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.

How To Solder

Understanding Solder Wire.

There are different types of solder for different jobs. There are usually two types of solder. The one type is made up of an alloy of 40% tin and 60% lead and the other one is 60% tin and 40% lead. The latter is usually preferred for electronics.

There has been a drive to eliminate the lead content in certain countries due to the toxicity of the lead. Lead has been known to be absorbed through the skin and inhaled by the fumes given off during the soldering process. This lead is usually replaced by either copper, silver, bismuth, antimony, indium, zinc, etc.

Solders used for electrical or electronics are resin cored and solders used for sheet metal plumbing are usually acid cored. Plumbers also use soldering bars which are acid free. One usually uses spirits of solder when soldering with these solder bars. Jewelers usually use borax acid for soldering jewlry.

A soldering paste, usually known in the trade as soldering flux, or resin flux is often used when doing electric or electronic soldering. However this is not always a necessity.

Applied to components before soldering some times to ensure a clean joint when soldering. Prevents oxidization when heating and helps to clean the joint.

Three basic rules for soldering.

1. Ensure that you have enough heat available to do the soldering. If the temperature is too low it causes crystallization of the solder which leads to a poor electrical connection.
2. Do not over heat. Overheating causes the solder to oxidize. This also leads to a poor electrical connection. In both cases these connections will deteriorate in time.
3. Ensure that the surfaces to be joined is clean and grease free. Dirty surfaces lead to what is commonly known as dry joints. This comes from the term known as wetting or tinning the joints before soldering. This means applying solder to both surfaces before joining. If the surface is dirty the solder will not bond properly, which leads to a dry joint. In other words it wont wet or stick to the dirty surface..

Soldering Irons.

There are numerous types of soldering irons out there. The professional irons for electrical  or electronic work are  usually temperature controlled. This is usually between 180 - 200 deg Celsius.
Powers ratings usually range anything from 15 watts to 500 watts. The 15 to 60 watts is usually used for electrical or electronic work. The heavy duty ones are usually used for heavy electrical wires or light metal sheet work. Extra heavy sheet metal soldering irons are usually flame heated.

This soldering iron is temperature controlled. Adjustment of the temperature is carried out by changing the tips. The tip has a magnet on the end which loses its magnetism after the predetermined temperature. Once the magnetism is lost the magnetic switch inside the soldering iron opens and the iron is switched off. As the iron cools the magnet regains magnetism and the switch is once again activated to reheat the iron.

How to solder electrical wires or electronic components.

1. Ensure that the soldering iron is up to temperature, and that both surfaces to be joined is clean and grease free.
2. Clean the tip of the soldering tip with a damp cotton cloth.
3. Apply a small amount of solder to the tip of the iron to ensure decent heat transfer to the job.
4. Heat up wire #1 with the iron. Hold the solder wire against the wire until the solder flows freely along the wire, covering the surface evenly.
5. Repeat the same procedure to wire#2 
6. Hold the two surfaces together and gently apply solder while heating with the soldering iron. Once the solder flows freely remove the soldering iron. Always hold the soldering iron against the job until its hot enough. Do not dab the iron against the job. If you hold the iron too long the solder will go dull on the joint. Stop the process, remove excess solder and repeat the process again. Never carry solder to the job on the soldering iron. By the time the solder reaches the job the oxidization process will have started. A decently soldered joint is always bright and shiny.

Desoldering wick used to absorb excess solder when unsoldering.

Solder Sucker used for sucking up excess solder when unsoldering components.

                    

 

Tripping Resdual Current Device Fault Finding.

Have you ever had a residual current device in your house that trips and you cant find the fault?

Picture of an residual current device or earth leakage unit as known in South Africa.

 
It is always best to understand the basics on how this unit works to assist you in trying solve the problem before calling an electritian.

There is a toroidal transformer which measures the magnetic field inside the unit, which  is linked to an electronic circuit. Both conductors pass through the transformer as indicated in the drawing below. Under normal conditions the magnetic field, generated by the current passing through the transformer is cancelled as they are flowing in opposite directions.

This unit is able to detect when there is an imbalance in the current flowing through it.

 However when current is carried through to ground, via yourself or for any other reason, an imbalance is caused. This imbalance is detected by the transformer which triggers the electronic circuit.

The electronic circuit, which is calibrated to react when a difference of more than 0.03 amps is detected, activates a electrical actuator which then isolates the unit from the main supply.

A current of more than 0.03 amps and some times even less passing through a person can cause ventricular fibrillation, which can lead to death. However the shorter the period of exposure, the lesser the risk.

Some units also have incorporated over current protection units. This means that you might be drawing too much current, which could also cause nuisance tripping without having a ground fault.

Tests one can do before calling an electrician.

1. Unplug all appliances and reset the RCD unit. If the unit does not trip again, suspect an overloaded unit or an appliance fault.
2. If the unit stays on, plug in the appliances one by one until the unit trips again. If the unit trips again when plugging in an appliance, suspect that as the faulty one immediately. Some times this tripping can also be caused by an intermittent fault on appliance. One will have to be patient in trying find the appliance if it has an intermittent fault.
3. If the unit trips again with all the appliances unplugged, set all circuit breakers into the off position. Note that the main supply must be left on with this test.
4. If the unit trips again, suspect a fault between the neutral conductor (black wire) and ground (green and yellow wire) or a faulty RCD unit.

At this stage it is best to call an electrician. However, should you feel you are a hands on type of person and feel you want to diagnose further, this is what you need to do. However please note that you do so at your own risk. I accept no responsibility for your safety whatsoever. As I have stated earlier it can take less than 0.03 amps to kill you!

1. Switch off the main switch. Open your distribution box.
2. Count your number of wires coming in to your RCD unit. There must only be a maximum of two. Do not do this test if there is more than two wires as you then have a three phase installation. Doing this test will cause damage to your appliances.
3. From the load side of your RCD ( the bottom) disconnect the black wire. Tape it up with insulation tape to make it safe. Replace your cover. Set all trip switches, RCD unit and main power to the on position. If the unit trips again your RCD unit is faulty. If the unit remains on you have a neutral to earth fault.
4. If the unit remained on, switch of the main power and remove the cover again. Remove the tape and reconnect the black wire. Ensure that the terminal is tight.
5. Follow the wire till you reach the other end. This wire should lead to a bar where all the other black wires connect to. This is called the neutral bar. Disconnect the wires one by one repeating exactly the same procedure as you did in step 3. Obviously you will not remove the wire from the RCD unit this time. When you reach the wire that stops causing the unit to trip, you have found the faulty circuit. Leave this wire disconnected and taped up, close up and switch on everything. Check around to see what is not working. Remember this could take quite a while as it could be your hot water cylinder that is faulty. This could take a few hours to find out.
6. Once you have traced your faulty circuit, call your electrician and explain the fault.

Note, that working with electricity is extremely dangerous. You must always Isolate your distribution board before attempting any work. You must close the board before switching on. Never ever disconnect green wires and operate your distribution board like this. The green wire is a wire that ensures the protection and safety to yourself and other users.