3. General Methods 13
3.4. Results
From the data obtained from the magnitude estimation procedures, it was possible to plot the relative fluctuation strength as a function of each varying parameter. An example of these plots
1Each experimental section is designated with the type of stimuli followed by the parameter varied. Therefore, AM-fm represents the fluctuation strength as a function of modulation frequency experiment for AM tones.
Figure 3.1.: Screenshot of the computer interface used in the final experiment design
is shown in Figure 3.2. The actual plot pertaining the experiments will be shown in Chapters 4 and 5.
In order to combine the data points from the two standards, a correction factor had to be applied to the data that used the second standard as a reference. The correction factor was obtained taking into account the data where the second stimulus in the pairs corresponded to the value of the first standard. The correction factor was calculated such that the median of the values that used the second standard was the same as the median of the values that used the first standard. An example of this can be seen in Figure 3.2 panel (b), where the value of the first standard corresponds to a modulation frequency of 4 Hz.
Finally, another correction factor was applied to all the data points, in order to normalize the maximum mean value of the medians of the standards to 100%. The curve of the mean values correspond to the black line shown in Figure 3.2.
Modulation frequency [Hz]
Figure 3.2.: Relative fluctuation strength as a function of modulation frequency (adapted from [9, pp.248]). The two standards had modulation frequencies of 4 and 0.5 Hz. The data points show the median and interquartile ranges per standard. The black line represents the mean values of the medians of each standard. Panel (a): data for AM tones with center frequency of 1 kHz, sound pressure level of 70 dB and modulation depth of 40 dB. Panel (b): data for FM tones with center frequency of 1.5 kHz, sound pressure level of 70 dB and frequency deviation of 700 Hz
This chapter describes the experimental design for the evaluation of the fluctuation strength attribute. First, the initial design (as in [7]) is presented. Then, the iterative process of the pilots execution is detailed, stating the progressive changes made to the initial design. Finally, a series of conclusions that lead to the final experimental design are presented.
4.1. Initial Design
4.1.1. Subjects
In total 9 subjects participated in the pilot experiments. Participants were between 20 and 30 years old. All of them had self-reported normal hearing.
4.1.2. Stimuli
Table 4.1 presents all the stimuli used for the different experimental sections, stating which parameters were fixed and which were varied.
4.1.3. Procedure
The pilot experiment was divided in two phases, the training phase and experimental phase, as stated earlier in Chapter 3. The experimental phase remained the same as explained before, using the stimuli set defined in Table 4.1. The training phase for the pilot experiment is described as follows.
4.1.3.1. Training Phase
The objective of this phase was to make the concept of fluctuation strength clear to the participants and to familiarize them with the range of stimuli. Past studies [1] have pointed out the need of such a phase to familiarize subjects with the sensation. However, this must be approached with caution, as the intention of the training phase is to show participants what the sensation is, not teach them how to answer to specific questions regarding the stimuli.
Stimulus Comparison First, a subset of AM tones (Table 4.2) was presented to the participants sequentially according to their ID in pairs (i.e., first stimuli 1 and 2, then 2 and 3, etc.). After
Section Parameters
Table 4.1.: Description of initial set of stimuli used per experiment section
each pair presentation, participants were asked whether a difference in the fluctuation strength among stimuli was detected. In the case of a negative answer the pair was repeated until a positive answer was obtained.
Table 4.2.: Initial subset of AM stimuli for training phase
Long Interval Afterwards, a long interval was presented to the participants. The long interval
Presentation
Table 4.3.: Initial long interval composed of AM stimuli for training phase
4.2. Iterative Improvements
Not all participants were subjected to the same experimental conditions, and not all of them used the same version of the experiments. Table 4.4 presents the conditions and in which one of them the subjects participated.
Table 4.4.: Participants experimental conditions and versions
The experimental procedure was varied during the pilot experiment to accommodate perceived errors during the realization of them. The first version of the experiment yielded unsatisfactory results with regard to the relation between fluctuation strength and modulation frequency (participants 2, 3 and 4). The procedure was then modified, adding two more AM tones with modulation frequencies of 64 and 128 Hz. The idea behind the addition of these two tones was that, if the participants had stimuli that give a distinguishing roughness sensation, it would be easier for them to distinguish between a fluctuating and a rough tone. Additionally, FM tones were included in the training, since up to this point only AM tones were used in the training phase. This constitutes the second version of the experiment.
Participant 5 was the only participant that was subjected to version 2 of the experiment.
