Speakers are somewhat ubiquitous nowadays. These sound-producing devices are found in our televisions and computers; at performance venues; in public spaces such as malls, places of worship and airports, and even in our pockets and palms with the popularity of the smartphone. If the mechanics behind how speakers work is a mystery, then this article is just for you!
How do speaker produce sound? Loudspeakers produce sound waves by causing a thin diaphragm to vibrate and disrupt the air pressure around it in the form of the intended sound wave. An amplified audio signal (alternating current) that has the same waveform as the sound wave is responsible for vibrating the speaker diaphragm.
In this article, we’ll discuss, at a beginner’s level, how speakers produce sound.
A Primer On Sound And Audio
Before we get into the bulk of this article, let’s discuss two forms of energy: sound and audio.
Sound is made of mechanical wave energy. A sound wave transverses through a medium (gas, liquid, solid) and causes variations in localized sound pressure level according to the waveform.
We typically hear sound waves transmitted through the air but can also hear them in water and other liquids or through the solids around us. Note that sound waves do not cause mass movement of air particles but rather causes the localized movement of the air molecules, creating moments of increased pressure and decreased pressure relative to the ambient pressure in the medium.
Here is an example of a simple 1 kHz sinusoidal sound wave. We see that for half the period there is an increase in SPL relative to ambient pressure and for the other half, there is a decrease in SPL relative to ambient pressure.
Audio, on the other hand, is made of electrical energy (whether stored or kinetic) can be thought of as an electrical representation of sound as electricity.
Analog audio can be stored on tape, vinyl and other media. It is represented by a waveform defined, typically, by voltage.
Here is an example of a simple 1 kHz sinusoidal audio signal. We see that for half the period there is a positive voltage and for the other half, there is a negative voltage. This tells us that audio is made of alternating current.
Digital audio is a representation of stored analog audio in a digital/binary format.
Here is an example of a simple 1 kHz sinusoidal digital audio signal. We see that the audio is sampled 48,000 samples/second and appropriate bit-ratings are assigned to approximate the level of the signal at each sample. What we end up with is a very close representation of the analog signal.
So how do we, ourselves, and our technologies interact between sound and audio? The answer is with transducers.
A transducer is simple a device or system that converts one form of energy to another form of energy. Remember that sound is mechanical waves energy and audio is electrical energy.
Our ears pick up the vibrations in the medium caused by sound waves and convert these vibrations into electrical impulses that our brains can understand. In other words, our ears are transducers.
Microphones work similarly with diaphragms that move in reaction to the sound waves around them. Their capsules (cartridges, motors) use this diaphragm movement to create electrical audio signals that represent the sound waves.
Loudspeakers (and headphones) work in the opposite manner. Their transducer elements, known as drivers, are designed to receive audio signals and move their diaphragms to reproduce these audio waveforms as sound waves.
For a more detailed article on the differences and similarities between sound and audio, check out my article What Is The Difference Between Sound And Audio?
With that primer, let’s get into it!
The Speaker Driver
So the transducer element of the loudspeaker we are concerned with is called the driver.
Though there are certainly variations in the design of speaker drivers, the vast majority will be of the dynamic variety (also known as moving-coil or electrodynamic).
To learn about how each type of speaker driver works, check out my article What Are Speaker Drivers? (How All Driver Types Work).
For this “beginners” article, we’ll stick with the most common speaker driver type that we’ll find in nearly all our loudspeakers.
Here is an illustration of the cross-section of a typical speaker driver.
We have the following components:
- Diaphragm (Cone)
- Voice Coil
- Dust Cap (Dome)
- Top Pole Plate
- Pole Piece
- Bottom Pole Piece
The Dayton Audio RS225-8 (link to check the price on Amazon) is an example of a moving-coil speaker driver.
So how does this seemingly complicated transducer with multiple components produce sound? Let’s simplify.
