The various features of the speaker system can be broken down into several sections in order to simplify explanations. The sections break down as follows:

Basics & Terminology | Cable Issues | Power, Efficiency & SPL
Phasing, Polarity & Frequency Response | General Cabinet Features

The problem is, at any point in time, users are likely to be employing any one or number of these different speaker systems and they all need to be treated a little differently. You might, for example, encounter no difficulty in boosting the bass EQ for a set of modern, full-range PA cabinets, but try that with an old set of columns or cabinets of lesser quality and the result could be wall-to-wall bits of cone paper (more likely, burned voicecoils). So the bottom line seems to be, "know thy speaker system".

BASICS
Electro-magnetic tranducers - woofers, compression drivers and tweeters - all have four things in common;

• (1) a voicecoil
• (2) a magnet
• (3) a cone or diaphragm
• (4) a frame or some other structure to hold everything together.

TERMINOLOGY

• "Driver" - a word originally representing the compression driver on a midrange or high-frequency horn, but which today often gets used in place of "woofer", "tweeter" and "midrange speaker". This always leads to confusion in conversations; eg. "What kind of driver is in that 3-way enclosure?". Now you have to ascertain if it's the low, mid or high frequency "driver" they're asking about. Therefore, to avoid confusion we will not use the word "driver" very much.
• "Bin" - another word that seems to get used for multiple things - "mid bin", "sub bin", full range bin". We will use the word "enclosure" and describe the type.
• "Woofer" - the low-frequency speaker in a full-range (2-way, 3-way, etc.) enclosure, or the sole speaker in a subwoofer enclosure.
• "Horn" - a veritable field of study in itself. Let's just say for our purposes that this means a complete horn/driver unit, "mid" (midrange) or "HF" (high-frequency) as the case may be.
• "Compression driver" - the sound-producing element in a horn/driver unit.
• "Tweeter" - a small horn and driver or a high-frequency component which is not attached to a horn.
• "Enclosure" - the speaker box - in this case, complete with its components
• "Speaker baffle" - the panel at the front of the enclosure to which the woofer, tweeter, etc. are attached. This is also where you will find the tuning port(s) as a rule.
• "Port" - one or more holes in the speaker baffle which allow air to move in an out of the enclosure and tune it according to the woofer's resonance in order to optimise low-frequency output.
• "Crossover" - a network which separates the low-frequency signals from the high-frequency signals in a 2-way enclosure, or the lows, mids and highs in a 3-way enclosure. Here we are referring to "passive" crossovers. Active crossovers are covered under Signal Processors.
• "Hz" - Hertz or cycles-per-second represents the rate or frequency at which something (a diaphragm or cone in this case) is vibrating to produce sound. Most music represents many sound frequencies from the low (eg. low "E" on a 4-string bass guitar which is 41.2Hz) to the very high (eg. the fine "sizzle" ontop of cymbal sounds which is around 10,000Hz).
• "SPL" - sound pressure level is the industry-standard measure of loudness. It is measured in decibels or "dB".
• "Frequency Response" - Speakers reproduce different sound frequencies with different amounts of loudness. The frequencies are expressed in Hz and the variations in loudness are expressed in dB. Frequency response is plotted on an XY graph with frequency along the horizontal plane and loudness up and down the vertical plane.
• "Impedance" - this is an electrical measurement of the physical characteristics of the speaker. It is used to show the interaction between the speaker and amplifier. In simpler terms, it can be viewed as the degree of "difficulty" that an amplifier is going to encounter when driving a speaker enclosure at various frequencies. Although factors in addition to electrical resistance are involved, impedance is expressed in ohms.

Woofers, for example, tend to increase their impedance at higher frequencies. The higher the frequency goes, the higher the impedance goes and, as a result, the less power the amplifier can deliver. Tweeter and horn diaphragms suffer somewhat less from this process because their surface areas are small and therefore encounter fewer air molecules. Conversely, an 18-inch woofer's impedance may start going up at just a few hundred Hertz. At 1,000 Hz, its impedance could be as high as 12 ohms with the amplifier delivering 60 to 70 per-cent less power to it at that frequency.

If you look at the frequency response graph of a woofer you'll notice considerable "high-frequency rolloff" which reflects the way the line slopes downward thoughout the higher frequencies. That's impedance at work. To verify your findings, look at a woofer's Impedance curve (looks a bit like a frequency response graph in reverse). You will see how the line begins to curve upward as you look from left to right. That is the woofer's impedance climbing as the signal frequency goes higher.

