How Wireless Speakers Work

At its most basic level, sound is one way we perceive vibrations. When an object inside the Earth's atmosphere vibrates, it pushes against and pulls on surrounding air molecules. In turn, they affect surrounding air molecules. It becomes a chain reaction.

Imagine a bell. When it rings, its surface actually flexes inwards and outwards. When the bell's surface flexes inwards, it pulls air molecules toward the surface of the bell. They pull on other molecules around them, and those pull on even more molecules. We call this area of decreasing air pressure rarefaction.

Extending outward from the vibrating object is a wave of air pressure fluctuations. If you happen to be within range of the wave -- the energy dissipates over distance -- your eardrum vibrates in reaction to the changes in pressure. Your eardrum is connected to tiny bones in your middle ear. These bones connect the eardrum to the cochlea in your inner ear. The cochlea is filled with fluid and tiny little hairs. As the bones vibrate the cochlea, fluid flows against the hairs, which triggers nerve signals that move from the ear to the brain. Your brain then interprets these signals as sound.

Sound can travel through solids, liquids and gases. If you put your ear down against a table and have someone gently scratch upon it, you'll hear it loud and clear. That's because sound generally travels more efficiently through solids than gases. This is also why sound doesn't travel in the vacuum of space -- there aren't enough particles to collide with each other to propagate sound.

The nature of the sound depends upon how forcefully and frequently the molecules bump into each other. Molecules that really bash into each other create louder sounds -- a large mallet striking a huge gong is going to make molecules collide more forcefully than a tiny bell would. Faster fluctuations in the atmosphere create sounds with a higher pitch than those that have slower fluctuations. The gong's sound won't just be louder than the tiny bell, but also deeper in pitch -- the bell creates faster fluctuations in air pressure than the gong does.

Speakers -- wireless and wired alike -- create sound through vibrations. All it takes is a little electricity and magnetism.

A typical speaker has several parts. The part that vibrates to make the sound is called a cone or diaphragm. It's a flexible surface that can be pushed out or pulled inward by the rest of the speaker mechanism. It's the diaphragm that creates the changes in atmospheric pressure that we perceive as sound.

An electromagnet called a voice coil attaches to the center of the cone. A permanent magnet -- a magnet that keeps its magnetic field without electricity -- sits behind the voice coil on the other side of the cone. This means that a speaker uses two different types of magnets, which is what gives speakers the power to push and pull against the atmosphere rapidly.

Electromagnets take advantage of the relationship between electricity and magnetic fields. As electricity flows through a wire, it generates a magnetic field. Coiling electrical wire around a core -- like an iron nail -- creates a magnet when the current is on. Turning off the electricity causes the magnetic field to dissipate.

Magnets have two poles -- a north pole and a south pole. Permanent magnets always have the same north and south poles. But an electromagnet's poles can switch depending upon the flow of electricity. Forcing the flow of electricity to reverse also reverses the position of the electromagnet's poles.

This is important because with magnets, similar poles repel one another and opposite poles attract. By altering the flow of electricity through the voice coil's electromagnet, the permanent magnet's magnetic field will either push or pull on the voice coil. Since the voice coil attaches to the diaphragm, this will cause the diaphragm to pull inward or push outward.

Speakers alter the flow of electricity within a voice coil thousands of times per second, creating the precise vibrations necessary to create sounds ranging from deep booming bass notes to the high pitch of a piccolo.

In traditional speakers, electricity flows from an amplifier within the source -- such as a stereo system -- to the speaker over two wires. This allows the source to alternate the flow of electricity to the speakers, which causes the electromagnet's poles to switch. Wireless speakers have to create the same effect without the benefit of wires. But how do they do that?

Wireless speakers have no direct connection to a stereo system or other source. Instead, the system must send a signal that the speakers can receive and convert into electricity in order to drive the voice coil inside the speaker itself. There are a few ways to do this.

One way is to use infrared signals. This is similar to how many remote controls work. The stereo system has a transmitter that sends out a beam of infrared light. Because infrared is outside the spectrum of visible light, we can't see it.

The transmitter's job is to take the fluctuations of electricity -- the same ones that would control the speaker if it were wired to the stereo -- and convert it into an infrared beam. The beam carries information through pulses. An IR system can send out millions of pulses per second. The wireless speakers have sensors that can detect these transmissions.

Once detected, the sensor sends electronic signals to an amplifier. Its job is to increase the strength of the sensor's output. Without the amplifier, the signals would be too weak to drive the voice coil within the speaker. This is why many wireless speakers still require wired power to work.

