biomasa, transpiración cuticular y peso específico

The investigation of the modal density has been based around three listening tests, revealing an increasing understanding of the perception of this parameter in a variety of scenarios, from the absolute threshold omitting mode shapes with artificial stimuli to realistic models and stimuli and finally perceived quality.

As Chapter 4 showed, there has been widespread use of room metrics based upon the modal distribution which solely take account of the room dimensions, without consideration of the position of either the listener or the loudspeakers. Through the modelling of such a scenario, it is possible to observe the smoothing of the room response as the density increases. Obtaining a threshold in such a scenario, where it is no longer possible to distinguish between the room’s response to two differing densities, allows us to determine something along the lines of a ‘subjective counterpart’ to the Schroeder Frequency. Using test tones which interact with the room only at a their specific frequency and a narrow band around it, an absolute

threshold has been obtained.

It is noted that this threshold at the lowest frequencies, is around four modes per bandwidth. This correlates closely with the Schroeder Frequency, which suggests three modes per bandwidth is sufficient to move into the statistical region. The important issue to be wary of here is the linking of the transition into a statistical region with the subjective case of a density having been met where reproduction can no longer be distinguished from a very high reference case. Furthermore, this result appears to be in agreement with the optimal spacing revealed in Chapter 4. Here, the optimal spacing which would result in the shortest decay, which also relates to the smoothest response, is 1/3rd of a bandwidth of one mode, resulting, of course, in an average of three modes per bandwidth.

We may also observe that the spacing investigation revealed that sensitivity to decays was greater at higher frequencies. This is confirmed by the threshold of den- sity results - a much higher number of modes per bandwidth are required at 125Hz and 250Hz. It is suggested that the individual peaks and dips at higher frequencies are perceived as more obvious degradation, and therefore a much smoother ove- rall response (obtained only when mode-shapes are omitted) is required before we become unaware of the degradation.

Whilst a useful starting point in our investigation, one should carefully consider the use of thresholds based upon scenarios unlikely to occur in real rooms. More accurate modelling of the room, including the source and receiver positions, is consi- dered more revealing in the search for a subjectively relevant density threshold. The interaction of mode-shapes results in irregularities in the frequency response. There is no longer a direct correlation between the modal density and the smoothness of the response. In large rooms, the typical definition of statistical region is reached at lower frequencies than in smaller rooms. It may therefore be assumed that low frequency treatment is less necessary where a large space exists. The listening test undertaken here reveals that the subjective response to a range of small and large rooms show almost identical patterns. It is clear that there is no linear relationship between density and the perception of modal artefacts.

It would seem natural that detection of differences between two modelled rooms is based upon their differences in terms of transfer function. It is for this reason that the final listening test was undertaken, in order to determine if there is an increase in perceived quality in rooms of higher density. The results presented show large judgement errors, and it is suggested that this is due to a) the complexity of the subjective task, asking subjects to evaluate ‘perceived quality’ and b) that the modal density does not sufficiently affect the quality of reproduction so as to be

noticeable.

To conclude, it appears that we have too often considered modal density as a relevant subjective parameter. We should instead be focussing on the individual frequency response. Part III of this thesis is motivated by this finding.

5.8 Summary

Modal density thresholds have been investigated in the pursuit of perceptually rele- vant objective room response parameters. The importance of considering the inter- action between room, source and receiver has been shown. When this is accounted for, and realistic stimuli are auralised, it is not possible to obtain thresholds. Ins- tead, the specific frequency response becomes the dominant factor. It is suggested that it is the unique frequency response and not specifically the density which is responsible for the quality perceived by a listener. If these responses are to be rated in terms of quality, further detailed testing is necessary.

The results of these tests reveal two main conclusions. Firstly, it is the overall evaluation of a room which reveals its quality, rather than the insistence on whether a difference between two samples can be heard or not. Secondly, we can conclude that modal density is not a particularly relevant parameter in terms of understanding perception of low frequency reproduction quality. Based on findings in the previous chapter and previous research, we consider the perception of modal decay from here on. Therefore, the following chapter will further study this decay time in an attempt to define new thresholds, and later, Part III will consider the question of perceived quality, rather than absolute detection of differences.

Chapter 6

Thresholds of Modal Decay

6.1 Introduction

The previous two chapters have investigated parameters which are related to the modal distribution. In attempts to define optimal values, the spacing experiment looked for the shortest perceived decay and the density found a threshold with reference to a smoothing of the response - thereby reducing the decay time. The common factor is the importance of a short decay time for audio reproduction to be perceived as high quality. In response to this, and with the research gap identified in the literature review for further investigation, this chapter attempts to define a subjective threshold for modal decay.