You'll also notice something else on the impedance graph - a tall, narrow "spike" over on the left side, down in the low frequencies. This reflects the woofer's natural "resonance". What happens is, speakers, like all things which work by vibrating, have various physical factors that cause them to favour certain frequencies. When a woofer receives an amplifier signal at its resonant frequency, it wants to move farther in and out than at other frequencies. But in the process of doing so, the woofer's voicecoil cuts added lines of force from the magnet and generates extra "counter-EMF".

As mentioned earlier (see Slew Rate & Damping Factor under the Power Amp) counter-electro-magnetic force is the voltage induced in a voicecoil because it is moving back and forth in the magnet's field. This raises the impedance whenever the speaker tries to reproduce that specific frequency. Additionally, the air load of the enclosure (sealed or ported) is the main force on the speaker cone, hence the size of the enclosure and its ports affect impedance. Even applied power affects impedance. Normally the woofer's magnet acts as a heatsink for the voicecoil, but if high power levels are applied for long enough, the voicecoil warms up the magnet and, as a result, the voicecoil gets even hotter. This increases its resistance then up goes the impedance and down goes the applied power.

• { TIP - If you notice the bass response sounding a little weaker as time passes during a performance, it could be due to this heating effect. Your first reaction might be to boost the low EQ frequencies slightly which is fine, but watch out for signs of amplifier clipping.}

Then, how do manufacturers determine a speaker enclosure's impedance? First it is designed to operate at that impedance ON AVERAGE, then it is measured while operating to verify the load.

• { TIP - When shopping for subwoofers, be sure that you find out the manufacturer's recommended crossover frequency. This is important because the impedance of a subwoofer tends to go up quickly when it receives signals higher than those in its recommended range. As a result the average impedance will be higher than you would expect and the amplifier will put out less power. In other cases, the impedance may actually go down within a range of frequencies above the recommended range which would reduce the average impedance and possibly endanger the amplifier. Needless to say, it's wise to make sure that your electronic crossover is working at the right frequency (most of them have Frequency controls which are not very accurate). Have a technician check it out or you can do it yourself if you have a pink noise generator and a good quality realtime frequency analyzer.}

• first, because dual speaker connections whether on an amplifier, a mixer/amplifier or a speaker enclosure are all wired in parallel. Some people think that if you run separate speaker cables from each speaker output on the amp or mixer/amp to the enclosures you somehow "avoid" putting the speakers in a parallel circuit. Others think that if you run a speaker cable from one cabinet to another you put the cabinets in "series" and that just adds the two loads together (eg., two 4-ohm speakers in series = 8 ohms). But the truth is that everything gets put in parallel. In fact it's quite difficult to put speaker enclosures in series - you need a special wiring harness.
• The other reason for needing to know how to calculate parallel loads is because amplifiers don't like running into loads which are too low. As mentioned in the Amplifier section, they will usually shut down if the load is too low and some of them may actually sustain damage.

The solution goes as follows:
1/R = 1/4 + 1/4 + 1/8 + 1/16 = 4/16 + 4/16 + 2/16 + 1/16 = 11/16.
Therefore R/(1) = 16/11 = 1.4545 ohms.

If you're not into finding "lowest common denominators", just get out a calculator and turn everything into decimal equivalents as follows:

1/R = .25 + .25 + .125 + .0625 = .6875
Therefore R/(1) = 1/.6875 = 1.4545.

To keep life as simple as possible, most people put enclosures of the same impedance in a parallel circuit. If you do this it's all just a matter of dividing that impedance by the number of speakers. Example; four 16-ohm loads in parallel = 16/4 = 4 ohms. Similarly, two 8-ohm loads in parallel = 8/2 = 4 ohms. The following is a quick reference listing of some commonly used parallel loads: ("R" = ohms)

• 2 x 16R loads = 8R
• 2 x 8R loads = 4R
• 2 x 4R loads = 2R
• 3 x 16R loads = 5.33R
• 3 x 8R loads = 2.67 R
• 3 x 4R loads = 1.3R
• 4 x 16R loads = 4R
• 4 x 8R loads = 2R
• 4 x 4R loads = 1R

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• { TIP - In ceratin rare cases you might find that coiling a speaker cable the right way can get rid of radio frequency interference. Once in a while, powerful radio signals effectively get picked up by the speaker lines. This interference then goes back into the amplifier's grounding, is amplified and comes out the speaker system. In a few instances it has been reported that winding the cables into coils and taping them in place actually helped to reduce the problem.}

For obvious reasons, it makes sense to pay attention to what speaker cable you are using. In general, the less wire, the better. The heavier the gauge, the better. We suggest that you print out the "CABLE LOSS CHART" and spend some time studying it. This could dramatically improve your system performance!!

