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In How I Met Your Mother S02E01, Robin & Marshal are shooting in a shooting range and having a conversation.

enter image description here

How can the conversation be audible to them when they've still got the ear muffs on?

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    I have worn all kinds of hearing protection and haven't yet worn anything that can actually block all sound. It's more like turning the volume down on the world. – Todd Wilcox Mar 31 '17 at 13:13
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    Lots of ear defenders are designed to block some frequencies (gunshots) more than others (human voices). – MissMonicaE Mar 31 '17 at 14:28
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As a sports shooter of now almost 23 years I can tell you: It's not an issue! You hear each other a bit muffled but still very well understandably. You only have to raise your voice because of the gun noises around you, not because of the muffling by the ear protection. We normally use this type here:
Picture of ear protectors.

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I don't see this as much of a tv conceit. The earmuffs usually used on shooting ranges do not go anywhere near blocking out all sound. It is quite standard to carry on conversations while having them on I've done so every time I've been at a shooting range with someone else. I assume we tend to raise our voices a bit, but I don't think it's even all that much.

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    I feel like it'd be unsafe if they blocked all sound. I mean, if you're dealing with guns, you probably wanna know what's going on around you, outside of your (relatively) narrow field of vision. – Fund Monica's Lawsuit Apr 2 '17 at 2:30
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There are special electronic ear protectors that can do that.

Guns.com

The premise of electronic muffs is quite simple. While the passive muff provides hearing protection, battery powered electronic components inside the electronic muffs include microphones, amplifiers and speakers that gather, amplify and transmit low volume sounds — like voices — inside the earpiece.

Price differences in muffs generally dictate quality of components and advanced capabilities. For instance, lower cost muffs may have only one or two microphones and a basic stop-type amplifier that simply shuts off upon high decibel sounds. This type of shutoff causes split second time lapses before the amplifier resumes and low tones are again transmitted to the ear. Cheaper muffs with fewer mics often make it difficult to ascertain the direction of the sounds as well. Higher end models use more advanced technology that allows the continuous flow of low volume sounds while at once blocking dangerous noises. They also contain multiple mics and speakers, making it easy to determine the direction and distance of the sound, which is crucial in hunting, tactical and competition applications.

enter image description here

There's a more complex description & explanation at earplugsstore.com

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    Unpowered ear protection allows conversation as well. Electronic muffs just make it a little easier. – TigerhawkT3 Mar 31 '17 at 23:45
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One thing to note is that the sound of a gunshot and the sound of 2 people talking have an entirely different frequency, volume and length. The human voice has a frequency of around 85-250 Hz, a volume of around 60 db and usually a length of a couple of seconds. A gunshot from a small caliber weapon is around 150-2500 Hz in frequency, has a volume of around 130-160 db and usually lasts only a fraction of a second. These are sounds with entirely different footprints and as such can relatively easily be attenuated independently. There is in fact a study from Finland: https://www.ncbi.nlm.nih.gov/pubmed/7761796, that states that large-volume earmuffs can be used to protect against gunshot sounds.

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    You are mixing up the pitch of a human voice and it's frequency content. The frequency content of speech spans roughly from 250 Hz and 4 kHz. It is very difficult for passive attenuation to differentiate between speech and gun shots. – StrongBad Mar 31 '17 at 13:10
  • Indeed, Plain Old Analogue Telephone only transmits 300Hz–3kHz, and most digital speech codecs try to mimic that (for one, because it's good enough™, and secondly because when digital telephony was introduced, it would have sounded weird if you would get different bandwidths depending on whether the person you called had a digital or analogue line). If human voice really were in the range of 85Hz–250Hz, you couldn't hear voices over the telephone. The reason why gunshots and other similar sounds are dangerous is because our ear is especially sensitive in areas around 2500Hz, because that – Jörg W Mittag Apr 1 '17 at 8:13
  • … is where a lot of the information content in human speech lies. – Jörg W Mittag Apr 1 '17 at 8:13
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Earmuffs for hearing protection are designed for just that: hearing protection. They are intended to bring volumes down to a safe level that won't cause injury or damage the wearer's hearing. Each model is rated for a given reduction in volume, listed as NRR (noise reduction rating), generally in the range of 20-40 decibels. If they blocked out all sound, they wouldn't be useful for their normal use cases, such as shooting ranges, airport tarmac, construction sites, and other noisy areas. These areas have other sounds that are important, such as cease-fire orders, conversation with coworkers, and so on.

The non-total nature of hearing protection muffs often surprises people who try to use them to block out quiet but distracting noises, such as nearby conversations or music. I know of no passive device (i.e., not producing white noise or other sound to drown out the outside noise) that can do this.

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