How do speakers work? Passive speakers vs active speakers

Image Credit: Sound On Sound

The loudspeaker is a staple of music. You’ll find them in every professional studio, and every bedroom studio thanks to the rise of digital music. But how do speakers work?

Studio monitors and headphones are a universal tool that music producers and recording artists utilise every single day. They’re the most vital component of any music production setup. However, how speakers work isn’t so widely known. And when we head out to a live gig or party, loudspeakers and PA systems draw the crowd in from the back of the dance floor.

This article is going to show you the ways of the loudspeaker. We’re exploring how speakers work to broadcast sound; the difference between active and passive speakers; how amplifiers work; and why the frequency response of a speaker is an important thing to consider when you’re looking for studio monitors or a PA system.

How does a speaker work?

To explore this section, let’s break down how sound works and then talk about the different components of a speaker.

How does sound work?

Firstly, sound waves are vibrating waves full of pressure in the air. Put simply, we hear sounds when air particles compress and rarify fast enough. Furthermore, higher frequency sounds have faster air pressure changes.

Therefore, a speaker cone produces sound by moving back and forth and pushing air particles – changing the pressure in the surrounding air.

The parts of a speaker box

The parts of a speaker are made up of the following:

  • Firstly, a dust cap and cone move air and produce sound.
  • Behind the cone, the spider/suspension holds the cone in place while allowing it to oscillate.
  • Then, a magnet and a voice coil interact and convert electrical energy into motion.
  • Holding these in place is the basket.
  • And behind the basket comes the pole and top plate.
  • Finally, a frame mounts everything together.
The parts of a speaker box.

Firstly, a dust cap and cone move air and produce sound.
Behind the cone, the spider/suspension holds the cone in place while allowing it to oscillate.
Then, a magnet and a voice coil interact and convert electrical energy into motion.
Holding these in place is the basket.
And behind the basket comes the pole and top plate.
Finally, a frame mounts everything together.
Image Credit: myMedia Installer

So, how do loudspeakers actually work?

Speakers convert electrical energy into motion – that is, electrical energy moves components within the speaker. Otherwise known as “mechanical energy”, this motion compresses air to convert the motion into sound energy/sound pressure.

You’ll often hear music professionals state that speakers and microphones are opposites to one another. This is true because while a speaker converts electrical energy into motion and then sound, a microphone converts sound waves into electrical energy!

As we mentioned above, sitting inside a speaker box is a magnet. And when an electric current travels through a coil of wire it produces a magnetic field. So, as an electrical current travels through the voice coil and produces an electric field – that electric field interacts with the magnetic field of the magnet behind the coil.

As your electrical signal travels through the voice coil – in every way reflecting the original musical waveform moving up and down – the voice coil is attracted and repelled by the magnet. In turn, the speaker cone (attached to the voice coil) also moves back and forth. This back and forth oscillation creates pressure waves in the air that we perceive as sound.

How do passive speakers work?

Passive speakers are what you’ll find when looking for consumer hi-fi systems and in clubs. Let’s break down a passive speaker system.

Firstly, there’s an audio source. In a live sound scenario, this could be a band playing on microphones or a DJ behind CDJs. Then a preamplifier separates the different sources/channels (guitars, drums, etc.) and allows you to determine the level of each source. If the sound source is CDJs, separating the channels isn’t necessary.

Up next comes the power. A power amplifier takes your weak electrical signal from the preamp and boosts it enough so it can actually drive the speaker cones. But before the signal reaches your speakers, your output must go through a crossover filter. The crossover splits your whole signal into its different frequency bands – it sends high frequencies to the tweeters and sends all other frequencies to the far bigger mid/bass units. 

But if you have a three-way passive speaker system, you can use your crossover unit to split the signal into three parts – highs, mids, and lows. Having a separate driver for each frequency group allows you to deliver far more power for each group – providing your amplifier has enough output wattage.

How passive speakers work

Your source audio travels to your pre-amplifier, then into a power amplifier. From here it travels into a signal crossover where the frequencies are split into treble, mid-range, and bass frequencies. Then these frequency groups travel to either the low-end woofer, the mid-range woofer, or the high end tweeter dome.

Whether active or passive, the input power of a speaker pushes its volume. In other words, it’s the wattage that a speaker can handle that determines what volume levels that speaker can reach. Despite this, higher wattage doesn’t necessarily translate to a louder output though – as we’ll talk about later, speakers are massively inefficient. However, you can determine how loud a speaker is with a combination of wattage and its frequency response (more on this later).

How do active speakers work?

Put simply, an active speaker is an all in one system that doesn’t require external amplification power as passive speakers do. Your studio monitors are active speakers as you only need to plug them into your mains power supply!

