Audio headphones have been around since the early days of radio, in the early 1900s. Radio operators needed a way of hearing the Morse code they were trying to transcribe, and with several radio operators in the same room, isolating the sound coming out of one radio’s speakers from another radio’s was difficult at best.
The U. S. Navy was the first to use audio headphones in their radio rooms. The idea of putting the speakers nearer to the radio operator’s ears appealed to the Navy, improving efficiency and even more importantly, reducing errors.
Early audio headphones were simple devices, with small speakers mounted in unpadded housings that were held to the head by strips of spring steel. The requirement for high sensitivity precluded using any sort of sound dampening, making for crude sound quality. Nevertheless, the use of headphones was far superior to loudspeaker units which would cause confusion.
Modern headphones are much more sophisticated than those early ones. They are designed to provide high fidelity sound to the listener, while being comfortable to wear for prolonged periods of time. They come in three basic styles:
Supra-aural headphones – These are also known as on-ear, open-back or open-air headphones. They sit lightly on the ear, allowing the user to hear the ambient sound around them.
Circumaural headphones – These are also known as around-ear or closed-back headphones. They totally cover the ear, blocking out external noise so that the audio produced by the headphones can be heard better. They are heavier than the supra-aural headphones.
In-ear headphones – Often called ear buds, these are warn in the ear canal or opening of the ear. They are intended to create an airtight seal, blocking out other sound. Extremely lightweight and unobtrusive, they are the most common type of headphone in use today.
In addition to these three basic styles, there are active noise canceling headphones. We’ll discuss how those work later in this article.
While early headphones were used solely for radio communications, modern headphones are used for many sound applications. More than anything, they are used in cases where private listening is needed, either to prevent disturbing others, or so that the listener can hear what it is they are listening to more clearly.
A Word About Sound
What we call sound is actually vibrations. Those vibrations can pass through any material, as they migrate by causing individual molecules to vibrate. This vibration is passed from one molecule to the next, forming waves that are consistent with the frequency, volume and tone of the sound. As such, sound cannot pass through a vacuum, but only through solid materials in that vacuum.
What hits our ear drum is actually moving (vibrating) air. This air causes the ear drum to move in much the same way that a speaker cone or microphone element moves. The nerves in the inner ear interpret that movement and pass on electrical nerve impulses to the brain, where it is interpreted as sound.
If you can eliminate the vibrating air reaching the ear drum, you can essentially eliminate the ability to hear that sound. However, the vibration can pass from material to material. So, simply blocking off the ear canal isn’t enough to eliminate the ability to hear the sound, as whatever is blocking the ear canal can vibrate as well. If materials which don’t vibrate well are used to block the ear canal, such as Silicone rubber, then the sound is attenuated (lowered).
Noise Deadening vs. Noise Canceling
Headphones go a long way towards providing a private listening experience, where the listener can immerse themselves in what they are listening to and ignore distractions. Even so, no headphone is perfect in eliminating ambient noise. Circumaural headphones and noise canceling headphones go a long way towards this, by using a combination of technologies. Supra-aural headphones basically make no attempt to eliminate ambient noise, rather are designed to allow you to hear both the sound coming through the headphones and the ambient sound around you.
In-ear headphones can only eliminate ambient sound to the ability that they are able to seal off the ear canal to outside sound. This is why the better ones usually come with various rubber ear pieces, so that you can pick the best fit for your ear. While many people think this is for comfort, in actuality it is for improving the ability of the in-ear headphone to block out the surrounding sound.
There is one fallacy to this concept; that is, the bone around the ear can also vibrate from those sound waves, transmitting the sound to the ear drum through the head. Even if the ear canal is perfectly sealed off by a material which can’t pass any sound, it is possible to hear, albeit at a much lower volume.
Noise deadening is the ability of a headphone’s design to block off those sound waves, preventing the vibration from reaching the ear. This is accomplished by using a combination of materials in the construction of the earphone, which don’t effectively transfer the vibrations of the sound from one material to the next. Open cell foam rubber is one of those materials, as it does a very poor job of transferring sound vibrations.
For sound blocking to work, the headphone must totally seal of the ear from the ambient sound, as well as the bone around the ear. For this reason, circumaural headphones are the most effective sound blocking headphones. A good pair of circumaural audio headphones will cut the ambient sound level by about 20 dB.
For reference sake, 3dB is normally considered to be half or double the original sound level. This is a logarithmic scale, so that 6 dB becomes four times and 9 dB is 8 times. So, a 20dB reduction in sound level is less than 1/4th of the original sound level.
To go farther than that, active noise canceling has been developed. This uses electronic circuitry to create an “anti-sound” that effectively eliminates a large part of the original sound. Noise canceling headphones are also always noise deadening as well to increase their effectiveness.
How Noise Canceling Works
Noise cancelation is an active means of dealing with ambient noise. It takes advantage of the high speed at which modern electronics operates. Essentially, it reproduces the sound that it “hears” in the environment and reproduces the opposite of it, causing the sound to cancel out.
As we’ve discussed, sound is a wave. For simplicities sake, it is most often shown as a parabolic wave, although true sound rarely is as neat as a parabola. Nevertheless, this is useful for demonstrating how noise canceling works. In the diagram below, we see a typical analog sine wave, representing that sound.
Noise canceling circuitry in headphones starts by listening in on the ambient sound through a microphone. The circuitry itself creates an opposite or mirror image of the sound wave, making positive negative and negative positive. In the diagram below, the blue line is the original sound wave and the red line is the negative of that wave, or “anti-sound” created by the noise canceling circuitry.
In this sound wave, the frequency of the sound is determined by the length of the sine wave, reading left to right. The closer together the crossing points are, the higher the frequency and higher the note. The volume is determined by how far the wave deviates from the center or zero line. As you can see, both the frequency and volume of the original signal have been faithfully reproduced, except being exactly opposite; what is called “180 degrees out of phase.”
When you have two waves that are 180 degrees out of phase, they cancel each other out. So, the actual sound reproduced by the speaker is nothing; or as a famous song once put it, “the sound of silence.”
This out of phase sound wave is added to the sound that is being sent to the headphone’s speakers by the piece of equipment that it is attached to. In this way, the speaker is actually reproducing both the music (or whatever else is sent to it) by the equipment and the out of phase sound. However, the only part that is heard is the music, as the out of phase sound is combining with the ambient sound to cancel it out.
While active sound canceling works extremely well, it’s not perfect. There is a slight delay in how the circuitry processes the signal, making it take some short, but measurable period of time. That results in the noise canceling circuitry adding about another 20 dB of sound attenuation to the ambient sound, over and above the 20 dB from the sound deadening.
Together, the sound deadening and noise canceling provide a remarkable 40 dB of ambient noise attenuation. While not perfect, it is the best isolation from ambient noise you can get, without creating a totally isolated sound studio.