The amplifier sends electricity to the voice coil, alternating the flow of electricity as directed by the signals the sensor sends along. The alternating current will cause the voice coil's electromagnet to change polarity rapidly. The magnetic fields of the electromagnet and the speaker's permanent magnet do the rest of the work, pulling and pushing the voice coil and causing the speaker's diaphragm to vibrate.

There are several drawbacks to this type of wireless speaker. One of the big ones is that an infrared beam requires line of sight. That means there needs to be an unobstructed path for the infrared beam to follow from the stereo system to the speaker. Anything blocking that pathway will prevent the signal from reaching the speaker's sensor and the speaker will remain silent.

Another problem is that infrared signals are pretty common. Devices like most remote controls use IR technology. But even lights and human beings give off some infrared radiation. This can cause interference, making it difficult for the speaker to detect a clear signal from a stereo system. Even the most casual music fan might find it a chore to listen to a system that delivers a choppy or inconsistent experience.

There are other ways to send signals wirelessly. Next, we turn to the world of radio.

Radio waves represent part of the electromagnetic spectrum. Light is also part of this spectrum. The visible spectrum of light has a wavelength range of 390 to 750 nanometers (a nanometer is one-billionth of a meter). Infrared (IR) light has a longer wavelength range of around 0.74 micrometers up to 300 micrometers (a micrometer is one-millionth of a meter). Radio waves are the big kid on the block -- the wavelengths range from 1 millimeter to 100 kilometers.

Radio waves have a few advantages over other types of electromagnetic radiation. But to get radio waves from a stereo system to a speaker you'll need a few components. A transmitter connected to the stereo system converts electrical signals to radio waves by sending alternating current through an antenna. The radio waves broadcast out from the antenna.

An antenna and receiver on the wireless speaker detect the radio signal, and the receiver converts it into an electrical signal. An amplifier boosts the power of the signal from the receiver so that it can drive the speaker. The speaker still needs a power source just like an IR wireless speaker. Unlike an IR system, a wireless speaker that detects radio signals doesn't need to be within line of sight of the stereo system.

Radio waves broadcast in different frequencies. A frequency is the rate at which a radio wave oscillates -- how long it takes for a radio wave to go from peak to trough to peak again. It takes more time for a longer radio wave to oscillate than a shorter one. Radio frequencies are important because radio transmissions using similar frequencies can interfere with one another.

That interference can be a major problem -- many of the communications systems we rely upon today rely upon radio transmissions. For this reason, many countries have established rules that limit the types of radio frequencies various devices are allowed to generate. This limits the potential for signal interference.

In the United States, the bands of frequencies allotted to devices like wireless speakers include 902 to 908 megahertz, 2.4 to 2.483 gigahertz and 5.725 to 5.875 gigahertz [source: Schotz et al.]. Within these ranges, wireless transmissions shouldn't interfere with radio, television or communication signals.

Within these ranges are different protocols, such as Bluetooth. The Bluetooth protocol allows devices to connect together. Bluetooth can also allow a manufacturer to include controls on a speaker that go beyond volume and power. Because the Bluetooth protocol allows two-way communication, you could have a wireless speaker that lets you control what track is playing or what radio station your system is tuned into without making you get up to change it on the main system.

The main advantage of a wireless speaker system is pretty obvious -- there are no wires connecting the speakers to the sound system. You can put your speakers anywhere within transmission range and not have to worry about tripping over or hiding wires that lead back to your sound system. This can simplify setting up a home theater system.

Wireless speakers are popular in outdoor speaker systems. If you want to set up a speaker system on a deck, patio or swimming pool area, a wireless system may be ideal.

There are a few disadvantages that come with wireless systems. Because wireless speakers still require power, it's likely that you'll need to plug each speaker into a power source. Battery-operated wireless speakers exist, but may not have the audio oomph you want from your sound system. If you need to plug each speaker into a power source, you may still find yourself limited in how you can set up your home theater.

Interference can be another problem. There are lots of devices that emit radio waves. If those radio waves are on the same frequency as your sound system and speakers, you could get some garbled signals as you listen to your music. Dropped signals can also be a issue -- if a transmitter or receiver stops working or drops out, it will affect your listening experience.

Another problem is bandwidth. Wired speakers can carry a lot of information in the form of electrical signals. Wireless signals can't really compete. Music may seem less full or rich. It's a subjective element that can be difficult to put into words. If you're an audiophile, you may find wireless speakers lacking from a performance standpoint.

I wouldn't classify myself as an audiophile. I love music and I can differentiate between a great sound system and an average one. But I'm not able to tell the difference between a great sound system and a top-of-the-line system. I would consider a wireless system if it meant I could have a decent listening experience and have the freedom to place the speakers wherever I liked.

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