The Hum is a low, faint rumble or murmur that is in some places barely perceptible and in other places easily noticed. I noticed the Hum years ago, and have long pondered its sources. I do not have a PhD in acoustics, but I’m a mechanical engineer and studied acoustics in college, and later at Denison Hydraulics I conducted sound tests of hydraulic pumps in special rooms with sensitive measuring equipment. This is an important part of engineering; the study and reduction of noise. Pumps, gears, engines, fans, saws…engineers can make them quieter, but there is a limit. As one senior colleague remarked, “A 200 horse power (axial piston) pump can never be made as quiet as a refrigerator, unless you bury it with ten tons of wet sand!”
In short, the Hum is the sum of all acoustic energy within earshot. Within earshot can be a surprisingly long distance and include thousands of sources! Each source contributes very little, but all taken together is sufficient to produce a ubiquitous sound without a discernable source direction. It is noise pollution similar to light pollution. A single street light does not cause an orange glow on the clouds, but many thousands together do. I doubt the Hum is solely electrical, or geothermal, or flowing magma, cell phone towers, tinnitus, cosmic radiation, ocean waves, gravity waves, aliens, or HAARP. To hear the Hum best, lie in bed at night (or go to the basement) with all windows and doors shut, and all noise sources in the house turned off, wait until the refrigerator/furnace/AC cycles off. Search your hearing at its faintest, way down. You might think, “That’s just the din of traffic in the distance.” That’s the Hum! Oddly, the same can be heard way out in the country, seemingly far out enough to escape it. There is something about that frequency range that travels well.
The main contributor is motor traffic, but includes all vehicles, trains, planes, ships, motorcycles, roof fans, thunder, humming electrical wires, wind, crashing waves, helicopters, mowers, blowers, etc. All this noise together is what’s called pink noise; it spans a broad range of frequencies. When in a quiet home the walls filter out masking noise and higher frequencies, but lower frequencies penetrate. When outside the Hum is still in the ears but because of other sounds the Hum is masked, much like odor masking. The stronger drown out the weaker. Most do not recognize it as traffic noise because the acoustic signature is altered with the higher frequencies absent. We’ve all had the experience of waiting on a red light and another car pulls up with powerful stereo thumping rap music. The car traps the high notes but radiates the bass notes in a way that you can’t recognize the song but you can hear the noise.
Another example of such a thing: when I camp in a quiet woods thick with mosquitoes, I notice that I can not identify the sound of any particular mosquito; indeed I can not perceive a mosquito 10 feet away. But because there are so many of them all around me they I hear a noticeable wing buzz from no particular direction. Similar to the cosmic background radiation.
Wherever I go I listen for the Hum and with a map take note of the distance to the closest 50+ mph roads. It is always louder the closer I am to interstates, even at 3 a.m. For example, when camping in the woods in Geels, Michigan I noticed the Hum coming from the South West, the average direction of I-75, six miles distant. Recall that a freeway is a line source of noise, not a point source like a leaf blower.
The Hum is often described as the sound of a diesel engine, which is indeed the major contributor. But heavy trucks at freeway speed are sources for more than just engine noise (1). Consider a large tractor/trailer pulling a typical 53-foot trailer whose sides and top are made of a thin, stiff board material. These panels are big drum heads, and are excited by the engine vibes, the turbulent air and roadway irregularities. Stand in view of a freeway with a gap in the pavement. Note that when a big rig passes over the gap, the panels sound off like mega woofers. Boom-boom! That is a lot of acoustic energy, and it travels for miles. Bridges are rumble sources too, just stand on a bridge during traffic and you can feel it vibrate underfoot.
Moving vehicles emit lots of tire noise. Think of a trombone and its bell shape on the end to bring the sound out. Remove the bell and the instrument is much quieter. That’s acoustic impedance and there’s an equation to describe it. Now think of a tire on the road, it has two bells of sorts, due to its round shape. 18 tires are 36 bells, and now think of the thousands of big rigs on the roads. A major source of noise near my house is Walton Blvd. It is a 45 mph choppy concrete road with lots of uneven cracks which cause a cacophony of “whap-whap-whap” tire noise. (2)
Have you noticed how loud light piston powered propeller aircraft are? The noise is mainly from the prop tips, they are moving so fast. Turboprop aircraft really put a hum in the sky with their big props thrashing the air. A business jet climbing out after take off is as loud as thunder. At any moment there are hundreds of planes in the sky ferrying people around the globe. They plow through the air over 500 mph with their big engines roaring. That they are so high lets their sound carry quite far. You might think that not even a shred of noise from a 767 cruising above Ohio could reach my ears in Michigan, but it is another contributor to the Hum.
Have you ever been in an office building with an AC on the roof that is a bit out of balance? It makes the whole building rumble and shake a bit, you can hear it and feel it in the floor. You sort of acclimate to it, but it is annoying and hurts concentration.
And have you ever been driving on the freeway and open one rear window with the others closed? The cabin pressure buffets so strongly it hurts the ears. That is a Helmholtz resonator and there is an equation for it, too. Oddly, the front windows don’t cause the same thing.