WIRE GAUGE (AWG)	LENGTH (FT.)	SPEAKER LOAD (OHMS)	POWER LOSS (% OF WATTS)
8 OHM LOADS
18	25	8	6.82
16	25	8	4.35
14	25	8	2.75
12	25	8	1.67
18	50	8	12.94
16	50	8	8.41
14	50	8	5.39
12	50	8	3.29
18	100	8	23.44
16	100	8	15.76
14	100	8	10.34
12	100	8	6.41
WIRE GAUGE (AWG)	LENGTH (FT.)	SPEAKER LOAD (OHMS)	POWER LOSS (% OF WATTS)
4 OHM LOADS
18	25	4	12.94
16	25	4	8.41
14	25	4	5.39
12	25	4	3.9
18	50	4	23.44
16	50	4	15.76
14	50	4	10.34
12	50	4	6.41
18	100	4	39.33
16	100	4	27.98
14	100	4	19.10
12	100	4	12.21
WIRE GAUGE (AWG)	LENGTH (FT.)	SPEAKER LOAD (OHMS)	POWER LOSS (% OF WATTS)
2 OHM LOADS
18	25	2	23.44
16	25	2	15.76
14	25	2	10.34
12	25	2	6.41
18	50	2	39.33
16	50	2	27.98
14	50	2	19.10
12	50	2	12.21
18	100	2	58.95
16	100	2	45.43
14	100	2	33.08
12	100	2	22.24

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There is some feeling in the consumer market that a power rating means you "must" use that many watts. This is not true in the case of PA speakers. Additionally it is sometimes assumed that a speaker's power rating directly reflects its sound output. This is also not true. If you apply the maximum power to a speaker it will (should) provide the maximum sound-pressure level specified by the manufacturer. If you apply less than that much power you will get less than the maximum SPL, but how much less?

Consider the following; if you reduce applied power by 50%,the SPL will be reduced by -3dB reflecting a decrease in perceived loudness of roughly 30%, possibly less depending on who you ask. If the system was producing 110dB, the SPL would still be around 107dB with 50% of the applied power gone (believe it or not!). And if you think that's unusual, consider the reverse situation - doubling applied power only nets +3dB or roughly a 30 to 40% increase in perceived loudness - more about this later.

THE DISTORTION ISSUE
The main reason for PA users thinking that you "must" apply 100 watts to a 100-watt speaker or 500 watts to a 500-watt speaker, etc., is because of distortion. The problem is that a mere 50 watts can blow up a 100-watt speaker if the ampifier is sufficiently distorted (again, believe it or not!). Therefore the reasoning seems to go that if you have the maximum power available to the speaker, you will have plenty of power "headroom" and you will never turn up the volume far enough to distort the amp and blow the speaker because by then the system will be too loud.

The problem wirth this reasoning is that "too loud" is just too vague and most people like things extra loud anyway, especially the bass. What often happens is somebody increasing the low-frequency EQ below the enclosure's low-frequency limit (if the the frequency response specifications say "50Hz - 20kHz", usually 50Hz will be the low-frequency limit). Now, the amplifier's remaining power headroom gets gobbled up in an effort to make the woofers do the impossible and distortion is just around the corner. But that's not all; the woofers are actually trying to do the impossible and reproduce frequencies that are too low for them. This causes a rapid drop in their power-handling capacity and they blow, even if the power is not distorted and the amp is putting out substantially less than maximum power.

• { TIP - If your power amps have "High-Pass Filters" or "Low Frequency Cut" buttons or "Subsonic Filters", use them. If your amplifiers don't have such a feature but you do have a graphic EQ, pull down the EQ faders which are of lower frequencies than the speaker's low-frequency limit. Do these things and your system will sound cleaner, louder and better.}

POWER RATINGS
Finally we have speaker power ratings. They are expressed in watts, everyone knows that, the question is, what kind of watts. Once upon a time it was "RMS" and now it's "PGM" (program) power. One of the reasons for the change away from RMS (aside from the fact that it was a technical misnomer) was all the marketplace misunderstandings about translating RMS ratings into applied power.

