Here we'll try and hit the high points of what you need to know about speakers.
A microphone "backwards"
Speakers date to the late 19th century, along with microphones. In a dynamic mic, you let a diaphragm (the capsule) vibrate a coil of wire in a magnetic field to generate a tiny AC signal.
In a speaker, you send an AC signal to the coil and it vibrates the diaphragm (the cone) by moving in a magnetic field. At times people have used speakers as microphones in the studio, in particular to capture the deep lows of kick drums. Yamaha even made a custom mic for this which looked like a drum with a speaker in it!
Looks like a little drum. . .
With a speaker in it!
One more thing to keep in mind - a generator is a device that moves a coil of wire through a magnetic field to induce a current. I mention all of these different things because they will all come into play later. It is kind of cool to see the same basic invention get used for lots of different applications.
Speaker Parts:
Magnets
In the beginning there were field coils - so named because they used big coils of wire that generated a magnetic field for the voice coil to work in. They generally used the high voltage supply as a power source and could be quite powerful. Drawbacks were cost, weight, complexity and exposed high voltage wiring. Over time permanent magnet structures would be identified that were strong enough to replace them. First AlNiCo, and later on Ceramic (Ferrite) magnets which were much less expensive and simpler to use. All else equal, bigger magnets tend to give the speakers better low & high frequency response and better overall efficiency.
Coils
The coil of wire we place in the magnetic field is called the Voice Coil. It has to be able to handle the current (heat) of the amplifier circuit - it is the load - while it tries to push & pull the cone assembly to make sound. The voice coil is wound around a former; a tube of some material to give the coil dimensional stability. Generally the former will be something lightweight. The former is then glued to the center of the cone. The former is often paper but over time more advanced materials were adopted to help speakers handle more and more power such as Aluminum, Copper, Nomex and Kapton. The coil sits in the magnetic field, and the tighter the tolerances, the more efficient the speaker, in general. The inverse square law applies; field strength falls with the square of the distance. The coil plus the magnet it sits in are commonly called the motor. You want a strong magnet and a lot of wire on the voice coil to maximize efficiency. The coils are fairly delicate and lots of different things can damage them. Here's a handy chart from dBM:
Cones
The cone is pretty obvious; it's what you generally associate with a speaker, although that doesn't mean they are well understood. They generally aren't. Their primary purpose is to try and match impedance with the air around so that as the voice coil moves them, they transfer as much of that energy as possible as sound waves. Sadly, they are tremendously bad at this, managing only perhaps 3-5% efficiency on average. The rest of that power turns to heat. Exponential horns can be 25-50% efficient, but have different issues.
To better understand impedance matching with the air, hold a piece of tissue paper in your weak hand, just let it hang there. Now punch the tissue with your dominant hand. What happened? Nothing probably. This is because you have a poor impedance match - almost none of your punching energy went into the tissue. This is our poor speaker, punching at the air. <sad speaker noises>
What makes things more complex for speakers is that the cones are essentially transmission lines; a signal is fed to the center (where they are glued to the voice coil) and it propagates outward to the surround (the outer edge of the cone). In an ideal world, the cone would be weightless and perfectly stiff. The cone would move like a piston. In the real world, they have mass (and intertia), they flex and they have nodes (resonances) based on their geometry. All kinds of stuff happens to that initial signal. They don't move with the coil as well as we'd like and they add all sorts of harmonics to the program material as they wave and flap around. Smooth coned speakers tend to have the most of this - and it can give them a sonic signature of sorts. Some guitar players refer to them as "greasy" - they have kind of a dirty, bluesy thing going on. Not hifi at all*. Other speakers have ribbed cones; the ribs are there to help break up the nodes as well as stiffen the cone. They tend to have better high frequency response, but there are no hard & fast rules. Most Celestions have ribbed cones, including the famous G12 or 'Vox Blue". The Jensen "N" and "Q" series have ribbed cones and some of the "R" series as well. Both the G12 and the "N"s tend to have big clear high & low end as a result. (big magnets + ribbed cones) * there are hifi speakers with smooth cones but they are typically using advanced materials
A pair of Celestion G12s here from an old Vox AC30 showing their cloth dust caps, ribbed cones and lightly doped surrounds.
