Why do cities “hum”

If you live in a big city such as New York or Chicago, you might have noticed the city actually has a non-stop background humming noise. This was first discovered in 1973, around the same time awareness about noise pollution started to rise, and it’s known as simply “the hum” (and no, this is not the title of a sci-fi movie).

If you’ve been living there long enough you might actually consider it silence, but when you pay attention to it, it’s not. 

Cities are noisy for a variety of reasons, some of the most obvious ones being construction, emergency vehicles blasting their sirens, bars playing loud music, etc. But even when you remove all that, there’s still something there…

So what is that humming noise that never ceases? Where does it come from?

A/C units and forced ventilation

A/C units and forced ventilation systems are the main causes of this non-stop city noise. 

Unlike construction or an ambulance, which we can not only hear but also clearly see and identify where the noise is coming from, A/C units and forced ventilation systems are trickier, because even though we might be able to see them, it’s impossible to tell whether they are on or not, and therefore whether they are making any noise or not.

I’m not talking about your own A/C unit, which you definitely know when it’s on or off, and can also be an important source of noise at home. No. I’m not even talking about your neighbor’s unit. I’m talking about the sum of all the units of your neighborhood, from the building around you, and even some that aren’t that close to you.

A/C units

There are several different types of A/C units, each of which has its own pros and cons when it comes to noise. All of them however cause a certain amount of vibrations, which we perceive as noise.

Window A/C units are small independent units, and as such they should not generate big vibrations. That is not always true though, since window A/C units are usually one of the cheapest A/C options, and therefore can sometimes (not always) be very poorly constructed, which means they generate way more noise than they should.

That being said, even high-quality window A/C units generate some amount of noise, and on a hot summer night, when most of the units might be turned on, this creates a very special scenario in which an entire building becomes the source of this humming sound. The source, in this case, is not easily identifiable, you can’t look at the facade and say “It’s that one, that’s where the noise is coming from”, instead our senses get confused, as it seems as if the building itself was vibrating. And to a certain point, it is.

Central A/C units are a different story. These are bigger machines that usually live on the rooftop of a building.

Central A/C units are arguably better in many ways, including noise. This does not mean they are quiet, but the noise they produce is concentrated at one point, that is where the condenser is located, usually the rooftop. In very high buildings where the distance between the rooftop and the lower floors is too much to cover, these might also be located on intermediate technical floors.

This means that at street level central A/C units don’t have as big an impact as the window units, but on the other hand, they will impact higher floors of nearby buildings more directly.

Forced ventilation

Forced ventilation is necessary in many situations in order to extract air from an interior space to the outside and keep the inside livable and healthy. Examples of spaces that need forced ventilation are restaurant kitchens, closed or underground parkings, the subway, gyms, workshops where paints and/or other chemicals are used, etc.

In other countries even residential kitchens and bathrooms that don’t have a window facing outside are required to have forced ventilation, but not in the US. In the US forced ventilation is mostly necessary in industrial or retail spaces.

In a nutshell, forced ventilation machines take air from the inside and push it to the outside. They usually exhaust this air either through the facade of the business (on the ground floor), or they canalize it all the way to the rooftop and release it there. Or like with A/Cs, if the building is very high, at intermediate technical floors.

The thing is, when you start moving air around, it makes noise, just like wind. And when you do it through a conduct that noise only gets amplified. The faster air flows through a conduct the more noise it makes. You can easily test this yourself by switching your car’s climatization system from minimum airflow to the maximum.

In order to move large amounts of air quickly and quietly you’ll need huge conducts, but that’s expensive in more than one way. First, the larger the conduct the more material it’s required to build it. Second, the larger the conduct the more space it will take up, space which then cannot be used and cannot be rented or sold.

Other sources of humming noises in the city

There are other sources of continuous noise that contribute to “the hum” we experience in big cities.

Highways

Traffic noise is one thing, honks, a motorbike passing by at full speed, etc. All those types of noises are easily discernible to the human brain. The noise from regular streets is also quite intermittent, it comes at goes depending on the hour of the day or night, if a traffic light turns red or remains green, etc.

But there’s a particular type of traffic noise that’s different, the humming noise of a distant highway. If you live next to the highway, well things can get pretty noisy, and it’s easy to tell why. But as you move further away from it the direct noises disappear and turn into a lower, continuous hum that is arguably worse than traffic horns for a couple of hours a day.

This noise can be heard from miles away, and is one of the reasons why living on a higher floor is not always quieter, as higher floors pick up residual noise from much further away than lower floors.

Factories

Factories are another great example of background humming noise. Many factories work with chemicals and other hazardous materials, and therefore need to use forced extraction to ensure that their employees work in a safe environment. We’ve already seen how forced ventilation creates noise, but in this case at an industrial level.

The fact that most factories run 24/7 only makes things worse, since the background noise never stops and we become unable to consciously identify it, but our brain picks it up nonetheless, and never gets a break, which can lead to tiredness, stress, headaches, and more.

Ports

Ports can also be a source of many types of noise, but in particular large ships docked for long periods of time are a source of the type of humming noise we are covering in this article. If you think about it a large ship is basically a large building, the only difference being that it floats. But other than that they are the same, people live and work in them, and they have kitchens that need forced ventilation, rooms that need heating and A/C, etc. 

Seeing a cruise ship out your window might be nicer than seeing a factory, but when it comes to noise, they are pretty much the same.

Conclusion

Although there are a number of elements that contribute to what’s known as “the hum” of modern cities, A/Cs and forced ventilation are probably amongst the top responsible, and in my opinion are also some of the most dangerous, as the source of noise is often not apparent to us.

Excessive noise is generally speaking bad and detrimental to our health, but low-frequency humming noises that literally never stop are in many ways some of the worst. Sure, a hum won’t tear your eardrum like a loud explosion, but the long-term effects of humming noises can be just as severe, causing fatigue, stress, migraines and more.