A speaker converts electrical energy to mechanical/acoustical energy. It uses a coil of wire (which acts as an electromagnetic of varying strength), set inside a magnetic gap formed by a pole piece and a top plate. A permanent magnet produces the magnetic field. The following demo shows the main components of a woofer. Holding your cursor over the individual buttons will highlight the respective component. Clicking on the button will bring up a brief description.

Voice coil motivation:
When a current is passed through the coil of wire, called the voice coil, it generates a magnetic field. This electromagnet interacts with the field in the magnetic gap and the voice coil moves. The direction of movement depends on the direction of current flow through the VC. Since audio is an AC waveform, current flows in one direction and then changes polarity, the VC moves either forward or backward from its point of rest. The diagram below shows how the VC is connected to the cone of the speaker. The cone is the part of the speaker that actually makes the sound by alternately creating an area of high and then low air pressure.

Magnitude of cone movement:
When an amplifier drives a speaker, it is driving the speaker terminals with AC voltage. If the volume is at its minimum position, the speaker doesn't move. If the driving voltage is low, the speaker moves a little. As the voltage increases (when you turn up the volume), the cone moves further from it's point of rest. Higher power amplifiers can drive the speaker with higher voltage and therefore produce more SPL (volume).

The following demo shows how the magnitude of cone movement increases when the volume is increased (and a higher voltage signal is driven into the speaker). Click on the slider handle to lock and unlock the handle. Move the mouse left or right over the white bar to change the volume level.

The next diagram shows a detailed view of the voice coil and the magnetic gap.

Please note that speakers DO NOT produce power. A speaker rated at 1000 watts is not necessarily going to be more efficient than a speaker rated at 50 watts. If they are manufactured by the same company (so that they are rated by the same standards), the speaker rated to handle higher power will be able to produce more sound pressure level because it can be driven with a more powerful amplifier without fear of damage. Many times, a manufacturers cheapest woofers will be more efficient and may be a better choice for a low powered system.

Coaxials and Triaxials:
It is very difficult to build a single driver capable of accurately and efficiently reproducing the entire audio spectrum. It is much easier to use multiple drivers, each reproducing its own narrow band of frequencies. Coaxial speakers are 2-way speakers which employ a larger driver (for bass and midrange) and a tweeter (for reproducing upper midrange and treble). A triaxial speaker is a 3-way speaker with a woofer, a midrange and a tweeter. Both types of speakers usually include the required crossover components for the midrange and high frequency drivers. The diagram below shows a 3-way design and a graphical representation of the frequency response reproduced by each driver.

This diagram shows how the voice coil position relates to input voltage. You can see that the voice coil moves above the point of rest when the voltage is positive. When the voltage is negative, the voice coil is below its point of rest. When the voltage is at 0 volts (ground), the speaker is at its point of rest. You should also notice that the magnitude of displacement is directly related to magnitude of the input voltage. More voltage means more displacement. Higher power amplifiers can generate more voltage at their speaker terminals.

Thiele/Small Parameters (not a complete list of parameters):

SD:
Effective piston area of the cone. It will vary slightly from one 10 inch (or any other size) to another but does not vary enough to make a difference in the performance of the driver.

Xmax:
By definition it is the peak linear travel of a driver. If you measure the distance that the voice coil can travel in the gap (in one direction) while the number of turns in the gap doesn't change, you have the Xmax. If you go past this point, the actual windings in the voice coil start to leave the gap. The diagram below shows the voice coil at its maximum travel at point of max linear travel and just past its point of max linear travel.

BL:
BL is determined by the flux density (B) in the magnetic gap and the length (L) of voice coil wire in the gap. A higher BL will generally mean a speaker will have a higher relative sensitivity (efficiency). This doesn't necessarily mean that all speakers with a higher BL will produce a higher SPL. Often speakers with very high BLs have a smaller Xmax.

The diagram below shows two different motors (that's what they're called). Motor 'a' is what you might find in a speaker with a relatively low power rating and a relatively short Xmax but its efficiency will be relatively high. Motor 'b' will have a higher Xmax, higher power handling due to the larger voice coil windings and a lower efficiency. The difference in the xmax is due to the difference in overall length of the voice coil. Xmax=voice coil length minus the gap height. The difference in efficiency is due to a different number of windings in the gap. Remember that the voice coil is an electromagnet. The current passing through the coil generates a magnetic field which is distributed along its length. On the shorter voice coil, more of the generated field is in the magnetic gap producing a slightly stronger motor but with a shorter stroke.

'R_E':
This is the DC resistance of the voice coil. It will be lower than the rated impedance of the speaker. A 4 ohm speaker may have a DC voice coil resistance of 3.3-3.8 ohms.

Resonance:
Free-air resonance is the frequency at which the speaker oscillates most readily. At this frequency, the speaker can be driven to its mechanical limits with less power than at any other frequency. Resonance in an enclosure is different and will be covered later.

'f_s':
Free air (not in an enclosure) resonance of the driver. All speakers have a resonant frequency. At this frequency, the impedance increases significantly.

'f_c':
This is the resonant frequency in a sealed enclosure. The resonant frequency will tend to be higher but the impedance will not go as high.

'n_o':
This is the reference efficiency. It is usually expressed as xdB when driven by one watt and measured at a distance of one meter. Ex: 89dB/1w/1m. See note below for more information related to reference efficiency.

Note:
You have to be careful when looking at reference efficiency (sensitivity). You can make a speaker really efficient by designing the voice coil to fit entirely in the magnetic gap. This would likely yield a sensitivity of 104 or so. This speaker may work very well if powered by a low powered amplifier because of the high efficiency but would not be able to produce high SPL at low frequencies because it would have a very small xmax. Actually, if the voice coil length was the same as the height of the magnetic gap, it would have no (zero) xmax.

