2. IMPEDANCE: 2 OHM, 4 OHM, 8 OHM, 16 OHM
All speakers have an impedance measurement just as they have a diameter measurement. Not all speakers are 15 inches in diameter and not all speakers are 8 ohms. The most common 12, 15 and 18 inch woofer used in speaker cabinets by clubs and DJ's is an 8 ohm woofer. If you have two of them in one cabinet you can wire the two woofers in parallel to create a 4 ohm cabinet, or you can wire them in series to create a 16 ohm cabinet. The parallel wired 4 ohm cabinet will produce much more volume, (6dB to be exact), and would be considered much more efficient. The series wired 16 ohm cabinet will need much more power, (4 times as much), to produce the same volume as the 4 ohm cabinet, so it would not be a good choice for economic reasons. Since the 16 ohm cabinet will need more power to produce volume, many lower end companies mass produce these cabinets and heavily advertise the high power that they can handle, as if that makes it a better speaker than the more efficient and lower power speaker. This is the most misleading specification we see in advertising. One club audio company that advertises as much or more than anyone else makes a dual 15 cabinet with two 16 ohm woofers wired parallel to create an 8 ohm cabinet. They advertise it as having a huge power handling capability, but in reality it is much less efficient when used with the same amps that would easily power other cabinets loaded with two 8 ohm woofers wired in parallel. Another mass advertised speaker has two 4 ohm woofers wired in series to create an 8 ohm cabinet. This is still not as efficient as the two 8 ohm woofers wired parallel.
This is where you must make a choice. Do you want a good sounding, efficient speaker cabinet that you can power with a small affordable amp, or do you want a speaker that produces much less volume and you have to buy a larger amp, but you have a little less chance of blowing.

While we are talking about speaker impedance we need to mention something about amplifier impedance. Most amps tell you the power output at 8 ohms, 4 ohms and 2 ohms (if the amp is capable of handling 2 ohms). An amp is most comfortable driving an 8 ohm load. That's why the 8 ohm rated power output is the lowest and the distortion level at 8 ohms is also the lowest. When you connect a 4 ohm speaker to the amp, it's a harder load for the amp to power, just like pulling a 5,000 pound trailer is harder on your car than pulling a 2,000 pound trailer. The power output rating is higher than at 8 ohms, but the distortion level is also greater. Many of the popular low price amps today are not capable of powering a 2 ohm load. It would simply heat up the power transistors too much. For amps that can handle a 2 ohm load, it is the hardest load for the amp to handle and causes the most heat build up which leads to a higher level of distortion.

If you have ever installed your own car audio system, you may have noticed that 4 ohm and 2 ohm loads on the amps are common. That's because there is so much road noise when driving your car that the higher distortion level of the amp usually goes unnoticed.

4. POWER: RMS, CONTINUOUS, PEAK
I have a good friend that has tested speakers in the U.S.A. for over 30 years and he gave me the best definitions for these 3 power ratings.
He sends a sweeping signal of all frequencies to the speaker and slowly increases the power. When he feels like he is getting near the maximum power that the speaker can handle, he extends the time before raising the level again. When he hears the slightest distortion or smells the coil getting hot, he backs it off 5 watts and lets it run for a few hours. If it endures that test, it gets a "Continuous Power Rating" of the power it handled for those hours.

RMS is the product of a math formula and it would rarely ever be the same as the continuous power level measured manually. In our industry the two terms are often mistakenly used to describe the same thing. I would rather have an honest manual measurement than a math formula.

Peak power usually means the highest amount of power that the speaker can handle for less than a second at a time. Although you can test a speaker for this measurement using the same method above, it is more common today to see speaker manufacturers double the continuous power measurement and call that the peak power even though it may not be technically accurate.

Just like the other measurements of speakers we mentioned above, power handling is very misleading. We tested a pair of speakers from one of the biggest advertising club audio companies that were rated at 320 watts. When they arrived, they had a spec sheet that showed the PEAK power handling at 320 watts but the continuous power handling capability was rated at 160 watts. During the test, the horn driver began distorting at around 100 watts when the music had heavy bass playing. You might wonder how a speaker that starts distorting at 100 watts can be advertised as a 320 watt speaker.

There is no standardized method of measuring these speaker specifications. Everybody does it however they please. Some companies choose to rate conservatively which is better for you but doesn't look so good on paper, while other companies grossly exaggerate their specifications to look great in advertising but it's not so good for the customer. The best way to measure the power handling capabilities of a speaker is to play the same music through it that the customer will be playing through it. Since that is not usually possible, most companies send computer generated signals to the speaker and see how many watts it can handle before it distorts. (That would be the point at which the voice coil has been thrown past the edge of the upper magnet plate). If that signal is full range and not just one frequency, and played for 4 hours, you will get a close idea of how much power the speaker can really handle.

Unfortunately, many budget speaker companies will send a signal that is just one frequency and crank up the power for 30 seconds to get a high power handling reading that is not an accurate representation of what the speaker will do with full range music playing through it. This type of reading may be legally "correct", but not what you and I would consider "true". Another method we have seen used by many budget speaker companies is to take the power handling capabilities of each individual component in the cabinet and add them up. If they were each measured conservatively and you made a great crossover to protect each one, this would be a better method than the single frequency signal method, but still not as accurate as a full range signal played for 4 hours.

