Wind turbine infrasound as a weapon

Description: Industrial wind turbine infrasound is not the best weapon, but it is a weapon. This German video documents the harmful effects of the infrasound produced by industrial-sized wind turbines. The dangers of infrasound have been known since the 1980s when the U.S. military heavily invested in infrasound (below 20 Hz) as a weapon.


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Flight Level
March 19, 2019 3:12 am

Back then propeller synchronization on EMB 120 turboprops was unreliable and often written-up. The resulting low frequency “Wah-Wah” beat, unavoidable on the long run no matter how hard one tries on the levers, drove nuts everyone on board with passengers and even flight attendant vomiting in butter-smooth weather for no reason.
It also shortened the fuse of already grumpy captains, a situation all lower forms of life on board quickly learned to fear.

Crispin in Waterloo
Reply to  Flight Level
March 19, 2019 5:40 am

I certainly remember those. I have been on incredibly noise aircraft with two high-mounted real engines. Sitting between them while they established a beat frequency was awful. Plus the sound level was very high – enough it was hard to hear the flight attendants. I used to worry about the crews.

Reply to  Crispin in Waterloo
March 20, 2019 7:22 am

Multi-engine turbo props! They can sing you to sleep or drive you over the edge. Been aboard enough C 130s to know that.

Reply to  2hotel9
March 20, 2019 7:16 pm

The worst noise I’ve experienced is that as a passenger in the C-7 Caribou. The C-130 is whisper quiet by comparison. I likened it to being in a metal bin with a thousand demons banging on the sides with crowbars.

Reply to  Ubique
March 21, 2019 7:46 am

123s were pretty bad, too. Flew in an interior stripped DC3 that was pretty loud. C 130 only ever bothered me when engines were not sync-ed well, only last for a bit til pilots got her settled down. Like a throb that moved from side to side, not quite loud enough to hear but still right at the edge. Once squared away I love it, push back and go to sleep to that meaty thrum.

Reply to  Flight Level
March 19, 2019 9:23 am

The DC-9 used to do that as well. If you had a seat near the rear, somewhere between the two JT-8Ds you’d get that beat frequency for most of the cruise, and at other times.

Miserable thing.

Reply to  Flight Level
March 19, 2019 1:47 pm

SF340A model (ge ct7-5a engine) with dowty props had similar issues.
SB340B model (ge ct7-9b engine) with hamilton std props was better.

March 19, 2019 3:18 am

Green New Deal radicals have always been open about how people should live in cities (where they can be easily “controlled” and often for nefarious reasons) only and very few should live in the outside rural areas, only to tend the needs of those in the ruling class and serve the previously mentioned urban areas. The help accomplish this, using wind turbines could drive People from such areas via infrasound and the like. Wind turbines are harmful to human health and the environment…from killing raptors and bats, causing unnecessary erosion and infrastructure support (power lines, access roads, etc) and a slight upon the scenery, yes, the GND’ers are poised to put us all under a modern day version of Feudalism.

Reply to  Spuds
March 19, 2019 5:30 am

See the movie “Logan’s Run”…and never trust anyone over 30… Same basic message.

Coach Springer
Reply to  Spuds
March 19, 2019 5:52 am

Well, Obama was all about converting society to urban cells and condemning clingers. I sometimes refer to these wind fields as rusting monuments to urban vanity.

Patrick MJD
March 19, 2019 3:39 am

Next thing: Weaponising stupid! Oh, wait!

Pop Piasa
Reply to  Patrick MJD
March 19, 2019 8:08 am


Reply to  Patrick MJD
March 19, 2019 8:23 am

The trick with weaponizing stupid is you have to make it attractive and able to do dance moves 🙂

Grady Patterson
Reply to  LdB
March 19, 2019 2:33 pm

“I got it all together now
“with my very own Disco Clothes
“My short’s half-open to show you my chain
“and the spoon for up my nose …
~ Frank Zappa

Reply to  LdB
March 19, 2019 4:24 pm

Just tell them it has electrolytes.

Ron Long
March 19, 2019 3:48 am

Well, I know from personal experience that the impact of infrasound must be true. I stood underneath the very large wind turbines, in the complex NE of Casper, Wyoming, and felt nauseous as the giant blades turned. Wait, maybe the nausea was due to the stench of the dead birds littering the ground? People, go stand underneath one of these choppers and see for yourselves.

