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			Filters and Crossovers

Most speaker systems (or systems of speakers, such as in a car audio installation) have several speakers, or “drivers”, of different sizes, power ratings, etc.  This is due to specialization:  It is very difficult to make a driver that handles all frequencies well. So, you end up with large heavy drivers, or “woofers” to reproduce bass frequencies, medium size drivers, or “midranges” (Sometimes also known as “squawkers”.  Horrible name – it must have come about back when even midranges usually sounded lousy…) for midrange, and small drivers, or “tweeters”, for high frequencies.  

There is a problem with this arrangement: You need some way to direct the signals on which a particular speaker can do a good job to that speaker, and keep out those signals on which it cannot do a good job.  Luckily, this is not too terribly hard or expensive to do, and a type of electrical or electronic circuit known as a “filter” can be called upon.  A filter, quite simply, lets signals in a desired frequency range pass through, and blocks, or “filters out” signals not in that range.  For example, you may want your subwoofer to only work from 80 Hz on down.  So, you need to “pass” low frequencies below 80 Hz, and block frequencies above that.  The circuit that will do just that trick is called (no surprise) a “low pass filter.  Same thing for a tweeter.  You need to let the high pass through, but keep out the mids and lows.  So, you use a “high pass filter. If you want to direct a range or “band” of frequencies to, say, a midrange, then you wants to block the lows and highs and let a band of midrange frequencies between them through.  Voila!  This takes a “bandpass” filter.

Subsonic filters

In some home and many pro-sound applications, a so-called “subsonic filter” is used to filter out, or block, frequencies below the area the speaker can be effective in.  A subsonic filter is really just a specialized high pass filter.  In this case, it is set to “pass” any frequency  “higher” than the very low stuff that the woofer can’t handle in this box. This works for car audio too, and a few amps and crossovers have subsonic filters built in.  However, a simple crossover that can be set low enough and has a steep cutoff will work fine.

			Excursion

Excursion is simply how far the woofer’s moving assembly travels at a particular power level and frequency.   This is often simply referred to as “travel” or occasionally as “throw”, as in “long throw woofer”.   It is affected by both the woofer’s design and the box the woofer is in.  The more excursion, the more air the woofer is moving to create sound.  Excursion may be given as “one way” travel:  Positive travel is from rest position to a forward (outward) position.  Negative travel is from rest position to a rear (inward) position.  Excursion can also be rated as  “total travel”:   The distance from a forward position to a rear position.  In other words, add the negative and positive travel (disregarding the “-“ sign in front of the negative number.)  In music, the negative and positive may or may not be equal magnitude.  For testing and computer analysis, the negative and positive excursion are usually assumed to be equal magnitude, even if they are not actually equal.

			Impedance Explained

Impedance is best explained by imagining the wire in the voice coil as a coiled up hose with water running through it.  The water is the electric current, if you will, and as it runs through the coiled up hose it does the work of creating sound.  That work is the amount of watts delivered to the speaker’s “motor”.  More on that later.  Now imagine that your amplifier is the pump pushing water through the hose.  Think of it as a sort of engine, pushing the water.  It is easy to see that if the “hose” is long and thin, there will be a lot of resistance to the flow of water.  In other words, the hose tends to “impede” the flow of water.  You have just discovered “impedance”.  Now, more loops of hose coiled up can do more work, but longer the hose, the less water flowing, so you may lose more than you gained.  So, how do you increase the water flow?

Well, one way is to use a higher pressure pump.  In our analogy, the water pressure is the voltage the amplifier can put out. So, more pressure (voltage) delivers more water (electric current), and more work  (watts) is done at the speaker.  There is just one problem:  The higher pressure pump tends to cost a lot more money.  The same is especially true in car audio with amplifiers:  Getting enough voltage, when the original source, the car battery, is only supplying 12 – 14 volts DC, is no small task. 

Well, you say, why not use a bigger diameter hose?  Aha!  This is just what is done in car audio, in most cases, compared to home audio. And it is why most car speakers are “ rated at 4 ohm impedance”, while most home audio speakers are “8 ohm.”   (Home amps have a 110-120 volt ac source – higher yet if turned directly into DC, so it usually has to be backed down!) 

So…  you say now…  If this is such a great idea, why not just use a really big hose coil?  Or multiple hose coils?  (Dual coils, or multiple speakers, side by side in parallel.)  It turns out there is a problem.  Any engine needs a certain optimum amount of load, or impedance, to work against.  Too much load is like trying to take off from a stop in high gear.  It may not hurt the engine, but it is not very effective.  Too little load is like punching it in low gear when you are already doing 70 mph.  The engine races and may be damaged.  So, if the pump (amplifier) tries to pump too much water (current), into a low impedance (the current flows too easily), it may fail.  Or, the protective circuit may sense danger and shut it down. Of course, the final solution if you really want to pump a lot of water is to use more pumps or a higher capacity pump.  You can go with more pressure, more capacity to pump water and hoses designed or set up to handle that, or a combination of the two.  This is what matching your speaker impedance to the amp is all about.  If your speakers present a very low, such as 1 ohm, impedance to the amp, then you need  a “pump” (amp) that can put out a lot of current without overheating itself. 

			Resistance/Impedance?

Resistance is the simplest form of impedance.  Go back to our water hose, above, and consider that music does not create a constant, steady flow from the output of the amplifier.  (That is known as “DC”, or direct current, and is very boring to listen to, indeed!)  In the case of DC, for practical purposes, water going through the hose (current through the wire) sees only resistance (sometimes known as “resistive impedance”).  If the pressure is constant, the flow is constant because the resistance to the flow is constant.  Not too exciting, unless it is from a failed amplifier and is enough to smoke your speaker’s voice coil! Music, however, is sound waves.  But while it is in the wires, it is in the form of waves of electricity.  Waves of electricity are sometimes known as “AC”, or alternating current.  Waves, in any substance, have the interesting property that sometimes they bounce or reflect back at you, to some degree.  If some of the current (water) did not get to it’s destination, it must have been “impeded”, right?  (For another example, if you are going into a club, and the bouncer pushes you right back out, you got pretty well “impeded”, right?)    It turns out that in the operation of the speaker, sometimes there is a tendency for some of the signal to get bounced back to the amplifier.  This adds to the steady resistance already present.  (The steady resistance affects DC and AC signals, alike – Your friend was also trying to pull you back away from the club entrance!)  So the impedance of the speaker is the resistance plus the “bounce back” factor.  Plus… There is another effect.  Higher frequency waves tend to get blocked more and more as the frequency goes higher.  Think of it as the hose getting smaller for higher frequency waves.  The flow of current literally gets “choked off”.  This effect is called inductance.  In fact special coils of wire specially made to create this effect, such as to purposely block high frequencies from a speaker, are sometimes known as “chokes”. All wires have at least a small amount of inductance.  Wrap them around in circles, such as in a speaker voice coil, and they have more.  (Electricity doesn’t like to turn corners.)  Wrap them around a cylinder of iron or steel, and there is a lot more.        Add all these up:  The steady resistance, the “bounce back” factor, and any inductance.  This is the speaker’s impedance, and it is different at different frequencies. The “working average” is often called “nominal impedance”.