Step 2 is the hard part.

Matt Ginzton writes here.

Headphones That Let Sound in but Not Out

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When our son Dominic was born, I knew that my wife and I (and he) would start having diverging sleep schedules for a while, and I suddenly discovered a need for something I’d never considered before: headphones that let ambient sound in but not out (and also umpteen pieces of more traditional baby paraphernalia, natch). The reason being: if they’re resting or sleeping and I’m not tired but want to stay in the same room, can I work or entertain myself without bothering them, and also without completely isolating myself from them? If I’m listening to music and Vanessa talks or Dominic squawks, I want to be able to hear them.

This is the opposite of what people usually want when using headphones: usually, you want the best fidelity possible for the sound playing in the headphones, without outside noise or distractions. Allowing ambient noise to intrude on the music is not normally the goal. So, it’s not a problem with many ready solutions.

I’m familiar with the basic headphone designs: in-ear, and open- and closed- back on-ear. Open-back headphones tend to be comfortable and provide a good soundstage, but don’t isolate your ears from the environment (in either direction), so they’re not suitable for noisy environments (you’ll hear the noise mixed with your music) and they’re not suitable for quiet environments with other people nearby (they’ll hear a tinny, audible, annoying version of what you’re listening to). Closed-back and in-ear headphones tend to have a physical seal between your ears and the outside world, with the headphone drivers on the inside of that seal, so they isolate you and your music from the environment — good if you don’t want to annoy nearby people, or if you don’t want to hear ambient noise, but not so good if you need to react to what’s going on around you.

In addition to these different physical and acoustic designs for allowing or blocking sound transfer, there are also active noise-canceling headphones, which are of the closed or in-ear variety (to physically block most sound transfer), but further have microphones and some active electronics to pick up ambient noise, negate it, and play its negative mixed into your audio stream, to really minimize ambient noise. These are popular for use on airplanes or other loud environments.

What I’m looking for here can’t be an open design because those let too much sound out, and it can’t be a passive closed design because those let too little sound in. Instead, what I want is basically a variant of the active noise-canceling design with a reverse switch, so that it uses the same circuitry but instead of negating ambient noise and mixing it with the audio stream, actually amplifies ambient noise (to a degree proportional to the volume of the audio stream) and mixes it with the audio stream. Trouble is, I don’t know of, and couldn’t find, any noise-canceling headphones with a reverse or “uncancel” feature. (When I attempted looking for this, I did find a few people looking for the same thing, mostly for use while running/biking near traffic, and most of the time they were told to use open headphones, since outgoing noise transfer isn’t a concern in that case.)

While I couldn’t find any noise-uncanceling headphones specifically designed or marketed as such, I did find two families of products that do essentially the same thing. The first is video gaming headsets with voice chat support; in addition to headphones they have a microphone for chat, and while that’s there for the purpose of sending what you say to other people, they also mix your own microphone input back into your ears so you can hear yourself talking. The second I wouldn’t have come up with on my own, but was suggested by a gun-nut friend when I posted a question about this on Facebook: electronic ear muffs designed for use around intermittent loud noise sources like heavy machinery and, er, guns, which attempt to block dangerously loud noises but otherwise keep you immersed in your environment. By design, they’re more concerned with incoming sound than outgoing, but the best way of protecting your ears is to seal them away from external sound and then electronically add back only what they think you want to hear, and that sealed design minimizes outgoing sound leakage too.

Having learned this, I tried both of these approaches. I bought Astro’s Mixamp 5.8, which has quite the bag of tricks: it connects to any headphones and makes them wireless; it also uses signal processing tricks to try to make a Dolby Digital signal sound like it’s coming from a multi-speaker surround sound system even though the headphones only have 2 drivers; it also mixes the microphone input back into the headphone output, and even has a mixer dial to fade between “chat” and “game” audio. But you don’t have to use this with a game system; you can also connect the transmitter to a music player, in which case “chat” will just be your own voice, and “game” will be the music.

I also borrowed an Impact Sport Electronic Earmuff from another gun-nut friend. It’s battery powered and by default self contained; it feels like wearing construction earmuffs until you turn it on, at which point you hear a faint hiss as it channels ambient noise from external microphones into speakers on the inside of the sound seal. But it also has an “aux in” jack which lets you feed in music or other audio from any sound source with an analog output.

How’d they work? The Mixamp, not so well for this purpose — it has many other nice properties and does a good job of what it was actually designed for, but I couldn’t get the mic on any headset I tried to pick up enough ambient noise to un-isolate me from the room. Now, the chat headsets I tried have directional mics designed to to pick up the wearer and nothing else, so they’re completely not designed for the purpose I was using them for, and I can’t really blame them for doing a poor job. I also tried a couple of separate omnidirectional mics, but couldn’t get them to pick up any sound at all when used with the Mixamp. I’m no microphone expert, and don’t know what kind of microphone you’re supposed to use with this, but I’m guessing the output level of the ones I tried was too low. Anyway, I think the theory is sound, and if I had the right mic (omnidirectional, sensitive enough, and with the right output level), the Mixamp would do what I want, but I couldn’t get it to work in practice.

On the other hand, the electronic earmuffs? Uncanny. When you first put them on, you can hear almost nothing (like passive ear protection, they’re designed to present a strong acoustic seal). Then you turn them on — and auditorily, they disappear, and you can hear what’s going on around you. Then you plug in an external audio source, and suddenly you’re marching to the beat of a drummer only you can hear, while still hearing what’s going on around you. Exactly the effect I was looking for, and, I repeat, the effect is uncanny. I had no idea things like this existed. Now, this specific set of earmuffs is not exactly perfect for my needs — they’re stiff and not comfortable to wear for long periods, and they’re certainly not audiophile sound quality — graded as headphones they’re not great, but as something that lets me hear ambient noise while mixing in a private audio track no one else can hear, they totally win. And these are just one example of a product category; there are many others available from other companies, some of which are probably more comfortable and/or better sounding. (Some of these are designed for all-day wear for police and soldiers, so they’d better be comfortable.) All at a price, of course. But they do exist.

Moral of the story: these electronic earmuffs are pretty cool. And it’s good to have gun-nut friends.