Tom Abbott
Reply to  Ron Long
March 19, 2019 5:37 am

“Well, I know from personal experience that the impact of infrasound must be true. I stood underneath the very large wind turbines, in the complex NE of Casper, Wyoming, and felt nauseous as the giant blades turned.”

I get nauseous just looking at a picture of windmills.

The windmill promoters never thought things out to their conclusions. Windmills and industrial solar are dead ends. It is obvious now to reasonably intelligent people and will be obvious to all at some point in time.

Meanwhile, we waste countless dollars going down this deadend road.

Look at all the trouble just a few Climategate Charlatans have caused for humanity! They have us going down the completely wrong path and have scared the daylights out of millions of people.

The CAGW Lie is a BIG Lie. The negative fallout from this BIG Lie is enormous.

Reply to  Tom Abbott
March 19, 2019 10:02 am

Solar may be viable in certain regions but wind is problematic on so many levels it beggars the imagination that anyone could be a wind advocate.

Reply to  Yawrate
March 20, 2019 3:25 pm

The only region solar is viable in is near Earth orbit and beyond. On planetary surface it is simply a niche market solution for very small, minor applications. Wind is a loser. Without substantial government subsidies wind generation craters. Big, deep hole. Wind requires coal fired, nuclear, hydro and gas fired generation facilities to maintain output to the grid. Take wind out of the equation and energy gets cheaper and more abundant. Oop, there it is.

March 19, 2019 4:18 am

Of course infrasound is weaponized – see the Scotsman report on 07.03.19 on the navy shadowing a Russian vomit weapon cruiser, which was targeting N.Sturgeon, the first minister!
Now, the British navy counters with brown-noise weapons used against Somali pirates. Don’t ask what these weapons do!
As noted, before Putin the seas were flat and since then the Russians have put waves everywhere.

Sea-sickness is just not what it used to be!

Ron Long
Reply to  bonbon
March 19, 2019 7:53 am

Bonbon, also note the Russians testing infrasound weapons against the US Embassy in Cuba, with devastating effects!

Reply to  Ron Long
March 19, 2019 2:57 pm

US Embassy staff in Cuba all went home because they were hearing what turned out to be crickets. LOL.

Well it made some good Russophobia headlines for a few weeks and fooled a few idiots.

Ron Long
Reply to  Greg
March 19, 2019 6:52 pm

Greg, when I search for reliable reports on what transpired with USA personnel in the Cuba Embassy I cannot find anything about “crickets”. What I can find is a recurring diagnosis, by reputable Doctors, of brain wave abnormalities of a sort normally associated with concussions. Combined with the sudden onset and pain level of the events, a sound weapon has been suggested, one either very directional of of unusual frequency. Can you please provide me with a reference to the “crickets” comment, thank you. “…fooled a few idiots” surely is not referring to me?

Geoff McPherson
Reply to  Greg
March 20, 2019 2:52 am

Incorrect check the recent published acoustical and medical literature

Geoff McPherson
Reply to  Ron Long
March 20, 2019 2:50 am

9kHz actually but the effect was horrible.
Probably used during Vietnam and underwater sound sources at 9kHz with suppressed harmonics well intentioned sure to warn dolphins of obstacles had dolphins aggressively attack the source…all published

Reply to  bonbon
March 19, 2019 2:31 pm

They found the brown note ?!?!

Reply to  Gamecock
March 19, 2019 3:54 pm

Here is a sound file at (called InfraSoundInterferenceChange1minute.mp3 stereo file) that I’ve made.
It is basically 66Hz tone for 1 minute and noise, after one minute the tone drops ~14%, however all through the is a 18Hz and below beat (or interference) signal. This sample is best heard on large speakers or headphones.

How does it make you feel. It made me feel nauseous making it (but I listened to it for nearly 1 hour while producing it!)

Reply to  tom0mason
March 19, 2019 5:00 pm

OOOOPS wrong file this file
That file is the mono start file. …
Sorry but I posted my start file (have I been listening to it too much and it’s croddled my mind already?)

Reply to  tom0mason
March 19, 2019 5:26 pm

Actually quite soothing.

Reply to  Robert
March 19, 2019 7:32 pm

Glad you like it.

