This is the last link in the chain of which you have control. It also happens to be probably the most complex issue when it comes to choice, placement, and a host of other issues.
When it comes to speaker selection, we’ll assume that you have listened to a variety of models, and have found several that sound good to you. We’ll also assume that you are going to buy existing full-range systems from any one of several makers. The first thing you need to consider is: how are they going to be used? What kind of rooms are you most commonly going to be in, and how large are the audiences going to be. A small (50 seat) club which is a wide room requires a different type of cabinet than a large, long venue, or an outdoor show.
You also need to take into consideration how loud you need it to be at various points in each venue.
In small rooms that are not very deep, you may be best served by front-loaded cabinets, which have a wide dispersion angle. This means you can use fewer cabinets on each side of the stage, while still having the majority of the audience in the “near field” (hearing sound directly from the speaker rather than merely the reverberant field). If you have found 2 or more systems that you like and can’t decide which to get, compare the efficiency ratings of each. Pick the one with higher efficiency – you will require a lot less amplifier power to achieve the same SPL (Sound Pressure Level, or “loudness”). For instance, one rated at 100dB is going to sound twice as loud as one rated at 97dB. If you pick the one rated at 97dB, you will have to run your system 3dB louder to compensate – which requires TWICE the power. Every 3dB increase in loudness requires a doubling of amplifier power! But beware – amplifier companies often don’t give you the full specs, especially on lower-priced amps, or amps from the musical instrument manufacturers. An efficiency rating should give you not only the SPL, but also at what input wattage (usually 1 watt), distance from the speaker at which it was measured (usually 1 meter), maximum distortion level at measured SPL, and over what frequency range (with +/- listed as well). The “standard” distance is now 1 meter – a calibrated SPL meter is placed 1 meter away from the speaker, on-axis to take the measurement. Next is the frequency range at which the measurement was taken. It is easy for the manufacturer to get deceitfully high efficiency ratings if they limit their measurement to only one frequency. This should also give you how much variation there is in this frequency range. The high and low frequency limits are typically the points at each end of the spectrum where the response is 3dB down (-3dB). There will be frequencies within this range that vary from a perfectly flat response, and that maximum should list listed as well (such as +1dB). So a full specification for a full-range speaker system should look like: 101dB(a) SPL, 1W/1M, +1/-3dB.
How does this translate to “how loud will it be 100ft away?” That also depends on a lot of things. It of course depends on how much power you put into it. In the above example, to get to 111dB at 1M, you will need to put in ten times the power, because an increase of 10dB requires ten times the power. So, 100W input will get you to 111dB at 1M. So, how does that translate over the distance? It’s hard to give a general rule because all rooms are different. If you used this cabinet outdoors, where there is no sound reflection, each doubling of distance results in a 4-fold decrease in loudness (6dB) (the “inverse-square” rule). So, at 2 meters, 111-6 equals 105dB. Double the distance again to 4 meters and you get 105-6= 99dB. At 8 meters, you end up with 99-6=93dB. At 16 meters (around 50 feet), you are left with 93-6=87dB – not very loud for your 100 watts of amplifier power. Also consider that you need to have headroom, or you will end up with clipping on every drum hit, or loud passage in the music. The more headroom you have, the more dynamic range you will have. To have a measly 10dB of headroom, you are going to require ten times the power if you are using that one speaker. 100W x 10 = 1000W. Whoa. There’s a problem. There are a couple of things to look at to fix this.
Look at a speaker system that has higher efficiency. If you go with one that’s 3dB more efficient, you can cut your power requirements in half. You can also look at a speaker system that concentrates more of the sound in the area you want. This is where dispersion angle comes into play. A front loaded cabinet has a very wide dispersion angle, and thus is good for a small, wide room, but doesn’t do a good job of concentrating the sound into a narrower angle. A horn loaded cabinet, on the other hand, is built to control dispersion, and as a side benefit, typically has a much higher efficiency. Ah ha! It sounds like this might be something to look into.
Everyone is familiar with the high frequency horn that has a 1″ or 2″ driver on the back of a flared “horn.” This horn directs the sound waves, and also increases the efficiency of how well the driver’s diaphragm movement is coupled to the air in the room. They will have a horizontal and vertical dispersion rating, a ‘cutoff’ frequency, and hopefully, a graph showing how well it controls dispersion at various frequencies (a common horizontal dispersion number is 90 degrees). As the frequency increases, the dispersion tends to narrow substantially. And, since the dispersion angle decreases, that means that the same amount of sound if focus on a smaller area, resulting in an imbalanced frequency response. This results in a situation where someone who is sitting slightly off-axis, but well within the 90 degree coverage angle, will hear less and less from the horn as the frequency increases. By the same token, people in the back of the room, who are only slightly off-axis (by a 10 or 15 degree angle), are going to have their heads cut off by the high frequencies.