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Crosscurrents is our award-winning radio news magazine, broadcasting Mondays through Thursdays at 5 p.m. on 91.7 FM. We make joyful, informative stories that engage people across the economic, social, and cultural divides in our community. Listen to full episodes at kalw.org/crosscurrents

The Hum, a worldwide mystery sound explained

Worldwide Hum Map.png
The World Hum Map and Database
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An image of the Worldwide Hum Map, a catalogue of reports of The Hum around the world.

The Hum is a mysterious low pitch noise that bothers up to 2% of people worldwide. It’s been annoying residents in San Francisco's Sunset District, where recently there’s been an outbreak of reports. In this story, we demystifies this strange global phenomenon.

Dale’s House Vibrates

Last year I did a story about the bridge that was called “Tracking Down a Mysterious Hum in San Francisco.” It was that story which inspired Dale Tutaj to reach out to me about his own mysterious hum. Dale lives in the Sunset District. Over the phone, he tells me that this is the third house he’s lived in in the neighborhood that hums and vibrates. It’s annoying, he says, and keeps him and his young daughters from getting sleep.

I go to visit Dale, and he takes me down into his garage to show me “the hum.” Huddled in the corner, we watch graphs form on his laptop screen, measurements he’s taken of the tremors in his home. The graphs are being generated by a ‘data logger,’ an expensive vibration measuring device. Dale’s an engineer by trade (he works in energy efficiency), so this kind of technical stuff comes naturally. He takes the graphs the data logger generates, analyzes them, and puts them on a website he made to raise awareness and find support. The data logger is attached to one of his water pipes, where he suspects the hum is coming from, but he’s not sure.

Dale with his Vibration Data Logger.jpeg
Nikolas Harter
Dale shows me his data logger, the device he's been using to try to measure his hum.

“It seems like it’s stronger on the street side, the intensity or magnitude,” Dale says. “But whenever I think it’s obvious, I take a measurement that’s counter to that. It’s just confusing, baffling.”

Dale tells me he comes down here in the middle of the night to chase the vibration/hum when it keeps him up, which is often. I noticed the bags under his eyes the moment I walked in. His wife, Cristina, can’t hear or feel it herself. But she’s convinced, like Dale, that it still affects her, and their kids. Whenever it starts, their three-year-old daughter wakes up, and her heart rate monitor spikes. Dale thinks it’s affecting their neighbors too.

“We’ve seen that at every place,” he says. “Bathroom lights come on from the neighbor’s building, or across the street they always have their TV on early in the morning on those really bad days. It seems so obvious.”

There are a handful of people in the Sunset District who also hear a hum like Dale. They’ve found Dale through his website and the social media app Nextdoor. But most people he talks with don’t take it that seriously.

Dale's Map of Hum Hearers.png
Dale Tutaj
A map Dale made of all the people who reported to him that they hear a similar hum in Sunset District.

Chasing The Hum

I agree to look into this hum for Dale. I want to find out if other people in the neighborhood really are experiencing the same thing as him. To learn more I head over to the house of Nelson Saarni, who met Dale through NextDoor.

Nelson’s a musician, so he’s actually made recordings of “the hum.” On his couch, I put on headphones, and plug them into an eight-track recorder on his coffee table. He plays it back, and at max volume I can just barely hear something that sounds like some kind of ominous creature breathing.

Then Nelson shows me the post he made on Nextdoor, in which he describes the hum he hears and asks if anyone else in the hood is hearing something similar.

“I looked at it a few days later, and just saw this tremendous chain of replies,” Nelson says. “And I didn't even read them. ‘Oh, God,’ I thought.”

Nelson’s post got more than 100 comments from about 60 different people. I end up personally speaking with nine of them, and they all do describe experiencing something similar. They’re usually the only ones in the house who hear it. They say it’s only heard indoors, even in some rooms better than others, and it’s low-pitched, like a distant truck idling. Some also feel a vibration, like Dale, but most just hear a hum. However, they all hear it at different times of day, and at varying intensities. Which tells me that while these folks all seem to be hearing some sort of similar hum, it’s probably not coming from the same source.

“I can’t imagine something in nature making a sound like this.”
Nelson Saarni

Back at Nelson’s house, he holds his bass guitar up in the air to show me how it interacts with electromagnetic fields, which he suspects are the source of his hum. The guitar makes a low buzzing sound, like “the hum,” but louder and more electric. The power lines and plugged-in appliances around us all generate their own electromagnetic field, which then registers as sound through the pickups in Nelson’s guitar.

“This is essentially acting like an antenna,” Nelson says, as he waves his guitar in the air. “It’s interacting with the magnetic field of our world, our electronic world.”

Wherever his hum is coming from, Nelson’s sure that it’s just another consequence of living in a built environment. This world we’ve made that runs on engines and electric current.

