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Earcons as a Method of Providing Navigational Cues in a Menu Hierarchy
These pages describe an experiment to investigate the ability of earcons to provide navigational cues in a hierarchical menu structure. In some situations graphical feedback cannot be used to provide these cues. In completely auditory interactions, such as telephone-based interfaces or those for visually disabled people, it is impossible to use graphical cues. In other systems where graphical feedback is available, the display may already be completely occupied by information that extra graphical cues would hide. For example, an interface for people with speaking difficulties who need to access a library of pictographic images. The link between these different situations is that a hierarchical structure must be represented without graphics. Papers on this work can be obtained from my publications list (numbers 15, 21, 29, 31, 33 and 35).
An overview of the experiment will be given here with demos of the earcons created. For more details see the papers. To download a HyperCard stack similar to that used in the first experiment click here. This is 5.2 MB to download and requires a Macintosh with reasonable sound output hardware (the sounds in the stack are 22kHz, 16bit). It does contain all of the sounds used but they are not stereo, otherwise the stack would have been 10.2 MB, which is a bit excessive. The stack has been BinHexed and compressed with StuffIt.
The aim of the experiment described here was to discover if a larger hierarchy could be represented by earcons. Figure 1 shows the hierarchy used. It had 25 nodes on four levels with four missing nodes on Level four (two of which are marked as A and B). This made a hierarchy of 27 nodes, three times larger than that tried previously. It was loosely based on the structure of the file system on Brewsters computer.
Figure 1: The file-system hierarchy used in the experiment. A and B show the two new earcons presented to participants during testing.
HypothesesThe main hypothesis was that participants should be able to recall the position of a node in the hierarchy by the information contained in an earcon. If this was correct then high overall rates of recall would be expected. Participants should also be able to listen to an earcon and position it in the hierarchy even if they have not heard it before by using the rules from which the earcons were constructed. This would be demonstrated by high rates of recognition when participants were presented with new earcons.
The hierarchyThe hypercard stack contains the hierarchy from the experiment. You can click on the arrow buttons on the stack to take you up or down in the hierarchy. There are also left and right arrows that take you across the hierarchy. Each time you move to a new node in the hierarchy a new sound for that node is play. In the rea experiment these continued playing, in the demo here the sounds only play for 4 seconds to save space. The description of the sounds are given in the next section. Try navigating around the hierarchy by using the stack. The picture at the top of this page will help you if you get lost.
Sounds usedThe earcons were designed using the guidelines proposed by Brewster et al. (1995). The sounds were all played by HyperCard on an Apple Macintosh via MIDI using a Yamaha TG100 sound synthesiser and presented to participants via loudspeakers. The sounds used at each level of the hierarchy will now be described:
Level 1: For the top level of the hierarchy (Main in Figure 1) a constant sound with a flute timbre was used (see Table 1). It had a central spatial location and a pitch of D3 (261Hz). A flute timbre was used at it is a pure sound close to a timbreless sinewave. The earcon was designed to be neutral sounding. As mentioned above, in the demo stack you can download from this page the sounds are not in stereo to save space.
Level 2: Each family had a separate timbre, register and spatial location. Table 1 shows these. Register was lowest on the left and highest on the right following the conventional musical pattern. The stereo position of the earcons also moved from left to right mirroring their position in the hierarchy.
Table 1: The timbre, spatial location and register for Levels 1 and 2 of the hierarchy.
The continuous sound was inherited from the Level 1 earcon but the instrument, pitch and stereo position were changed. Three parameters were used so that if the listener could not remember which instrument went with which node he/she could still use register or stereo position.
Level 3: At this level rhythm was used to differentiate the nodes. Each left node had one rhythm, each centre node another rhythm and each right node another. Figure 2 shows the rhythms used. From Figure 2 Graphics, Letters, Earcons and Doom all had the left node rhythm, Microsoft Word, Reports & Papers, Buttons Experiment and Adventure were centre nodes and General Programs, Manuals, Scrollbar Experiment and Arcade Games were right nodes. Each of these rhythmic groups repeated continuously once every 2.5 seconds. As Figure 3 shows, the first note in each group was accented. The last note of each group was also lengthened slightly. These two help make each group into a complete rhythmic unit ( Brewster et al., 1995) .
Figure 2: The rhythms used for Levels 3 and 4 of the hierarchy.
At this level the earcons inherited timbre, spatial location and register from Level 2. This meant, for example, that Graphics used the left node rhythm described in Figure 3 and it was played with an electric organ timbre, on the left side of the stereo space and in the register of C4. Letters used the same rhythm but, in this case, the timbre was a violin, stereo position was centre left and the register was C3.
Level 4: A faster tempo was used to differentiate the items. The rhythmic units from Figure 3 now repeated once every second. In addition to this the effects reverb and chorus were applied to all of the earcons. These gave the earcons a much fuller sound. This time rhythm was inherited from Level 3. Each of the nodes in Level 4 used the same rhythm as its parent node but the earcons were repeated more frequently.
Prof Stephen Brewster
Department of Computing Science,
University of Glasgow,
Glasgow, G12 8QQ, UK
Tel: +44 (0)141 330 4966
Fax: +44 (0)141 330 4913
Last Modified: December 29, 2000