From time to time I fit electronic hearing protection for musicians or hunters. The idea is that electronic hearing protection would provide slight amplification for soft level sounds but hearing protection (sound attenuation) for overly loud sounds. These devices are really just hearing aids in disguise – they are typically, but not always, digital, have a microphone(s), an amplifier, and a receiver.
There is a Personal Sound Amplification Product (or PSAP) that is analog that is quite useful for musicians and is based on the K-AMP analog circuit from 1988. This amplifies softer sounds and acts as an attenuator for louder sounds. And unlike most other devices, it does not distort overly loud inputs.
To my knowledge all other forms of electronic hearing protection are digitally based.
The issue arises when we look at the specification sheets for these devices. One case uses the same hearing aid reporting techniques to develop “specs” on these sound attenuators as we do for hearing aids- namely ANSI S3.22.
In ANSI S3.22 there is a measure called “attack time”. This is the amount of time that the circuit takes to enter into a non-linear or compression phase when an overly intense sound is detected. Unlike peak clippers, the compressor takes a moment to click in. Examining the specifications, we see numbers like 5 msec or so which is quite fast, but not instantaneous. A lot of impulse noise damage can occur before the electronic hearing protector activates and reduces the gain. In fact, for impulsive noises, permanent hearing damage can occur in the microsecond (fraction of a millisecond) range.
In order to understand why electronic hearing protection can work and still be protective even with sudden impulse blasts- gun shots or perhaps percussive music- one needs to understand where the number 5 msec comes from.
According to ANSI S3.22 in order to test attack time, the device is first presented with a 55 dB SPL signal, then an 80 dB SPL signal, and then with a 55 dB SPL signal again, this last one being to test for “release time”. (In the new version of ANSI S3.22 attack time and release time is being removed for a number of reasons, and relegated to the annex). For fast acting compressors, with this jump from 55 dB SPL to 80 dB SPL, attack times of 5 msec are indeed quite common, but it’s an artifact and a function of this jump from 55 dB-80 dB.
If the jump were from 55 dB SPL to 120 dB SPL, which is more realistic for music (and for hunting), this identical hearing aid would now have a value of less than 1 msec- perhaps even in the microsecond range.
So a “spec sheet” that says 5 msec may sound like an awfully long time for a gain reduction circuit to activate, for more real life impulses, may be more than sufficient to minimize the effect of these occasional, but annoying impulse blasts.