A.2. Aural Style Sheets

Sound is, in the view of some Web designers at least, a vastly under-utilized aspect of communication on the Internet. Most applications for aural presentation of Web content today revolve around people with hearing loss, but in the future, I anticipate that we will see far more use of spoken language, background music, and mixtures of voice and music to enliven some user experiences.

The CSS2 Recommendation from the W3C defines a whole range of sound (aural) presentation qualities that can be defined in CSS. Collectively, these make up the components of an aural style sheet.

Here's a snippet of an aural style sheet, borrowed directly from the W3C's Web page on aural style sheets:

h1, h2, h3, h4, h5, h6 {
  voice-family: paul;
  stress: 20;
  richness: 90;
  cue-before: url(ping.au);
}
p.heidi {
  azimuth: center-left;
}
p.peter {
  azimuth: right;
}
p.goat {
  volume: x-soft;
}

Let's go over this style sheet fragment, line by line.

All headings will be spoken using a voice-family called "paul." A voice family is much like a font family; it contains a collection of minor variations on a voice. The headings will apply a stress value (determining the "contour" of a voice, i.e. the degree of difference in inflection in various parts of the sentences), of 20, which is pretty low.

The code defines a richness of 90, which is very high. Richness determines how far a voice carries, and affects what we might think of as "loudness." Before any heading is pronounced, an "auditory icon" called ping.au will be played. You can define cues to be played before and after any sound segment.

Any paragraph marked as an instance of the class "heidi" will appear to originate from the listener's left, in a vertical center of space. Paragraphs that are instances of the class "peter" will come from the listener's right side. All paragraphs spoken by the "goat" voice will be extremely soft.

You get the idea. Again, a full treatment of this topic is beyond the scope of this book, but I wanted you to gain something of an appreciation for the scope of what can be done.

With aural style rules, you can control the following characteristics of a voice or the spoken presentation of the information on your Web page:

• volume

• whether to speak words or spell them out

• pausing

• cue sounds before and after

• mixing (playing two sounds simultaneously)

• spatial location of the sound in 3-D space

• speech rate

• pitch and range of pitch

• stress

• richness

• how to speak punctuation (pronounce it or use it as pause cues)

• how to speak numerals (separate digits or numerical values)

The properties that control all of these factors are listed in Appendix B.