There’s a great scene in This Is Spinal Tap where the musicians tout an amp as louder because the dial goes to 11. It’s hilarious, of course, and obvious that the dial numbering really has nothing to do with the output itself.

However, it had me thinking about a few misconceptions in audio that can affect the end result of many of our installed systems. I want to lead with the disclaimer that I’m not an audio engineer or acoustician. My intent here is not to explore every nuance or get down in the weeds on technical details of specific technologies.

Instead, I wanted to offer up some basic principles we all can use to create a platform for better audio design and performance, all while keeping the signal above the noise.

That being said, we do need to start by understanding how loud an audio system needs to be for someone to actually hear it. This is where basic signal-to-noise ratio is important. You first have to know how noisy the environment is at the time the audio will be playing, or the noise floor. Then you need to have a signal volume that exceeds that noise floor in order to be heard. AVIXA standards recommend signals need to be 25-30 dB ABOVE the noise floor in order to meet professional standards.

Now, in order to achieve these volumes, we often think of amplifier power as the first variable to adjust to get more signal. Although amplifier power will ultimately be important, we first need to know the sensitivity of the speakers being used.

A speaker’s sensitivity is how much sound pressure (SPL) it creates per watt of power at 1 meter. If we know the target SPL needed, we can work backwards to the speaker efficiency, and then to the amplifier, in order to start to create a basis of design.

The trick here, though, is that we also need to know the distance to the listener(s). In free-field conditions, sound from a traditional point-source speaker drops 6 dB per doubling of distance, so the farther the speaker is from the listener, the more power you will need to achieve the desired output.

Using some basic AV math, we now have some power calculations that may be helpful, but here is where things get tricky.

If you want a deeper dive on dynamic range, you can also get AVIXA’s reference standard here.

However, signal-to-noise ratio doesn’t guarantee intelligibility.

In other words, we can accidentally create a system that is loud enough to hear, but not clear enough to understand.

There are many things that go into the intelligibility of an audio system, and there is actually a rating system for intelligibility called the Speech Transmission Index. STI ratings range from 0 to 1, from words that sound completely incoherent to those that are highly articulated.

There are a lot of things that can affect STI, even when the SNR is correct. Again, this isn’t meant to be comprehensive, but directionally we should think about a few main culprits of poor intelligibility.

STI can be affected by coverage. If speakers create an uneven coverage pattern in the room, then SPL levels can vary by chair. Frequencies can also drop off at certain angles of those coverage patterns, meaning that there are also hot spots for specific tones as well. Coverage patterns can be designed by looking at the installed height of the speaker and the speaker’s dispersion. Simple speaker coverage in lower-end systems are typically edge-to-edge, whereas in high end systems, center-to-edge, with designs that also fall in between.

Raising the speakers up can create wider coverage, but also requires adding power to maintain the volume, and adding power (as we will explore in a moment) can add risk.

More speakers, closer to the listener(s), running at lower volumes can help if properly designed.

Intelligibility is affected by sound reflecting off of the surfaces in the room. In other words, it bounces around, and the listener hears the same sound at a reduced volume several milliseconds later. This may not sound like a huge deal, but it greatly affects the intelligibility of the sound itself. This is why raising speakers and adding power can introduce more risk. Adding more energy into the space can actually decrease intelligibility by creating more reflections.

Geometry also matters. If you have influence on room dimensions, have them adjusted slightly. A 20-by-17 room and a 20-by-16 room have similar footprints, but rooms with common ratios (20 and 16 are both based on 4) have more potential to stack frequencies and cause poor audio performance.

This is where we all can bemoan the glass walls in conference rooms, hard tile floors in malls and other highly reflective surfaces that do nothing to reduce reflections and negatively affect audio performance.

This is where finishes in rooms become extremely important to the desired outcome, and why earlier coordination with architects and designers is something we all should be asking for.

There are two main ratings we need to pay attention to in this regard. One is the Sound Transmission Class (STC) and the second is the Noise Reduction Coefficient (NRC).

A lot of emphasis is placed on STC by architects, but unless we’re building theaters or studios, this rating is not usually our main concern. STC is the rating that tells us how much sound goes through the room to adjacent spaces. There are ways to improve STC, (thicker materials, air gaps, staggered studs, special fasteners, etc.), and these typically affect how the room is actually constructed.

The Noise Reduction Coefficient (NRC) however tells us how the sound is absorbed when it hits the material itself, and this is what affects the experience in the room itself.

Materials that absorb sound can be specified to mitigate reflections and, in simple terms, “tune” the room. Acoustic panels and fabric, wall systems, and even plusher furniture and friendlier flooring can change the NRC of surfaces dramatically.

At the end of the day, the systems we install should be producing clarity, not just volume, and knowing some of these basics greatly increases our value as AV professionals. Good design starts with the basics, and when things get really tricky, it’s time to bring in the heavy hitters.