Last but not least, please keep in mind that as the cone moves "out" in front to push air, the rear of the cone is moving "in" and pulling air. They are doing the same thing at the same relative intensity but they are 180 degrees out of phase. This is a pretty important idea that plays into micing/recording as well as cabinet design. It is also why we want to have the surround seal the speaker edge so the sound waves from behind don't radiate up front.
"Spider"
Probably my favorite part just because of the name, the Spider is an impregnated fabric disk that is glued to the speaker frame on the outer edge, and then glued to the voice coil on the inner edge. The spider and surround together locate the voice coil so that it only moves up and down. It also helps keep dirt out of the motor. The spider is a spring - it resists the motion of the voice coil and it can affect the low frequency response of the driver.
A typical spider
Basket
The basket provides a handy scaffold for all of the other parts. They can be stamped or cast. Most common speakers are stamped steel. A manufacturer makes a blank speaker "bowl" and then the speaker company either stamps or has stamped, their own pattern of openings. The famous Jensen "hot dog slots" are a classic example of that. Celestion guitar speakers are stamped. EV, JBL and other higher end drivers often use cast frames. As long as they don't flex you are in good shape.
"Surround"
These can be paper, rubber, paper with "doping", cloth or other materials. This is the outermost upper edge of the speaker cone, where it glues down to the basket. It's not really part of the cone, it is usually where we see the response get dampened. It's the "end" of the transmission line. Depending on what we want to have happen, we will treat the surround in various ways (or not). Often if there is a problem frequency (a resonance aka "cone cry") we will try and dope the surround to get it under control.
"Dust Cap"
While the dust cap is really just there to keep dust out of the motor, some designs take advantage of the location and use it to augment high frequency response. While an old Jensen R type might just have a felt disk here, EVs and JBLs often used Aluminum for both heat dissipation and additional high frequency response. Players who favor distortion generally don't want Aluminum dust covers, but to each their own.
Putting it all together
Diagram: Norman Crowhurst (the OG!)
The magnet structure shown is AlNiCo, what many refer to as a "horse shoe magnet". There is a pole piece up the center of the voice coil and then a "U" shaped magnet to complete the magnetic circuit. AlNiCo magnets tend to be taller and thinner, Ferrite magnets tend to be flatter and larger in diameter. It's a function of the material properties, but we're getting to the same place - putting as many lines of flux as possible in the voice coil gap.
Impedance
This is what a typical impedance curve looks like for a random speaker. They all do some variation of this. I wish I knew where this diagram came from because I would love to credit it properly; it's the best one of its kind I have ever found. #kudos
Couple of quick call-outs on the diagram:
1 - Nominal impedance is usually measured around 400Hz. That mystical "8 ohms" doesn't really exist anywhere else. People can get a little crazy about impedance matching, but it's a bit of cups and balls in real life. Just smile and wave while they rant.
2 - You can see here that DCR is about 20% below rated impedance. You can use it as a rough guide for sorting unknown speakers though. 6 ohms DCR is probably 8 ohms impedance. 12 ohms DCR is 16 and so on. Just remember they are not the same thing and you'll be fine.
3 - "Back EMF" may be a new concept for you. While we are powering the coil and it is moving, it is also a coil moving through a field - which is what a generator is (or a dynamic mic). And in fact, your speaker generates its own reverse voltage that gets fed back to the output stage via the output transformer. It's little mind blowing at first. This is part of what you lose when you use a reactive load box - they cannot simulate this reverse EMF. They probably could with some kind of active circuit, but I've never seen anyone who tried to pull this off.
4 - Resonant frequency here it is a physical thing; the cone is suspended/located up top by the surround and at the bottom by the spider. It is on springs, and any time you have a weight on springs there will be some resonant frequency it wants to naturally bounce at. If you are a motor head (hello!) you will recognize that the cone assembly is "sprung weight". Same idea. On a car you have the car body typically sitting on 4 springs with front and rear springs having different rates to help keep the car from bouncing around, and then each of the 4 has a shock absorber to further help dampen oscillation. Here in our speaker we have 2 springs (surround and spider) and no shock per se, although the surround and spider can have things done to affect their compliance. What you tend to see in guitar speakers is the resonance is moved to the bottom of the guitar's frequency range to boost the bass. Marshall tone junkies will debate the merits of 55Hz cones and 75Hz cones all day, or until the beer runs out. That's what they are really debating - where is the resonant peak of a certain speaker model and how does that work with their playing/sound and cabinet of choice.