Reply to  tom0mason
March 20, 2019 7:19 am

Felt like I was below decks in a cargo ship.

Delts started aching. Could have been from Monday workout.

March 19, 2019 4:40 am

The local water utility dug out a large tunnel to transport water from the lake as a secondary source of water to a suburb many miles away, during the drilling operations where much of the horizontal boring was at depths several hundred feet below grade, many people in the area had an elevated sense of dread and anxiety. That stopped when the drilling stopped.

Taking that experience lead to exploring a similar occurrence with a family member in the north Texas area. Around the same time most evenings, (3AM-5AM) particularly in the summer, the same laying in bed awake with “the boogies”. After checking civil infrastructure maps of the area, a major pipe to fill the community’s massive water tower ran right through the neighborhood proximate to the house.

Don’t know if it was a coincidence, but telling someone “Don’t worry, its just infrasound” doesn’t help any.

March 19, 2019 4:44 am

Many years ago, the CBC did a program about psychic phenomena. It featured the work of Michael Persinger among others. One of the items included a bit about a chanting Buddhist monk.

There is a way of chanting where you can produce two tones at the same time and those two tones can give rise to beat notes. A person sitting in front of a chanting monk observed the face of the monk distort and look like ancient images of demons. It turned out that the monk was generating infrasound and that was distorting the eyeballs of the observer and making him see strange things.

We’ve known for a long time that infrasound affects people and not just through their ears. As pointed out in the video, the profit motive of the wind industry gives rise to corrupt science to combat any evidence of what used to be commonly accepted. This, of course, is no surprise to CAGW skeptics. It’s something like trying to erase the Medieval Warm Period.

Steve O
March 19, 2019 4:45 am

So, the corporate wind turbine operators say it’s not an issue, but independent scientists say that it is. I wonder whose side the greenies take?

March 19, 2019 5:11 am

From what I recall US Army abandoned the project because it effected the operators as much as it did those targeted. Work on such has continued for psy-ops application, using leave behind devices, and for crowd control, set up and move away from it and push said crowd towards devices. Does not effect all people the same, some people it does not both at all.

I have spent time around wind farms and yea, it is nausea inducing.

March 19, 2019 5:34 am

It’s probably an innate fear of a pre-historic underground monster – see “Tremors”.

Subwoofers are used in horror flicks to generate exactly those feelings!

jack Morrow
March 19, 2019 5:58 am

Riding in the back of the DC-9 will do the trick too!

james francisco
March 19, 2019 6:29 am

I worked on the F111B for about three years. The noise from it was painful without ear protection. I noticed a change in myself that is hard to describe when I left that job to become an instructor. I believed it was due to the absence of the noise. I loved that job working on the instrument and autopilot systems of the F111.

Ron Long
Reply to  james francisco
March 19, 2019 10:07 am

james, I reviewed an old-fashioned Yuba dredge gold placer operation just north of Beale Air Force Base, California. One day we saw an SR-71 Blackbird getting into position for takeoff so we stopped and got out of the vehicle to watch. Bad mistake! When it went to full power, from about three-quarters of a mile away, the sound was like a punch in the stomach and your whole body vibrated! One experience that I have checked off and don’t have to do again!

Reply to  Ron Long
March 19, 2019 2:40 pm

Guy at the range a few weeks ago was shooting a Weatherby .30-378.

I was wearing ear plugs AND ear muffs, for noise reduction of about 50 dB. When he shot it, the noise was fine at my ears, but I felt concussion in my chest. It was okay, but I don’t think I could have taken it many times. Fortunately, he was finishing up and stopped.

We humans have limited experience with the extremes of sound.

Richard Patton
Reply to  Ron Long
March 19, 2019 2:54 pm

You haven’t heard f/a 18’s on afterburner from 3/4 mile away. I lived 3/4 mile away from the end of the runway at NAS Fallon and saw and heard both F/A 18’s and a Blackbird take off. While the SR-71 was an awesome looking plane it was NOT impressive in either sound or speed of takeoff. It had the acceleration and noise level of a B-52. It has a lot more mass that one would suspect. The F/A-18’s were low mass in comparison and could really jump. It is like the difference between a horse and a hare. A hare will beat a horse in the first 50 to 100 feet but after that the horse leaves the hare in the dust. The SR-71’s beauty is in its ceiling (in excess of 80,000′) and speed (in excess of Mach 3).