The results did not show any significant improvements with regard to the confusion between fluctuation strength and roughness. However, by talking to the participants it was discovered that the training phase was not able to make the concept of fluctuation strength clear. Participants
were only asked to tell whether there was a difference of fluctuation among the presented stimuli, without explaning what fluctuation strength actually was. As such, several participants associated the rate of change of the stimuli (modulation frequency in this case) with a bigger fluctuation in the presented sounds. Hence, they tended to deem as highly fluctuating the sounds that had a high modulation frequency. Participants 4 and 5 explicitly stated that they were counting the number of cycles in the stimuli, due to confusion on what to answer.
Taking all these comments as feedback, version 3 of the experiment was elaborated. In this version, explicit instructions regarding the rough tones were given. It was indicated that the sensation of fluctuation was unrelated to the apparent ‘speed’ of the stimuli, and that the answers should be intuitive, based on the arising sensation and not rationalizing any judgment about it (for instance by counting cycles). Using this approach participants were able to understand better the fluctuation strength concept, some of them even coming with analogies to the sensation itself (the sound of an ambulance alarm, the sound of a washing machine). The actual instructions used in the final experiment can be found in the prompts of the experimental protocol (Appendix A).
The final version, number 4, of the experiment added a small test experiment before starting the actual experimental sections. This was added as a suggestion from participant 8, who indicated that although the training phase was effective in making the fluctuation strength concept clear, it did not show the participant how to do the expected judgments using the magnitude estimation procedure. Moreover, a latin square randomization approach was used, rotating the order of the experimental sections for each participant. The purpose of this was to distribute possible learning effects of participants among the experimental conditions. Finally, the modulation frequency sections (AM-fm and FM-fm) were split into two separate sections, each one with 2 repetitions per pairs instead of 4. Therefore, two smaller sections for the AM-fm and FM-fm were used in the final experiment. This was done due to the longer duration of the modulation frequency sections when compared to the other sections. By keeping all the sections relatively short (around 6 minutes each) it was expected that participants attention and focus would be retained from one section to another.
4.3. Results
The following figures show the results of the pilot experiments, together with the results reported by Fastl and Zwicker [9]. Comparing the two plots per experimental condition, it can be concluded that both graphs are similar from a qualitative point of view. Therefore, the pilot experiment was deemed as successful in obtaining the relevant subjective data from participants.
A more detailed description of the particularities of each experimental condition curve will be given in Chapter 5.
Modulation frequency [Hz]
Figure 4.1.: Relative fluctuation strength as a function of modulation frequency for AM tones with center frequency of 1 kHz, sound pressure level of 70 dB and modulation depth of 40 dB. The two standards had modulation frequencies of 4 and 0.5 Hz. The data points show the median and interquartile ranges per standard. The black line represents the mean values of the medians of each standard. Panel (a): data adapted from [9, pp.248]. Panel (b): own results
Center frequency [Hz]
Figure 4.2.: Relative fluctuation strength as a function of center frequency for AM tones with modulation frequency of 4 Hz, sound pressure level of 70 dB and modulation depth of 40 dB. The two standards had center frequencies of 1 and 0.25 kHz. The data points show the median and interquartile ranges per standard. The black line represents the mean values of the medians of each standard. Panel (a): data adapted from [9, pp.250]. Panel (b): own results
4.4. Conclusions
From the obtained data it can be concluded that the main problem when dealing with the perceptual attribute of fluctuation strength is its ambiguity and confusion with the perceptual attribute of roughness. The proposed training phase was effective in clarifying the concept to participants, by adding stimuli with a clear rough sensation, and by clearly instructing them on what the sensation is about. It should be noted that only with regard to modulation frequency this confusion arises, the other parameters do not present this particularity and as so it was not necessary to adapt the experimental procedure with them.
Sound pressure level [dB]
Figure 4.3.: Relative fluctuation strength as a function of sound pressure level for AM tones with modulation frequency of 4 Hz, center frequency of 1 kHz and modulation depth of 40 dB. The two standards had sound pressure levels of 70 and 50 dB. The data points show the median and interquartile ranges per standard. The black line represents the mean values of the medians of each standard. Panel (a): data adapted from [9, pp.249]. Panel (b): own results
Modulation depth [dB]
Figure 4.4.: Relative fluctuation strength as a function of modulation depth for AM tones with modulation frequency of 4 Hz, center frequency of 1 kHz and sound pressure level of 70 dB. The two standards had modulation depths of 40 and 4 dB. The data points show the median and interquartile ranges per standard. The black line represents the mean values of the medians of each standard. Panel (a): data adapted from [9, pp.249]. Panel (b): own results
Modulation frequency [Hz]
Figure 4.5.: Relative fluctuation strength as a function of modulation frequency for FM tones with center frequency of 1.5 kHz, sound pressure level of 70 dB and frequency deviation of 700 Hz. The two standards had modulation frequencies of 4 and 0.5 Hz.