Back in the 1970's when everybody was learning about sound systems, you might hear someone say, "Oh I know that speaker. The rating is 100 watts RMS but you can hit it with two times that much power." Go around the corner and you might hear someone else say, "RMS times three, that's how much power you sock into that speaker - in fact any speaker." (argh!). Meanwhile speakers, horns and tweeters were blowing up like popcorn and repairmen were the only ones making a profit - at least that's how it seemed. Thankfully, "program" power ratings are more reliable when used the right way. Now, when you see "pgm" you know it means "APPLY NO MORE THAN THIS MUCH POWER". Life is simpler and safer.

Oh yes, it's worth noting that applied power is shared by speakers. Two 100-watt seakers powered by the same mono amplifier or one channel of a stereo amp can handle a total of 200 watts (you already knew that, right?).

EFFICIENCY
Some woofers, horns and tweeters are capable of producing more sound pressure for the amount of applied power than others. These are said to more "efficient". Efficiency is expressed as the number of decibels measured at a distance of one meter (3.281ft.) from the enclosure or component, with one watt of power applied. A slight increase in efficiency can mean a great deal to the performance of the system.

• For example, if an enclosure is just 3dB more efficient than another one, you can get the same amount of sound pressure from it with only half as much applied power!

SOUND PRESSURE (spl)
Sound pressure is measured in decibels (dB) and represents the real end product of everything covered thus far. In basic terms, sound-pressure level or SPL is loudness. Here are some SPL facts:

• SPL dissipates -6dB each time you double your distance from the source.
• SPL varies by +3dB if you double the applied power to a given enclosure.
• SPL varies by +10dB if you increase applied power by a factor of ten to a given enclosure.
• SPL varies by +3dB if you double the number of similar, close-coupled, in-phase, equally-powered enclosures. ("Close-coupled" means close together, either side-by-side or stacked vertically). The following is a chart of sound pressure levels as you double the number of similar, close-coupled, in-phase enclosures and the total applied power. Note; efficiency is assumed to be 100dB at 1 watt at 1 meter.
• The average ear interprets a 3dB SPL variation as roughly a +30% or - 20% variation in perceived loudness.
• The average ear interprets a 10dB SPL variation as roughly a +100% or - 50% variation in perceived loudness.
• The quietest sound we can hear is around 12dB, but that would have to be in a very silent place. External sounds occurring at a level below 12dB, get drowned out by the sound of blood rushing in the vessels in our ears. A more normal quiet place, the public library, tends to have a 40dB noise "floor" (constant average noise level) caused by people rustling pages, moving about or whispering, traffic outside, air conditioning, etc. Here again, some sounds which are quieter than the noise floor are likely to be at least partially drowned out. And if you wonder why it is necessary to shout at a rock concert it's because normal human speech averages roughly 70dB while the PA averages 90 to 120dB in various audience areas.
• On the opposite end of the audible sound range, the maximum SPL we can handle before it becomes painful is around 130dB, however this can vary between individuals. The hearing loss resulting from such encounters is usually temporary unless the experience is repeated too frequently (more about hearing loss under RUNNING THE SYSTEM).
• In the workplace, the Dept. of Labor considers a "safe" SPL to be 90dB for 8 hours a day.
• Rock concert PA enclosures can produce better than 130dB SPL's at one metre. If the front row is 4 metres or 13.12 ft. away, the SPL at that distance could be over 118dB, that is if only one enclosure is working.

Looking at it, certain things become apparent to you right away. First, it's clear why humans enjoy lows and highs on home stereo so much - at low sound-pressure levels we have difficulty hearing them compared to all the midrange frequencies, especially around 3kHz where our hearing is most efficient. But it is also clear that, with increases in the SPL, the need for a "smiling" EQ setting to provide those extra highs and lows is decreased as our hearing response smoothes out.

This is good news because "smiling" EQ settings gobble up power headroom in both the amplifiers and the speaker system. That situation may be alright at low levels when there is ample headroom but, as the SPL goes up, the EQ should be flattened out to prevent distortion (and remember to cut the EQ below the enclosure's low frequency limit if the system levels are being maxed-out).

SPL DISSIPATION
In the open air with no wind, or in a large anechoic chamber, SPL dissipates -6dB each time you double your distance from the source. Surprisingly, this rule does not change with horn loading. Even the mythical "long throw" horns obey this law. If their sound travels farther it's because their output or "source" SPL is higher.

Although the -6dB rule varies indoors with the size and physical makeup of the venue, and outdoors with wind velocity and direction, the fact remains that sound pressure does diminish over distance at a rate which is seldom less than -6dB as you double your distance from the speaker system, and is usually more - especially in clubs with sound-deadening architecture, furnishings, etc.

