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Hertz So Good

“Write a feature on loudspeaker comparisons.” That seemed harmless enough. After all, I once worked for a loudspeaker manufacturer that specializes in loudspeakers for churches. I also interned under Bill Thrasher, so one would think that I should know a little something about loudspeakers…

The reality is, loudspeakers come in more sizes, shapes, features and configurations than I could ever hope to cover here. I found nearly forty major manufacturers, regional companies and countless guys with table saws and nail guns assembling loudspeaker enclosures in their garages, some of which sound really good, I’m sure.

For these reasons, our loudspeaker comparison chart covers only some of the major manufacturers. We also decided to stick with the workhorse, the tried and true two way front-loaded box. While I do not have actual stats (and the editor does not think making up stats is appropriate), I’m sure the two-way front loaded enclosure is the most common type of loudspeaker.

Let us take a few minutes and discuss some of the features listed on the chart.

Front loaded vs. horn-loaded enclosures
There are some fine horn-loaded cabinets out there, but we are focused on front loaded, meaning the woofer is mounted to the front of the enclosure. If one looks at the enclosure with the grille removed, one could see the woofer cone. The main advantage to front loading is cost. A horn-loaded device has acoustic benefits and some disadvantages too, the big one being price.

How big should a woofer be? Exactly what is a woofer, anyway? A woofer is a cone, usually paper, with a coil of wire attached via a former to the back. The coil is suspended in a magnetic field. When voltage is applied to the coil, it becomes an electromagnet. The two magnetic fields attract and/or repel, forcing the magnet with the smaller mass, the coil and paper to move. A 500 Hertz tone means the cone, coil and former has to move 500 times a second. The cone pushes air to produce sound. Larger woofers require more force to move larger amounts of air. The power rating on a woofer tells how much energy it can take before literally burning out.

All things being equal, a woofer with a higher RMS Watts rating will get louder and will last longer than a lower rating. The efficiency of the device, measured in decibel output from one Watt measured at one meter, tells how the device uses the power it has been given. A device rated at 93dB 1W/1m will be louder than one with an 87dB 1W/1m rating, if both devices receive the same amount of power from the amp.

I have read about electronic organs which have woofers as large as 32 inches in diameter. The largest I have ever seen was twenty-eight inches. A woofer this large has so much mass that it cannot respond quickly enough to produce notes above 50 or 60 Hz. A twelve or fifteen inch woofer is much more practical. A fifteen inch woofer is good from 60 or 70 Hz up to 2000 Hz. For this reason, in certain applications, a fifteen inch woofer and a horn are all the sound reinforcement that is needed. Some subwoofers use fifteen inch woofers with a good measure of success. A fifteen inch will go lower than a twelve inch woofer. 100 Hz is the practical bottom end of a twelve inch. It will, however, produce higher pitches than a fifteen. Twelve inches are good up to 2500 Hz. Most engineers agree that twelve inch woofers sound better in the vocal range than fifteens, but the fifteen is more versatile, in that it can stand alone or work in conjunction with a subwoofer. Seldom do we ask a woofer to work through its entire effective range. Most of the time, a woofer is only asked to work up to 1500 Hz, then the horn takes over.

I suppose if we wanted to be technical, the horn is actually the wave guide attached to the front of a compression driver. Granted, but for this discussion, we will call a compression driver with a horn flare a horn. We will, however, continue to call components by their proper names. When we talk about a one inch or two inch horn, we are actually referring to the opening between the compression driver and the horn flare. There is no law stating that the opening has to be one or two inches. I have used 3/4 inch as well as 1 and 1/2 inch and read about a three inch horn. Again, that is the diameter of the opening between the compression driver and the horn flare. It has nothing to do with the length of the horn, or the front of the horn. A horn flare keeps the acoustic energy that passes through it contained in a given area. Good horn flares also evenly distribute that energy over that area. A poorly designed horn flare will allow “hot spots” of energy in the coverage area.

We mentioned the depth of the horn. We often refer to the “format” of the horn flare as small, medium, or large. The depth of the horn helps guide the acoustic energy. A deeper horn will have a longer throw distance. Also, the depth of the horn helps with pattern control. A large format horn will throw sound a long distance and have crisp edges to the pattern. Pattern control and its nerdy cousin, directivity factor, describe the cabinet’s ability to place sound where it is wanted. Just as importantly, they keep sound away from where it is not wanted.

Directivity with a woofer requires a larger enclosure, and cannot be altered. Pattern control with a horn flare is much easier. It is possible to manufacture multiple pattern horn flares that fit the same compression driver and can be changed out in the field. Installers like that very much. For instance, a manufacturer might make a 90 by 40, a 60 by 40 and a 150 by 60 all for the same enclosure and one can simply order the one they need.

Many horn flares can be rotated 90 degrees and the screw holes will still line up on the cabinet. A 90 by 40 is shorthand for 90 degree horizontal dispersion by 40 degree vertical dispersion. That is an acceptable pattern for speakers on a stand, but perhaps not for permanent installation. Manufacturers gain product flexibility by building several formats of horns with multiple patterns in each format that can bolt onto their line of compression drivers.

Generally speaking, the diameter of the diaphragm inside the compression driver will be twice the horn’s smallest opening, called the throat. A two inch horn will have, usually, a four inch diaphragm. A four inch diaphragm will generally produce lower notes than a two inch diaphragm. Also generally speaking, a larger diaphragm will have a higher RMS rating than a smaller one. Because compression driver diaphragms vibrate so fast, up to 20,000 times a second (20k Hz), they have very low power ratings and extremely short travel distances. They also have fairly short life spans. In other words, fully expect to blow a horn at some point in your tenure as a worship tech.

Nestled between the woofer and horn will be a crossover. Crossovers come in two flavors—active and passive. An active crossover will be an electrically powered device located in a rack near the power amplifiers. Active crossovers require one amp channel for the woofer and one for the horn. Passive crossovers are placed inside the loudspeaker and do not require additional power to operate. Active crossovers give more flexibility and control, but require more involvement. On the other hand, a passive crossover is totally invisible to the sound operator.

Quite visible to the operator will be such things as pole sockets and handles and perhaps even flying hardware. Many articles have gone into details about rigging and flying speakers safely, so there is no need to do so here, except to remind you not to hang speakers that were never intended to fly. On wooden boxes, a pole socket and handles is a clue that the box was not intended to be flown. Some plastic enclosures do have handles and fly points, so be sure to check the dreaded owner’s manual before hanging any speaker. Lost the owner’s manual? Call your dealer or sound contractor—even if you did not purchase the cabinet from them, almost all will be happy to help. Safety is that important!

By no means have we exhausted the subject of loudspeaker enclosures and components, but hopefully this has helped resolve some confusion, without creating too much new confusion, about the work horse of the speaker industry: the humble woofer and horn combo box.

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