Active speakers have a built-in power amplifier in the speaker cabinet. Here’s how the signal chain works – it’s very similar, and it all happens inside the speaker rather than outside of it.

Your electrical signal travels into the internal crossover. Like in a passive system, this splits your signal into high frequencies and everything else (depending on whether the speaker is a two or three-way speaker). But there is a slight signal chain difference between active and passive systems.

An active signal chain sits inside the speaker box. The speaker splits the signal into its treble, mid-range, and bass frequency groups and sends each group to a dedicated amplifier. In turn, the amplifier send the signal to a dedicated speaker cone.

Sticking with our DJ as an audio source, a DJ mixer handles the necessary pre-amplification stage.

Your home studio will follow this path: your microphone plugins into your audio interface. Your audio interface converts the electrical analog signal to a digital one. Then, this digital signal travels in and back out of your computer to your audio interface once more. Finally, your interface re-converts the digital signal to an electrical one and sends it to your active studio monitors.

In a passive system, your signal reaches the crossover after your power amplifier. But in an active system, your signal reaches the crossover before power amplification. This slight difference in signal flow allows for more precise frequency amplification at the power amp stage. Therefore, active systems have far more accurate signal reproduction – which is why studio monitors are active systems.

How do speaker amplifiers work?

Whether an external amplifier or inside an active system, an amplifier creates a brand new output signal which is an amplified copy of your input signal. The amplifier then sends your signal on its way through the signal path.

It’s the power supply of your amplifier itself that generates the new output signal. Whether battery-powered or from mains electricity, an amplifier also smoothens out the electrical signal to ensure it’s even. Without this crucial step, you’ll hear an inaccurate reproduction of your source.

The input signal modifies the output signal in real-time. Because the electrical signal of the input is weak, a pre-amplifier is necessary to boost the signal and to make sure it has enough power to actually affect the output signal of the power amplifier. Some external amplifiers have multiple pre-amplifiers inside them to build up the desired voltage too!

Remember how we said the electrical signal reflects the original musical waveform of your source? This is made possible by the varying resistance that the electrical input signal applies to the output of the amplifier! As a result, this reproduces the compression and rarefactions in the audio signal which drives the oscillation of the voice coil inside the speaker cabinet!

RMS Power vs. Peak Power

If you ever shop for a speaker system, it’s important to understand what peak power is. More importantly, you’ll want to know why RMS power (root mean square) is a better measurement of power.

PA speakers push a certain amount of watts in a pair. And many manufacturers advertise a systems’ peak power – which isn’t the actual power that a speaker can handle. Peak power is what the system can handle when there are spikes in the audio. In contrast, RMS power is the measure of continuous power. RMS power is the measurement of continuous power that an amplifier can output or a speaker can handle.

What is the frequency response of a speaker?

Essentially, a speakers’ frequency response is how loud its output will be at different frequencies.

To test the frequency response of your studio monitors, simply sweep the pitch of a sine wave from 20 Hz to 20 kHz in your DAW. The ideal result is that your speaker sounds the same in all frequency groups, though this is pretty uncommon. When a speaker does sound the same reproducing all frequency groups, this is a flat frequency response.

A flat frequency response means no frequency dips or boosts are present in the frequency range. Moreover, making music on active studio monitors allows whoever listens to the final product to hear it as you intend. If your studio monitors have apparent frequency cuts/boosts below 10 kHz, you’ll find it difficult to make music the way you want it to be heard unless you put workarounds in place.

But if your track sounds good on speakers that have a flat response, it will sound good on any playback system.

Why is a flat frequency response important?

But many speakers, whether studio monitors or live PA systems, are not flat. Many lack the required power in treble and bass frequency groups (particularly the sub-bass region of 20 Hz to 40 Hz). And many speakers do have peaks or dips in their frequency response too.

A consequence of this is that some instruments in your music will sound louder or quieter in the final master than you want them to. However, let’s look at the facts here. There is a reason that speakers have these unfortunate characteristics.

The threshold of human hearing stretches from 20 Hz to 20 kHz. That’s 10 octaves of frequencies! And seeing as faster air pressure changes produce higher frequencies, a tweeter dome must oscillate very quickly to produce them. And bass frequencies below 300 Hz require a speaker to push a lot of air. After all, you feel bass more than you hear it.

And this is precisely why tweeters are small domes and woofers are large cones. To conclude, to accurately produce the full threshold of human hearing with no alterations, a speaker would require multiple drivers, cones and domes. And at that point, speakers become unaffordable for many.

I share music production tips, tricks, and tutorials for RouteNote. When I'm not writing, I'm listening to and making music, occasionally DJ'ing, and reading - but I haven't figured out how to do them all at the same time.

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