Geoff McPherson
Reply to  Richard Patton
March 20, 2019 4:15 am

Picky maybe but F14s seemed to make F18s seem quiet. I never heard them at the same time to compare.An F14 at 100m at whatever speed on a rainy night when one was in a tent on Mogadishu Beach was rather intense

Richard Patton
Reply to  Geoff McPherson
March 20, 2019 11:01 am

Actually, I think that F-4’s on afterburner take the prize for noise. Of course, I may be biased. When I was stationed on the USS Midway my berthing compartment was one deck down and 60′ away from the jet blast deflectors where the F-4s would kick in their afterburners just before launch. It got so loud in the compartment that I couldn’t hear my music with headphones on and cranked up all the way. I learned to sleep with earplugs, which preserved my hearing. When I left the Navy they were surprised how good my hearing still was. It still is. I can hear the tick-tock of my wall clock which my audiometer claims is below 20 dBa (that is the lowest it goes).

Reply to  Ron Long
March 19, 2019 11:47 pm

I understand perfectly. One day long ago I was waiting on a taxiway at Dulles with a Concord on the runway. I was maybe 40 feet from it as it hit afterburners and took off.



Reply to  Ron Long
March 20, 2019 11:04 am

K. You guys beat. I was in TAC, and just heard C-130s, and an occasional C-141 or C-5.

March 19, 2019 6:51 am

Except there’s no evidence they are producing infrasound and despite repeated studies, no evidence they are in any way harming human health.

Flight Level
Reply to  griff
March 19, 2019 7:19 am

One propeller does not produce that much infra sounds. Two or more can. It’s called beats. That’s for one.

Second, on huge airfoils the resulting tip turbulence vortex is quite noisy. Now stand aside and watch, one blade comes the other goes. Both generate sound, both are mobile sources, right ?

Put in some doppler, the incoming blade sound will be perceived higher the outgoing lower. The mix will obviously induce beats. Which given the relatively low rotation speed can be in the realm of a few Hz or less.

This is one of the well studied mechanisms by which respectable propellers can shake the control tower windows with the typical “Boom Boom” as a/c maneuvers quite far away.

And marks the presence of choppers miles before they show as low frequency sounds travel pretty well over the terrain.

And if you don’t think infra sounds have ill effects then obviously you never experienced them.

Further up I have posted on how this feels and what are the effects on passengers.

John Endicott
Reply to  Flight Level
March 19, 2019 10:05 am

And marks the presence of choppers miles before they show as low frequency sounds travel pretty well over the terrain.

So that’s how Radar always knew when the choppers were inbound before everyone else on M*A*S*H. He was sensitive to infrasound. 😉

Reply to  John Endicott
March 19, 2019 11:17 am

We can always tell when the Chinooks are passing overhead from the local Fleet Air Arm station by the woof-woof beat produced by the two rotors. A completely different sound from the Lynx and other single rotor helicopters.

Reply to  StephenP
March 21, 2019 8:01 am

Choppers don’t sound the same as they did back in the day, except for ‘Hooks, they sound just the same! I kinda miss that whopwhopwhopwhop from the old Slicks and Cobras and Loachs. Hell even JetRangers don’t whopwhop anymore. Different rotors and different engines.

Reply to  John Endicott
March 19, 2019 2:42 pm

“So that’s how Radar always knew when the choppers were inbound”

That, and it was in the script.

Geoff McPherson
Reply to  Gamecock
March 20, 2019 3:55 am

From experience after Blackhawk Down someone sensitized to beats can detect a simple Blackhawk in half the time of someone standing next to them. Not the cochlear but other options exist

John Endicott
Reply to  Gamecock
March 20, 2019 5:46 am

That, and it was in the script.

Obviously (hence the winking smiley at the end of my post). But it’s nice to put some “logic” into it rather than mindlessly accepting it just because “it’s in the script”.

Reply to  Gamecock
March 21, 2019 12:00 pm

Playing golf today.

I heard a chopper.

Looked up, and said, “It’s a Chinook!”

Golfing buddy, “HOW DID YOU KNOW THAT ?!?!?”

“Twin rotors.”

