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EMERGING TECHNOLOGY BY JES SE FI SH M A N, C T S - D Doing the AV Math HOW MUCH POWER DO I NEED? M y March 2013 column provided a quantitative answer to the subjective question of, "What is loud enough?" In this month's edition, our example is extended further to answer the question, "How much power do I need?" To briefly sum up the previous article, we know that, in order for a loudspeaker to be deemed "loud enough," we need its signal to be 25 dB greater than the ambient sound level in the room. Extending this acoustic example to electrical systems, we want to figure out exactly how much power is required at the loudspeaker to achieve those signal levels. The equation for calculating the Effective Power Required (EPR) is shown below. It may look complicated, but it is another example of plugging in the proper variables and adjusting as needed. Most professional quality loudspeaker manufacturers publish specifications on their products, which are necessary in order to calculate how much power is required. Here are the values we are specifically seeking in this application: • Sn = signal level required at the listener — from the previous article, we determined that was 80 dB • Dl = distance to furthest listener • Dr = reference distance (this should be the same value as the distance at which sensitivity is measured, see below) • Ls = loudspeaker sensitivity — this is a measurement taken by placing a microphone at a specific distance away from the loudspeaker and measuring the dB level of the output when 1 W is sent through it. Loudspeaker manufacturers typically use an industry standardized standard distance of 1 meter, but it is always important to verify this by looking at the specifications. This is, in essence, the efficiency of the loudspeaker: a more efficient loudspeaker (from a power perspective) will have a higher sensitivity value. It is important to note a brief disclaimer: many factors can skew the results of this measurement. But for brevity and for the purpose of this article, we will assume that manufacturers hire an independent third party to measure all loudspeakers with the consistency required to allow us to trust the results. The last piece of this puzzle is called "headroom." Headroom is the amount by which the capabilities of a system exceed the necessary requirements of the signal. An analogy for this might be the "red line" on your car's tachometer. While your car may be able to handle a little bit of extra RPM, sustained amounts of use at those levels will damage the system. For this example, we will use a figure of 6 dB of headroom. You might fi nd more headroom in a theatre, and maybe less in economical systems where audio performance is not as great of a concern. For this example, we will use actual data from two different loudspeakers. Loudspeaker "A" has a sensitivity rating of 87 dB SPL, measured at 1 meter away with a 1-W signal sent through it. Loudspeaker "B" has a sensitivity rating of 91 dB SPL (also measured at 1W/1m). The results for each loudspeaker are shown underneath the Effective Power Required formula image at the side. Loudspeaker "A" would require 170 W, while loudspeaker "B" only requires about 68. Even if we revise our headroom down, it is unlikely we will achieve a very power-efficient system. It is important that an acoustician take a look at even the simplest designs in order to determine if there is anything that can be done that will bring the ambient noise to a manageable level. For example, it would be more manageable if we had a baseline ambient noise level of 35 dB wherein a signal could be considered "loud enough" at 60 dB at minimum for the farthest listener. If we can begin our equipment selection using these more realistic values (see option "C" at the side), we will be able to bring the power required down to a much more manageable 1.7 W (not a typo) for the less efficient loudspeaker — one that also saves money, which is a plus. CPM Jesse Fishman, CTS-D, is a senior AV systems designer at Westlake Reed Leskosky (wrldesign.com), integrated architects, engineers, and technology designers. He has more than 10 years of experience in the AV industry. MAY 2013 / COLLEGE PLANNING & MANAGEMENT 89