“I can’t imagine something in nature making a sound like this,” Nelson says. And he’s right.

This is where the story gets weird.

As it turns out, there have been numerous “hums” reported all over the country, and all over the post-industrial world.

The first “hum” that made big news was in Bristol, an industrial town in Britain. That was way back in the 1970’s. There have been others in Kokomo, Indiana, in Denmark, and more recently there was a hum in Windsor, Ontario (Canada). That hum stopped after a nearby steel mill was closed. I found this one University of New Mexico study, which says that up to 2% of people in any populated area may be able to hear a hum. They all describe the same thing, a low-pitched sound that’s only heard indoors, and that sounds like a distant piece of machinery, or an engine idling.

“And so in reading these accounts, it's like, ‘Oh, I hear the same thing,’” Nelson reflects.

I ask him if that’s comforting, and he says, “Maybe it's a little disconcerting, because what was something that was just one of the minor mysteries of life is now like this major international scandal. Maybe there's this huge problem. Maybe this is driving people to insanity.”

Nelson In Closet Taking Measurements.jpeg
Nikolas Harter
The closet is the place in his house where Nelson hears his hum the loudest, and where he tries to capture it with his microphones.

The international nature of The Hum hits home for me after I do a shoutout on Reddit for an Acoustical Engineer, and I get a response from Alex Mendoza, who lives in Sydney, Australia. I wanted to speak with an expert, to help me wrap my mind around all of this, and Alex turned out to be the perfect person. He loves acoustical mysteries. He works for a firm that gets hired out to investigate and measure noise levels, and mostly it’s pretty banal. They help companies and other entities make sure that whatever construction or activities they’re doing aren’t surpassing legal noise limits. Occasionally, however, he gets the chance to investigate a mystery hum like what Dale Tutaj and Nelson Saarni are experiencing. Like one time, his crew was called out to investigate a hum in someone’s home near a power substation, but when they went out with their microphones and meters, they couldn’t detect anything. So Alex gave the homeowner his card and said, “Call me when you hear it again.”

“And so maybe a few nights or a week later, he called me back and said, it's happening right now. So I jumped in the car around midnight, drove down south, took some measurements, and there it was.”

Even though he can’t hear it, Alex can see the hum on his meter. So he takes his microphone and he follows the sound. He does that for two or three hours, and eventually tracks the source of the hum to a kitchen exhaust fan.

Exhaust Fan In Australia.png
Google Maps
An exhaust fan in Sydney, Australia that caused a low-frequency hum.

He sent me a picture of this “exhaust fan,” it’s just a typical vent that you might see snaking up the side of any building. When air moves through a vent like this, it can be like blowing over the top of a gigantic empty glass bottle; it can create a sound so low-pitched that it’s barely in the range of human hearing. However, even though it’s barely within the range of human hearing, the sound travels. This vent was halfway down the block and had to pass through a brick wall to penetrate into the affected homeowner’s bedroom.

One thing I noticed with almost everyone I spoke with, and the reports I read online: the hum is quiet, and heard indoors, but not outdoors. It seems to be able to travel a long way despite being undetectable outside. What I learned from Alex is that this has to do with the physics of sound.

Sound is made up of waves that travel through the air. Imagine you drop something into a bathtub, and you see ripples bounce back and forth. That’s like how sound waves bounce around a room. A high-pitch (or high-frequency) sound would be like dropping a stone into the tub, you get little ripples. A low-pitch (or low-frequency) sound would be like dropping a bowling bowl into the tub, you get a big wave. The small ripples get stopped by the edge of the tub. The bigger wave— that could slosh right over.

That’s what’s happening with these hums, like the noise coming out of that vent. Since it’s creating a low-frequency sound wave, it more easily spills over, or more accurately “pierces,” into this person’s room. A “big” or “small” wave has nothing to do with volume in this case. Even if it’s loud, a high-pitch sound is still easier to block out than a quiet low-pitch sound. This creates a filtering effect which is why these low-frequency hums are only heard indoors. They’re covered up by all the higher frequency sounds outside, but then inside, where the higher frequencies get blocked out by the walls, low-frequency sounds become more noticeable.

That’s also why low-frequency noise has been shown to be particularly irritating, because of its throbbing quality, and because it’s virtually impossible to block out with earplugs or insulation.

In both of the cases Alex told me about where he successfully chased down one of these low-frequency hums, it was coming from an exhaust fan. At first this got me excited! I thought to myself, Maybe I ought to go start looking for exhaust fans around Dale Tutaj and Nelson Saarni’s houses! Until Alex confirms what Nelson suspected, which is that this type of sound can be generated by any number of things.

