“Anechoic” \an-nah-kō-ik\ adjective: free from echo and reverberation
Step inside an anechoic chamber at Shure, and you are immediately transported to a world like none you’ve experienced before. The metal mesh floor gives a little under your weight. There are hundreds of pointed grey fiberglass wedges covering the walls and ceiling. When the doors close behind you, you are in a place so quiet, so echo-less, that you can eventually hear the rushing of blood in your eardrums.
The First Anechoic Chamber
An anechoic chamber is a special room containing sound-absorbing and sound-attenuating material in the walls, floor, and ceiling. These materials absorb 99.5% of the sound to ensure the room is free from echoes and reverberation. Shure headquarters in Niles, Illinois, has multiple anechoic chambers.
The word anechoic was coined by legendary acoustician Leo Beranek in the 1940s. He managed Harvard’s electro-acoustics laboratory, designing communications and noise reduction systems for World War II, and designed the first anechoic chamber there. Bell Labs’ Murray Hill, New Jersey, anechoic chamber was built during the same period, and for many years, it made the Guinness Book of World Records as the world’s quietest room. Orfield Laboratories in Minneapolis, Minnesota, eventually edged out Bell Labs. In 2015, a chamber at Microsoft’s Redmond, Washington, headquarters claimed the title, which it still holds today.
Anechoic Chambers: Not Just for Audio Products
Manufacturers of products ranging from industrial machines to automobiles use the services of commercial laboratories with anechoic chambers (Orfield Laboratories is an example), but the need for testing can stretch all the way to the military. The largest known anechoic chamber in the world belongs to The Department of Defense. It operates the Benefield Facility at Edwards Air Force Base with 4.6 million square feet of space, capable of accommodating a U.S. military airplane, even the B-1 bomber.
Some companies have more than one chamber. Apple spent $100 million in 2010 to build 17 anechoic chambers to test their devices.
Anechoic Chambers at Shure
Each anechoic chamber at the Niles headquarters is a box floating within a room. The outer walls rest on the main building foundation and have no contact with the chamber. Eckel Industries manufactured and installed them at a cost of $250,000 each when Shure moved its headquarters from Evanston to Niles in 2004.
The chambers are located in the Technology Annex and float on multiple spring pads eight feet below the main building foundation. The spring pads have a resonance of 3 Hz. The eight-foot deep concrete pit is isolated from the main building foundation.
Mike Joyce is the product lab manager at Shure and the person responsible for all microphone tests conducted in the anechoic chambers. Tests like these are a necessity for the accurate measurement of microphone specifications. The audio test signal must be completely absorbed once it passes the microphone. Any sound reflections from the walls, ceiling or floor will corrupt the test data.
He explains the testing this way: “The microphone’s transducer converts audio waves into electrical energy and is the most critical part of a microphone. You can compare it to the engine of a car. If you want to test a car’s performance, you’ve got to test the engine. That’s what we’re doing with transducers in the chambers.”
Thirty-eight years ago when Mike joined Shure, he was told that the ideal frequency response test would be to “float a speaker and a microphone in the air and stop all the wind for miles around so that you could test without any reflected sound. Anything short of that is a compromise.” The next best tool, he says, is an anechoic chamber for controlled and repeatable testing.
Here are some facts about the engineering and the performance of the chambers:
- Sound-isolation layers between the outer walls and the chamber:
- 3″ Air gap
- 3″ Foam insulation
- 5 Layers of ¾″ drywall on all sides and the top
- Cable floor weight limit: 2,000 pounds
- Exterior dimension of chamber: 24′ long x 22′ wide x 22′ tall
- Interior dimension of chamber (fiberglass wedge tip to fiberglass wedge tip): 16′ 4″ long x 14′ 4″ wide x 14′ 4″ tall
- Sound absorption provided by fiberglass wedges and cloth clovers: 99.5%
- Internal ambient noise level (50 Hz–20 kHz): 5.4 dB SPL
- Minimum internal ambient noise level: 0 dB SPL
- Maximum internal ambient noise level: 19 dB SPL
What It’s Like in the Shure Anechoic Chambers
You might think that spending time in an echo-less chamber would be a peaceful and pleasant experience, but for some it is not.
Take the anechoic chamber at Orfield Laboratories in Minneapolis. Steven Orfield, the company’s founder and president, challenges people to sit in the chamber in the dark for 45 minutes. He told the Daily Mail: “When it’s quiet, your ears will adapt. The quieter the room, the more things you hear. You’ll hear your heart beating; sometimes you can hear your lungs, or hear your stomach gurgling loudly. In the anechoic chamber, you become the sound.” Some visitors find the anechoic chamber experience so disquieting that there have been reports of nausea, dizziness, panic attacks and even aural hallucinations.
Of course, no one at Shure is ever subjected to a torture test like that, but Mike Joyce notes that some people visiting the chamber experience anxiety and claustrophobia. It is, after all, a space like no other with wedges pointing right at you from every direction.
“Our sense of balance relies on visual and audible references as well as the equilibrium of fluids in the inner ear,” says Shure Associate Director of Acoustical Engineering Ken Platz. “Our anechoic chamber has a suspended floor that flexes slightly. The slight sensation of motion when our eyes and ears can’t confirm that motion disrupts some people’s internal gyroscope.”
We asked a pair of stouthearted Shure Associates to spend five minutes in the dark inside one of the chambers and report on the experience. Here’s what they had to say:
Digital Marketing Specialist
“The first weird thing about the anechoic chamber was the weight and thickness of the door. The other thing I noticed right away was the ringing in my ears. Because it was completely silent, I thought I’d experience a feeling of peace, but the ringing killed it, and it got worse the longer I sat there. When I tried to stand up, I started to feel nauseated, and that stayed with me through the last few minutes I spent in there. When I exited the chamber, everything sounded louder. By the time I got back to the Associate entrance a short walk away, I felt normal again.”
Cheryl Jennison DaProza
Media Relations Specialist
“Becca and I turned off the light, shut the huge padded door and spent the next few minutes in a quiet darkness that is probably as close to the grave as you can get. If not for the feeling of the wire floor beneath my feet and the breathing of my associate a few feet away, it would have been complete sensory deprivation. I didn’t feel like I was hallucinating or ‘going crazy’ as some have, but I wouldn’t volunteer to spend more time in the dark there by myself. The most remarkable thing wasn’t necessarily what happened in the chamber, but what I noticed once I stepped outside of it. The world became a noisier place than I realized! HVAC systems were suddenly an assault on my ears, and everything seemed amped up.”
Perhaps the phrase “quiet as an anechoic chamber” should replace “quiet as a mouse.”