Special Interview: Noral Stewart Talks Acoustics and Noise Control
Let’s go back to the beginning: How did you first become interested in Acoustics?
“It was something that sort of hit me in the face. I never grew up thinking anything much about acoustics. I went to NC State to study mechanical engineering. I know for a fact that when I was a high school senior, I did not know what an octave was.
I was not doing very well my freshman year in my mind, and in the fall of my sophomore year, I got called to the departmental office along with a few other guys. They asked who was interested in a summer job in the acoustics group at NASA Langley — and of course, I was.
We went up there, and the NASA staff (group headed by Harvey Hubbard) taught us classes in acoustics that summer. I was working on a project on a jet engine compressor stage. One day, while we were running the engine at different conditions, the technician, who was an older guy, suddenly started saying that something was wrong. His microphones weren’t picking up anything. My manometers still showed pressure, but the dang thing had gone quiet. Nobody anticipated it.
That got me hooked. I went back to school, and a young professor starting an acoustics program arranged classes for us and more summer jobs. I was on my way.”
What other pivotal moments shaped your approach to Acoustics?
“As a consultant, it is very important to work on your ability to communicate. Right after my bachelor’s, I wasn’t working in acoustics. I had a job that forced me to write letters, meet with high-level management, and sometimes tell them they didn’t know what they were doing — gracefully. I consider this a very important part of my education.
The company I was working for started a scholarship program, and I got selected for a scholarship for a master’s degree. I wasn’t planning on doing a thesis, but I wound up working on a project that needed security clearance, and thankfully, I already had it! I worked on punch press noise, and I finished my master’s in 13 months.
Then the university offered me a position on a new research project on noise control in the textile industry. My employer — the Bell System — told me to take it because ‘there’s not going to be any future for you here.’ Because they knew the company might be coming to an end.
My early work was noise control on machinery and factories. That gave me a good analytical basis. I started consulting when I was still at the university. When I started my own firm, I didn’t have mentors or even a consulting partner. I had to learn things like environmental noise, community noise, and architectural acoustics on my own. I decided early to narrow the scope — I couldn’t do everything like sound systems and advanced vibration, so I had to narrow it down so I could be an expert in those things.
Back then, I wasn’t in a position that I am today. I didn’t have the experience, so I had to analyze everything. I wouldn’t make recommendations without carefully analyzing them, and I wrote reports so clients would understand why.
There were some lean years in the 80s, but we stuck it ou,t and by the 90s, things started looking up, and here we are today.”
Are there any super fun, memorable, or unusual projects?
“A lot of strange things. One project in ’83, someone measured 65 dBA as an almost steady level in a rural area and said it was road noise from a two-lane rural road. Didn’t sound right. Not something you would expect. When I arrived to investigate, it was quiet. Locals said the guy was there in winter; the area had heavy concentrations of wild geese. Now you can guess who was the culprit of the 65 dBA levels. People make measurements without paying attention to what they’re measuring.
I worked on airport noise cases early on in Charlotte and Raleigh-Durham. I was working for the neighbors, and Andy Harris was the airport consultant. We developed a strong respect for each other, and I later teamed with him on a project in our last consulting years.
I worked on a potential problem at a 1740s-era home that was the home to two signers of the Declaration of Independence. I got to sleep in their bedroom. I also did work on projects for the White House, which I can’t talk about here, but those were unique experiences.
In later years, after Joe Bridger joined me and started concentrating on architectural projects, I moved toward environmental and community noise: power plants, racetracks, amphitheaters, cooling towers, chillers; all kinds of outdoor environmental noise issues and outdoor-indoor isolation issues.”
The technology we use in Acoustics has dramatically changed over the years. What changes have you witnessed over the course of your career?
“When I started at NASA, we used graphic level recorders and water manometers. In the 70s, for graduate work, we used heavy B&K meters — twisting knobs, reading one frequency at a time. You had to carry the meter and a clipboard to take notes, everywhere through textile mills. You then write a report with a pencil and hand it to a typist.
As I started consulting, we didn’t have personal computers or e-mail. You got your mail at a post office box early in the morning because that’s how you communicated with everyone. Needless to say, you couldn’t get nearly as much work done.
Later, we moved to a Larson Davis 800B meter, which could be hooked to a mini-computer and data cassette. We basically guinea-pigged those meters. We constantly kept finding problems, and Larson Davis was very responsive. Once, a dog pulled the power cord out of the mete,r and I asked Larson Davis why it couldn’t automatically switch to battery when this happens? The next day, we had a new meter that could do exactly that.