“Anything from industrial facilities, manufacturing plants, industrial machinery, ventilation plan, as well as you've got all kinds of ducts and exhaust pipes and motors, and things that are happening all the time everywhere, making all kinds of noise at all different types of frequencies. Traffic noise— even like you've experienced [in San Francisco] with the bridge itself,” Alex says. “Essentially, if you’re within the built environment, especially in cities, you’re bathed in noise 24 hours a day. It never goes away.”

In addition to these examples, anything with moving parts that’s plugged in, like a fan or small motor inside of an appliance, can vibrate and induce a low-frequency hum in the right conditions.

Picking sounds out of this sonic soup can be difficult; it can take a team of acoustical engineers like Alex days, sometimes weeks, to locate the source of a hum. This can easily cost thousands of dollars. That’s why Alex’s firm is usually hired by companies and businesses, and not the affected residents themselves. That story he told, about getting up in the middle of the night to chase down someone’s hum— he wasn’t able for liability reasons to even tell that homeowner where their hum was coming from.

“Because we weren't engaged by him,” Alex explains. “We were engaged by the power company because he had complained to the power company. And so whether he can talk to them and get the report that I did is a different story. It's out of my hands.” Alex says that the company just wants to know whether what they’re doing is legal. “[The power company] just wants an answer whether they need to do something or not.”

Looking For The Department of Strange and Annoying Sounds

There’s something similar happening in Sunset District, San Francisco. The low-frequency hums that folks hear there could be coming from the same source, or could be from all different sources. To know for sure, you’d have to go investigate each individual hum, which is prohibitively expensive for most people.

So if you are in this 2% of people who are susceptible, and one day you start to hear an annoying hum in your house and you can’t figure out where it’s coming from, what are you supposed to do?

To be clear, most of the folks I spoke with weren’t as annoyed by their hums as Dale Tutaj, who went sleepless for years. Most people will tune it out with white noise, or it just becomes part of their life, something they learn not to bring up with house guests because no one else can hear it, and it makes them seem crazy. However, Dale actually had complained to the city about it.

Dale submitted a noise complaint. How those work in San Francisco is you call 3-1-1, you get connected with a dispatcher, and then the dispatcher will send your complaint to the appropriate department according to what the complaint is about. If you’re complaining that your neighbor is blasting music too loud, then you get sent to the police non-emergency line. If you complain about noise from airplanes, they’ll forward you to the airport, and so on and so forth. There is no one whose job it is to hunt down mystery sounds for people. You can’t just call 3-1-1 and tell them, “I’m bothered by this noise but I don’t know where it’s coming from.”

Dale suspected that the source of his vibration/hum was his water pipes, and so when he complained to the city, the city dispatched a technician from the water department to investigate his pipes. They determined that the pipes were working normally, and closed the case.

Dale also made a website to raise awareness, contacted his city councilperson, contacted the EPA, the health department, talked to countless people about it, and reached out to different publications. None of it got him anywhere, and so, he moved. As in, he left the state entirely and went with his family to Wisconsin. I have since checked on him, to see how he is, and if he still hears a hum.

“It's a pretty drastic difference really,” Dale says. “I mean, like the sleep deprivation side of it, that has completely gone away. I feel like I'm twice as productive as I was before. Words are more accessible. I really felt like I was pretty miserable before, and it's crazy, like, my daughter's wake up and they're just happy.” Dale laughs, and it’s nice to hear him at ease.

I tell Dale that that must be a huge relief, and he tells me it is, and also that it’s frustrating. He feels like he and his family were traumatized.

None of the other folks I spoke with complained to the city like Dale. I asked them why, and they said either it didn’t bother them enough, or they didn’t think the city would do anything about it, or both. A couple of them said that they’d read on Nextdoor about how difficult it had been for Dale to get help and that that had informed their decision not to reach out.

They’re correct that there’s not much the city can or will do. However, I am obligated to say that San Francisco (true to its reputation) is still progressive on this issue. The San Francisco noise ordinance establishes an acceptable ambient level for low-frequency noise and says that you can’t go a certain amount above that level. I looked up the noise ordinances for San Jose, Sacramento, Los Angeles, and San Diego— not a single one even mentions low-frequency noise.

Many cities use a “plainly audible” standard for enforcement of noise at all frequencies. That standard states that if a sound is easily heard at a certain distance, then it should be mitigated. Cities like that standard because it can be enforced by any police officer. You don’t need highly trained people like Alex Mendoza, or expensive equipment. It’s cheap to enforce. Unfortunately, a “plainly audible” standard is not very helpful for a noise that only 2% of people can hear.

While it’s more comprehensive than a “plainly audible” standard, San Francisco law doesn’t take into account how low-frequency noise can be annoying even within ambient levels. In general in this country, we’re behind the research when it comes to noise enforcement, and we don’t provide the necessary resources to help people like Dale. Until that changes, people are left on their own to try to find the source of their hums. Oftentimes they won’t be able to figure it out.