I love the distant rumble of thunder. At any time there are hundreds of thunder storms on earth; the sound of thunder can travel for many miles and especially well across water, which covers ¾ of the earth. Globally there are about 50 lightning strikes per second, that is a lot of acoustic energy. I suspect that distant thunder might be the main source of the Hum in locations far enough from traffic.
The Hum is near the threshold of hearing, under 10 decibels and is easily masked. In some places it is so low that a person’s own heart beat will make it seem to throb. It is impossible (?) to measure with instruments. Human hearing is very sensitive, more so than any microphone at such low levels. That is one element that makes it so mysterious; the Hum is so difficult to examine scientifically and that puts it on the level of subjective perception. We can not do experiments on the Hum, such as eliminating certain sources of noise and then remeasuring.
It is the price we pay for living in a motorized world. I would like to travel around the globe and find where I can or not hear the Hum. However, contrary to folk lore, the Hum can not cause nausea, ear aches, nose bleeds, dizziness or anything but annoyance or nocebo (3). Anything causing such symptoms is much more than a faint hum. I have long advocated a quieter world. Let us install more concrete noise barriers along freeways, and muffle the noisiest offenders – but alas, there is only so much we can do and still have usable machinery. Large jet aircraft are much quieter in recent decades, but it is nearly impossible to muffle a mower blade or open prop.
So to experience an ultra low noise environment, twice I closed myself in to the anechoic (without echo) chamber at Ohio State University. This chamber has thick, massive walls covered with noise absorbing material. It is amazingly silent within, no Hum. Another experiment I conducted is to pay a visit to a busy interstate on a night without wind, far from the city, preferably in an agricultural area. I used I-70 about 25 miles West of Columbus, Ohio. It is best to do this in a season without crickets chirping. Take note of the sounds of passing traffic. Then, back off a kilometer and listen again. Repeat repeatedly and note how the noise diminishes to a dull murmur, but still identifiable as the freeway under study. (4)(5)
(1) It is not only the exhaust that is the noise source! I have noticed the inlet can be surprisingly loud. I used to rent a farm house a mile from a conveyor belt factory near Marysville, Ohio. The factory air compressor must have had an outdoor inlet, because I could hear it cycle on. Bububububububububububub. Another example; when I had a two-stroke trail bike I had the seat and air filter off for cleaning. Out of curiosity I started the bike and blipped the throttle a few times. The pulsing inlet noise from the air box was really loud! BWWAAAAH! Same thing for diesel engines; but they have an unrestricted inlet (no throttle plate) and send a lot of inlet pulsing noise out past the air filter. Here is how to see it literally under your own nose. Take a tall drinking glass and fill it 1/3 of beverage. Put the cup to your closed mouth preparing to drink, hold it still to let the liquid settle down. Rapidly draw in a sip and rapidly stop. Notice this causes a surface wave to travel away from the mouth toward the bottom of the glass. It was caused by sucking, not blowing out liquid, analogous to an engine air inlet. It also happens on the inlet of a hydraulic pump, which is shaped like a trombone bell for easy inflow. We use computer programs to model and minimize the fluid pulsing. Loud pumps do not sell well.
(2) What really annoys me is HD V-twin bikes with straight pipes who roll up to a stop, pull in the clutch and rev the engine to announce their presence and imagined machismo. That is anti social noise and it makes me want to torture their ears in return with a 44 magnum blank gun. Many places that ban motorcycles are just trying to ban painful noise.
(3) A nocebo (Latin for "I will harm") is something that should be ineffective but which causes symptoms of ill health. A nocebo effect is an ill effect caused by the suggestion or belief that something is harmful. The term 'nocebo' became popular in the 1990s. Prior to that, both pleasant and harmful effects thought to be due to the power of suggestion were usually referred to as being due to the placebo effect.
(4) I have long observed that 2- and 3-axle cargo delivery trucks commonly have undersized or leaking rusted mufflers and are offensively loud, such that I have to roll up my car windows to avoid pain when one is nearby.
(5) Each spring my friend visits Madison, Indiana on the Ohio River for a spicy food festival. He camps in Clifty State Park with its lovely trails and waterfalls. Also on the river nearby is Clifty Creek power station, which he describes as constantly sounding like a “droning C-130” airplane. It must be water pumps, turbines, fans, and generators. I want to camp there, but not listen to that. Years ago, we made a list of fictitious noise sources that would bother a quiet camper, below.
Lynyrd Skynyrd tribute 100 keg rave, free kazoos!
Steam train "tug-o-war" competition and Civil War Gatling gun demonstration
Range Warden’s "home on the downrange" artillery accuracy competition
Buckeye "nitro burners" radio control car night race
Lumberjack chain saw competition with "Hosea's hundred hammers" Amish barn raising
Alpine yodelers hand bell and hound baying chorus wsg Evangelical bagpipers morning praise
”Jolly Green Giant” helicopter rotor balancing workshop
“Dawn’s first light” crop dusting school
Peterbilt truck jake brake adjustment workshop
WW2 air raid siren show and auction
Confederate air force “Screamin’ Stuka” dive bombing reenactment