Negative frequency sound

1. Can sound frequency be negative?
2. What are negative frequencies?
3. What happens when frequency is negative?

Can sound frequency be negative?

Scientists have demonstrated for the first time how 'twisted' sound waves from a rotating source can produce negative frequencies - akin to turning back time.

What are negative frequencies?

Negative frequency is an idea associated with complex exponentials. A single sine wave can be broken down into two complex exponentials ('spinning numbers'), one with a positive exponent and one with a negative exponent. That one with the negative exponent is where you get the concept of a negative frequency.

What happens when frequency is negative?

The meaning of negative frequencies is just mathematical(not physical) similarly to the imaginary part of a complex signal. In real world, the negative frequency does not exists and the spectral content on negative frequencies must be added to the spectral content at the positive frequencies, to save energy.