“Untangling” Noise Canceling Headphones: The Facts and Fiction of Headphones and their Assistive Technology Applications

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Here is a post by Travis Snyder, who provides Assistive Technology to people of all ages all over San Francisco.

I see it a lot-- people recommending “noise canceling headphones” and either getting bamboozled into purchasing a product that is not what they’re looking for, or dropping fat stacks of cash on a set of headphones that their client doesn’t need (and will probably just be inconvenienced by). The terms “noise canceling” and “noise dampening” sounds like a difference of degrees, but they areactually two totally separate technologies with different applications as assistive technologies. In the first installment of my series on noise reduction technologies, I am here to explain the difference and help you distinguish between the most commonly employed noise reduction technologies by assistive technologists.

First off, noise canceling and noise dampening-- how are they different? Simply put, noise canceling headphones use a battery powered process to actually reduce a sound out of existence, whereas noise dampening headphones lessen the volume of the outside world by preventing the sound from getting to your ears. Noise dampening is really easy to describe. Just cover your ears with your hands. That is what noise dampening headphones do. They just use science and engineering to do it better.

However, while the technology is fairly simple around noise dampening, the marketing is not. So there are a few things to know. First, there are essentially two kinds of noise dampening headphones-- one with speakers that can play sounds (such as music) and ones without. The former are called “headphones” and the latter “earmuffs.” A lot of people aren’t aware of that distinction, but it’s a pretty common convention among suppliers to distinguish between the two. So, while it might be rather pedantic to correct a colleague over a difference in terms in conversation, keep in mind the difference next time you’re searching for headphones to save time and make sure you’re buying the right thing!

Second, noise dampening headphones go by many names. Too many if you ask me. And they can be pretty misleading. A few off the top of my head are “noise dampening”, “noise isolating”, and “noise reducing”. The most insidious one though is “passive noise cancellation”. There is no such thing as passive noise cancellation! That is simply a marketing ploy to get us to buy the “half priced” noise canceling headphones that don’t cancel noise at all! All noise canceling headphones require a power source (usually a usb chargable battery) to perform noise cancellation. Period. End of story. Science has not yet figured out a way to cancel noise without a power source (i.e. passively). So if your headphones don’t have an on/off switch they are not noise canceling!

Noise Canceling headphones are a different animal. They take advantage of a physical property of sound called phase cancellation that can occur when two sound waves collide and are added together. Take a look at the picture. If we think of the diagram in terms of what we hear, then the height of the wave represents how loud a sound is to us, while how far apart each successive hill or valley is from the next represents how high pitch or low pitch it seems. It’s important to know that when two sound waves collide they tend to combine, causing them to add their properties together. If when they combine, the hills and valleys of the two waves are matched together, then the highs get higher and the lows get lower causing the sound to appear louder (like in the first diagram). But if the positive value of the hills align with the negative value of the valleys, then the two sound waves cancel each other out causing the wave to flatten and the sound to seem quieter (see the second). That’s what Noise Canceling Headphones do. They have microphones that record the sounds in the environment around you and produce that wave’s opposite so the unwanted sound is canceled out.

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Noise cancellation is great in theory but it can’t block out everything. If you get a chance, put on a pair of noise canceling headphones and turn them on. You’ll notice a few things.

First, you can still hear. Cancellation is not absolute. That’s because an actual sound wave is a bit more convoluted than a drawing of one. There are a lot of natural factors at play in the real world that alter sound waves making it difficult for even the best noise canceling technology to produce a canceling wave that will reduce an incoming wave to a perfect zero.

A second thing you’ll notice is that with any pair of noise canceling headphones, it takes a few seconds for them to begin working when you first turn them on. That’s because it takes time for the headphones to record, process, and playback the canceling sound wave. For that reason, noise canceling headphones don’t have much of an effect on noises that occur suddenly such as speech or sudden environmental sounds like a car horn or the clanking of train tracks. What they are good for is quieting prolonged sounds, particularly lowish sounds (under 1000 hz), that go on at a constant pitch like the thrum of an engine or the buzz of an appliance.

The final thing you’ll notice is that as soon as you put on the headphones, even before you’ve turned them on, that everything will seem quieter. This is because all noise canceling headphones have, to at least some degree, noise dampening built in by their very nature of being placed  over your ears. It might seem obvious but noise canceling headphones use noise dampening too!

Anyhow that’s pretty much what you need to know about noise canceling versus noise dampening. In summary, be wary of the names and of the prices. You’re not going to get out of a store for much less than a couple hundred bucks if actual noise cancellation is what you’re looking for. However, much of the time it’s not really what you need. Hmm… what you need... Sounds like another part to a forthcoming mini series on headphones. ;-)

Until that time… Shop smart!  

Travis