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Sizing Speakers for Indoor and Outdoor Events

For corporate presentations of any size or complexity, the audio portion has become a necessity rather than a nice option. The judicious use of “sound design” brings a patina of professionalism to even the simplest visual presentations. Still, the soundtrack is irrelevant if the audience cannot hear it clearly.

Poor “sound reinforcement” can be due to many factors. The original sound material can be of poor quality, or it could have been used incorrectly in the presentation software. However, one common reason for poor sound reproduction is choosing the wrong loudspeakers for your event.

Outside in, inside out

There are a number of variables to consider when choosing your sound system. We will assume your presentation will be running on a laptop that is connected to a projector, and will look at how best to prepare for a small inside meeting and a larger outdoor event.

Despite your conference room presentation being inside and in a room less spacious than, say, an indoor stadium, you still need to consider how the sound is going to be projected and heard. In a small room, perhaps 12 by 20 feet, the power supply in the projector – which can be anywhere from 5 to 50 watts or more – will be sufficient to power two passive speakers that will fill the room with sound.

You will also have the option of using powered speakers, and most modern projectors have a variety of audio outputs from which to choose so connections will not be a problem. You can consult other articles here at CRE for information on audio (and video) output standards, cable types and specifications.

When the length of the room exceeds 20 feet or so, you will have to consider augmenting the two forward-firing speakers in the front (your default setup every time) with additional speakers halfway down the room at the sides. It is when the rooms start getting very large (ballrooms, convention centers), or the event moves outside, that the “ballpark estimates” for amplifiers and speakers must give way to more precise calculations.

Sound calculations

If math is not your strong suit, the sound reinforcement experts at CRE can assist you in devising the proper setup for your event. However, for your do-it-yourselfers, there are some (fairly) basic equations that will help you calculate the wattage of the required speakers (see Resources).

An important thing to remember is that speaker “size” is not a reliable indicator of its strength. A speaker the size of a shoebox can easily have higher output, which is measured in watts, than one twice as large in external dimensions. It is speaker strength, in fact, that you are concerned with, not physical size.

By the time your event grows enough to be held in an outside setting – a convention tent, a restaurant banquet room, an arena – your need for expert advice has also increased dramatically. Because every event and setting has unique considerations, including a wide variety of acoustic properties to understand and contend with, you should enlist expert assistance if you are uncertain about how to proceed.

Generally speaking, however, outside events have to deal with the fact that, unlike indoor ones, there are no walls or other surfaces off which the sound can bounce. This is a plus as far as controlling “standing waves” and reflected sound, but a minus as far as devising full audience coverage. For most every outdoor event, there will have to be an array of loudspeakers whose positions must be carefully determined – front, side, back, up and down.

As the event size grows, so must your expertise at sound reinforcement. If you are mathematically sophisticated enough to use the equation for Required Amplifier Power (Resources), you can do so. If you need assistance, you need only contact one of our sound reinforcement professionals who will be happy to help you.

Glossary

Sound Design

The use of music, sound effects, electronic tones and acoustic samples to create background and foreground soundtracks that add to the effectiveness of the accompanying visual media
Sound Reinforcement

A group of related tasks that result in sound being captured, processed, routed and amplified in a live setting
Standing Waves
Standing Wave
Sounds that reinforce each other, like two speakers wired in phase; as they bounce back and forth between walls, they build up areas of high and low pressure that are consistently in the same places in the room
Resources

Equation: Amplifier Power Required in Watts

This equation yields the required electrical power (power output from the amplifier) to produce a desired Sound Pressure Level (SPL) at a given distance, along with an amount of headroom to keep the amplifier(s) out of clip.

dBW = Lreq - Lsens + 20 * Log (D2/Dref) + HR

Where:

W = 10 to the power of (dBW / 10)

Lreq = required SPL at listener

Lsens = loudspeaker sensitivity (1W/1M)

D2 = loudspeaker-to-listener distance

Dref = reference distance

HR = desired amplifier headroom

dBW = ratio of power referenced to 1 watt

W = power required

Example: You are designing a system where the farthest listening position from the loudspeaker is 100 meters, and the desired Sound Pressure Level is 85 dB SPL. The loudspeaker chosen for the job has a sensitivity rating of 95 dB. With the minimum recommended amplifier headroom of 3 dB, then you need to choose an amplifier that can supply at least 1,995 watts to the loudspeaker.


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