The reality is that people who are impacted often just have to cope the best they can. So I went to talk with somebody who can help them do that.

Habituation, and The Hum Explained

I went to speak with Doctor Tracy Peck. She’s an audiologist and director of the Hearing and Speech Center in San Francisco. She specializes in the treatment of tinnitus, and she told me she sees many parallels between low-frequency hum hearers and sufferers of tinnitus.

Before I go further, I should say that I know from my research that folks don’t necessarily like to be told to seek treatment for their hums, which is understandable, and I’m not recommending tinnitus treatment per se. What Dr. Peck gave me, rather, was just a deeper understanding of how the brain works, which could be helpful. She tells me that once we start to pay attention to a new sound, say like, a hum you just noticed—

“The perception of that sound can actually be perceived as being louder. If there is that connection between it sounding amplified, and that emotional connection is a negative connection, where it's a fearful or worrisome reaction, it tends to create this… negative feedback loop.”

While it’s likely a combination of factors, this is the strongest explanation we have for one of the biggest mysteries surrounding the hum: why some people can hear it while others in the very same household will not (according to studies out of the University of Salford in England). When the hum triggers an emotional response rather than getting tuned out, that could be the beginning of this process of internal amplification. That process is difficult to interrupt, both with low-frequency hums and tinnitus.

Even many doctors don’t know that it’s possible, Tracy explains. “When I was reading some of the research that a lot of people have gone to see different professionals for both of these things, and been either kind of dismissed, or told there's nothing [they] can do about it. It's not true, there actually are a lot of things you can do.”

The reason that many doctors will say there is no treatment for tinnitus is that, for most forms of the disease, there is no drug one can take, there is no surgery one can have that will get rid of it. Similar to a hard-to-locate hum, you cannot fix the source of the issue. However, you can manage the symptoms to the point where it becomes negligible. They do that through a process called habituation.

“Habituation is going through this retraining process to help the brain learn to filter out whatever the bothersome sound is,” Dr. Peck explains. “We habituate to sounds all the time. If you move to a house that's near the airport, and when you first move in, the sound of those planes taking off is pretty loud, right? Pretty noticeable, pretty novel at first. And then over time, your brain learns, ‘Okay, that's not important. It's not dangerous, it's not something I need to alert to. I'm going to filter it out,’ and it's going to go into the background.”

It’s the same concept as why you don’t feel the clothes on your skin all day long. Our brains tune out, or habituate, non-important information, and amplify what we need to pay attention to. Dr. Peck explains that this goes back to our caveman days.

“Like if you hear a rustle in the bush, is that something that is going to try to come and eat you, and you need to run away for safety? Or is it possibly something that is of interest and you want to actually go towards it. So there are emotional connections to sounds.”

We can’t always control our emotional responses, especially for low-frequency noise, which, as I’ve mentioned, has been shown to be particularly annoying. It’s important to note that it’s not a person’s fault if they’re bothered by a hum. Research has shown that dismissing hum hearers can increase their emotional response to the sound, which could literally make the sound louder. That’s why habituation treatments exist: to retrain the brain to tune it out.

The patients that Dr. Peck sees have in fact learned to get their brains to filter out their tinnitus to the point where it is no longer bothersome. The sound becomes barely perceptible unless they pay attention to it.

To explain briefly, habituation breaks down into two elements. First, there’s mixing. Mixing is where you mix the hum (or other noise) you hear with another more pleasant sound (also referred to as “masking”). That can retrain your brain to then tune both of those sounds out. That’s easier to do on your own at home. Second, there’s mindfulness, which is a little more difficult to do on your own. It’s basically just noticing one’s thoughts around the sound, and trying to have a different emotional response to it.

Here are a few articles that list apps that can help someone habituate. There are many apps out there, and one study suggests that the best app would depend on the person’s needs. Some focus more on the sound mixing/masking, while others are focused more on the cognitive or mindful aspect.

You can also try using a white-noise machine (or app). I spoke with a few people who said that helped, but it’s not a long-term fix like habituation is. If you’re experiencing a hum in your life and it doesn’t bother you, that’s great, don’t worry about it.

If you are really bothered by one of these low-frequency hums, and you’ve already given some effort to discover the source, I wouldn’t necessarily suggest you go visit an audiologist like Dr. Peck. If you do, be patient, and come prepared to explain, and advocate for, yourself. If they’re open to it, I’d recommend sharing with them an article such as this one.

Here’s a link to a resource that can help you locate the source of your hum. Nearby vents, compressors, electrical motors, and pumps are common sources of hums.

If you’re interested in finding out more, being connected with other hum-hearers, or would like assistance, don’t hesitate to reach out! You can email me at nik.harter@gmail.com, just put “hum” somewhere in the subject line. I’d be happy to hear from you.