This post continues from our previous post concerning the Noise Inside Your Helmet. To understand how hard it is to block low- frequency sound, let us consider the standard off-the-shelf, ear-muff style, over the ear (circum-aural) ear protection. Earmuffs work passively by forming an air-tight seal around your ear, and along with the hard exterior shell, act as a barrier to prevent the harmful pressure variations from reaching your ear drums. Inside, there is usually foam material, which absorbs the higher-frequency sound that might enter the ear cups and otherwise bounce around inside.
On the surface, a helmet seems to have everything you might need – a hard, rigid external shell, some crushable foam, and soft foam padding. All the elements needed for noise cancellation? Except for one thing – no airtight seal. The neck opening in your helmet is a wide-open source allowing the air-pressure variation (noise) to freely enter into your helmet. The outer shell, foam and other materials do indeed stop high frequencies, which is why it is often difficult to hear someone speaking to you, but this neck opening allows that intense low-frequency sound pressure to enter your helmet and your ears.
Easy solution – just make an airtight seal around the neck with the helmet…but how do you get the helmet on and off and how long can you hold your breath? Impractical!
Finally, lets do a quick demonstration of the low-frequency noise problem and the need for an air-tight seal. If you have a set of over-ear hearing protectors at home, get them now. You will also need a PC or Bluetooth speaker of some kind, the larger the better. Go to a webpage such as https://onlinetonegenerator.com/ and generate a test signal of 100 Hz (the peak of motorcycle helmet noise) over your speakers. You should hear a noticeable difference in sound level between having your ear protection on and off. Next, with the ear-protectors on, slightly lift the edge of the headphone so there is a tiny crack – and the sound will come in much louder.
A serious issue with passive hearing protection is that it tends to be less effective at low frequencies than active methods.
Active Noise Cancellation (ANC) works best on frequencies below 1 kHz. In fact, standard consumer ANC headphones generate most or all of their noise cancellation below 1 kHz actively. The remaining noise cancellation, which is above 1 kHz, is due to the passive effects, by blocking sound (e.g. the hard outer cover) and by absorbing it (e.g. foam), and mostly, keeping a close to air-tight seal around the ear. The same principles apply to in-ear buds.
In the next blog entry, we will look at our motorcycle in-helmet ANC system performance.