• { TIP - In large venues or outdoors, you may need to optimize the SPL of you speaker system so that audience members at the back can hear properly. Under these circumstances it is the mids and highs which need to be optmized most because they contain the most musical information including the vocals. To accomplish this, stack the horns vertically. If you have full-range enclosures with the horns at the top, stack them "head-to-head" so the horns will be as close together as possible. Be careful that they are aligned properly. The idea with "close coupling" (what we're doing here) is to have the sound waves from the close-coupled sources radiating in unison so that they reinforce each other. If the sources aren't aligned, the waves will be out of phase with each other to some degree and the coupling effect wil be lessened. If the alignment is bad enough, sound cancellation can take place.}

SOURCE SPL (dB at 1 meter) @ DISTANCE(meters) = RESULTING SPL(dB)
[Note: 64 meters = apx. 210 feet]

130dB @ 2m = 124dB, @ 4m = 118dB, @ 8m = 112dB, @ 16m = 106dB, @ 32m = 100dB, @ 64m = 94dB
125dB @ 2m = 119dB, @ 4m = 113dB, @ 8m = 107dB, @ 16m = 101dB, @ 32m = 95dB, @ 64m = 89dB
120dB @ 2m = 114dB, @ 4m = 108dB, @ 8m = 102dB, @ 16m = 96dB, @ 32m = 90dB, @ 64m = 84dB

MULTIPLE ENCLOSURES
SPL varies by +3dB if you double the number of similar, close-coupled, in-phase, equally-powered enclosures. ("Close-coupled" means close together, either side-by-side or stacked vertically). The following is a chart of sound pressure levels as you double the number of similar, close-coupled, in-phase enclosures and the total applied power. Note; efficiency is assumed to be 100dB at 1 watt at 1 meter.

NO. OF ENCLOSURES @ TOTAL APPLIED POWER (WATTS) = RESULTING SPL @ 1METER (theoretically)

1 @ 1w = 100dB, 2 @ 2w = 103dB, 4 @ 4w = 106dB, 8 @ 8w = 109dB, 16 @ 16w = 112dB
1 @ 10w = 110dB, 2 @ 20w = 113dB, 4 @ 40w = 116dB, 8 @ 80w = 119dB, 16 @ 160w = 122dB
1 @ 100w = 120dB, 2 @ 200w = 123dB, 4 @ 400w = 126dB, 8 @ 800w = 129B, 16 @ 1600w = 132dB

• { TIP - It is interesting to note that the difference in sound pressure between one enclosure with 100 watts applied and sixteen enclosures with a total of 1600 watts applied is only 12db! In other words, the average ear would hear slightlymore than a 100% increase in percieved loudness if the two systems were compared in a blindfold listening test at a distance of one meter.

  However, the difference in SPL at a greater distance would be more noticeable. This is because the acoustic output of the sixteen, close-coupled enclosures would combine over distance to reinforce the SPL so that the difference between one enclosure and sixteen would be greater than 12dB at a distance from the source. This reinforcement effect helps to keep the PA audible.

  For example audience noise level at a concert can be so high that the PA may be drowned out if its SPL is only a few dB quieter. If, for example, you were in the audience, 64 meters (210 feet) from the system and the audience noise level around you was 90dB, you would probably still be able to hear the PA which puts out 130dB at one meter (see "Source SPL @ Distance = Resulting SPL" above). But the 120dB source would result in only 84 dB at 64 meters and would be drowned out.}

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Sound wave cancellation can mean no sound or at least a serious decrease in sound-pressure level and usually a change in sound quality. Consider that if one of the two woofers in an enclosure, let's say a box with two fifteen-inch woofers, was accidentally wired with reverse polarity to the other one (manufacturers take pains to check for this) the SPL from those woofers could be attenuated by minus 10dB or worse. To put this in some perspective, it would be the equivalent of losing 90% of the power applied to them.

Things are not quite so bad when you have two similar enclosures side-by-side and one of them has reverse polarity (speaker cables soldered-up dissimilarly are usually the culprit here) but the SPL can still be attenuated by around minus 6dB which is roughly comparable to a 75% applied power loss. Similar speakers with reverse polarity to each other are said to be "180 degrees" out of phase - as far out of phase as you can get.

PHASE ERRORS
Acoustic phase errors can range from a few degrees to over 100. The culprits here are matching and positioning. If, for example, you were to place two different types of subwoofer side by side, they would tend to encounter cancellations at certain frequencies, but reinforcement at others due to their differing phase characteristics. The net result would be uneven frequency response and probably less overall SPL than if the enclosures were identical.

