Flat attenuation and flat frequency response are two different concepts related to sound and hearing.
- Flat Attenuation: Flat attenuation refers to the consistent reduction of sound across the frequency spectrum. In other words, when using hearing protection devices (such as earplugs or earmuffs) with flat attenuation, they provide nearly equal reduction of sound levels across all frequencies. This means that the relative loudness of different frequencies is maintained, but at a lower overall volume.
For example, if you have flat attenuation earplugs and are exposed to a noise level of 100 dB, they would reduce the noise level by the same amount (let's say 20 dB) at all frequencies. This maintains the balance and natural perception of different frequencies while still providing protection from excessive sound levels.
- Flat Frequency Response: Flat frequency response refers to the accurate reproduction of sound across the frequency spectrum without any significant variations or emphasis on specific frequencies. It means that a device or audio system with a flat frequency response will reproduce all frequencies with equal or consistent amplification or attenuation.
For example, in audio equipment, a flat frequency response means that the system reproduces all frequencies (bass, midrange, and treble) with equal clarity and without any boosting or reduction of specific frequency ranges. This results in a balanced and accurate sound reproduction, where no frequencies are disproportionately emphasized or attenuated.
In summary, flat attenuation relates to the consistent reduction of sound levels across frequencies when using hearing protection devices, while flat frequency response refers to the accurate and balanced reproduction of sound across frequencies in audio equipment or systems.
It sounds like both flat attenuation and flat frequency response are a good thing, and generally that is correct. But using the term "flat frequency response" AS IT RELATES TO EARPLUG ATTENUATION (and not electro-acoustic reproduction devices as mentioned earlier), and ear canal resonances, is determined by first measuring the acoustic response curve of the ear canal at the eardrum, with the hearing protection device in place. The ear canal has it's own open ear response curve, and this response curve generally peaks around 2700Hz and a bell-curve response (+17dB peak approx) for about a 3 octaves range. Applying a broad-band stimulus, if you subtract the attenuated response curve of the ear canal at the eardrum, from the non-attenuated open ear canal response, you should achieve the ear canal's natural un-aided gain (response) to speak of it as flat frequency response. Looking at the equal loudness growth contours (loudness growth contours) you will notice at loud inputs, the sensitivity shown by the louder (eg. 95dB) phon lines look much like the ear canal's natural (un-aided gain) resonance! With common vocabulary, our ears own resonance, or some factor of it, is the amount that we need to reduce to retain normalcy in a moderately loud to loud environment. Our ears own resonance is what first causes loud signals to clip our uncomfortable limits.
On the contrary, flat attenuators are measured using very soft sounds in an anechoic chamber and comparing the aided thresholds to the un-aided thresholds, which are the softest sounds that a person can hear (ie. with and without the earplugs in place). Since we don't need hearing protection for very soft sounds, and because our ears are non-linear (sensitive changing across different inputs), it doesn't make sense to say a flat attenuator is better, or more natural than a flat response earplug just because it brings all frequencies down equally. It's different, and it may sound better to you....and that can always be subjective and not up for dispute. But it doesn't address the elephant in the room, and that is that we need help for loud environments. Further, the flat attenuating earplugs are generally only measured out to 8KHz (ANSI 1977) but because of their shape and structure, roll off much more above 8KHz where musicians detect much of the fidelity and softer passage of music are easily missed. We call this the space between the notes and the sibilance and overtones. Finally, numerous studies have been conducted on flat attenuating earplugs that have not been able to reproduce many of the original inventors claims regarding the amount of attenuation derived compared to the packaging. Is this due to design changes over the years or different test protocols - who knows? But, it's more important to focus on the actual sound quality, and look for any post effects of low attenuation. If you get ringing in your ears or head under water feeling (temporary threshold shift) after exposure with earplugs, then increase your filter's attenuation!
If you wish to learn more about the ear canal resonance and measurements taken at the ear drum, search using the terms REAG (Real Ear Aided Gain), REUR (Real Ear Unaided Response) and REOR (Real Ear Occluded Response) which should give your a good start.