Noise Cancellation Information

When in the market for headphones to pair with your favorite playback device, you may have come across the term Noise Cancelling, Active Noise Cancelling, or even Passive Noise Cancelling Headphones. Seems like a terrific invention! Who wouldn't want to block out or cancel unwanted noise? Not so fast. Unfortunately this term has many meanings depending on the context, and marketers of such devices do not always follow the same definition. The following is an attempt to decode this ostensibly wonderful device and figure out exactly what noise cancelling headphones consist of. 

The following are a few terms to aid with the hopefully helpful explanation of Noise Cancelling Headphones. I highly encourage one to familiarize themselves with these definitions. 

Sound-  Vibrations caused from a source which travel through the air (creating literal waves) sometimes referred to as sound waves.

Frequency- The rate per second sound waves travel through the air. 20 Hz (Hertz - unit of measuring sound a la cup, meter, mile, pound etc.) means there are 20 vibrations occurring per second. 20 Khz or 20,000 hertz means there are 20,000 vibrations per second. Typically a lower occurrence results in a deeper sound where as more vibrations per second results in a higher pitched sound. A man's voice is typically 85 to 180 Hz where a woman's voice resonates between 165 Hz to 255 Hz. 

Hearing- Ears capture sound waves (air vibrations) and channel them down towards the eardrum. Sound waves hit the cartilage and physically move it causing it to vibrate. The vibrations are interpreted by the brain as sound. Humans are capable of hearing Frequencies ranging from 20 Hz to around 20 Khz. 

Example- Think of a long piece of yarn with a soup can attached on each end. Pull the yarn taut and speak into one end. The vibrations emitted from ones voice are captured by the soup can and cause the yarn to vibrate all the way down to the other end.  These waves (vibrations) are then amplified by the Soup can at the other end. The vibrations travel through the air and hit your ears where your brain interprets the sound.

Noise- Sound, either wanted or unwanted depending on the context. For instance, noisy neighbors is a form of unwanted Sound waves or vibrations traveling through the air. Listening to your favorite song, watching a movie, or perhaps having a conversation is a form of wanted sound (You want to hear these sound waves).

Ambient Noise- Often referred to as Background noise. Such noise pertains to any sound or audible vibration other than that of the primary sound (wanted or desired sound). Examples of ambient noise include traffic, motors, alarms, white noise (commonly referred to as static), as well as animals or people talking (a crowded venue). 

Electrical noise (often shortened to just Noise)-  Everything that has electricity flowing through it produces electrical noise, or a certain frequency. Imagine this frequency as a kind of humming. Some electrical devices emit a very High frequency that is above most humans ability to hear. Some frequencies are lower than one's ability to hear.  The analog (none electrical) equivalent of this would be a dog whistle. Blow into a dog whistle and the frequency is higher than most humans hearing range, but well within a dogs range of hearing.  

Noise Cancellation- Active or Passive methods of drowning out ambient (unwanted) noise. The aim is to reduce the overall level of unwanted noise.

Active Cancellation- This methods listens through the use of a small microphone to the ambient sound and creates an opposite sound wave signal. This anti-signal is then played back to reduce unwanted ambient noise. This method is very useful to cancel out continuous low-frequencies (slow moving- low pitched) such as the hum of a motor/generator, or from air turbulence caused by a ceiling fan. This method is not effective at removing frequencies that constantly change. 

Passive Cancellation- This method resorts to physically blocking out unwanted sound. Foam ear plugs or those giant bulky earmuffs seen worn by construction workers both rely on using material to block sound from hitting the ear drum. 

Electrical Noise cancellation-  devices such as MP3 Players, portable CD players, TV’s etc. emit their own frequencies based on the various components used. Cheap components emit more unwanted frequencies (electrical noise) whereas high end components are made to eliminate the source of unwanted electrical noise. Does electricity itself really even make a sound? Have you ever heard that ZAP! from an electrostatic shock? That's caused by electrons moving from your body to an outside source causing the air to vibrate thus producing sound. 

Playback Device- MP3 player, Walkman, portable DVD player, Iphone, computer, TV, any device that plays multimedia. 

So where does electrical noise come from?