You can also design speakers for very high power handling and high SPL but those speakers would likely have a very low reference efficiency. Speakers designed for high SPL in cars generally have a larger xmax and therefore lower reference efficiency but would easily out perform the speaker (in the previous example) with the higher reference efficiency at low frequencies.

Speakers that are designed to operate in very small enclosures are usually less efficient than speakers designed for larger enclosures. To make the speaker perform in a small enclosure, the suspension has to be stiff. This will raise the resonant frequency. To get a lower resonant frequency, they must add mass to the cone of the speaker. This added mass and the stiff suspension kill the efficiency.

The diagram below is a graph which shows how impedance relates to frequency.

The following demo shows how the speakers respond when testing them with a battery. When you press the buttons, the battery connection is made and the speakers move in the direction dictated by their wiring.

NOTE:
I've heard that at least one speaker manufacturer (Gauss) that makes their drivers with the polarity opposite of the above diagram. Guass is a company that makes drivers for use in commercial PA cabinets, so you won't likely see them in cars.

Rated Efficiency:
Since there are nearly infinite number of ways to measure the efficiency of a speaker, many manufacturers will use the method that gives the highest efficiency for their speakers. To give a more accurate comparison of speakers of equal size, you can enter the Thiele/Small parameters into the following calculator. Vas is in cubic feet. The fields which contain 'speaker#1' and 'speaker#2' can be used to enter the model number of different speakers. They have no bearing on the calculations. They are simply there so that it's easier to remember what speaker the specs are for.

Understand that this calculator is not designed to be 100% accurate. It's just here to show how the SPL increases or decreases with changes in the system.

• For clarification of output data:
• Gain from multiple woofers tells how much of a change in SPL you use different numbers of woofers.
• Reference power is the power that the manufacturer used when they measured or calculated efficiency.
• Gain from power is the gain in SPL you get from increasing the power above the 1 watt reference. doubling the power will give you a 3 dB gain if all else remains constant.
• Cabin gain is the reinforcement you get from your vehicle's interior. The values I chose are from my experience. They certainly won't be accurate for all vehicles.

• What you should realize after using the calculator is...
• If you manually enter a cabin gain of zero, and a power output of 1 watt and enter 1 in the 'woofers' field, the output will equal the reference output.
• If you double the power to 2 watts, you gain 3dB.
• If you set the power back to 1 watt but double the cone area (number of woofers), you still gain 3dB.
• If you double the power to 2 watts and use 2 woofers, you'd get 6dB gain over the reference efficiency of a single woofer.

Note:
If 'cabin gain' is set to '0', the speaker output is what you'd expect if the woofer were in its enclosure in the middle of a large open space with no reflective surfaces. Your car isn't a big open space and it reinforces the overall output of the system. In the vehicle, the cabin gain will vary with the vehicle (which I guestimated for various vehicles) and will vary with the frequency (which I don't use in the calculations at all). Like I mentioned before, this is not going to be 100% accurate but... if you had a single woofer in your vehicle and drove the speaker with a 1 watt low frequency signal, this calculator could help you predict how your output would change if you added more speakers or power. The cabin gain would be the difference between the reference given by the manufacturer and the SPL that you'd actually produce at 1 watt in your vehicle.

Preventing Damage:
I get a fair amount of email concerning torn or punctured surrounds. Most are from the screwdriver or drill slipping when pressure is applied to the screw head. There are a couple of things that can help prevent or lessen the chance that this will happen. First, use a bit that fits the screw head well. Don't use a worn bit. A good fitting bit will not have any 'slack' when inserted into the screw head. I use bits that have teeth on the sites of the bit to help grip the inside of the screw head. You'll have to find a bit that works well with the screws that you're using. Second, use a bit holder that has a sliding sleeve to help prevent the screw from separating from the bit. When starting the screw, the sleeve will be slid over the bit and screw as is shown in the top example. As the screw grabs the wood and starts to dig in, the sleeve slides up out of the way to allow you fully tighten the screw. Since the damage is usually done when trying to start the screw in the wood, this little tool will prevent almost 100% of the accidents. I don't recommend using drywall screws (as is shown below). I recommend using pan head screws for most speakers.

After Damage has Been Done:
If you have already damaged the surround, it can be repaired with little or no change in performance. Everyone has an adhesive of choice. I prefer contact cement for this job. It can be applied in a very thin layer and remains very flexible. The following images are a before and after of the repair. When applying the adhesive, I use a cotton swab that I've cut the cotton off of. You can use anything that will allow you to get the adhesive into the repair site. You want to apply the adhesive to both sides of the damaged edge of the surround. Generally, coating the applicator and simply inserting it into the tear will assure proper application of the adhesive. After the adhesive is applied, realign the foam to where you want it. You need to work fairly quickly because the adhesive will start to set up in a minute or so. Allow a couple of hours before playing it at full power. If you apply the adhesive properly and get the surround back together as it should be, the speaker will be as good as new. I've never had one fail after being repaired. The repair below does not look very strong but I could not pull the repaired area apart after the adhesive set.

Before (above)

After (above)

This link is a 250hz tone recorded at -1dBfs for 1 second and then it drops to -2dBfs for 1 second. It repeats 3 times. This was put here to give you a reference for the calculator above. Most people don't know what a 1dB difference sounds like.

Speaker Mounting Depth:
The following image shows how bottom and top mount measurements differ.