How do speakers work

Sound moves in pressure waves. When air particles are compressed and rarified fast enough, we hear it as sound. When a speaker moves back and forth it pushes on air particles which changes the air pressure and creates sound waves. Speakers work by converting electrical energy into mechanical energy motion. The mechanical energy compresses air and converts the motion into sound energy or sound pressure level SPL. In speakers, a current is sent through the voice coil which produces an electric field that interacts with the magnetic field of the permanent magnet attached to the speaker.

Like charges repel each other and different charges attract. As an audio signal is sent through the voice coil and the musical waveform moves up and down, the voice coil is attracted and repelled by the permanent magnet. This makes the cone that the voice coil is attached to move back and forth. The back and forth motion creates pressure waves in the air that we perceive as sound. The ultimate test of fidelity for a speaker is how similar the waveform in the air the pressure wave is to the electronic signal the sound recording that was sent into the amplifier.

There are several factors that determine how accurate the listening experience will be including the frequency response , the amount of distortion , and the directionality dispersion of the speaker.

A typical test for frequency response sends out a sweep of frequencies from the bass to the mids, and up to the treble range to see if the sound from the speaker is the same in all these areas.

The ideal frequency response for a speaker is very flat. Speakers use alternating current AC that changes direction polarity just like sound waves in real life. A speaker also referred to as a loudspeaker, a name from back in the day uses an alternating current AC electrical power signal and are driven by a stereo or amplifier.

In a matter of speaking, speakers are just an electric motor of sorts: they are powered by an electrical signal and change it into a mechanical output: moving air to create musical sounds. Speaker impedance, measured in Ohms, is the total resistance to the flow of electric current through a speaker voice coil. Unlike standard conductors, as the voice coil is tightly wound in a coil the makes this complicates things because it adds inductance. Inductance is different from resistance as it changes as the frequency changes and this is called inductive reactance.

In other words, when the magnetic fields of the voice coil are created they oppose the flow of electrical current a bit. If you like fancy math, you can see here how speaker impedance is calculated. It is the geometric sum of the resistance in the voice copper wire winding and the resistance caused by its inductance at a given frequency. Image showing how to measure speaker impedance with an Ohm meter.

This measures only the direct current DC resistance of the wire in the voice coil, not the total impedance of it with music playing due to inductance.

The practice began long ago when radios and speakers were first installed from the factory when cars were built. In both cases these Ohm ratings became common for home and car speakers. While a subwoofer or mid range speaker needs to be used in a sealed structure tweeters do not. In some cases like for surround speakers they may be smaller than the front main speaker cabinet pair.

An example of a typical speaker frequency response graph is shown here. Speaker frequency response is the measured performance of a speaker, in decibels dB of volume, over a range of sound frequencies.

While some speakers include a graph or other specifications to help you understand how they perform, not all do. More expensive speakers may do so, however. If you have the right equipment you can also measure it yourself at home using a real-time analyzer RTA program and a high-quality microphone for this purpose. They are all around us -- in our TV's, computers, alarm clocks, cars, stereos, headphones, etc.

Magnet Structure -- T wo pieces of oppositely oriented magnets that produce a radial field from the inner to outer magnet. Voice Coil -- C arries the current so that it is always moving in a plane perpindicular to the magnetic field; thus the force always acts on the same axis.

Spider -- V ibrates rigidly with the voice coil and translates the mechanical energy to the cone. Cone -- P roduces pressure waves from its surface due to the oscillation of the spider. Basket -- H olds the components together firmly, preventing motion in parts like the magnet structure. That is, can the loudspeaker be used as a microphone which detects sound? In the video of the jumping wire demonstration, we moved a wire in the magnetic field and observed a voltage on an oscilloscope.

The jumping wire is reversible. Essentially the same effect should occur for a loudspeaker! Incoming sound causes motion of the cone and thus the voice coil. The motion of the voice coil in the magnetic field should produce a voltage across the coil, but the effect may be too small to observe. The loudspeaker is now directly connected to an oscilloscope.

Touching the cone also produces a voltage. For more mature audiences, we connect the amplifier to another loudspeaker, which is used to produce sound that is detected by the original loudspeaker, as you can see. It is interesting that a loudspeaker acting as a microphone can actually be useful. This was done during World War II for the first detonation of a nuclear weapon. A normal microphone could have been overloaded or damaged by the blast wave.

A conventional loudspeaker has a circular coil of wire in the gap of a circular magnet. The coil is attached to a cone. When an oscillatory electric current is passed through the coil, there is an oscillatory magnetic force on the coil, which moves the coil and cone, thus causing sound to be emitted.

The loudspeaker can act in reverse, as a microphone, although it is not very sensitive and is subject to noise. A conventional loudspeaker is not the only kind of loudspeaker!

An unusual and interesting type is an electrostatic loudspeaker. This works in a fundamentally different way than a conventional loudspeaker, and has its own advantages and disadvantages. We will demonstrate and explain an electrostatic loudspeaker in another video.