amplitude envelope

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Our natural environment is rich in complex acoustic signals that provide us with information crucial to our welfare and survival. There are a number of different acoustic properties our brains use to extracting such information, but our research mainly focuses on one property in particular: amplitude envelope. Amplitude envelope refers to the changes in the amplitude of a sound over time, and is an influential property as it affects our perception of timbre. This is an important property of sound, because it is what allows us to effortlessly identify sounds, and uniquely distinguish them from other sounds. For example, we are easily able to identify a piano sound, and tell it apart from a trumpet sound.

Our research examines two kinds of amplitude envelopes: percussive and flat (see figure 2). Percussive envelopes are characterized by an abrupt onset followed by an immediate exponential decay. This amplitude envelope is characteristic of various impact sounds: two wine glasses clinking together, hitting a drum, slamming a door, etc. Flat envelopes, on the other hand, are characterized by an abrupt onset, an indefinite sustain period and an abrupt offset. These sounds are man-made and are used in a variety of different electronics, such as the dial tone when you’re making a phone call. Although tones with a percussive envelope are encountered much more frequently in the natural world, flat tones are often used in experimental settings because their qualitative properties are easy to manipulate and control.

Percussive tones are characterized by a lack of sustain with decay. Flat tones are characterized by an indefinite sustain period with abrupt offset.

In our lab’s previous research, the use of percussive vs. flat tones have had differential effects in multiple research domains. In an audiovisual integration experiment, visual stimuli of marimba impact gestures influenced duration ratings of percussive tones, unlike flat tones. In a memory association task (involving associations of melodic sequences and various household objects) participants remembered 60% more of the sequence-object associations when using percussive tone sequences compared to flat sequences. Therefore, there is a meaningful difference in using sounds with different amplitude envelopes.

Currently we are investigating the differential effects of percussive and flat amplitude envelopes in a temporal order judgment (TOJ) task and surveying the types of sounds used in auditory research by classifying the temporal structure of sounds used in articles published in the journal Music Perception.  In that survey (under review), sounds are classified into four categories: Percussive (i.e. piano, drum, etc.; Fig 2a), Flat (Fig 2b), or Other (i.e. vocal, clarinet, etc.). A fourth category, Undefined, was included when the information required to classify was unavailable.

A surprising 34.6% of studies did not adequately specify the temporal structure of sounds employed. Flat tones and percussive tones are the next most common in the survey, followed by non-flat tones.

Despite the importance of temporal structure on an overall sound, this parameter is often not specified in empirical research on auditory perception. This underspecification suggests that temporal structure is an area ripe for exploration, and has motivated the MAPLE Lab to focus a variety of studies on the ecological validity of sound using percussive and flat amplitude envelopes.

We would like to expand this approach to other articles in other journals, such as the Journal of the Acoustical Society of America; Attention, Perception & Psychophysics; Journal of Experimental Psychology, etc. It is likely that those journals used even more flat tones than percussive tones.