The helmet protects the ears from direct airflow noise, but it also extends out into the high-speed airflow much more than ears do.

The overall picture is that a helmet’s thick material blocks high frequencies. But it exacerbates and amplifies low frequency sound and white noise. As well, a helmet confuses the ear’s capabilities for identifying direction of sound that’s incoming

If a helmet is helpful is a question of how fast the motorcycle is moving and what kinds of sounds the rider needs to hear.

It’s complicated, but wearing no helmet might be safer at low speeds because the driver is more aware. No helmet, is undoubtedly not safer at high speed because brains are fragile

Edit: a simple experiment for anyone is to put on a full size motorcycle helmet anywhere, and then you can understand how much your hearing is dampened by it. But I guess it’s probably no worse than the experience of someone driving a car, which is soundproof by design

Not at all. 30-40mph you can hit around 90 db inside the helmet. You’re not that much better with a helmet off either. Air moving over surfaces is loud, if not the helmet you are going to get it from the wind hitting your ears.

There are certainly helmets that try to optimize for noise but there is no single one fix beyond ear plugs.