A very basic answer- Electrons, when moving from one atom to another within a wire, transistor, or some other electronic component, actually cause physical vibrations to occur. Keep in mind that these vibrations are minute and are not powerful enough to vibrate the device off a table, but are strong enough to travel up the wires to the headphones themselves. The actual wires and electronic components within a device vibrate. These vibrations are often referred to as frequencies. In other words, How Frequently does the vibration occur? This is measured in Kilohertz or How many Thousands of times per second.  Humans begin to hear things around 20 Hz or 20 vibrations per second.  Music from a CD is played back around 16,000 vibrations per second. So why such a high frequency? Well, the more times per second vibrations occur the softer/less stutter the sound is perceived to have. This also allows for multiple sound sources to be played alongside each other and remain discernible. However, after a certain point the human ear just can not tell a difference. *Studio Master quality music is often recorded at 96kHz or 96,000 vibrations per second to ensure all of the instruments/vocals are fully captured. Sometimes unwanted frequencies show up when the components used to playback sound are cheap or just not as good may begin vibrating themselves causing unwanted noise.

This is where Noise-cancelling technology comes into play in a few different ways.

Active Noise Cancelling Headphones often aims to reduce two types of noise. The first being Electrical noise and the second being ambient noise. In both cases the headphones actively create a specific electrical frequency (a vibration which can be inaudible to the human ear, but not always) aimed at canceling unwanted noise, sound, or vibrations.

This is a bit complicated to explain so bare with me. GRRRRR!! are you bearing with me?!

• Every number has an opposite number. 1 has the opposite number -1.   -5 is opposite of 5, 99 is opposite of -99 and so on. If you add opposite numbers together you get ZERO or they CANCEL each other out.
• If you eat a giant cake with a value of 1000 calories, but then run for two hours and burn 1000 calories, then you have neither gained nor lost any weight.
• If you Deposit $500 into your checking account and quickly spend $500 on those late night QVC infomercials, then you have not saved any money nor have you accrued any debt. (maybe just lost a little self respect.)
• If you have a high pitched note and play the opposite low pitch note they CANCEL each other out and no sound is heard. 

This is essentially what “active” or powered (requiring batteries or electricity to operate) noise cancelling headphones do. They create the opposite electrical noise frequency to cancel out unwanted sound. However, as mentioned previously, this method is best suited to cancel out constant low frequencies such as a humming sound. This allows only the frequencies from the music, movie, TV, audio book, podcast etc. to pass through the wires to the headphone's speakers, which in turn cause the air to vibrate and hit your eardrums. (air vibrations bang on the ear drum to create sound.)

Sounds great so where does this aforementioned confusion come into play?
At this point Active Noise Cancelling headphones split into two different sound cancelling objectives. One objective being to cancel out electrical noise and the other being ambient noise. That is not to say that some headphones don't attempt both, but rather figuring out what the specifications on the packaging actually refer to. 

In the case of ambient noise cancellation the headphones do a terrific job at reducing unwanted continuous frequencies, but are not well suited towards sounds that slightly defer from each other. For instance, such headphones would not be able to create the opposite sound wave from a person reciting their favorite passage. This is due to each word having a distinct frequency. Even though a dogs bark may sound rhythmic and repetitive, each time the dog barks the frequency is slightly different. Because of this some headphones simply add a generic static or white noise in the background. With the Headphones powered on and no music playing it would sound reminiscent of the black and white random flicker of dots or "snow" channel on TV. 

To demonstrate how this approach works try the following.  Place one index finger into each ear creating a nice seal. Any sounds you hear should be muffled. If you begin to talk it should sound like there is an echo. With your index fingers in your ears, begin to hum one note. Simply create a constant hum of the same note (frequency) and all of the outside noise should become even harder to hear. If your fingers could produce sound at the same time as you are humming, then you would be doing what this type of noise cancelling headphone aims to do.  

The idea is that the purposeful humming is only heard or only catches your attention when nothing else is playing. However, when you begin to play music, that humming is lost in the background. It is still there helping to block out the surrounding environment, but your brain focuses on the music itself. The same idea applies to glasses.  Put on glasses and you can see the frame, but after awhile your brain just throws that information away because it's not needed (until you think about it and look at the frame and can’t stop looking at it, but before you know it you've forgotten they were even there to begin with). If headphones cancelled out every frequency then you would not even hear the music itself. Phones also employ this method to reduce static and increase the clarity of one's voice. When a talking with someone on a phone there is a microphone that listens to the ambient noise and cancels it out, but one's voice is left alone. 

Passive Noise Cancelling Headphones (oft referred to as Noise Isolation headphones) take a slightly different approach. Instead of trying to cancel out or remove unwanted noise from the playback device they simply seal your ears from the outside environment much in the same way as traditional ear plugs.  Again, it would be like putting each index finger into each ear, but without humming.  The only sound you should hear is that of the music, TV, audio book, etc.  In other words just picture if traditional foam ear plugs could play music.