Familiarity aids identification.

Reply to  Gamecock
March 22, 2019 5:35 am

Up until the late 1990s I was fairly accurate IDing aircraft by sound, especially helos. They have all changed with the slow shift to newer rotors and power plants. Blackhawks and Apaches never whopwhopped, always roared, so they are still pretty identifiable just from sound. All the new generation birds used for civ air-amubulance service sound much like Blackhawks only not so meaty. I miss A 7 Corsairs, beautiful ship and very distinctive sound. And A 10s! Nothing sounds like that, especially smoking along at 3/4 throttle just above treetops, or when they tip up on a wing doing a 180 and firewall the throttles. Awesome.

Reply to  Flight Level
March 20, 2019 6:31 am

Same effect with twin inboard engines on boats when the engines are not at the same RPM. ONE NEEDS TO adjust the throttles to eliminate the beats from the props.

Reply to  griff
March 19, 2019 8:16 am


A rotating blade most definitely can create infrasound; in fact, there is a subwoofer driver that is exactly that:

I work in acoustics for a living, and have often dealt with infrasound from fans and HVAC systems. There’s no peer-reviewed study for you to point to because anyone who’s ever worked in the field (an expert) knows it is a baseline fact – the laws of physics kind of demand it.

Patrick MJD
Reply to  ShanghaiDan
March 19, 2019 10:05 pm

Griff always pipes up and talks about subjects he knows nothing about and then gets shotdown in flames by someone who actually knows the subject.

Griff is the comedy gift that keeps on giving!

Reply to  griff
March 19, 2019 10:08 am

Did you not watch the video? Infrasound is produced when the blade pass the pylon. They measured the pulses. They also did tests that showed infrasound affects the brain and heart muscle.

John Endicott
Reply to  icisil
March 19, 2019 11:10 am

Did you not watch the video?

this is griff we’re talking about. If he puts as much effort into comprehending it as he does the articles he links to then it’s doubtful that he comprehended it even if he did watch it.

Alan Tomalty
Reply to  griff
March 19, 2019 11:30 am

Griff you will back intermittent renewables and the CO2 scam until your death ; because you have succumbed to the religion of Al Gore’s Church of Climatology. Despite all the evidence against intermittent renewables and lack of evidence for global warming by CO2, you cling to your beliefs like a true believer of a cult.

Craig from Oz
Reply to  griff
March 19, 2019 7:17 pm

Big Tobacco used pretty much the same argument.

Didn’t work well for them.

Reply to  griff
March 20, 2019 3:51 am

Griff, what about the recent Finnish study of wind turbine infrasound problems. Also may I recommend Peter Mitchell’s excellent paper March 2016, of the problems of wind turbine infrasound in Australian wind turbines. Remember that the wind farm operators try to pretend that there aren’t any problems by measuring only the sound hearing range.

Reply to  griff
March 20, 2019 7:24 am

Except that once again you are entirely wrong, lawsuits are moving forward on this based on studies it is bad for people and animals. Nice lie though! Keep’em coming.

March 19, 2019 6:59 am

The film is interesting, but we should always be skeptical of documentaries like this in that they will – of course – be cherry-picking their pieces to show. The film of natural disasters at the start really has nothing to do with the message, but pre-warns the audience that there is something dreadful going on.

Having said that, there were some very interesting points about how the sound is being measured: The very noticeable differences when looking at specific wavelengths vs a general background noise level is something that really should be considered. This is one area where you could say that the technology to measure the vibrations have moved on, but there seems no excuse for not re-visiting the regulations based on the new measurements.

Also, the evidence that the human ear responds to low level frequencies even though they don’t stimulate the “hearing” cells is very important. I am less convinced by their neurological assumptions about affecting areas of the brain related to stress as this is exactly where there is little evidence and a lot of (published) speculation, but to ignore the effect of wavelengths just because they can’t be heard is very poor. We have a lot of physiological response to wavelengths of light that we can’t see, after all.

The issue of whether you can stop any development that has any kind of measurable effect (regardless of whether you can say it is a harmful effect) is just the kind of thing the environmental activists love as they can scream “precautionary principle”. I am not enamoured of using the same argument against wind turbines, but if you are going to promote any kind of development with the kind of government support that wind turbines have garnered you should really have done some better work on impacts – and be prepared to re-visit the impact as technology and experience develop.