The magnetic structure is made of a main magnet and several pole pieces. The pole pieces extend the magnetic poles of the main magnet and form a small ring-shaped cutaway just slightly larger than the voice coil. This concentrates the magnetic field around the voice coil while still allowing it to move freely inwardly and outwardly.
The basket is essentially the housing that holds everything together.
The surround and spider make of the suspension, limiting the movement of the voice coil and the diaphragm (and dust cap) to the inward-outward directions only. Restricting lateral movement keeps the voice coil from hitting the magnet and causing issues.
The dust cap keeps foreign particles from entering the driver.
Now it’s time to explain the voice coil, which is where things get really interesting.
The voice coil has two electrical leads attached to it (a positive and a negative) and receives audio signals. Each terminal is connected to an end of the voice coil.
Below is a picture of two terminals on a speaker driver (photo taken from my quick and dirty experiment turning a loudspeaker into a microphone).
Note that we can also see the magnet, spider, basket, surround, diaphragm and dust cap in the picture.
Remember our discussion on audio signals. These alternating currents flow through the terminals and the voice coil of the speaker driver.
This alternating current causes an alternating magnetic field to be induced in and around the voice coil due to electromagnetic induction.
The changing magnetic field of the voice coil, which is proportional to the audio signal, no interacts with the permanent magnetic field of the magnetic structure.
A varying magnetic field will mean that at some instances, the voice coil will be “attracted” to the magnetic structure and at other instances, be “repulsed” by the magnetic structure.
So then the voice coil oscillates relative to the audio signals waveform (made of information between 20 Hz and 20,000 Hz or cycles per second). As you can imagine, the voice coil oscillates very quickly!
Recall that the suspension ensures the voice coil only oscillates along one line (inward and outward).
Now for the final part.
A voice coil oscillating by itself will not produce much sound at all. To really make the speaker produce sound, we need a thin, lightweight diaphragm with a relatively large surface area attached to the voice coil.
This large diaphragm will be capable of pushing and pulling a lot of air and produce significant sound waves into the medium.
Of course, the diaphragm is connected to the voice coil and so these sound waves will be representative of the information contained in the audio signal!
A Note On Enclosures
As a side note, a speaker diaphragm moving by itself with produce sound waves in both directions that will effectively cancel each other out due to phase cancellation.
For this reason, speakers will have enclosures to offset the phase issues and produce more coherent sound wave propagation.
For more information on speaker enclosures, check out my article Why Do Loudspeakers Need Enclosures?
Speaker Level & Amplification
Speakers come in a variety of shapes and sizes. Some speakers require stronger audio signals than others and all speakers require stronger audio signals than those that are stored in playback media/devices and in live mixers, DAWs and the like.
Line level signals (nominally +4 dBu) are used in processing, recording, storing and playing back audio.
Related article: What Are Decibels? The Ultimate dB Guide For Audio & Sound.
Line level does a few things for us:
- Provides a standard for equipment manufacturers to follow.
- Makes equipment synergistic.
- Produces great signal to noise ratios (relative to low-level mic level signals).
- Allows manufacturers to focus resources on quality rather than on producing circuits that can handle higher levels of signal (stronger signals will produce more heat and, therefore, require heavier-duty components. Small signals allow manufacturers to build more intricate circuit designs with greater precision).
However, line level signals typically will not be enough to effectively drive a speaker driver.
So, then, we need amplifiers to boost line level signals up to speaker level signals in order to provide enough signal to move the speaker’s diaphragm and produce sound waves.
These amplifiers are generally known as power amplifiers and can be found as standalone units or built into the loudspeaker itself.
The Crown Audio XLi3500 (link to check the price on Amazon) is an example of a power amplifier.
Crown Audio is featured in My New Microphone’s Top 11 Best Power Amplifier Brands In The World.
The main take away here is that, in order to produce sound waves, a speaker will need an amplifier.
My New Microphone has plenty of great resources to further your knowledge on this topic.
One article to check out is Why Do Speakers Need Amplifiers? (And How To Match Them).