5. CROSSOVERS: ELECTRONIC AND PASSIVE
A crossover limits the frequency range that reaches a speaker. Almost everyone will say too much power blows speakers. It would be more accurate to say that too much of the wrong frequency blows speakers.
First and most important note; I have never found anyone that disputes the opinion that an electronic crossover is better than a passive crossover. Here's why: an electronic crossover selects the frequencies to be passed to the speakers at a low line level before the amplifier. At this low level, there is no distortion and absolutely no loss of signal. A passive crossover is usually mounted inside the speaker cabinet and selects the frequencies to be passed to the speakers at a very high level after the amplifier. The main problem we have seen with these passive crossovers in budget speakers is the design of the crossover itself. To save money, they usually scrimp on this component a little too much. We have seen many crossovers that were designed to handle only 50 watts installed in a cabinet that was rated at 120 watts. This is the main reason we see so many budget speakers distort earlier than their more expensive counterparts.

There are 4 types of passive crossovers:
LOW PASS, HIGH PASS, BAND PASS, NARROW BAND PASS.

LOW PASS crossovers allow low frequencies to pass through to the speaker while gradually reducing the power of the higher frequencies and cutting them off. Coils are used in low pass crossovers because they cut out the higher frequencies.

HIGH PASS crossovers allow the high frequencies to pass through to the speaker while gradually reducing the power of the lower frequencies and cutting them off. The power reduction increases as the frequencies become lower. Capacitors are used in high pass crossovers because they cut out low frequencies.

BAND PASS crossovers are a combination of the two above. They only allow a certain band, or range of frequencies to pass through to the speaker. This type of crossover is most often used on mid range speakers where the extreme highs and lows are cut off so they do not reach the speaker and cause it to distort.

NARROW BAND PASS crossover must be used when the band of frequencies chosen to reach the speaker is less than a decade apart, that is, if the high crossover frequency is less than 10 times the low crossover frequency. A narrow band pass crossover has the same function as the normal band pass, however, it is wired very differently.

Crossover points or frequencies are achieved by using different values of coils and capacitors. There are different rates for the reduction of power, called SLOPES. The most common are 6dB, 12dB and 18dB slopes. A 6dB slope will reduce the power at a rate of 6dB per octave starting at the chosen crossover frequency, creating a gradual slope of power reduction. The 12dB and 18dB per octave slopes work the same way except that a 12dB slope has twice as much reduction per octave as the 6dB slope and an 18dB slope has 3 times the amount of reduction, thus cutting off those frequencies 3 times as fast.

If you are making your own crossovers, here are a few starting tips:
Air core coils are best for higher frequencies. Iron core coils are best for lower frequencies. Solid iron core coils are best for low frequencies at high power (over 300 watts). Polypropylene are the best capacitors. Mylar capacitors are next best. Non-polarized electrolytic are the most commonly used.

7. HORN DRIVERS: COMPRESSION AND PIEZO
The least expensive of the two is the Piezo driver. Most people think of a small round tweeter when you say Piezo, but there are also larger, threaded horn drivers that have a piezo element instead of a voice coil. Piezo drivers have a vibrating ceramic disk that is usually capable of producing only 2 or 3 frequencies

Because there is no voice coil in a Piezo driver, it is usually harder to blow than a compression driver with a voice coil. Most people don't even use a crossover with a Piezo driver. Right now most people would be wondering why anyone should use a compression driver if they cost more and can be blown easier. The answer is simple, they sound better. The Piezo is most commonly referred to as being harsh sounding because of its 3 frequency limit. Yes, it can produce high's and a lot of volume, but it has never produced high's as smoothly as a good compression driver.

There are two types of compression drivers used in our industry. The less expensive of the two has a Phenolic diaphragm and the more expensive unit has a Titanium diaphragm. The Titanium diaphragm is much better for reproducing the extreme high's heard when a drummer crashes his cymbals. Both units have a heavy magnet and a voice coil with wire windings just like any woofer. Both units need a good crossover to protect them from harmful low frequencies.

9. SUB-WOOFERS: FRONT LOADED, HORN LOADED, BAND PASS
FRONT LOADED sub-woofers are cabinets that have the woofer facing the front just like a common full range DJ speaker cabinet. These units usually produce more bass within the first 30 feet of the cabinet, but lose volume as you step farther away.

HORN LOADED sub-woofers have the woofer mounted inside a large cabinet where you can't see it. The most well known horn loaded sub cabinet is the Cerwin-Vega Earthquake. This cabinet is designed so the sound created by the woofer travels a precise distance through a precise opening before it is released from the cabinet.
The distance and opening size are calculated by math formulas based on the woofers specifications. These units usually produce less bass within the first 30 feet of the cabinet than a front loaded cabinet would, but the sound travels much farther.

BAND PASS sub woofers are designed with the woofer mounted inside and have two chambers of precise size with one chamber having one or more ports to release only a certain band of low frequencies. The size of the two chambers as well as the size and number of ports are the result of math formulas based on the woofers specifications.

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