5 - On paper, we design tube output stages to work into a straight (resistive) load line. In reality we are working into a squirrelly load like the one above. The net effect is our true load line is really an ellipse. The extra math really doesn't do anything material for us, so we just assume a resistive load anyway. At the end of the day we build and listen and tweak. Net net, running a tube amp into a resistive load is fine - it is literally how they are designed. Issues can arise when people run the things flat out (since they are quiet now) and they treat the thing like a fuzz box. But don't lose sleep because you ran your amp into a resistive load. Just use the amp in a sensible manner - like you would in the real world with a cabinet etc. and you'll be fine. If you run it at 110%, you should expect some extra maintenance. This should not be a surprise to anyone. Make sure you have the proper fuses installed and your bias etc are good. Run hard, maintain harder.
Cabinets / Room placement
Closed back or open back cabinets will act differently and where you put them in a room (or on stage) will result in different responses as well. The big resonant frequency peak will move around based in part on the cabinet you put the speaker into. You can graph it. You can read up on full space, half space and quarter space and how they affect speaker placement. The cheaters guide is to place the amp on the floor for more bass and in a corner for even more bass. Get it up on a stand or a barstool to reduce bass. High frequencies tend to "beam" (go in a straight line) where low frequencies are more omni-directional. The upshot here being that you should really try and be close to on-axis with your amp - playing at stage volume will cause ear fatigue and if you are above or to the side of the speaker, the amp will start to sound muffled and you'll turn up the treble. Which is a bad idea (tm). Try to hear what the audience is hearing so you don't give them a migraine. It will also be better for you. Another good approach is to run through your settings & your set and come up with everything but your volume offsite. Not perfect, but a good start from a time when you were less rushed and distracted and your ears were fresh.
Speaker cabinets and their tuning are really a lifestyle choice, there is so much detail here. Closed backs tend to beam (send sound out straight) while open backs tend to fill the room, sending sound in all directions. The size of your cabinet will impact the bass response and where your amp develops standing waves (resonances). Just as speakers can make a dramatic difference in your sound, so too can the specifics of the cabinet you put them in.
One thing that is good to keep in mind though is that most open back guitar amp cabs are around 10" deep. If you hang your arm at your side to carry an amp it will be around 5" from your hip. At least mine is. Much deeper and the gear becomes pretty uncomfortable to carry. This is my pet theory as to "why". Try to carry an Ampeg B15 with one arm; you will want to spit. They are just too deep to carry comfortably. Wonderful amps, but no fun to carry. (yes I know they have casters) Measure your favorite combo, I bet it's 10" deep - or close. The Twin Reverb & most professional sized "blackface" amps, the Vox AC30 combo, and so many more fall into this range. It's no accident. That's my story and I'm sticking to it.
The net effect of this 10" cabinet depth is that the out of phase sound from the rear of the cabinet is traveling about 20" to get to the front. 10" out the back, then around 10" to get up to the front. We could then assume that the frequency that has a 20" wavelength would be cancelling out up front as our out of phase rear wave combines with our in phase front wave. Wavelength = Speed of sound / Freq and at sea level and room temp the speed of sound is 1,125 ft/sec (using English here since our speaker dia and cabinet measurements are in inches) 1.66 Ft (20") = 1125/F so F = 677.71Hz This is about where we would expect maximum cancellation. So a nice little mid scoop. By varying the opening in the rear you can get some control over how much sound makes it out the back to come cancel out - so you can tweak the cabinet eq. This is without even getting into porting etc. Real life is not quite this neat, and there will be phase effects and some kind of curve to the cancellation above and below the frequency of interest (the "Q" if you're used to parametric EQs) . You can sweep your amp & cabinet if you have a spectrum analyzer. See what you find. Randy Aiken swept a 4x12 Marshall for fun: https://www.aikenamps.com/index.php/frequency-response-of-a-marshall-4x12-cabinet
Putting it together:
If we have a 12" driver, we know that wavelengths longer than 12" will lose intensity, this is just physics. That will happen at 1 ft (12") = 1125/F which of course is 1,125Hz. Yes, those last few octaves are all a struggle. Your guitar's low E has a fundamental of 82.5Hz, so you have octaves at 165, 330, 660 and 1,320Hz. Most of what you're hearing on the low E though is is the 2nd harmonic or 165Hz. So your speaker really needs to squeeze out those bottom 3 octaves somehow to make things work. The low mids are critical to a good tone that sits well in the mix and isn't too bright or too boomy. (go listen to "Back In Black", a lot) If you have a DAW, look at the spectral analysis (stock EQ in Logic has this for example) and look at the frequencies that are most prominent.
