The purpose of a loudspeaker or speaker is to convert electrical AC waveforms into moving air so that our ears can hear them. You could almost say they enable us to hear electricity.
As the name suggests, they were first used to hear speech, notably in the first telephones, and then later, for music reproduction. The first use of a speaker was by Johann Reis in 1861, but it took a while until 1924 for Kellogg and Rice (yes, really, Crispie was sick that day!) for it to develop into a proper dynamic moving coil speaker we know today.
Over the years, many technologies have emerged, all converting electrical signals into moving air. The main ones are moving coil, electrostatic, capacitive, ribbon, and plasma. We are going to look at moving coil as it is by far the most common.
Shown below is a cross-section through a moving coil speaker showing the main parts (used with permission), and below that, a real speaker viewed from both sides with the same names used. You cannot see the actual moving coil as it is inside the magnet and covered by the dome. The clearance between the coil and the magnet is minimal and without the dome, metal filings will be drawn in and stuck.
How it Works
The principle behind a speaker is elegantly simple: An electrical signal through a coil of wire will produce a magnetic field of a direction that changes with the polarity of the AC signal. If this coil is inside a fixed permanent magnetic field, the two magnets will repel each other when the poles are the same and attract each other when they are different. The result is that the coil moves in and out in sympathy with the AC signal.
If you attach a suitably shaped lightweight object (such as the paper cone), it will move with the coil causing the air in front and behind to move in and out, and the moving air reaches our eardrums, and we hear it as sound. This all sounds remarkably simple, but there are huge obstacles to overcome before we get too Hi-Fi.
First, as the air gets pushed forward from the front of the cone, the air at the back gets sucked in. This makes the air want to rush around from the front to the back, giving us an air “short circuit.” This short depends on the wavelength of the signal and results in the bass or low-frequency end being lost. To prevent this, we can put the speaker in a box, solving the short circuit. However, we have now created other problems—the box has its own resonant frequency and starts to boom on a certain note.
Also, the air in the box now has to compress, reducing the speaker’s efficiency. For this reason, speakers are never used on their own. It’s used as a part of a system, usually consisting of several different sized speakers to deal with different frequency ranges and a set of filters called the ‘crossover network’ to steer the low, mid, and high frequencies to the appropriate speakers. Because the physical size, the mass of the cone, and the box’s volume all affect the speaker’s frequency range, a Hi-Fi system will generally divide the load up into at least three bands, and sometimes even four. These are:
- Subwoofer – from below 20Hz to about 200Hz
- Woofer – from 20Hz to 2kHz
- Mid-range – from 150Hz to 5kHz
- Tweeter – from 2kHz to 20kHz
There is a lot of overlap, producing a flatter more even sound across these.
If you buy a speaker system (i.e., in a box with all speakers and crossovers pre-wired), the manufacturer will usually specify the frequency range impedance and power handling ability. These are often not believed to vary greatly with frequency, and cheap units may even have complete lies. There is only one power standard, and that would imply continuous power—formally RMS power. Folks in the east make up all sorts of standards to fool you with, i.e., music power, peak power, etc.
Individual speakers will have their own specifications, and they are also not going to mean much if not used in context, i.e., mounted or unmounted, boxed or not, etc. As a guideline, these are the ones you may encounter:
- Power: Only RMS is to be trusted. A 100w RMS speaker should be able to handle 100W sine wave continuously
- Impedance: As the coil is inductive, this is meaningless and will only be true at one frequency, usually 1kHz, and is reactive. Generally 4, 8 or 16Ω.
Generally, if you connect speakers in series, you add the impedances, and in parallel, they divide, i.e., two 8Ω speakers in parallel will give you 4Ω. This is simple Ohms law: Rseries = R1+R2 etc. and 1/R parallel = 1/R1 +1/R2.
- Frequency response: Depends on the enclosure but will give some idea of the range.
- Self-Resonance: The frequency where the speaker becomes self-resonant, used in calculating the box volume.
Matching Up to the Amplifier
Valve amplifiers have output transformers, and these are tapped and will demand the right speaker impedance to be connected. Solid-state amplifiers generally have much lower output impedances than the speaker and don’t care about matching impedances. However, an amplifier has a limited power supply voltage, and this will limit the swing of the output signal, e.g., an amplifier with a +/- 30V PSU will have an output swing of almost 60Vpp or 21Vrms (I say the saturation voltages of the output transistors will reduce this a bit).
Theoretically, this 21V signal is available to drive the speaker, assuming the speaker impedance is nominally 8Ω P= V2/R = 55W. This naively assumes the output transistors and power supply have the grunt to deliver this. If the amplifier was indeed a 50W amplifier and we put two 8Ω speakers in parallel, we could be badly mistaken to assume P is now 212/4 = 110W. But this will never happen as the PSU cannot deliver this.
Using the above knowledge, how on earth do car Hi-Fi amplifiers get rated at 100W if the power battery voltage is only 12V? After all, P=122/8 is only 18W!
Assuming a standard type amp (no inverters or transformers), they just parallel up speakers to get a low enough impedance for the voltage to deliver into. That means R=V2/P i.e., 144/100 = 1.4Ω. So they parallel up speakers to get this low value. Note that the current is becoming large, I=√(P/R) = √(100/1.4) = 8A and decent wire size is becoming important.
What if we have a genuinely powerful amplifier, say one rated for 1kW? How are we going to get all that power going into the speaker system? We would have to use many speakers and, with clever series and parallel combinations, we can end up with the right impedance at that power level.
For our 50W system, how much current is going into the speaker? Once again, using simple Ohms law (I=V/R), we have 21/8 = 2.6A. Note that the lunatic fringe may be using a huge “monster cable,” but this will have little effect on losses in the wire.
Now, you see speakers are both simple in concept but very complex in application.
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