The researchers at Carnegie Mellon University and Disney Research drew inspiration from wind instruments in devising these mechanisms, which they call Acoustruments.
    
The idea is to use pluggable plastic tubes and other structures to connect the smartphone's speaker with its microphone.
    
The device can then be controlled by acoustically altering sounds as they pass through this system.
    
Just as a simple slide whistle or flute can produce expressive music, these Acoustruments can add a wide range of functionality to a smartphone, including proximity and pressure sensors.
    
And because no electrical circuitry is involved, the plastic Acoustruments can be made rapidly and inexpensively.
    
"We're providing people with tangible interactivity at basically no cost," said Gierad Laput, a PhD student in Carnegie Mellon's Human-Computer Interaction Institute (HCII).
    
"Using smartphones as computers to control toys, appliances and robots already is a growing trend, particularly in the maker community," said Laput, lead author of the study.
    
"Acoustruments can make the interactivity of these new 'pluggable' applications even richer," said Laput.
    
People who want to control a smartphone application while their eyes need to be focused elsewhere, for instance, might be less distracted by a tangible knob or button made possible with Acoustruments than with trying to glance at a touchscreen.
    
Applications that use smartphones as virtual reality displays make it impossible to use the touchscreen controls at all; Acoustruments enable users to make adjustments without disrupting the virtual reality experience.