Ok, let's look at a real speaker for a sec to see this stuff in come together in action. Here is the spec sheet for a Celestion Blue: https://celestion.com/product/celestion-blue/
They are quoting a frequency response of 75Hz to 5KHz, and a resonant frequency of 75Hz. Normally a speaker's stated response is +/- some dB range. For HiFi it's often 3dB. Here our response swings all over, and they are charting 20Hz to 20kHz. It can be confusing.
But let's follow what they are saying: between 75 & 5k we have a dB range of about 92dB to 108dB or ~16dB. Then they are quoting 100dB 1W/1M as the efficiency, so if we assume that carries for this chart (so we'll use 100dB as our center line), our frequency response is then 75Hz - 5kHz +/- 8dB. If this was a HiFi speaker we would not take it seriously. 8dB is a huge range; 16dB is comical in the world of HiFi where flatness is valued.
But the Blue is arguably the most important guitar speaker ever made and it still commands a hefty price tag to this day. And it is because it has a "sound". When Groove Tubes patented a speaker simulator, the speaker design they based it off of was - yup - a Celestion blue (aka G12). The Vox blues and silvers are the same speakers with different paint. Don't get confused. The Celestion Greenback was the G12 fitted with a ferrite magnet instead of AlNiCo. It's impact is all around you.
Chart: Celestion
You can see a few things going on:
1 - Our response peaks around 2.5KHz; nearly 10dB higher (10X the power and twice as loud) as the speaker is at 1kHz. It's a huge presence peak, probably enhanced by harmonic distortion generated by cone breakup. That 2-5kHz peak is a big part of our signature sound here. Call it "chime" and you are probably on the right path.
2 - By 5kHz you can see we are about 10dB off our peak and the response plummets from there. We just cannot wiggle a 12" cone more than 5,000 times a second very well. This is why HiFi tweeters are tiny. There are some nodes and harmonics up high but they are ~30dB below our peak, a whisper. We hear them as "air".
3 - You can see low frequency response start to drop off around 1kHz which is what you would expect based on the driver size as we calculated above. Celestion appears to have played with the motor parts to keep the bass working pretty well down to around 200Hz (recall our low E 2nd harmonic is 165Hz) and below that we are losing around 6dB per octave. (a single pole HPF essentially) They then probably tuned the motor to resonate at 75Hz to keep the low response as good as possible in the passband. HiFi speakers often tune a port to give a bass bump below what the speaker could normally do and allows them to use smaller cabinets. Some guitar amp manufacturers have played with porting, notably Dr Z. I'm not sure it really ever caught on though for guitar. (Bass is another story)
Recording
There are some useful generalizations when it comes time to record - your clearest highs are found where the voice coil is attached to the cone. The classic old school mic setup was to have your assistant move the mic while you sat in the control room. The mic would be on-axis (perpendicular to the cone) and you would place the mic where the hiss was the loudest. This is where your voice coil and cone meet. It's easy to roll off highs, it can be a challenge to eq them back without adding hiss or other artifacts.
Traditional Mic placement (photo: Trace Davis)
Your lows develop as you move the mic towards the surround. Taking advantage of this as you think about microphone placement can be helpful to achieve various effects. You might also play with mics like the RE20 that don't exhibit proximity effect. Or drop a mic in the back of an open back cabinet to get the lows there (remembering to flip the phase!) The more you know about the speaker and what it is doing, the more tools you have in your toolbox. There is no "right" answer, just getting the sound you want.
Conventional wisdom says to close mic your amp, but if you have a decent room, try moving the mic back - the "sound" really doesn't develop until the waves have travelled a bit and recombined. (amp sound, reflected room sound, highs and lows from different parts of the cone, etc) You are hearing it this way, why not let the mic hear it this way? There are some very famous records that even leveraged room mics to get their vibe; the old Billy Squier records achieved a unique sound this way. Listen to the guitar on "Lonely is the night" for example: https://youtu.be/d5U1VgV7puw You can hear the room, as if you are there, and the track is instantly recognizable because it doesn't sound like everything else (that is close mic'd)
I hope this elementary overview helped clear some things up and will assist you in finding the best speakers and cabinet for your performances.
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