Outdoor Acoustics 101
Every integrator has experienced the trials and tribulations of dealing with indoor acoustical problems caused by room dimensions and the acoustic properties of wall and ceiling materials. The most common indoor acoustical problem is that of “standing waves” that can cause large bass sound pressure dips and peaks at various locations in the room. It’s a real challenge to deal with indoor room acoustics issues, but that’s not what we’re going to cover here. Let’s move the conversation outdoors.
What happens when there are no walls and ceilings? Should you expect fewer acoustical problems or more? The issues will be different, but not necessarily fewer.
In this article we’ll focus on how the typical outdoor acoustical environment, amplifier power and speaker placement influence sound. We’ll explore how to get better tonal balance and clarity as well as great bass impact.
Can You Hear Me Now?
With no room surfaces to contain the sound and with the high ambient noise typically found outdoors (birds, traffic, kids and other annoyances) the first issue you face outdoors is the amountof sound.
First, let’s establish how much sound is enough. Outdoor background noise is usually in the 55dB – 60dB (A-weighted) range. A “background” music level where the listener is aware of the music but conversation is possible is 60dB – 65dB, just over the ambient noise level. “Foreground” music listening is in the range of 66dB – 75dB; at this level music starts to interfere with conversation. “Party” loud is 80dB and up. For reference, a live rock concert is typically 90dB – 95dB.
Sound outdoors follows the Inverse Square Law, where each doubling of distance drops the output by 6dB (that’s a lot). If a speaker is playing at let’s say 70dB at 1 meter (~3 ft.), at 2 meters (~6 ft.) the sound pressure will be 64dB, and so on. It is not uncommon for listeners to be 8 meters (about 25 feet) away from the speakers; in this scenario the sound pressure level would be only 52 dB, not loud enough to overcome typical outdoor ambient noise of 55dB to 60dB. Yes you could turn up the volume so that the listeners at 8 meters can hear the music, but the listeners at 2 meters will be blasted with too-loud sound.
If you’re just doing a simple deck and two-speaker set up where the listeners are always near the speakers, the Inverse Square Law isn’t much of a problem. But for larger multi-area scenarios you’ve got to keep it in mind. Using multiple strategically placed speakers will go a long way to solving the problem.