We then got a RadioShack computer, I think around 1983, with two floppy disks. One with an operating system and one with 40 kilobytes of data storage.
Then, in the early 90s, I bought a Larson Davis 2800 — 13 pounds, about the same as my newborn son. Everything I used to haul in a station wagon or a pickup truck in grad school could be done with that instrument. FFT, recording, third octave, just about anything I wanted to do.
Of course, today you can do it in one pound or less, even on a cell phone with the right microphone. People today don’t appreciate how difficult it was to get and process data in those earlier days. We have come a long way.”
In talking with clients or other acoustical professionals, are there any common misconceptions about Acoustics that you often come across?
“Everybody gets confused over sound blockage and sound absorption. It’s always difficult to get people to understand the effects of why you hear things at night and not necessarily during the middle of the afternoon from distant sources. People don’t understand atmospheric effects.
That reminds me of an interesting project. I had a case where a neighbor suddenly heard noise from an industrial plant they hadn’t heard before. The plant hadn’t changed anything, making noise. There was a building between the industrial plant and the resident that recently shut down. So, I asked if anything inside that building was making any noise. The plant said no.
So, no one knew why they could suddenly hear this noise. They heard it in the morning and sometimes in the evening. Okay, atmospheric effects. So, I started explaining the concept to them. After a while of me talking, the plant engineer says that the building that recently shut down didn’t make any noise, but it generated a lot of heat. See where I am going with this?
The building in between created a heat barrier that would move the sound up and away. Without that thermal barrier, the resident could hear the noise from the industrial plant. I hadn’t thought about the building giving off heat, but when I explained the atmospheric effects to the client, he figured it out.
Another thing I normally encounter is that people don’t understand the nonlinearity of perception. If you want it half as loud, you eliminate 90% of the sound. And whether you can understand speech depends not just on level but on background interference. So many people don’t understand that.
Low-frequency wavelengths are long and hard to control as compared to high frequencies, which can lead to problems. Take a hotel room with a PTAC unit at one end and a headboard at the other. The right dimension can put a 60 Hz maximum on your pillow. I’ve experienced that several times.”
Let’s switch gears a little bit. Would you like to talk about your involvement in ASTM and developing Noise standards?
“I joined ASTM E33 in 1985 when Rich Pepin got me to come to a meeting. Rich and Bill Cavanaugh were arm-twisters who got a lot of people involved in a lot of things.
At first, I was just another member, casting votes and discussing. The leaders then were Alf Warnock, Ron Mulder, and Howard Kingsbury. Around 2000–2005, I started taking more responsibility and was asked to take over leadership of E336. Alf and Trevor Nightingale were doing a major overhaul, but didn’t want to be the task group chair, so I was put in that position.
E336 is my main standard, but occasionally I have led others to. I was asked to chair the research subcommittee and was vice-chair of the E33 committee for 12 years. I was more or less asked not become the chair of E33 so I could concentrate on the standards instead of the administrative problems of the chair.
ASTM functions because good, qualified people are a part of it, to write standards, review them, critique them, debate them with one another, and come up with the best results we can. I am convinced that the ASTM methodology is about the best that it can be.
Anyone with an interest can join, participate, and be heard. As long as you participate, you will be heard. If you make a good argument, it will be heard and debated. And as a result, we put out good documents.
About 20 years ago, I was worried, where are the young people? But in recent years, I have been pleasantly surprised by the quality of young people such as yourself. I think they can keep things going.”
Do you think there are some underexplored or underutilized ideas in Acoustics today?
“I’m a little at a loss because I haven’t been heavily active in field consulting for a few years. But I know the office acquired acoustic cameras, and they’ve been useful. Others are finding good applications for them.
There is a ripe opportunity to develop better computer models — room acoustics using the wave equation, outdoor propagation modeling. I’m not satisfied with the quality of what we have today. With the programs we have today, there are a lot of assumptions that don’t always hold.
It takes people who know acoustics and computer programming. I’ve seen kids who were primarily programmers digging into acoustics enough to solve wave equations. That could be an advance.”
Do you have any advice for the next generation of acousticians?
“Make use of technology, but remember the basics. Understand what you are doing. Develop your ability to communicate. That basic understanding and the ability to communicate it to people who won’t know acoustics are very important.
Don’t get dependent on Artificial Intelligence answers. They can throw you down the wrong path. Same with books; every book I have ever read has had a mistake or two in it somewhere. Don’t take everything for granted.
I worry that some people today may not have had enough real training in analytics. There’s a lot you can do in architectural acoustics with a simple understanding. But if you encounter an unusual problem, you won’t know what’s going on unless you can go back to basics and figure it out.”
