One of the biggest concerns facing audio designers for any installation is ensuring you have enough speakers of sufficient wattage to be appropriate for the space. How much speaker wattage is enough, and is more too much? Will my speakers not only cover the area, but also cover it with enough volume? While much of this is open to interpretation and personal opinion, there are some basic guidelines.
The first thing to remember is that the size of the sound system required for a particular installation is different from considering the amount of amplification needed to safely drive a speaker. If all you are wanting to know is how big of an amp you should buy to drive your speaker without damaging it, then you should take a look at the “How much power can my speakers handle?” section of Crown’s “How Much Amplifier Power Do I Need?” white paper.
When you are considering the “size” or loudness of a sound system, what you are really determining is what is required to achieve a sufficient sound pressure level (or SPL) within a particular space. SPL, measured in decibels, is the perceived loudness of the sound within the space. SPL is affected by a variety of factors within the sound system, including wattage, power handling and speaker design to name a few. Because this perceived loudness is determined by the amount of pressure the sound waves generate in a room, size and distance will affect your decision. All else being equal, the larger the room is and the greater the distance the speaker is from the listener, the more wattage you will need to create a sufficient sound pressure level. In fact, a single speaker will typically lose 6 dB of loudness every time you double your distance from it.
That said, there is a lot of variability with the way loudspeakers are made, which is why understanding the intricacies is important.
Much of this variability is because not all speakers are created equal. The higher the sensitivity of the speaker, the higher the output (and thus the SPL) will be. Speaker sensitivity (or system sensitivity) is typically measured based upon something the JBL website refers to as the “1w @ 1m” method. This measures what sound pressure level a microphone picks up when placed one meter away from the speaker and then you push one watt of signal through it. The higher the decibel measurement of this spec, the better the speaker will be at converting wattage into volume.
Also affecting this decision is the variability of the signal. While certain styles of music or pre-recorded audio in a video clip might be highly limited or compressed, other sound signals will have a lot of variance in volume levels, with signal peaks of 20 dB or more depending on the style of music and the way the audio engineer sets up the mix.
Even this is a simplistic look at things. The other side of this coin is speaker coverage pattern; the “shape” of the speaker’s output, both horizontally and vertically, can greatly affect loudness. This is because a speaker’s loudness is not all or nothing. Audio gets progressively quieter along the edges of the speaker’s coverage pattern, meaning there are locations where the speaker will be louder, where it will be more even in volume and where it will be softer. Volume also varies by frequency. That is why you should look at the frequency response both on and off-axis. A speaker may be loud enough with sufficient coverage, but if it does not have enough bass in the right places, you will need to supplement.
Coverage pattern and frequency response are very important for applications such as immersive audio, but all installations should consider the effect these features have on loudness, especially installations with multiple speakers. This is particularly important for off-axis areas where the sound from two or more speakers blends. This blending can cause noticeable varying in both frequency response and loudness.
When creating an effective sound system design, you want to be sure that the sound not only reaches all portions of the room, but that the SPL and frequencies of the sound system are consistent to all of those areas. You will also want to account for the time arrivals from multiple loudspeakers. If the sound arrives later (and louder) than the point of primary focus, this will cause a noticeable and distracting “echo.” We tend to focus on the first received sound as being the “real” sound, and expect that from the visual focal point (pulpit, stage, etc.).
It’s important to remember that this speaker coverage occurs in three dimensions. One of the most often-overlooked aspects of an audio system design is the speaker’s throw distance. As I said before, the sound from a single speaker will typically drop 6 dB every time you double your distance from it. This is why sound designers look for a high decibel rating in the “1w @ 1m” test. If using a single speaker, you will want to be sure that the wattage of the speaker will provide sufficient loudness to cover the distance you need it to (without being too loud for those in front), or else you will need to supplement with additional fill speakers to reach the rear areas.
There are also some ways around this inconvenient fact of physics, the most common of which is using multiple speakers, either using a line array or built into a column speaker. When implementing an array or column speaker, it is still important to look at the throw distance of the design and ensure the speaker(s) will cover the entire area. Column speakers like JBL’s CBT Series not only provide control of the horizontal and vertical coverage patterns, but also provide a consistent SPL over a specified throw distance, making them simple to specify in a variety of installations.
There is obviously much more involved with audio system design, including intelligibility, architectural and acoustical challenges, and more. However, this should hopefully give you a good guideline on where to start when determining the loudness for your system.
Do have any tips on achieving a sufficiently loud sound system? Share them in the comments.