The data points show the median and interquartile ranges per standard. The black line represents the mean values of the medians of each standard. Panel (a): data adapted from [9, pp.248]. Panel (b): own results
Center frequency [Hz]
500 1000 1500 2000 3000 4000 6000 8000
Relative fluctuation strength [%]
500 1000 1500 2000 3000 4000 6000 8000
Relative fluctuation strength [%]
Figure 4.6.: Relative fluctuation strength as a function of center frequency for FM tones with modulation frequency of 4 Hz, sound pressure level of 70 dB and frequency deviation of 200 Hz. The two standards had center frequencies of 6 and 0.5 kHz. The data points show the median and interquartile ranges per standard. The black line represents the mean values of the medians of each standard. Panel (a): data adapted from [9, pp.250]. Panel (b): own results
Sound pressure level [dB]
Figure 4.7.: Relative fluctuation strength as a function of sound pressure level for FM tones with modulation frequency of 4 Hz, center frequency of 1.5 kHz and frequency deviation of 700 Hz. The two standards had sound pressure levels of 60 and 40 dB. The data points show the median and interquartile ranges per standard. The black line represents the mean values of the medians of each standard. Panel (a): data adapted from [9, pp.249]. Panel (b): own results
Frequency deviation [Hz]
Figure 4.8.: Relative fluctuation strength as a function of modulation depth for FM tones with modulation frequency of 4 Hz, center frequency of 1.5 kHz and sound pressure level of 70 dB. The two standards had frequency deviations of 700 and 32 Hz. The data points show the median and interquartile ranges per standard. The black line represents the mean values of the medians of each standard. Panel (a): data adapted from [9, pp.251]. Panel (b): own results
In this chapter the final experimental design and results for the evaluation of the fluctuation strength attribute are presented.
5.1. Design
5.1.1. Subjects
Twenty-four participants were recruited from the JF Schouten database of the Eindhoven University of Technology. Participants were between 19 and 31 years old. There were in total six females and eighteen males. All of them reported to have normal hearing, however this was not confirmed in any way. Subjects were paid for their participation.
5.1.2. Stimuli
The stimuli used in the final experiment have the same characteristics as the ones used in the pilot experiments (Section 4.1.2). Table 5.1 shows the stimuli used in the final experiments.
5.1.3. Procedure
Participants were assigned either to AM tones or to FM tones, both conditions having 12 participants. Furthermore, the presentation order of the experimental sections was varied, using a latin square design. The order of the parameters used is presented in Table 5.2.
The whole experiment had an approximate duration of 60 minutes. The experimental protocol followed during the experiment can be found in the appendix of this document (Appendix A).
The experimental phase of the experiment remained the same as the one of the last pilot experiment. The only sensible change compared to the procedure described in Chapter 3 is the split of the modulation frequency sections into two separate sections. The training phase had some changes, described below.
5.1.3.1. Training Phase
Adding all the improvements postulated in the pilot experiment section, the training phase was expanded and became a 3-part phase, described below.
Section Parameters
Table 5.1.: Description of stimuli used per experiment section
Order1 Parameters
1 fm, fc, {md or df}, fm, SPL
2 fc, {md or df}, fm, SPL, fm
3 {mdor df}, fm, SPL, fm, fc
4 SPL, fm, fc, {mdor df}, fm
Table 5.2.: Presentation order of parameters
Stimulus Comparison As in the pilot experiments, the initial part of the training phase consisted of comparison between stimuli. An additional stimulus was added (fm = 128 Hz) and a subset of FM stimuli was also added to complement the AM tones.
The stimuli presented in Tables 5.3 and 5.4 were reproduced in groups, according to their ID values. First, stimuli with ID values of 1 and 2 were reproduced. This pair presented the
values of fluctuation strength. Finally, stimuli with ID values of 3, 4 and 5 were reproduced.
This last group presented the difference between fluctuating and rough tones. After each group presentation, participants were asked whether the specific difference of sensation of the group was acknowledged. In case of a negative answer, the stimuli of the group were once again reproduced.
ID fm [Hz] fc [kHz] SPL [dB] md [dB]
1 0 1 70 40
2 0.5 1 70 40
3 4 1 70 40
4 32 1 70 40
5 128 1 70 40
Table 5.3.: Subset of AM stimuli for training phase
ID fm [Hz] fc [kHz] SPL [dB] df [Hz]
1 0 1 70 700
2 0.5 1 70 700
3 4 1 70 700
4 32 1 70 700
4 128 1 70 700
Table 5.4.: Subset of FM stimuli for training phase
Long Interval This part of the training phase presented participants with long intervals, which consisted of stimuli separated by 800 ms of silence. Two long intervals were reproduced, composed of AM and FM tones, respectively. These two intervals are described in Tables 5.5 and 5.6.
