absolute levels are measured, but by use of a different reference level (the Support ST measures).

In my view, the acoustic measures studied for performance spaces should at a minimum include T, EDT, G and C80, both in the audience area and on stage. For the audience area spatial measures are also relevan (like LFC and LEV). From measured G and C80, Gearly and Glate should also be found to provide an indication of how loud the reverberation is, not only the reverberation time (also see ‘How loud is my reverberation‘ by Griesinger).

In addition to obtaining these measures it appears highly beneficial to listen a lot to rooms with own ears, and as a substitute, listen to auralisations of potential responses for planned venues. The acoustic measures have not been developed much since the ’70s and to progress further I believe experiencing real spaces  and discussing them are very useful to test and develop new hypothesis regarding links between physical acoustic conditions and experiences.

By having a common understand of basic acoustic concepts among for instance acousticians, sound engineers and musicians it will be much easier to discuss and exchange ideas and experiences based on qualified listening in real rooms (with ensemble on stage, audience present, directivity taken into account etc etc). Such qualitative discussions may be a very fruitful complement to objective acoustic data. If musicians and sound engineer also can understand the essential use and relevance of acoustic data I believe we are heading towards exciting discussions and new exciting findings within the discipline of stage acoustics.

Get the basic measures right and be aware of their limitations.  Then make sure to have enough time and interaction to listen, discuss and explore acoustic spaces with open ears and an open mind! I believe this will make it easier for us (and more enjoyable as well?) to effectively create great acoustic spaces for everyone involved.

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