The rule here is simply to make sure that speakers doing similar jobs are similar. Well, that is if they are side-by-side. If they are not positioned immediately side-by-side, there will be much less phase cancellation which leads us to positioning.

POSITIONING
Positioning is important when you need to have similar speakers reinforce each other's outputs. As a result it is possibly a more common cause of phase-related sound problems than reversed polarity. However, the SPL losses are often lessened by sound bouncing off walls, floors and ceilings thus softening the effects of cancellations. Still, subwoofers positioned in some manner other than close together will suffer losses as a rule. The effect is less low-frequency sound pressure than you would expect.

PHASE TIPS

• To test for electrical phasing, touch the positive and negative terminals of a 9-volt battery to the + and - leads of a speaker cable plugged into the enclosure. This will cause the woofer cones to move slightly. If the cones move out, the cable and enclosure are in phase. If they move in, the cable or enclosure wiring is out of phase.
• Another test method requires a microphone and a VU meter (the level meter on your mixer may work if it is fairly precise). Face your enclosures towards each other, around six feet apart. While someone plays a single, low note through the PA, perhaps on synthesizer, hold the mic between the woofers of the two cabinets and slowly move it back and forth while watching the meter. If the reading gets higher when the mic approaches the half-way point between cabinets, they are in phase. If it decreases, they're out of phase.
• If it appears that something is out of phase, check the cables first; cable manufacturers are somewhat less careful about phasing than speaker manufacturers. The clearest indication that a cable is out of phase is when the wire leads are not soldered to exactly the same lugs on both connectors. Take the cable connectors apart. Most flat speaker wire has one of the leads marked with printing (the industry standard is "printing to positive" - centre tab if it's a 1/4" plug). In any case, the + & - connector lugs should be soldered to the same cable leads on both ends. If not, unsolder, reverse and re-solder the leads on one of the connectors (only). If the cable is not wired out of phase, the cabinet's input wiring must be reversed. Remove the connector panel and reverse the wires on the input connector. You may want to get some help from your local service technician for this one!!
• If you are having a problem with what seems to be low SPL and all the electrical phasing checks out OK, the problem could be acoustic phase cancellation where only sections of the sound range are out of phase - eg. the lows or mids. Cabinet alignment is critical for proper acoustic phasing. For example, two subwoofers side by side but facing in opposite directions would be 180 degrees out of phase resulting in at least a -3dB effect on SPL, the equivalent of losing 50% of your power.Two boxes with one facing off to the side would have a 90 degree phase error.
• As a matter of good general practise, close-couple all the enclosures side-by-side or stacked vertically and face them in the same direction. This maximizes SPL by minimizing phase cancellations.
• Some places can be veritable "SPL sponges", a phenomenon related to the architecture, building materials and/or furnishings. If this turns out to be a problem, try the following: (1) Position and aim the mid & high frequency horns, so that there is line-of-sight with the audience (if you can see a speaker clearly, you 'should' be able to hear it), This will probably require stacking pairs of full-range enclosures head-to-head so that the horns are close together. (2) Flatten the main EQ (set all faders at centre ) to give the system as much power headroom as possible (3) Make sure that all power amplifier level controls are at maximum (4) Make sure the Input Gain controls on the mixer channels are all set sufficiently high to allow a small amount of input clip indication, then increase the main mixer levels as far as you can without either offending the audience or causing feedback.}

Fixed gain means that the intensity of the test signals is the same from one frequency to the next (at least that much can be regulated). The results are detected by a microphone with very flat frequency response connected to a computer which plots them on an X-Y graph with frequency along the horizontal axis and SPL up and down the vertical axis.

Ideally the result would be a perfectly straight and horizontal line from left to right meaning that the speaker reproduces all frequencies at the same SPL. In real life, response graphs indicate all kinds of peaks and dips with the far right and far left ends bending down where the lows and highs roll off. The degree of accuracy of a frequency response graph can be ascertained by noting the "dB" markings down the left side. Response graphs can be calibrated so that every vertical marking is 5 or 10dB louder or quieter than the one above or below it. It is important for you to ascertain this dB scale when evaluating a response graph because it indicates the accuracy of the results.

Remember, a difference of 10dB means either double or half the audible loudness - a HUGE difference when applied to the sound of something (see GRAPHIC EQ under SIGNAL PROCESSORS). A response graph which is calibrated in 10dB increments may look deceptively smooth compared to some other graph calibrated in 5dB increments. Don't be fooled.

