Loudness of natural sounds – LoudNat

Loudness experiments will be conducted using different kinds of stimuli : stationary sounds with interaural differences (as for spatialised sources), sounds with temporally variable intensity, and, finally, sounds mixing these two characteristics. Models will also be tested in order to predict results from the knowledge of stimuli.

In the case of stationary dichotic sounds, experiments confirmed the reliability of the model proposed by Sivonen and Ellermeier (2006), with a slight reduction of the binaural gain suggested by these authors (4.5 instead of 6 dB).
In the case of intensity variable sounds, the loudness asymmetry was confirmed. It does not depend on context, level or measurement procedure. The overall loudness is based on an temporal integration around the maximum level time.

- model of loudness for intensity variable sounds;
- evaluation of the model for stationary sounds in the case of realistic stimuli (recordings of real sources);
- experiments using moving sources.

One paper in peer-reviewed journal (J. Acoust. Soc. Am. 134, EL321, 2013);
5 talks in international conferences (Acoustics 2012, ASA meetings 2012, 2013).

Loudness (the subjective level of a sound) is a basic character of a sound. Some models exist in order to describe this sensation from measurements of the signal, but they are restricted to some cases. Signals should be monaural, or equal at the two ears of the listener (presented through headphones, or in free field in a frontal incidence, or in a diffuse field). Moreover, if the level of the signal is varying over time, these models cannot predict the overall loudness of the sound.
Such limitations are major drawbacks because they prevent from using these models for environmental sounds. Indeed, sources in the environment often produce sounds with varying features over time (as an example, the sound of a passing car). The overall loudness depends on how the sound varies over time and this feature should be taken into account by a predictive model. Another important fact is that the position of the source, as well as the body of the listener can modify sounds at the position of the eardrums. These modifications are not the same at the two ears and some work is still to be done in order to understand how these two signals are combined by the listener to give him a single loudness sensation.

Therefore, the goal of this project is to extend the validity of loudness models so that they can be used for environmental sounds. The study will consist in psychoacoustical experiments. It will test several hypotheses about cognitive and perceptual phenomena which should be taken into account in order to adapt existing loudness models.

Different cases will be studied : first of all, attention will be paid to the loudness of stationary sounds in binaural listening, in situations leading to interaural differences. Some already published studies have shown very important inter-individual differences, though from a rather low number of listeners (less than 10). Experiments planned in this project will be realised on a higher number of persons, so that it will be possible to understand this inter-individual variability in a better way. From experimental results, a loudness model applicable to stationary binaural signals will be defined. It will be checked that this model can be used from dummy head recordings, which will ease its use by acousticians in the industry.
In the meantime, the loudness of non stationary headphone-presented sounds will be investigated. A preliminary study will consist in characterizing typical temporal profiles of environmental sounds. A special attention will be paid to two categories corresponding to sounds with increasing and decreasing levels. Then signals with such typical characteristics will be used as stimuli in psychoacoustical experiments, so that to study how a listener evaluates an overall loudness. Some short-term memory effects will be carefully examined, and a loudness model will be proposed for such sounds.
Finally, by combining the previous results, the third part of the project will study the loudness of non stationary signals in which binaural differences exist. Experiment will place listeners in real listening conditions, or will use signals measured with a dummy head. The results of these experiments will be compared with the predictions got from a model mixing the characteristics of the two previous ones. If necessary, this model will be adapted so that it will allow providing accurate results, which will make it suitable for practical applications.