7. Discussion 51
7.2. Model
7.2.2. Limitations
The use of the absolute value of cross correlation coefficients may pose problems when using the model with BBN AM tones, the other type of stimulus used by Fastl and Zwicker in their study. Although some FM tones used in this study present already negative cross correlation coefficients, they do not affect negatively the overall fit obtain with the model.
The increased frame size needed to achieve the frequency resolution for the model renders it unfeasible to implement in practical application. Currently a frame size of 220samples is needed, which corresponds to around 24 seconds for a sampling frequency of 44.1 kHz.
Finally, more modern modeling techniques, such as the use of the ERB perceptual scale and a Gammatone filterbank instead of Terhardt’s can be implemented, to improve the model and bring it to a more up to date state.
7.3. Conclusions
The use of a training phase before the experimental phase proved to be useful. On average, participants were able to understand better the concept of fluctuation strength. Moreover, the inclusion of the training phase reduced the confusion between fluctuation strength and roughness, compared to the pilot experiments.
The obtained perceptual data were found to be qualitatively similar to the data originally reported by Fastl and Zwicker. Although some differences existed between the two data, it was possible to isolate and identify the cause of discrepancy. Furthermore, the characteristic bandpass response of fluctuation strength on modulation frequency was observed in the obtained data.
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A.1. Procedure
Three subprocedures are defined, corresponding to the period before, during, and after the subjects participation.
A.1.1. Before
1. Verify that the sound interface is calibrated and working properly 2. Execute the corresponding experiment batch script1
3. Input participant’s assigned ID
4. Turn off computer’s monitor and wait for the participant to arrive
A.1.2. During
1. Present the participant with the informed consent form and ask him to sign it 2. Read the training phase prompt (A.2.1)
3. Present twice the pairs of non fluctuating and low fluctuating sounds, indicating before and after their reproduction the level of fluctuation they present
4. Ask whether a difference in sensation was felt between the sounds, if not repeat previous point
5. Present twice the pairs of non fluctuating and low fluctuating sounds, indicating before and after their reproduction the level of fluctuation they present
6. Ask whether a difference in sensation was felt between the sounds, if not repeat previous point
7. Read the rough tones prompt (A.2.2)
8. Play the remaining triplets of sounds in descending order according to their fluctuation strength
9. Ask for participant’s perception of fluctuation strength 10. Read the rough tones prompt (A.2.3)
1run_am_experiments.bat for AM tones, run_fm_experiments.bat for FM tones
11. Play the again the remaining triplets of sounds in descending order according to their fluctuation strength
12. Ask to the participant is the difference between the high fluctuating sound and the faster but less fluctuating rough tones is clear and agreed upon
13. Read the long stimuli prompt (A.2.4).
14. Play each of the long stimulus sounds, asking after each reproduction whether a difference between the sounds regarding their fluctuation strength was felt
15. Close the current experiment (pressing the ALT+F4 key combination) 16. Read the test trials prompt (A.2.5)
17. Observe the participant during the test trials completion. If he makes any mistake correct him
18. Read the before sections prompt (A.2.6)
19. Leave the experiment room and wait for the participant to finish
A.1.3. After
1. Copy the results files to a safer location, possibly to an USB Drive 2. Shutdown the experiment computer
A.2. Prompts
A.2.1. Training Phase
As you read in the informed consent form, this experiment is about an auditory sensation called fluctuation strength. Fluctuation strength is related to a fluctuating or circulating feeling that arises from certain sounds. For instance, ambulance sirens and washing machines have this quality, as their sounds have a certain movement, rotation or circulation associated to it.
Since the concept of fluctuation strength is difficult to explain with words, the first part of the experiment consists of a training phase, aimed at making clear what is fluctuation strength to you. During this phase I will be presenting you with different sounds with different levels of fluctuation strength. After the training phase is over, and the concept of fluctuation strength is clear, we will start with the experiment itself.
Now, we will start with the training phase.
A.2.2. Before Rough Tones
A.2.3. After Rough Tones
As you may have perceived, as we move from one sound to another between these three sounds, their rate of change or speed increases. You may have also noted that also the sound begin to resemble noise. However, the increase of rate of change does not mean that the sound fluctuates more, as the sensation between the first sound and the other two is difference. The first sound is considered to have a high fluctuation, whereas the other two sound are considered to have a lower fluctuation.
It is important that you focus on your sensation, on what you feel when you hear the sound.
Since the fluctuating is not related to the speed of the sound, I do not want you to count cycles in order to estimate a fluctuation strength sensation. Remember that the fluctuating sensation is sometimes that moves, that circulates.
Now I will reproduce the sounds again and I will ask you again what do you think about their fluctuation strength.
A.2.4. Long Stimuli
Now I’m going to present you with a couple of long duration sounds. Each sound is composed of shorter sounds, with silence pauses in between each of them. You will listen to approximately 10 sounds. What I want you to do is to pay attention to the sounds, and try to detect whether a difference of fluctuation between the sounds exist. I’m not interested in a specific pair of sounds, but in the set of sounds as a whole.
A.2.5. Test Trials
This is a test experiment, to familiarize yourself with the interface that you will use for the rest of the experiment. When you click on the Start button on the top right corner of the screen, the experiment will start. You will hear a pair of sounds, with a silence pause in between. So, it will be one sound, then a silence, then another sound. What I want you to do is to estimate how much does the second sound fluctuates with respect to the first sound.
In order to give an estimate, you will use the slider that is situated in the bottom part of the screen. For instance, if the second sound fluctuates more than the first sound, you will drag the slider to the right part of the area that it contains. In case both stimuli fluctuate around the same, you will position the slider close to the ‘Equal’ label. In case the second sound fluctuates less than the first, you will position the slider in the left part of the area that in contains. Lastly, if the second sound presents no fluctuation at all, you will position the slider below the ‘None’
label. When you are done with your answer, you can click the ‘Next’ button below the slider, and another pair of sound will be presented to you.
If by any chance you need to listen to the pair of sounds again, there is the ‘Repeat stimulus’
button to the bottom right part of the screen that you can use. Below this button there is also a progress bar, so you can see how much of the experiment is left for you to do.
Whenever you are ready you can click the ‘Start’ button and begin with these test trials.
A.2.6. Before Sections
We have just finished the training phase. The remaining sections constitute the actual experiment that we will be doing. These sections are similar to the test trials phase that you just did. Each section takes approximately 10 minutes. When you click the ‘Start’ button the section will begin.
Once you are done with it, the screen will close and another one that looks exactly the same will present to you. At this point you can choose to have a small break, go outside the isolation booth and stretch a bit, or you can continue if you wish.
I will leave now and close the doors behind me, so you are in a complete audio isolated environment. The doors are not locked, so you may exist at any time you want. Just bear in mind that the doors are quite heavy and you need to pull them strongly.