Every month, Google releases a report revealing updates from the prototype autonomous vehicles, detailing some of the changes and technologies being designed and redesigned for the self-driving machines. This month, the Google engineers are mostly discussing sound on two fronts. The first is when to use the horn and the second is the sound the vehicle makes when moving.
In the United Kingdom, inappropriate use of the horn is something the police can fine a driver for. The horn is reserved to warn other motorists when they pose a danger: it's designed to be used when the vehicle is moving and one needs to warn other motorists of your presence. It is not to be used when the vehicle is stationary (unless another vehicle poses a danger), to express anger at another person on the road, or to say hello or goodbye to people you know or might wish to know. Thankfully, the autonomous vehicle is not likely to desire to use the horn as a greeting, but the team of engineers have been teaching the vehicles how to use the horn. During the initial training, the engineers had the car only use the horn in the cabin rather such that the test driver could determine if this was a suitable time. Google programmed the car to use the horn differently as the report says: "We've even taught our vehicles to use different types of honks depending on the situation. If another vehicle is slowly reversing towards us, we might sound two short, quieter pips as a friendly heads up to let the driver know we're behind. However, if there's a situation that requires more urgency, we'll use one loud sustained honk."
As for the sound of the car, here Google did more than accept the noises that the prototype electric vehicles make. Instead, Google considered the whys and hows of how vehicles make their noise. Their engineers noted that other road users - particularly cyclists and pedestrians - use their hearing as well as their eyesight to alert them of a nearby moving or potentially moving vehicle. Google also noted that for people with impaired eyesight rely on the sounds of vehicles. To this end, the prototype car has been designed to hum in a certain way: when accelerating, the pitch increases and when decelerating, the pitch decreases, thus mimicking the sounds of other vehicles and providing audible clues as what the vehicle is doing. However, Google went further than simply accepting the electric motor hum from the prototype machine - instead they created "a little personality" and aimed to "create a unique voice" for the car. The team looked for inspiration in many places before settling on the distinct sounds of the machine. The team aimed for a friendly but futuristic noise. Unfortunately, there's no word as to the tone of the horn that the prototype car uses!