How do speakers work?

This post is all about those mysterious little boxes that musicians and fans alike can’t live without: speakers. Have you ever wondered how they work and what makes one speaker better or more suitable than another? If so, read on to learn all about our noisy friends. Oh, and don’t worry… although some of this is technical stuff, I’ve tried to keep it in layman’s terms (so that I can understand what I write!). 😉

The purpose of a speaker.

The fundamental purpose of a speaker is to recreate sounds as accurately as possible. To do this, a speaker transforms the source sound information that’s in the form of an electrical signal into sound waves in the air.

Speakers come in all shapes and sizes.

HeadphonesYou’ll find speakers all over the place, maybe even in places you wouldn’t expect. For example, your phone most likely has speakers, your headphones are speakers, your guitar amp has a speaker, your computer motherboard has a speaker on it, your microwave has a speaker, your fire alarms have speakers, your car has speakers, crosswalks have speakers, and on and on and on. The special speakers in your music studio are also just speakers, but they are often called “studio monitors,” not to be confused with video monitors that show you images from your computer (which also often have their own speakers built in!).

At their core, however, practically all speakers have the same general construction: a driver that moves a diaphragm, which in turn moves the air.

Create sound waves by moving air around.

Sound waves are created by waves of high and low pressure moving through the air. When you move your hand through the air back and forth, it pushes the air next to it, which in turn pushes the air next to it, and so on. And when your hand starts going back, it pulls the air with it, which in turn pulls the air next to it, and so on. A speaker does this same type of thing, but in a controlled manner and at a much higher rate. This repetitive pushing and pulling motion is the basic concept of a sound wave. Sound waves travel through the air and eventually lose all their energy and dissipate.

Sound, WavesHow fast and often the air is pushed and pulled is called the frequency of the sound wave, and it controls the pitch of the sound that you hear. How much the air is pushed and pulled each time is called the amplitude, and it controls how loud or soft something sounds. The higher the amplitude, the more energy it contains and the longer the longitudinal wave travels before it equalizes. Ah… now the term “guitar amp” makes so much more sense: “amp” > “amplify” > “increase amplitude.”

The reason sound waves are so amazing is because that pushing and pulling motion in the air can travel into your ears where it moves your eardrum back and forth, which then ends up pushing and pulling little hair cells in the different parts of your cochlea, resulting in nerve signals to your brain that are interpreted as a particular type of sound! Your ears are very much like microphones in this way. Microphones have diaphragms, similar to your eardrums, that get pushed and pulled by those sound waves, which then get turned into electrical signals. So, when someone gets a cochlear implant, it means that this part of their ear is damaged or not working, and they have some tech “installed” into their ear that works sort of like a mic, where the electrical signal from the mic is passed on to the brain (sorry… I’m way oversimplifying this process for the sake of brevity and to hide that I’m reaching the limits of my knowledge here). Our ears are so cool!

Speakers can act like microphones. What?!

I just said that microphones are basically just speakers set up in reverse. In fact, you might not be aware that every speaker can also act like a poor-man’s microphone simply by reading the electrical signal from the driver, instead of sending an electrical to the driver. Happy micThat’s because the diaphragm on your speaker will naturally move with the sound waves in the air. For example, if you say something out loud next to a speaker, the speaker diaphragm moves a little.

When the speaker diaphragm moves a little, it creates a corresponding signal in the driver (basically working in the exact opposite way), which can then be read as an electric signal from the jack in the back of your speaker. Don’t believe me? Try it for yourself. Take a throw-away pair of headphones and plug them into the jack you normally plug your mic into (maybe you want to turn off phantom power, if your audio interface has that setting on, just in case), and then start recording and speak into your headphone. Amazing!

There have been some interesting privacy concerns and security issues recently regarding this topic.

A driver can only do so much.

Now that we know the basics of how a speaker operates, we should talk a little bit about their limitations, such as how loud they can get and their frequency response. Since a driver can only push and pull the diaphragm so far, it is limited to how loud it can sound. If you push or pull a diaphragm more than it was designed to handle, it can bust.

Similarly, a driver and diaphragm can only reliably reproduce frequencies at good quality in certain ranges. Generally speaking, the cheaper the speaker, the smaller the frequency response range. But the frequency response range is also affected by the type of driver used. There are all different types of drivers, each with their own purpose and each with their own specialized frequency response ranges.

Why do some speakers have two drivers?

SpeakersIf you look at most speakers, you’ll see a big cone diaphragm and a smaller little cone. Ever wondered what that little cone is? You might know it as a tweeter, but why is it there and what relationship does it have to the bigger cone?

The bigger diaphragm does a good job at reproducing mid-range frequencies, but tweeters are specialized drivers that do a good job reproducing high-frequency sounds (aka they are really good at moving back and forth really, really fast). Some speakers have multiple drivers to reproduce a larger range of sounds. On the surface it seems simple, right? Nope.

Wouldn’t the speaker sound louder if both drivers are generating sound at the same time for the same frequencies? Yep! And wouldn’t it be bad if say the tweeter is a lot louder when it tries to play the same amplitude as a lower frequency played on the mid-range driver? Yep!

Here’s how speaker designers deal with it. Since each driver has its own range of frequencies it can respond to, and since you want to make sure not to miss out on any frequencies, there is naturally going to be some overlap between the two drivers. When a sound moves between frequencies in this overlap region that causes a different driver to start playing at the same time, this is called “crossover.”

Higher-quality speakers have a small crossover region and calibrated drivers so that the transfer between drivers is not noticeable. On the opposite end of the spectrum, lower-quality speakers have large crossover regions and drivers that are not finely calibrated with each other, which means that it can become really noticeable when a sound crosses that frequency range. The worse the speakers, the more noticeable the problems: certain frequencies from the speaker sound really loud and “different” than others.