Multi-Driver Speaker Units & Crossovers
So you may be wondering why some or all of your speakers have multiple drivers.
Do all the drivers receive the same audio signal and produce the same sound waves? The answer is “kind of but not really”.
Some multi-driver speakers have identical drivers that receive the same exact electrical signal to produce sound with.
An example of this type of speaker is the JBL ASB6128 (link to check the price on Amazon) dual 18″ driver subwoofer.
JBL is featured in My New Microphone’s:
• Top 11 Best Home Speaker Brands You Should Know And Use
• Top 11 Best Subwoofer Brands (Car, PA, Home & Studio)
• Top 11 Best PA Loudspeaker Brands You Should Know And Use
• Top 10 Best Loudspeaker Brands (Overall) On The Market Today
However, more commonly, we’ll find 2-way, 3-way or even 4-way speakers, where drivers of varying sizes with be used. A cross-over network of audio filters will be used to send specific frequency bands to certain drivers with little to no overlap.
For example, the largest driver (typically speaking) will be responsible for producing the lowest frequency band and the smallest driver will be responsible for producing the highest frequency band.
The drivers are often referred to as follows:
- Woofers: produce low frequencies
- Mid-range drivers: produce the mid-range frequencies
- Tweeters: produce high frequencies
Cross-overs can be generally be defined as follows:
- 2-way: a single split between low and high frequencies
- 3-way: a split between lows and mids and another between mids and highs
- 4-way: 3 crossover points to produce 4 different bands.
So in these speakers, each driver is designed to produce a specific frequency band of the full audio signal. The audio signal is filtered by the cross-over to give each driver its own band of the full audio signal. In this way, each driver is presented with a different “version” of the same audio signal.
The Dynaudio Core 59 (link to compare prices at select retailers) is a great example of a 3-way speaker. We can clearly distinguish which driver is the woofer, mid-range and tweeter.
Dynaudio is featured in the following My New Microphone articles:
• Top 11 Best Studio Monitor Brands You Should Know And Use
• Top 11 Best Home Speaker Brands You Should Know And Use
• Top 10 Best Loudspeaker Brands (Overall) On The Market Today
When all performing together, the drivers of a multi-driver speaker should be designed to reproduce the entire bandwidth of the audio signal (except maybe the very low-end which could be sent to a separate subwoofer).
For more information, check out the related My New Microphone articles:
• Differences Between Mid-Range Speakers, Tweeters & Woofers
• What Is A Speaker Crossover Network? (Active & Passive)
A Note On Stereo Speakers
Stereophonic sound is typically achieved via two mono speakers. One speaker, positioned to the left of the listener, is provided with the left channel audio of the stereo signal. The other speaker, positioned to the right of the listener, is provided with the right channel audio of the stereo signal.
However, some speakers (particularly sound bars) are designed to produce a stereo image in a single speaker unit. Of course, the speaker uses multiple drivers positioned at different locations to achieve stereophony.
A stereo signal applied to the sound bar is effectively split into left and right with varying amounts of each channel getting sent to each driver.
The JBL Bar 2.1 (link to check the price on Amazon) is one such stereo soundbar that comes complete with a subwoofer.
How do I make my speakers louder? Speakers are generally designed at specific sensitivity (sound pressure level output per power input), so “making a speaker louder” means increasing (amplifying) the strength of the input signal via an amplifier. Otherwise, passive amplification such as a horn can be used to increase the directivity and perceived loudness of a speaker (in certain directions).
How fast does a speaker vibrate? The speed or frequency at which a speaker vibrates is limited by it design and based on the frequencies of the audio signal that is applied to the speaker’s driver(s). Audible sound ranges from 20 Hz – 20 kHz. Many speakers will vibrate across a smaller bandwidth within the audible range. Others can vibrate to produce infrasound (<20 Hz) and/or ultrasound (>20 kHz).
Related article: How Fast Do Loudspeakers & Headphones Vibrate?