Maxed Out: A Noise-Canceling Primer

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Do you travel much? With me, things sort of go in phases. Sometimes I can go six months without any travel at all, and then suddenly I find myself jetting all over the place.

This reminds me of a computer I was working on around 25 years ago. In the schematics I saw a register called the LBW. I asked someone what this stood for and they said “London Bus Register.” The idea was that in London you can stand around waiting for a bus for ages, then a herd of them come all at once.

And that story reminds me of another computer. In the schematics I found a register called “Banana.” I tracked down the designer of this portion of the machine, who informed me that he had called this unit the “Banana Register” because “the data comes in bunches” (honest – I couldn’t make this stuff up).

There is a reason I asked about your travel habits...

 It Was a Gray Morning

This incident occurred in the late 1970s when I was a student in Sheffield, England. I was on a four-year Control Engineering degree in which we spent nine months at the university, and then six months in industry, then back to college, then back to industry, and so forth.

In order to get a position in industry the university set you up with a number of interviews. We were all terrified of not finding a position somewhere. My dad bought me a suit. I was poised for action…

My first interview was with a company located in a small town outside of London. This meant that I needed to catch a train from Sheffield to the center of London, about 170 miles away, and then catch an underground train out to the interview.

I awoke on the morning of the interview and looked at my alarm clock. Oh no! I had slept through the alarm. I raced to have a shower, get dressed and catch a bus down to the center of Sheffield to catch the next train to London.

While I was at the station waiting for the train, I called my mom and asked her to call the university and ask them to call the company to tell them I was going to be late.

I spent the entire journey on the train fretting and beating myself up (mentally – I didn’t want to mess up my suite by hitting myself physically). When I arrived in London I ran to the nearest underground station and caught the first train on the line that passed through the town in which I was to have my interview.

After about 20 minutes it dawned on me that I was going in the wrong direction. Arrgggghhh! I jumped off the train at the next station and caught the first train going the other way.

When I eventually arrived at the company I was emotionally drained to say the least. I asked the receptionist to inform the appropriate people that I had arrived. When they came down into the lobby I apologized abjectly for being late. You can only imagine my feelings when they replied, “What are you talking about? You’re a day early – we weren’t expecting you until tomorrow!”

But that’s not what I wanted to talk to you about.

Noise-Cancelling Headphones

Last week I flew out to Redmond, Washington, to give a presentation to a bunch of hardware design engineers at Microsoft’s headquarters. I’m not sure what they were expecting, but what they got was me bouncing back and forth through time, metaphorically speaking, from Hero (or Heron) of Alexandria, circa 10 – 70 AD, to the far future. I don’t like to restrict myself unduly.

I must admit that they looked a little glazed when I finished, but when we met up for a chat afterwards they were very complimentary. Of course they may just have been humoring me in case I was dangerous! The guy who took the podium after me commenced by saying, “I have no idea how to follow Max’s presentation,” which I’m taking to be a compliment.

The thing is that I had left my noise-cancelling headphones at home. These are Bose headphones that I purchased a couple of years ago. The idea behind noise-cancelling headphones is that each earpiece has a small microphone on the outside. The microphone detects the outside sounds from an aircraft’s engines and air system and so forth – these sounds are then amplified and fed into the earpiece itself along with whatever audio signal (music, film, whatever) you want to listen to.

“Hang on,” I can hear you saying to yourself. “Why would you want to amplify the external sound? Doesn’t that make it worse?” I guess this is best explained using the image in Figure 1. 

Figure 1. Diagram of noise-canceling headphones.

Note that this image only reflects what happens to the left-hand earpiece – the same thing would be happening independently on the right-hand side. The thing is that the amplified version of the external signal is fed back with the opposite phase, which means it cancels out the external sound and only leaves you with the signal you want to hear.

Let’s take this one step at a time. We start with two signals – the audio signal we really want to listen to (shown in green) and the external noise source (shown in red to the left of the earpiece). Of course, the fact that the headphones are padded means that only an attenuated version of the external sound will get through to your rear (shown in red to the right of the earpiece). But when you are on an aircraft, this is still sufficient to really impact what you are trying to listen to.

The usual answer (with non-noise-cancelling headphones) is to “wind up the volume” on the audio signal you really want to hear. But if you make this loud enough to overcome the noisy external signal, you are at risk of damaging your ears.

So what the noise-cancelling headphones do is to pick up the external sound using the microphone, swap it around so it’s 180 degrees out of phase with the original signal, and feed this into your earpiece (shown in blue to the right of the earpiece).

In the same way that out-of-phase ripples in water cancel each other out, so do the red and blue signals shown in the image above. The result is to leave only the (green) signal you wanted to listen to in the first place.

Unfortunately, this works best with repetitive signals like aircraft engines and suchlike – they have not yet perfected the technology to filter out a screaming kid or your wife!

Now, when the Bose headphones first came out, everyone was raving about them. I must admit that they do significantly reduce the ambient noise, but, truth be told, they didn’t do quite as good a job as I had hoped. I’m sure that today’s models are much improved, but “once bitten, twice shy,” as they say.

So while I was as the airport in Seattle I went into a small travel electronics store and asked the young guy behind the counter, “What are the best noise-cancelling headphones?” He replied that if I was looking for the best noise cancellation, then he would recommend the Sony MDR-NC500D headphones. However, if I was looking for the best possible audio quality coupled with a little less on the noise-cancellation front, then he would recommend an alternative whose name escapes me.

I opted for the Sony MDR-NC500D headphones and I am 100% delighted with the result. In one of the films I was watching on my iPad, there was a part where we were in a forest and the only sounds were crickets and birds and suchlike. Everything sounded as clear as one could hope. You really have to try these little beauties to fully appreciate just how amazing they are, but you will have to purchase your own – the only way anyone will get mine is after a fight when they pry them from my lifeless hands (grin).

On the one hand these were really rather expensive; I’m embarrassed to say just how much they cost. On the other hand, if you divide the cost of these headphones by the number of hours I will be using them over the course of the next few years, then I personally feel they are well worth it – after all, who deserves a little luxury more than yours truly?

Until next time, have a good one!

Clive (Max) Maxfield is founder/consultant at Maxfield High-Tech Consulting. He is the author and co-author of a number of books, including Bebop to the Boolean Boogie (An Unconventional Guide to Electronics) and How Computers Do Math featuring the pedagogical and phantasmagorical virtual DIY Calculator. To contact Max, click here.  


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