Understanding Loud Speakers.

Have you ever heard a group of car audio enthusiasts have a get together and listen to the terms they start slinging around? I have and I often end up having a good chuckle about it.

Although one should really use AC theory when explaining speakers, I will use DC theory to simplify explanations and to do basic analogies.

Just for your information however, the impedance of the speaker is measured in  ohms. This is affected by the frequency of the signal, the dc winding of the speaker voice coil, the capacitive reactance of the winding and the inductive reactance of the winding. And then to make things worse it is also affected by the movement of the voice coil as well. In other words impedance is basically the ac resistance of the voice coil. This means that you have all kinds of horrible things happening to a speaker impedance at different frequencies which makes things worse.

Another fact for your information is that amplifiers, commonly known as boosters, car radios etc, are tested by connecting the speaker outputs to a resistor and not a speaker. Therefore the specifications on the spec sheet are not valid anymore when you connect your speaker. The resistor is a constant load and the speaker is not as explained in the previous paragraph. This is why high fidelity enthusiasts always try match different combinations of speakers and amps for the best quality sound.

Understanding the different terms and their meaning.

Watts.

This is the unit of power. A speaker of 10 watts RMS means the maximum continuous power that that speaker would be able to handle. RMS in lay mans terms, means Root Mean Square Value of a sinusoidal wave which would have the same heating effect as a DC value of that power. And obviously the same term is used for determining the output of the amplifier.

PMPO. 

"Peak Music Power Output" This has no meaning whatsoever and is actually useless information. When doing any tests you have to work to defined standards. No standard has ever been defined for this test. In other words the manufacturer can use any conditions he likes for this test. For example he might connect a booster to 48V DC and do this test to achieve his PMPO value. However when connected to a 12V source you have a totally different value. There are so many ways to achieve these values but in the real world they are meaningless and unpractical. This just looks nice when selling the item to you.

Ohms.

This is the unit for DC resistance and Impedance.  It is critical that the impedance of the speaker is not lower than the lowest impedance allowed on the amplifier. Going higher is no problem except for loss of power but going lower can seriously damage the output components on your amplifier.

Volts.

This is the pressure that forces the electrons through the circuit. There are pressure drops throughout the circuit due to resistance.

 Amps.

This is the unit of current. In other words the electrons that  flow through the conductor. If 1 amp goes in to the circuit, 1 amp must com out.

Decibels.

This is the unit for measuring sound pressure levels.

Sensitivity.

This is normally used to determine the sound pressure levels of the speaker and is expressed in decibels per watt. A high sensitivity speaker is much louder than a low sensitivity speaker.

Crossover networks.

 A passive crossover network is a combination of inductors, capacitors and resistors in some cases as well. Although  they can enhance the quality of sound they can also cause havoc and power wastage and phase distortion between speakers if not correctly matched. Some times a simple filter such as a capacitor connected to your tweeter is better than a power wasting cross over network. Please note that this section does not cover active crossovers. Active crossover is usually fitted in to the pre-amplifier stage and has far more advantages than a passive system.

Picture of a passive crossover network. Courtesy of  MTZ Audio.

Now lets get down to brass tacks.

The definition of high fidelity is that the sound that comes out the speakers must be as pure as the sound that goes into the microphone. Unfortunately in real life this is just not possible, but we can get get close to it. However there are a tremendous amount of variables that effect the quality of sound, unfortunately far too many to mention here. I have studied books on making speaker enclosures and crossover networks, and these books have been in excess of over 200 pages and more.

Unfortunately you will never be able to even class a car installation as a high fidelity installation. No matter how well you tune the system there are just too many variables. For example, load 4 people into your car and you have reduced your acoustic volume.  Leaving clothing or towels in your car will absorb high frequencies. There are a million and one reasons why you cant reach high fidelity standards.

However in the real world it does not mater that you cant as your system must suit your preference. You are the one that has to listen too it. Not your friends, unless you are building a sound system for competitive reasons.

Have you ever heard of some one who cranks up the volume in a car till distortion and says, "Look, the system is too powerful for the speakers. I have a 100 watt speakers and it cannot handle the power."

The truth is it is not the speakers that cant handle it. It is the amplifier. This distortion is known as clipping. What it means is that the voltage supply on the output of the amplifier is not enough to allow extra current to flow through the speaker. This will prevent the cone from moving further forward on the low frequencies. The cone will hang their even though the input signal is carrying up higher. The cone will hang their and wait until the level of the  current is less than the current in the voice coil. Speakers can quite easily handle 3 to 4 times the input power as it is not a continuous power level but an instantaneous power level.

Courtesy of ONVILabs. This image demonstrates clipping. Te red line is the input signal and the blue line is the resultant clipped output signal due to the amplifier's inability  to supply a higher output signal.

One way to overcome this is to either add more speakers in parallel or to replace the speaker with a speaker of lower impedance. For example two 8 ohm speakers in parallel will give a combined impedance of 4 ohms. However always ensure that the total impedance is never less than the amplifier otherwise damage will occur. Another advantage of inserting two speakers in parallel is that you will increase the power handling capacity and the cone area if they are mounted in close proximity.

If you are competing for volume with a low powered amplifier their is another trick you can use. This is to use high sensitivity speakers. Speakers sensitivity usually range anything from 80 decibels per watt to 110 decibels per watt. Hearing tests have indicated that the smallest change in volume detected by humans on average is around 3 decibels. To achieve a 3 decibels difference you have to double your power every time. In other words there is not much difference in volume between a 10 watt amplifier and a 20 Watt amplifier. A 100w amplifier is only twice as loud as a 10 watt providing you are comparing with the same speakers.

Here is a table comparing  two speakers with different sensitivities. The  outputs in decibels  is compared to power input in watts. Note that the speaker with a 100 db/watt at 1 watt power input puts out almost the exact same sound pressure as a 90 db/speaker operated at 16 watts power input.

As you can see from the table if you used a 100 db/watt speaker with a 8 watts RMS amplifier you could generate almost as much sound as a 90 db/watt speaker on 64 watt RMS amplifier.

Here are some tips when installing your sound system.

1. Ensure that your system is always protected by a fuse in the supply wire.
2. Use as thick wires as possible.  Try go for no less than wires with a conductor area of less than 4 mm square for your speaker and supply if possible. If you are using a booster amp, take the supply direct from your car battery. Remember to install your fuse as close as possible to the battery to prevent electrical fires.
3. Always ensured that your speakers are phased correctly. Especially your woofers. Otherwise loss of bass will occur. Phased means that the positive of the amp connects to the positive of the speaker in both cases.
4. It is easy to find the positive and negative terminals of the speakers if the markings have worn off. Take a battery and connect it to the terminals. Mark the terminal that is connected to the positive terminal of the battery as positive when the cone moves forward.

And finally, happy listening.