Dispersing Acoustical Myths

Today’s post is by a good friend of mine, Greg Jackson. He uses his humor and dry wit to present an otherwise dusty topic… handling acoustical issues in your room.

The arcane science of acoustic design can be dated back to the earliest cave-dwellers, who have been found, through archeological excavation, to have hung animal pelts from the walls to keep their domiciles from feeling so “live.”

That’s not true. It was an allegory. About a cave. But it sounded good, no? Such is the case with a lot of the information floating around out there relating to acoustics: it sounds great, but it’s malarky. We see it a lot in regards to studio design, but it reaches a lot further than that. I had a guy call once that had a large, noisy piece of machinery in a room, and was wondering how many absorption panels he needed to buy to keep it from being heard outside. Answer? However many you need to stack under it to raise it a foot off the floor.

We’ve all experienced bad acoustics, whether we knew it or not. Gyms are an obvious example, but consider the restaurant that is so reflective, you have to shout to get your wife to hear you over the sound of utensils clanging together, or the doctor’s office where you can hear his conversation with the guy in the next exam room (that’s why the people in the waiting room were snickering when you left). The best way to dispel myth is with a little education, so, armed with the sword of truth, let’s hack to pieces the enemy forces of ignorance and let forth the battle cry “Scientia Potentia Est!!!” (Hey, I didn’t know GI Joe spoke Latin!)

The science of acoustic design has been around for a long time. The Greeks and Romans built their theaters and auditoriums in such a way that the speaker on stage could be clearly heard from every seat in the house, long before the advent of sound amplification. We still utilize many of the principals that they developed when designing a performance space, but with all the advancements made in science over the last couple of millennia, acoustics is a field that can get so complicated in its finer nuance that it requires a four year degree and/or decades of experience to fully understand.

I have neither.

But, I have clapped my hands in a lot of rooms, and I have gleaned a few nuggets of sage-like advice from people that are a lot smarter than me.

The first distinction we need to make when talking about acoustic treatment is the difference between TRANSMISSION and REDUCTION.

Sound TRANSMISSION is the movement of sound from one space to another. If you’ve lived in a cheap apartment, you have probably had lots of unwanted sound transmitted into your home. This is usually due to architectural problems and people being downright inconsiderate. In offices, a shared airspace above a drop ceiling is often the culprit. Other common pathways for sound transmission can be wall entries for electrical or plumbing, non-insulated walls, hollow doors and the openings under them, and windows. Sometimes, especially with low-frequency energy, the sound can even travel through the structural members of the building from one room to another. Most sound transmission issues are tricky and expensive to rectify, but where there’s a will (and a checkbook) there’s a way.

However, there is a great, inexpensive product on the market that can help in noisy office environments or doctor’s offices, and that is a sound masking generator. We have had great success with these in clinics, where patient confidentiality is of utmost importance. Sound masking generators emit white or pink noise (sounds like television static) at a very low level that “covers up” unwanted noise. This concept is known as “auditory masking.” When deployed, it kind of sounds like the rush of air through an AC vent. It isn’t noticeable to the building occupants, but a successful installation will improve privacy, productivity, and comfort.

Sound REDUCTION describes the process of manipulating the sound waves within a confined space, and is primarily what we deal with as AV guys. The goal here, usually, is to manipulate the reflections and echoes in a room to improve intelligibility, while maintaining a pleasing acoustic atmosphere.

When dealing with reflections, we start by measuring the reverberation time (RT60) of the room, also referred to the decay time. This is the amount of time it takes sound waves in the room to decrease in energy by 60dB, and can be measured by clapping your hands and using a stopwatch to see how long it takes the sound to die off. Preferable RT60 ranges for different spaces (from “dead” sounding to “live” sounding) are as follows:

Recording Studios – <.5 second
Board rooms, class rooms – .5-1 second
Small theaters, contemporary churches, clubs – 1-1.5 seconds
Traditional churches – 1.5-2 seconds
Opera, orchestra, underwater basket weaving facilities – 1.5-2.5 seconds

Anything above 2.5 seconds will begin to do real damage to intelligibility. In order to shorten decay times, we employ absorption panels. These vary in size, shape, and effective frequency range and are placed in strategic locations depending on the needs of the space.

The ability of a material to absorb sound is expressed as its noise reduction coefficient (NRC), and will range from a 0 for perfectly reflective surfaces (think glass or concrete) to a 1 for perfectly absorptive materials (think absorption panels). This number is an average of the absorption coefficients across several frequency ranges and is used for simplicity, but more detailed charts comparing frequency to amplitude can be obtained from the manufacturer of a given product. The NRC is the reason covering your wall in egg cartons or carpet is a joke. These offer a very low NRC (>.3) and won’t do much to deaden your room. They do, however, make great kindling if you have a fire.

***side note*** This is a great time to talk about the other things to look for in an acoustic product, those being fire protection, durability, and aesthetics. Keep all these in mind when investing your hard earned moolah. Spend the extra dollar per square foot and get something that won’t burn or get torn up.

The other bad force we have to combat in noise reduction world is ECHO, which is… echo. Duh. Sound goes out, bounces off of something, and comes back. The worst kind of echo, especially in most church buildings, is flutter. It occurs between parallel walls, and sounds… fluttery. Duh.

We can treat echo with absorption as well, but in instances where we want to maintain our room’s RT, we can employ diffusion. Think of diffusion like a prism. If you shine a beam of light at a mirror, it will reflect at the angle incident to it’s approach, and the reflected light energy will be equivalent to the source energy (assuming you wipe your grubby prints off the mirror first). When you shine the same beam of light through a prism, however, it is broken into multiple beams of light at a decreased energy level and dispersed at different angles, depending on its design. We can do the same thing to sound waves with diffusers to break up standing reflections without removing the sound energy from the room. How cool is that!?

Again, type, size, and placement of diffusion and absorption products will vary depending on the application. In rooms where this is extremely important (*cough* churches *cough*), it would behoove the client to enlist the assistance of a qualified acoustic designer to make suggestions (not your brother-in-law). If you are installing a new PA and you REALLY want to do it right, plan on spending as much on acoustic treatment as you do on the PA. In situations where this isn’t cost effective, at least pay attention to room acoustics when designing the space and do as much as you can. Every little bit helps.

So what about you? Have what’s the biggest acoustical fallacy you’ve ever seen? Tell us your story in the comments section.

Greg Jackson is a contract consultant for Moyers Group in Lubbock. He is heavily involved with the media ministry at his home church, Experience Life. You’ll also find him running around the campus of Texas Tech University as he completes his long-denied degree.