How to make drones sound less annoying

These days almost everyone has either flown a drone or listened to the nasty whining sound they produce. Although small drones (up to 20 kg) are about 40 decibels quieter than conventional civil aircraft, they produce a high-pitched noise – which people tend to find very annoying. One NASA study found that drone sounds were more annoying than those made by road vehicles. And my own research has found that the noise of drones is less preferable than that of civil aircraft – even at the same volume. Part of the problem is that drones often fly at relatively low altitudes…

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