Design Drivers
To help our users carol with confidence, Whispr needs to be comfortable and have good sound attenuation. Six metrics were identified to drive our design further. The following describes how the team determined and validated these features.
Table A. Design Drivers
Sound Attenuation Materials
Materials need to be chosen such that the user's voice is muffled adequately. To determine the materials, a physical experiment was conducted comparing several materials relative to one another. Acoustic fabric was selected because it showed the largest difference in measured audio level. See Parameter Analysis for more information on this test.
Noticeable Delay
Whispr’s electronics provide live audio feedback. This needs to be seamless such that the user will not notice any delay. Components were selected such that the total audio delay was less than 6 ms, the upper limit of what a human can audibly notice. The circuit was tested with users after it was built as well, with no noticeable delay reported. See Validation for more information.
Fit
Because Whispr is a wearable device, fit is an important consideration. The seal against the mouth is an especially important consideration. This was determined by using usability testing with a wide user base. Users put the mask on and were asked if they thought the mask was comfortable, and to give a rating out of ten. The average score was 6.5, which beat Amazon reviews for Belt Box's score of 6.4, a competitor to Whispr. See Validation for more information.
Reliable Straps
The straps need to be able to support Whispr’s weight without sagging. Failure mode and effects analysis and first principles analysis were used to determine how strong the straps needed to be. See Parameter Analysis for more information.
Size and Weight of Mask
The size and weight of the mask are important considerations. Minimizing both will make the user more comfortable, physically and emotionally. GRANTA software was used to determine which material to use for the shells. The material had to be compatible with injection molding, non-toxic, lightweight, and have low to moderate ductility (~10% maximum). ABS was selected because it best fit our goals. Packaging was also a concern to minimize the mask's weight. Whispr's electrical components were carefully soldered such that its wires fit within the small gap between the shell and IDU. Finally, the device was weighed with all components to ensure it is lightweight and meets our target of less than two kilograms.
Attachment of Components
The internal dampening unit (IDU) and external shell need to have a reliable attachment mechanism so it doesn’t fall or slip during use. Guide rails were implemented so that users could intuitively attach the IDU to the shell and know when it was secure.