Speakers are electromechanical devices (transducers) that convert electrical signals into sound. Locomotives equipped with sound will employ one or more speakers to reproduce the sound generated by the decoder. Only sound decoders will require speakers.
The speakers will be mounted either in the body, on the frame or in the fuel tank, or in the tender of a steam locomotive. For proper sound a baffle or enclosure is needed. These can be purchased from a hobby shop or constructed from plastic sheet or stiff cardboard. The baffle enhances the sound and the volume.
Speakers, or "voice coils", are transducers that convert electrical energy into mechanical movement. The mechanical movement of the speaker cone creates sound that you can hear.
A speaker is constructed by winding light gauge wire into a coil. The coil is then mounted on the speaker cone, and the entire assembly is surrounded by a magnet. When energy flows through the coil, the resulting magnetic field causes the coil to move. The movement creates sound.
The speaker is rated in terms of watts and impedance. Watts is the amount of power the coil can withstand, and the impedance is the load the speaker presents to the amplifier driving it. You cannot measure the impedance with a DMM as the coil is almost a short circuit.
Speakers are produced in a variety of sizes, impedances and power capabilities for numerous applications.
Impedance is a complex measurement that indicates the resistance, inductance, and capacitance of the voice coil. It cannot be measured with an ohmmeter, as it is an AC only quantity. Impedance is represented by the letter Z. The impedance is mainly from the inductive component of the coil.
Most general purpose speakers are 8 ohms, such as those used in home audio. For applications such as automobiles, 4 ohms is typical (same volume, less power needed). The value usually is marked on the speaker. You may see the omega symbol, or the letter R used to indicate impedance. (Especially in print, R is often used. If you see a value like 2R2, the letter (may be R, K, or M) is the multiplier and takes the place of the decimal. 2R2 is 2.2 ohms, and 2k2 is 2200 ohms.) The impedance is determined by the design, as a woofer will have a larger gauge wire to handle the power needed for low frequency reproduction, and the tweeters, which don't demand as much energy, will be made with a finer wire.
Speakers wired in series: Impedance equals the sum of the individual impedances.
For example, two 8 ohm speakers in series are 16 ohms total.
In parallel, 1/Z(total) = 1/Z1 +1/Z2.
or: Z1*Z2 / Z1+Z2
EXAMPLE: Two 8 ohm speakers yield 4 ohms total.
1/8= 0.125 ohms
= 0.25 ohms
1/0.25 = 4 ohms
TIP: Two equal resistances (or impedances) in parallel have a total value of half the value of one impedance (or resistor) As shown mathematically, two 8R loads in parallel equal 4R total.
The same formulas used for resistance in series and parallel also apply to impedances. Two equal impedances in parallel will have a total Z of half the individual impedance of one component.
It is important to match the speakers to the amplifier. For maximum power transfer, they should match. Using a 16 ohm speaker on an amplifier designed to drive an 8 ohm load will usually result in maximum volume being halved. The opposite is also true: 4 ohms means more volume with less power.
The real danger is driving a speaker that is too low in impedance. As impedance depends on frequency, and the speaker is really an inductor, the impedance decreases with the frequency. At low frequencies the speaker load could drop to a very low value. The danger is that low frequencies usually mean more power output from the amplifier to drive the speaker, and the lower impedance could overload and damage the amplifier. It could also result in distorted waveforms and damage to the speaker itself.
Phasing or Polarity
Phasing means that in an installation with multiple speakers, all speakers move in the same direction. This is accomplished by wiring all connections in the same manner. Speakers usually have an indicator for polarity at their terminals, so if one speaker is connected from it's plus terminal to the plus terminal on the amplier, the other must be connected in the same manner to it's amplifier.
Two terms describe speaker operation: Compression and Rarifaction. Compression: the speaker movement compresses the air in front of the cone. Rarifaction: The speaker cone moves in the opposite direction, reducing the air pressure in front of the cone.
Incorrect phasing will result in poor sound quality, as one speaker is cancelling out the other (Compression versus rarifaction.) If wiring two speakers in series, the minus terminal on the first speaker connects to the plus terminal on the second one. In parallel, plus to plus. The movement of the speaker cones is in phase, enhancing the sound.
Incorrect phasing tends to attenuate the lower frequencies.
Speakers for use with DCC should be of the impedance recommended by the decoder manufacturer. Too low an impedance can damage the amplifier, and the speaker. Too high, and the maximum sound levels will be reduced.
For mobile decoders, speakers will be by necessity, small. This introduces new challenges. The speaker must be mounted securely to avoid resonation with the body shell, with no interference from internal components or wires.