Breaking the (ecological) law: why illusory sounds don't make for safer cars

Electric cars are great for the environment, but they come with a problem - they're too quiet, and this makes them dangerous to pedestrians used to the loud noises of the internal combustion engine. One idea is to add noise to the cars, and Mark Changizi recently wondered whether perceptual psychologists could help design a better sound, using an illusion. I don't think it would work, and the reason is a nice example of why it's important to understand the relationship between the world and perceptual information about the world.

Events in the world create information. When a car moves towards you, for example, there are sounds coming from the engine, and the way these sounds change over time is information about the heading of the car. If it's getting louder over time, it's coming towards you; but if the pitch is decreasing at the same time, then the car is going to pass you, not hit you. Increasing volume and constant pitch, however, specifies a collision and the particular rate of change of these variables tells you about the time-to-contact. These patterns of change over time in the acoustic array are related to the way the car is moving via the laws of physics and are therefore informative about the details of the car's motion. Importantly, the relationship between these patterns is also governed by the laws of physics; increasing volume and constant pitch only go together when the car is really heading towards you, for example.

Every perceptual system has limits, though, and those limits are called thresholds. When a stimulus is outside a threshold (e.g. too quiet, or too high pitched) the perceptual system won't respond to it.  The problem with electric cars is that they are quiet at low speeds, and so the variation in pitch you need to detect a collision is small and possibly below threshold. You could make the cars louder, but this goes against one of the selling points of the cars - reduced noise pollution. Mark suggested creating an artificial sound, an illusion, in which the range of the variation in pitch created by the slowly moving car is amplified without just making the car louder. This is in principle possible, but it's a problem because it's breaking the law, and you end up with less information about the car than when you started.