Presentation
fm [Hz] fc [kHz] SPL [dB] md [dB]
order
1 0.5 1 70 40
2 32 1 70 40
3 2 1 70 40
4 16 1 70 40
5 4 1 70 40
6 1 1 70 40
7 0 1 70 40
8 64 1 70 40
9 0.25 1 70 40
10 128 1 70 40
11 8 1 70 40
Table 5.5.: Long interval composed of AM stimuli for training phase
Test Section In order to familiarize participants with the interface used during the experiment and with the magnitude estimation procedure, a small test section was added at the end of the
Presentation
fm [Hz] fc [kHz] SPL [dB] df [Hz]
order
1 0.25 1 70 700
2 64 1 70 700
3 32 1 70 700
4 2 1 70 700
5 1 1 70 700
6 0 1 70 700
7 8 1 70 700
8 4 1 70 700
9 0.5 1 70 700
10 128 1 70 700
11 16 1 70 700
Table 5.6.: Long interval composed of FM stimuli for training phase
training phase. This section consisted of four pairs (Table 5.7), which were presented to the participants in randomized order.
Pair Parameters
fm [Hz] fc [kHz] SPL [dB] md [dB] df [Hz]
1 4 1 70 40 —
32 1 70 40 —
2 4 6 70 — 200
4 6 70 — 200
3 4 1 70 40 —
0 1 70 40 —
4 4 1.5 60 — 700
4 1.5 80 — 700
Table 5.7.: Pairs used in training phase test section
5.2. Results
The following section presents the results of the experiments, compared to the data published by Fastl and Zwicker [9]. Overall the obtained data is qualitatively similar to the data by Fastl and Zwicker, although some differences do exist. Most notably, in the modulation depth response curve for AM tones and in the center frequency and frequency deviations curves for FM tones.
In the following paragraphs the obtained curves will be described, focusing on similarities and discrepancies with the literature data. Possible causes of this discrepancies will be discussed in Chapter 7.
modulation frequency below 4 Hz on average higher values of fluctuation were obtained. This leads to the fact that the response from the obtained data has a wider bandwidth than the data from the literature. Also, more variability seems to exist with the use of the first standard, evidenced by the difference of length between the larger interquartile range (IQR)s of the first standard and the smaller IQRs of the second standard.
Figure 5.2 shows the dependency of fluctuation strength on center frequency from AM tones.
In this case both responses present a similar flat response with large IQRs. Figure 5.2 shows the dependency of fluctuation strength on center frequency from FM tones. Here the difference is more dramatic, the data from the literature decreases monotonically with the increase of center frequency, whereas the data from this study remain mostly flat.
Figures 5.3 and 5.7 show the dependency of fluctuation strength on sound pressure level for AM and FM tones. Although the curves from the obtained data are not as linear as the data from the literature, in both cases fluctuation strength increases with sound pressure level.
Figure 5.4 shows the dependency of fluctuation strength on modulation depth from AM tones.
Here a difference exists between the obtained data and the literature data. Both response curves show an increase of fluctuation strength with the increase of modulation depth. However, the obtained data increases more quickly with modulation depth than the data from the literature.
Figure 5.8 shows the dependency of fluctuation strength on frequency deviation from FM tones. In this case a clear difference exists between the obtained data and the literature data.
For the obtained data, for small values of frequency deviation a significant fluctuation strength (around 50%) does exist. This causes that the curve presents a less steep slope when compared to the literature data. Both curves present an increase in fluctuation strength with an increase in frequency deviation.
Finally, to summarize Figures 5.9 and 5.10 compared the mean of the values of the two standard for each experimental condition. The discrepancies between obtained data and literature data can be further observed in these figures.
Modulation frequency [Hz]
Figure 5.1.: Relative fluctuation strength as a function of modulation frequency for AM tones with center frequency of 1 kHz, sound pressure level of 70 dB and modulation depth of 40 dB. The two standards had modulation frequencies of 4 and 0.5 Hz. The data points show the median and interquartile ranges per standard. The black line represents the mean values of the medians of each standard. Panel (a): data adapted from [9, pp.248]. Panel (b): own results
Center frequency [Hz]
Figure 5.2.: Relative fluctuation strength as a function of center frequency for AM tones with modulation frequency of 4 Hz, sound pressure level of 70 dB and modulation depth of 40 dB. The two standards had center frequencies of 1 and 0.25 kHz. The data points show the median and interquartile ranges per standard. The black line represents
Figure 5.2.: Relative fluctuation strength as a function of center frequency for AM tones with modulation frequency of 4 Hz, sound pressure level of 70 dB and modulation depth of 40 dB. The two standards had center frequencies of 1 and 0.25 kHz. The data points show the median and interquartile ranges per standard. The black line represents