In your music studio, you want to use speakers with as flat (aka even) a response as possible across all frequency ranges, no matter which driver is playing, so that you know that you are hearing exactly what your system is trying to play. That’s partially why studio monitors are generally much more expensive than regular speakers.

Remember, everything we’ve talked about so far has to do with all of the drivers operating off of the same single audio channel. What happens when you have more than one audio channel? Read on to find out.

Living in stereo!

Just like each ear on your head hears slightly different information in the real world, each speaker in a stereo setup has its own information on its own channel that tries to recreate what each ear would have heard when the sound was originally recorded.

In a stereo setup, there is usually a left speaker and a right speaker. Keep in mind that each speaker can have multiple drivers, as explained above. When both speakers play the same thing at the same loudness, both of your ears hear basically the same thing and your brain interprets that as if the sound is coming from the center or middle. If you turn up both channels equally, everything sounds louder, but it still sounds as if it is coming from the center. Likewise, if you turn both channels down equally, everything will sound quieter, but still in the center.

When the left speaker is a little louder than the right, your brain interprets that as if the sound is coming more from the left. And it works the other way around as well. In fact, it’s not simply an on or off situation. In other words, you can make a sound come from the hard left or the hard right, or anywhere in-between, simply by messing with the volume of each channel.

This concept is called “panning,” and it is used all the time when you mix your songs. It allows the mixer to position and spread out instruments and sounds to make them distinguishable and to give a nice wide, overall sound. The reason this works so well is because our brains are really good at isolating and pinpointing sounds coming from different directions. However, our brains aren’t nearly as good at distinguishing sounds when they all come from the same spot.

Can you have more than two speakers?

Yes, that’s where surround sound comes in. You might have a 5.1 surround sound system in your house. If that is the case, you have six speakers: 1 forward-left speaker, 1 forward-right speaker, 1 back-left speaker, 1 back-right speaker, 1 forward-center speaker, and… “.1” of a speaker? Yep, that “.1” in “5.1” represents a special type of speaker called a subwoofer.

16.2 Surround Sound

16.2 Surround Sound

The reason a subwoofer is a special type of driver and called out separately is because its job is just to recreate frequencies in the sub bass range. These are frequencies that a standard driver in a normal speaker cannot recreate. In fact, many people blow out their speakers by trying to send such low frequencies to their normal speakers, or at best they hear a horrible rumble or messy crackle when trying to listen to those frequencies. The frequencies are so low, in fact, that even our ears can have trouble hearing them. Sub bass frequencies are usually felt rather than heard, and they are the ones responsible for making those huge explosions or kick drums make you feel like you just got hit in the gut. Nevertheless, they are still sound waves like all the rest.

Surround sound doesn’t stop at 5.1. It can go to 7.1 and beyond. As technology progresses, we get setups with more and more speakers that try to recreate the original source exactly, as if we were there at the time it was recorded. Two big players in this new field are Dolby Atmos and Aura-3D.

No matter how many speakers you have, the important thing to remember is that each speaker needs its own channel. As a result, the more channels you have, the more data you need to send. For example, stereo is twice as much information as mono, and a 4.0 surround sound setup is twice as much information as stereo. This also means that you need a different mix for each format. For a stereo mix, your panning system will be a simple left-right knob or slider. For a surround mix, your panning system becomes necessarily more complex because you need to position the sound in 2D space. For Dobly Atmos and Aura-3D setups and similar, you would also need to specify a third dimension (height) to the position of your sounds (aka position in 3D space).

Make your speakers sound better!

When your speaker driver moves the diaphragm back and forth in order to move the nearby air to create sound waves, it might also incidentally move the speaker box itself. This is bad because when that happens, energy is lost to the speaker; instead of directly moving the air, some energy escapes and can end up moving other objects that the speaker is resting on, such as a desk. This might not sound too bad (pun not intended), except that when your desk gets that energy, it will move a little just like the diaphragm does. And then it will continue to pass that energy on to other nearby objects, which in turn start moving slightly, too. The net result is that lots of objects in your room suddenly turn into horrible-sounding speakers that highlight certain frequencies.

It’s a fun exercise to take a speaker driver and place it against something like a piece of paper you hold up or against a window. When you do, the paper or the window will act like the diaphragm and you’ll start to hear the sound from it.

Companies spend a lot of effort and expense trying to make speakers that don’t lose energy in this way by isolating the speaker system. In fact, some companies such as IsoAcoustics create stands for your studio monitors that isolate the movement of the speaker so that the energy isn’t transferred to your desk. Here are the IsoAcoustics stands that I personally use in my studio.

Position your speakers!

Putting your stereo or surround speakers in the right places is very important. Why? Well, remember how sound waves eventually lose energy and peter out to silence? It turns out that how close or far away you are from your speaker can have a HUGE effect on how much energy reaches your ears. This is because sound waves decay in an exponential manner, which simply means that it will sound dramatically louder if you get close to the speaker. That’s why you have to be very careful about putting your ear up to a speaker, even a tiny little one! If you get too close, you could blow out your ears!

If your speakers are not aligned properly, it will be as if everything is panned incorrectly.

Therefore, if you want an evenly balanced setup in your music studio, make sure all speakers are equally spaced from your head, based on your normal listening position. There is a lot more to it than that, but that’s the basic idea.

Speaking of balancing… I think it’s time to wrap this up.

Believe it or not, much more could be said about speakers, especially as they relate to reverb, isolation, mixing, and recording… but I’ll leave that for another post.

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