Doing the AV Math
- By Jesse Fishman
- March 1st, 2013
“Loud enough” is a subjective term. In a court of law, we would want to accurately define that phrase, and we can do so with math. Additionally, there are various new standards these days regarding whether or not the sound emitted from a loudspeaker is “loud enough.” These standards have to do with specific systems or uses, such as life safety/emergency evacuation, working environments (rf. OSHA), and a multitude of other applications. But what does “loud enough” mean in terms of a typical classroom? For the purposes of understanding classroom needs, we can begin with an interpretation. “Loud enough” is an attempt to describe the volume of one signal in comparison to others. This effectively means that whatever is being listened to (i.e., the signal) should be louder than the “noise” in the room.
For the most part, the Audiovisual (AV) and Acoustics industries have standardized a mathematical definition of how much louder the signal should be over the ambient sound level within a space. The “signal” should be 25 decibels of sound pressure level (dBSPL) greater than the ambient sound (or noise) level. To design an appropriate system, a measurement of a room can be taken in the case of existing spaces in order to determine how loud the signal needs to be. When we talk about sound, we are typically talking about comparisons; that is, how loud one signal is versus another. For example, when we say that a busy street is about 90 dBSPL, we are stating that the sound pressure level of that area is approximately 90 dBSPL greater than the standard threshold of hearing.
So loud enough has been defined, but to whom? The instructor? The student? For this example, one could say that the student who is farthest from the source of the sound should have the minimum of 25 dBSPL signal greater than the noise. A measurement could, of course, be taken during a time when the room was being used and determine the appropriate volume for that student, but what happens when the room is under design or construction?
Some assumptions and approximations about noise levels in the room need to be made. For example, a typical classroom has an ambient noise level of about 55dBSPL. To give this measurement some context, this would be the typical volume somewhere between a conversation in a home (50dbSPL) and a conversation in a restaurant (60dbSPL). This holds true for the most part. An acoustician can give a more accurate range of ambient sound levels in specific rooms. For example, some rooms may be located under an air handling unit, which, without the appropriate noise/vibration isolation, can generate a substantial amount of unwanted noise. In that case, we would need even louder signals from our loudspeaker.
Doing the math is straightforward addition: assuming the classroom has about 55dBSPL ambient sound volume, we need a volume that is 80 dBSPL at each student’s ears (55 + 25). However, there is something else working against the system: the loudspeakers are typically located on the front wall, and the farthest listener is 50 ft. away. A fundamental law of physics that applies to light and sound, known as the inverse square law, comes into play. Simply stated for sound, for every doubling of distance, we lose 6 dBSPL of signal. To put this in practical context, losing 6dBSPL is the practical equivalent of dropping the power down by 75 percent — a heavy loss. So if we stand 3 ft. away from the loudspeaker, and measure it at 80 dBSPL, at 6 ft. we have 74 dBSPL, at 12 ft. we have 68 dBSPL, at 24 ft. we have 62 dBSPL, and finally, at 48 ft., we have 56 dBSPL. We lost 24 dBSPL of signal in reaching that furthest student.
In order for classroom sound to be deemed loud enough, a calculation based on noise level and the distance to the farthest listener is needed. To compensate, we really just need to add the lost 24 dBSPL back into the system. With that in mind, it is now determined that 104 dBSPL is needed in order to account for noise level of the room and distance to the farthest listener. To give an idea of what that sounds like, it is like standing next to a running motorcycle. Again, to put this in more practical terms, 90-95 dBSPL is the point at which sustained exposure can lead to hearing loss, and 125 dBSPL is the threshold at which pain begins.
It is important that room acoustics succeed at getting the ambient sound level down to something easier on the ears to minimize the volume necessary to achieve the 25dBSPL headroom. For example, a 30 dBSPL ambient sound level would provide a great starting point. Providing appropriate acoustics and appropriate sound pressure level will help to also achieve good clarity and intelligibility in speech necessary for an excellent learning environment.
In the May issue, we will take a closer look at loudspeakers, and how much power is required to push those sound levels mentioned above.
Jesse Fishman, CTS-D, is a senior AV systems designer at Westlake Reed Leskosky, integrated architects, engineers, and technology designers. He has over 10 years of experience in the AV industry, having designed AV systems for classrooms, corporate and enterprise solutions, theatres nationally and internationally, and building and campus-wide networked AV solutions.