Buyers Guides > Audio

The Speaker System in More Detail

HISTORY

PA speakers, like mixers, power amps and processors, have gone through various evolutionary stages. Starting with "columns" in the early 1960's they progressed to stand-mounted "cubes" in the late sixties, then to bin/horn "stacks" in the seventies, then to advanced, passively crossed over, "all-in-one" enclosures in the mid-1980's to which were added actively crossed over "subwoofers" in the latter 1980's. Now we face the era of "actively controlled" speaker systems wherein multi-functional electronic processors, designed specifically for given speaker systems, monitor the amplifier outputs and adjust the input signals accordingly.

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.
It all works quite simply; a cylendrical voicececoil is attached to the center of the cone or the perimeter of the diaphragm. It is suspended in a circular slot in the centre of the magnet and the outer edge of the cone or diaphragm is attached to the frame. The amplifier's outputs produce electrical signals which vary in polarity according to the frequency of the source signals. When these reach the voicecoil, its electro-magnetic polarity varies accordingly which causes it to be attracted to and repelled away from the magnet. This causes the voicecoil to move back and forth which moves the cone or diaphragm back and forth and that causes variations in air pressure which make our eardrums move back and forth creating the sensation of sound. These back-and-forth movements occur rapidly and are thus referred to as "vibrations".

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.

TECHNICAL STUFF!!! - IMPEDANCE

Hold a piece of stiff cardboard out at arm's length and wave it slowly back and forth. Now do it faster. Notice how the air mass is impeding its movement? The same thing happens with cones and even diaphragms to a lesser degree. This is physical impedance - mechanical resistance in technical terms. It gets translated into impedance in speakers by a few other factors including capacitance, inductance, reluctance and reactance.
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, WHAT IS IMPEDANCE?

Well, it's partly the electrical resistance of the woofer's voicecoil plus the speaker cable, partly the mechanical resistance of the woofer's cone operating within the enclosure and/or horn, partly counter-EMF generated by the woofer's voicecoil moving in the magnet's field, partly the crossover's resistance, inductance, etc. and partly a few other things. One thing impedance is NOT is "fixed". The only time an "8-ohm" enclosure is likely to register exactly 8 ohms on a meter is either going to be when the meter puts a little DC (battery) current through it and the enclosure turns out to have that exact DC resistance (very uncommon) or in those instants when the audio program (music?) produces the frequencies which cause the speaker to operate at exactly 8 ohms.
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.

PARALLEL LOADS (MORE THAN ONE SPEAKER PER AMPLIFIER CHANNEL)

Calculating parallel loads is an important capablility for two main reasons;
• 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 FORMULA

• 1/R = 1/R1 + 1/R2 + 1/R3 + etc. ("R" = ohms)

EXAMPLES

Say you have two 4-ohm enclosures, an 8-ohm enclosure and a 16-ohm enclosure all in parallel. Of course you would never do such a thing because the lower-impedance speakers will get more power and be louder than the others (you figured that one out yourself, right?), but this is just an example.
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

THE SPEAKER SYSTEM - CABLE ISSUES

Speaker cable, as mentioned earlier, is a real factor in determining a system's overall impedance. Cable presents the overall load with additional resistance. It is even possible for speaker cable to add inductance.
• { 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.}
Resistance, however, is the main additive and it varies in direct porportion to the gauge and length of the wire. The effect that it has on the system's impedance is fairly easy to chart out; what we offer instead is the effect that cable has on the amplifier's delivered power. The reason we do this is because the percentage of power loss varies, not only according to the length and gauge of wire, but also according to the speaker impedance. As you will notice in the below "CABLE LOSS CHART" the lower the speaker load, the higher the loss.
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!!

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