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HORN-LOADED vs. BASS REFLEX ENCLOSURES
The era of horn-loaded "bass bins" has all but passed in general application P/A systems. The added efficiency which bass horns offer comes at the cost of smooth, extended bass response and/or compactness. Efforts to produce more compact bass horns in the latter 1970's netted low end response which rolled off abruptly below the bass horn's cutoff frequency (all horns have one) giving them a comparitively "hard" sound.

Modern uses of low-frequency horn loading in subwoofers have brought some improvements in performance, but horns still must obey nature's laws and roll off quickly below cutoff. In fact, some of the very large bass horns developed during the 1930's and '40's remain among the best in terms of low-frequency performance which possibly indicates that there aren't many secrets left to uncover in this technology.

Reflex technology is similarly well known. Its potential advantages are compactness and smooth, extended bass response. Higher-powered woofers and amplifiers have now made it possible to obtain higher sound pressure levels from reflex-type enclosures and this, combined with their superior low-bass response, has swept them to prominence in both concert and club applications.

BOX DESIGN SHAPE - Trapezoidal vs. Square
The reason for that "wedge-back" trapezoidal enclosure geometry is to facilitate the creation of semicircular arrays where multiple enclosures are arranged closley side-by-side. Created initially for "flying" in elevated clusters, trapezoidal enclosures have also found their way onto stages all over and are used in small numbers with great success.

Their shape looks a little horn-like leading sometimes to speculation about them having "longer throw" than similar, square enclosures - of course we know better now. However there is a potential benefit in this shape - when properly configured, it reduces internal standing waves which means smoother frequency response and possibly improved phase coherence, a factor which gives the enclosure a "tight","focussed" sound. On the other hand, not all trapezoid boxes automatically benefit from this shape, nor do all square boxes fare less than favorably in comparison to them.

ENCLOSURE SIZE - Is Bigger Better?
The size of an enclosure is often assumed to reflect its bass response and acoustic output potential. This is not automatically true. There is an optimum size for an enclosure which is dictated by the various design parameters of the woofer. If it is too big, the frequency response will be uneven with a "loud spot" over a narrow range of frequencies causing one or two notes to jump out every time they're played. On the other hand, if an enclosure is large in order to properly enclose the woofer or woofers, it could mean more and/or deeper bass response, but that depends entirely on the woofers.

Also, keep in mind that it is now possible to obtain surprisingly large amounts of full-range SPL from smaller enclosures. In fact a well-designed, high-performance compact enclosure can conceivably blow a much larger but inferior box away. Check the manufacturer's specifications. Look for the "maximum SPL" and frequency response figures. If the response numbers are good, the max. SPL may be all you need to consider - regardless of the enclosure's size. Once again, rent the speakers and try them out on a job. Specifications can be good guidelines, but reality is the final word.

PASSIVE CROSSOVERS
Modern passive crossover technology is miles ahead of where it was twenty years ago. Aside from the obvious features such as a variety of input connectors, you may find circuit breakers, fuses, lightbulbs and even transistors in addition to the usual capacitors, resistors and inductor coils.

All of these are desirable and for different reasons:

• On enclosures rated at 400 watts or more, input connectors should include Speakons or XLR's (see Outputs under Power Amp Features). 1/4-inch jack sockets are alright for lower-powered applications, but they only have around one square millimeter of contact area at the tip which means that resistance and heat will build up there when the number of electrons passing through exceeds the number posessed by the atoms in the contact area. Speakon connectors have many times more contact area having been designed especially for speaker usage complete with extra pins to accomodate a bi-amp or tri-amp "snake" (multi-element cable) and a twist-type locking system.
• A circuit breaker or fuse means an added margin of protection for the components. Although such measures are not foolproof, they do at least offer some protection from overpowering and possibly even distorted amp signals. You may find it inconvenient changing bulbs or fuses or resetting breakers, but resist the temptation to circumvent these safety devices with aluminum foil, tape, etc. As a rule they blow for good reasons and if you defeat them, the next thing to blow is likely to be voicecoils and that gets expensive.
• It seems odd to find a lightbulb inside your PA enclosure, but this feature can perform two functions. First, it acts as a time-delay fuse, usually for the mid and high-frequency components, so that large, fast power transients (peaks) will not instantly interrupt mid/high output. The lightbulb may also help to dissipate excess power headed for the mid/highs and should not cause concern if "sighted lighted" - it's just doing its job.
• The presence of transistors, resistors and capacitors may not make an impression on you, but inductor coils - coils of copper wire - are more obvious and are worth looking for. The presence of inductors in a crossover circuit means that the rolloff slope is at least 12dB per octave. Without them the slope will only be 6dB/octave with no rolloff on the woofer which is acceptable for some stereo speakers and studio monitors, but lacks the safety factor that *fast, 2-way and 3-way crossovers afford the components.*["Fast" in crossover parlance usually means a rate of 18 or even 24db per octave] On the other hand, some high-performance "touring" enclosures may only have DC blocking capacitors on the horns and/or tweeters. This is because they are designed to be bi-amped or tri-amped using electronic crossovers and separate low (mid) and high-freqiency power amps. Full passive crossovers ontop of this would defeat one of the benefits of bi- or tri-amping - i.e. no passive crossovers (even the very best ones create small amounts of phase distortion).

