Chris Plack, my friend in England, describes the problem clearly in his book The Sense of Hearing. Below is a figure from his book that illustrates what our ears do on a daily basis. In the figure below, Chris is shown being presented simultaneously with the sounds of a car engine, a speaker, and speech. The sound waves add to form a single waveform and in order to hear out the sound we want to attend to, the ear has to segregate the relevant sounds from the combined waveform.
To learn what cues the brain uses to perform this task, we are studying listeners' ability to hear loudness differences for a single tone presented in isolation and in the presence of 8 distracter sounds. Below is an example of the tone presented in isolation. It will be presented in pairs, and listeners have to judge whether the first or the second tone in the pair is louder. The difference in the level of the tone is 4 dB; most people can hear a 1 dB difference so the louder tone in the pair should be discriminated easily.
In the following sequence, you will hear 4 pairs of tones. Judge whether the first tone in the pair or the second tone in the pair is louder.
Four pairs of tones in isolation.
Answers:
1. First Tone
2. Second Tone
3. First Tone
4. Second Tone
Now listen for that same tone again and try to ignore the eight distracter tones.
Four pairs of tones in the presence of simultaneously presented distracter tones.
That was probably harder. I will have to check the answers after spectral analysis of the stimuli. I can't trust my ears on this one or the next one. I'm at near chance performance (50%) on this task. A study participant with absolute pitch was able to hear out the target tone and had a near perfect score after judging hundreds of pairs.
This time (below) the distracter tones will be presented but the target tone will be turned on with a roughly 1/3-second delay. All of the tones are terminated at the same time. Listeners find this task easier. The delay in the onset of the target tone makes it easier to attend to. I perform near 80% on this task.
Four pairs of tones in the presence of distracters with the onset of the target delayed.
We tend to judge sounds that come on together as originating from a single source. That's likely why it was harder to segregate the single tone from the distracters in the previous condition. We are also examining other acoustic cues, such as frequency modulation, that might assist in enabling listeners to hear out the single tone from the distracters.
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