Spectral Profiles

The spectrum of the sound from one source will differ from that of another source. Experiments in spectral profile analysis measure the auditory system's sensitivity to changes in the amplitudes of the spectral components of a complex sound. The schematic diagrams on the bottom of Fig. 16 display different spectra used in a spectral profile experiment. The spectra consist of several tonal components logarithmically spaced in frequency. As can be seen, the center

Figure 15 Both the basic MDI task and results are shown. The basic task for the listener is depicted at the bottom. The listener must detect a decrement in the depth of probe amplitude modulation (difference between low and high depth). When just the probes are presented, the task is relatively easy. When an unmodulated masker tone with a frequency different from that of the probe is added to the probes, thresholds for detecting a decrease in probe modulation depth are not changed much from the probe-alone condition. However, when the masker is modulated with the same rate pattern as the probe, the threshold for detecting a decrement in probe modulation depth increases greatly, indicating that modulation depth is difficult to detect when both the probe and the masker are comodulated. When the masker is modulated, but with a different rate (shown as a faster rate in the figure) than that of the probe, the threshold for detecting a modulation depth decrement is lowered. The waveforms are not drawn to scale (from Yost, 2000).

frequency component has a greater level than the flanking components. The listener's task is to discriminate between presentations of spectral profiles in which the center component has a level increment and complex sounds in which all components have the same level. The level of the increment in the center component is adjusted until threshold discrimination is achieved. A key element in these experiments is that the overall level of each complex sound is randomly varied over a very large range. The overall level of randomization makes it difficult for the listener to use the overall change in stimulus level or the change in level of just the level-incremented component (the center component) as the cues for discrimination. The listener appears to use the relative change in level that exists in the complex. That is, the change in level of the center component compared to the other, equal-level

Number of Frequency Components

Figure 16 Results from a profile analysis experiment in which the number of masker frequency components surrounding a 1000-Hz signal component increased from 4 to 42. The thresholds for detecting an increment in the 1000-Hz signal component (the center component) are shown in decibels relative to that of the rest of the masker component intensity. The asterisk indicates the typical threshold for detecting a level increment of a single, 1000-Hz tone. The level of the signal is expressed in terms of the signal-to-background level. (from Yost, 2000).

Number of Frequency Components

Figure 16 Results from a profile analysis experiment in which the number of masker frequency components surrounding a 1000-Hz signal component increased from 4 to 42. The thresholds for detecting an increment in the 1000-Hz signal component (the center component) are shown in decibels relative to that of the rest of the masker component intensity. The asterisk indicates the typical threshold for detecting a level increment of a single, 1000-Hz tone. The level of the signal is expressed in terms of the signal-to-background level. (from Yost, 2000).

side frequency components is the discrimination cue. As the data in Fig. 16 show, listeners are almost as good at detecting the change in relative level of the center component of the complex sound when there are 11 components as when this center component is presented as a single sound with no flanking components. When there are a few widely spaced components it is more difficult to judge the relative level change. When there are many, densely packed components, discrimination is also difficult, most likely because the components are close enough in frequency to directly mask each other. Studies of profile analysis describe the stimulus conditions that allow the auditory system to detect small changes in spectral shape that would be crucial for sound source determination.

In profile experiments the overall level was randomized. If other forms of randomization are used to make the stimulus conditions vary from trial to trial (e.g., the frequency content of a masker is varied from stimulus to stimulus), there is often considerably more masking than when the uncertainty about the stimuli is small. The extra masking resulting from stimulus uncertainty is referred to as informational masking, distinguishing this form of masking from that which occurs due to the direct interaction of the masker and signal.

All About Alzheimers

All About Alzheimers

The comprehensive new ebook All About Alzheimers puts everything into perspective. Youll gain insight and awareness into the disease. Learn how to maintain the patients emotional health. Discover tactics you can use to deal with constant life changes. Find out how counselors can help, and when they should intervene. Learn safety precautions that can protect you, your family and your loved one. All About Alzheimers will truly empower you.

Get My Free Ebook


Post a comment