The overriding theme of the human psychophysical research effort is to better understand those stimulus variables that enable us to determine the sources of sounds in our environment. That is, sounds from the many sources that surround us are combined into one complex sound field which is the sound that ultimately arrives at our ears. We do not perceive a single complex sound, but rather the sounds of the different objects (sources) that produced the complex sound field. Thus, it must be our "brain" that processes the complex sound field input so that the individual sounds or the sources are perceived. In order for the brain to accomplish this task there must be variables of the incoming sound that it can use to parse the complex sound field into the individual parts that are associated with the various sources. Research in the human psychophysics laboratories is conducted on three major topics: pitch and timbre perception, sound source localization, and temporal envelope or modulation processing. These three aspects of sound could be used by the auditory nervous system to help it in determining the sources of sounds that generated the complex sound field. In our work we use both real sounds in real environments, and sounds produced over headphones sometimes in situations that mimic a virtual auditory environment.
A defining characteristic of sound is often the pitch of the sound. Often two sounds will have the same pitch but will be perceived as different due to a difference in sound quality. Sound quality is often called timbre. For instance, two different musical instruments playing the same musical note will produce the same pitch but because they are different instruments they also produce different timbres. Our research is aimed at understanding how the various physical stimulus variables of sound lead to the perception of pitch and timbre and how the nervous system might process these variables. We also are interested in how these perceptions might aid the auditory system in determining the sources of sounds, especially when many sources are producing sound at the same time.
We can use our sense of hearing to determine the location of a sound source in three-dimensional space. When the location of a sound source is at one location in the horizontal (azimuth) direction, the sound will reach one ear before it reaches the other ear (the ear closest to the sound) and will be louder at the ear at which it arrives first. Thus, there is an interaural (between the ears) time difference and level difference for a sound source at a particular location. Both of these interaural variables will change when the location of the sound source is changed. A major component of our research effort is devoted to understanding how these interaural variables are used by the auditory system in determining the location of a sound source, especially when there is more than one sound source producing sound. It is also the case that most of the time the sound we receive from a source is combined with echoes of the sound (e.g., echoes off of the ground). One of our research efforts is understanding why it is that we are rarely confused when listening to sound when there are echoes. That is, we almost always are able to accurately locate the source of the sound and we are not even aware of the echoes.
Almost all sound sources produce a slow temporal change in loudness level as a natural consequence of the way in which sounds are produced. Each object is likely to generate a different temporal pattern of level modulation. Thus, these differences in the pattern of the slow modulation in level might be a cue used by the auditory system to segregate one sound from another. Many of our experiments are designed to try to better understand how the auditory system processes slow temporal changes in stimulus level (or in other sound variables). We are especially interested in how this processing might aid the nervous system in determining the sources of sound.