When worn over the head, the helmet shall not absorb excessive volume of sound so at to pose as an accident hazard against any vehicle approaching from the back of the user. For ensuring this, the helmets shall be tested for their sound attenuation properties. The sound transmission loss shall not be more than 10 dB (A).
The JUSTY Audibility Test Apparatus for helmets consists of a rigid angle iron frame to keep a test head-form with the helmet under test mounted over it one end and a horn placed at the other end. The horn is held such that that the sound waves from it strike the helmet at its back in the same horizontal plane.
Both the horn and the head-form are mounted at a level 1.2 m above the ground. The horn is operated by a lead-acid storage battery supplied with the apparatus.
A microphone and a digital sound level meter are provided for measuring the sound level. The microphone is mounted inside the wooden head-form at the normal ear level of the user. Two microphones are mounted in each head-form, one on the left and the other on the right. The selection between left and right microphones is done with the help of a change-over switch.