Times are changing, for example it was once thought that good mid/high horns had to be made of cast metal. Since then it has been dicovered that almost any ridgid, non-resonant material works well for midrange and high frequency horns as long as the driver is adequately supported. In fact it turns out that some cast metal horns have nasty resonances and are actually less desirable than plastic, fibreglass or wood horns. Consider as well that not all cast metal components are great performers; high-tech-looking cast cosmetics are surprisingly easy to manufacture.

So, in the final analysis, the only generalization which is likely to be fairly accurate is that woofers rated at 400 watts or more should have cast frames. This is necesssary to support a big magnet's weight, and a high-performance PA component should have a big magnet. The larger the magnet is, the more magnetic "flux" (power) it can hold. If the magnet isn't big enough, there won't be enough flux to overcome the mass of the high-powered voicecoil (you increase voicecoil power capacity by using heavier gauge wire which weighs more) and the woofer's efficiency will be compromised. Massive magnets need the support of cast frames because cast frames are more rigid than stamped frames which may deform in time due to the magnet's weight and the constant vibrations.

ENCLOSURE MATERIALS
Chipboard is generally used in the production of home stereo speakers and studio monitors. It is comparitively massive and rigid and resonates less as a result, hence less woofer energy is wasted by being converted to cabinet vibrations. Chipboard is cheaper than most grades of plywood (potentially good news price-wise), but it does not hold up on the road, eventually developing the "crumblies" in those areas where it has been dropped or hit with something. It also does not like getting wet.

Three-quarter-inch plywood - mostly 7-ply - is the material of choice for PA enclosures. It is durable and internal bracing in key areas can overcome its tendancy to vibrate slightly in large enclosures. Thirteen to eighteen-ply, 3/4-inch wood combines chipboard's mass and rigidness with plywood's durability and can make a fairly noticable improvement in almost any large enclosure's performance compared to one made of 7-ply. Unfortunatley, such multi-ply wood costs considerably more as do the enclosures in which it is used. It is also comparitively heavy and, considering that it pricipally benefits large enclosures (smaller ones have less wood to resonate and work almost as well in 7-ply) multi-ply wood can be something of a back-breaker.

Plastic has been used for certain small enclosures for many years. Its prime advantages over wood are lighter weight and the ability to be made into interesting shapes. Foam insulation or some other internal muting and stiffening material is usually found in plastic enclosures, partly for acoustic damping and partly because they tend resonate freely and may buzz without it. Large enclosures, especially high-powered subwoofers, are seldom made from this material as it needs to be reinforced with wood or thick fibreglass to prevent vibration. This negates any weight savings and makes the product more expensive. Plastic enclosures have the appearance of being weatherproof and some of them are. Check the manufacturer's specs to be sure. Some plywood enclosures are also weatherproof - again, check with the manufacturer.

• { TIP - You can weatherproof paper speaker cones to a fair degree by spraying them with Scotchguard front and back. You will lose a little efficiency and the sound may be slightly different, but the cones will survive dampness, although not a complete soaking.}

HARDWARE & FLYING HARDWARE
Once upon a time, hardware meant handles and corner pieces. It still does in the case of most PA enclosures, the exception being "flown" systems. Someone discovered many years ago that you need more than a chain through the handles to safely hang a speaker over people's heads. Even the beefiest-looking bar handle can work loose with vibrations over time. This proved to be a headache for enough people - both literally and in terms of legal suits - that someone began designing metal fasteners and plates, hardened steel eyebolts and other things to aid in the quest for safe "flight".

There are many ways to fly a speaker, but above all, you are putting people lives at risk when you fly them. Seek the advice of a rigging professional before flying speaker cabinets.

Further information may be obtained from ATM Fly-Ware 20960 Brant Ave., Carson, California, 98010-1040, U.S.A.