Methodology

The methodology proposed in this article conceptually understands the acoustic space as a filter. That is, when sound passes through a filter (space), it is modified, and our hearing is the result of that filtered sound. This filter will be the same as the pattern of sound reflections on the room surfaces at a given place in the source-receiver, and like all sound, it will be described as a function of sound pressure level, frequency and time.

How is the filter representing each acoustic space recorded?

Filters, by nature, are defined by their response to an impulse. This is explained as follows: if we produce something so short and spectrally broad that it excites a filtered system, we can only notice the filter by itself.

Theoretically, this impulse is known as Delta Dirac and corresponds to a function whose value is 1 at the initial time and 0 for the rest of the abscissa axis. Therefore, to simulate a Dirac delta in practice, we must perform an impulse and approach the sound by firing blanks, fireworks, electric sparks, or simply popping a balloon, which is probably the easiest method to implement due to its low cost and light weight. The shorter and more spectrally broad this sound pulse is, the more accurate the characterisation of an acoustic space will be. The acoustic response of a space to this sound is called the Impulse Response.

However, over the years, acoustical engineers have refined this type of measurement with much more accurate and faithful methods, as the potential drawbacks of doing so with impulse responses (such as with the balloon method) have been thoroughly investigated. The difficulties arising from the world of science are based on instabilities in the frequency range, sonic distortion, non-linearity and inconsistency in statistical repeatability factors.

Since the introduction of iso 3382.1 (iso, 2001) for room acoustic measurements, standardised methods for this process have been established. The most commonly used is a frequency sweep: it is reproduced by a loudspeaker system and recorded at several statistically consistent points by standardised microphones.

It is also important to note that the academic/scientific standard nowadays mostly uses ambisonic microphones and omnidirectional loudspeakers to deliver complex sound sequences produced by computational algorithms. However, these standardised methods require the implementation of a very specific technological system, both for playback and recording and for analysis, which often makes this process cumbersome, complicated and difficult to implement in some places and by some people.

In this sense, the balloon method is reliable in the frequency range of the information of a sound message of human hearing, as well as being particularly versatile in its application. It is also important to note that in large, open spaces, the balloon method can provide a significant signal-to-noise ratio which, if realised with loudspeakers, requires large amplification systems.