Of course, this pre-supposes that there is any benefit from wind-turbines which is a whole other kettle of fish. If the benefits go away, they even small costs are unsupportable.

Flight Level
Reply to  Rob
March 19, 2019 7:36 am

“Also, the evidence that the human ear responds to low level frequencies even though they don’t stimulate the “hearing” cells is very important.”

The whole body feels them and your guts express their strong will to take a walk. Bottom line, it drives people nuts and nauseous for no reason. How do I know ? Spent quite some hours between two propellers slightly out of sync. The resulting “Wah Wah” beat is something I’m glad not to endure again.

Patrick MJD
Reply to  Flight Level
March 19, 2019 10:09 pm

British bomber pilots in WW2 used to have their engines slightly out of synch too for exactly that effect which certainly could be heard and apparently felt on the ground.

Pop Piasa
March 19, 2019 8:15 am

Why do kids get such pleasure out of infrasound? When a car gets turned into a giant subwoofer I can’t see how they can drive coherently. Especially the ones with ear buds in on top of it.

March 19, 2019 8:51 am

If you want to learn a little about infrasound, search for articles using the key words XF-84H Thunderscreech.

Joe Crawford
March 19, 2019 9:06 am

It is well known that elephants communicate using infrasound ( for long distance communications. It would be interesting to know if anyone has noticed any strange behavior around wind turbines.

kent beuchert
March 19, 2019 9:10 am

The biggest death certificate for wind as a power genersator was penned several months ago by a study which showed how far short these large turbines are of meeting output and reliability expectations fostered by the wind hawkers. Power roughly twice as expensive as predicted.

March 19, 2019 9:55 am

I once visited a bank’s HQ which had obvious and unpleasant infrasound. I don’t know how the staff coped with it.
That had a central 170m long, three storey high corridor, and was probably no worse than a large cathedral, if that cathedral had been full of computers and people trying to work and talk quietly.

March 19, 2019 10:03 am

Solar may be viable in certain regions but wind is problematic on so many levels it beggars the imagination that anyone could be a wind advocate.

John Endicott
Reply to  Yawrate
March 19, 2019 12:21 pm

Solar may be viable in certain regions during the daytime on a cloudless day

Fixed that for you. Solar isn’t very viable at night or when the sky is filled with clouds – in other words all the times that the sun doesn’t shine.

The problem with both wind and solar, beyond their unreliable intermittency, is that they’re not very energy dense. It takes an awful lot of real estate to put bird choppers and solar cells on to gather as much as energy as you can get from one small coal or gas plant.

Eric Harpham
March 19, 2019 1:15 pm

Put “Dr. Marianna Alves-Pereira” into google and view the video of her lecture in Slovenia in May 2018 about “The Cumulative and Irreversible Harm from Infrasound”. Brilliant lecture and very thought provoking and worrying. I cannot claim the credit for finding this as I got the link from a fellow WUWT’er who commented on another article. You’ll need an hour to watch it so grab a beer or two.

Michael Keal
Reply to  Eric Harpham
March 19, 2019 3:34 pm

Thanks for that. Watched something on that quite a while back and was about to go hunting for it. Saved me the bother. Well worth having a look. Complements the above perfectly.

March 19, 2019 3:27 pm

Reply to  Sommer
March 19, 2019 3:38 pm

This presentation is very good!!

March 19, 2019 5:09 pm

There is a post on this subject, , which goes into this subject and uncovers that this issue has been known for decades. Please visit and understand more about this subject.
Here are some historic elements to the story:
Beginning in 1980, and published in 1985, study of a NASA-DOE wind turbine in North Carolina featured these findings:
“Vibration complaints were more severe inside the houses than outside,
There was a resonance response in the houses and in humans,
The bedrooms were more severely impacted than larger rooms,
And the effects were stronger when the house wall faced the turbine blades.
This report states the effects on the residents were “real, and not imagined,” and frequently took the form of “an intermittent thumping sound accompanied by vibrations,” and, “a feeling or presence was described, felt rather than heard, accompanied by sensations of uneasiness and personal disturbance.” ”
There is also the reference to the World Health Organization’s European Office declaration that wind turbine infrasound generates “numerous adverse health effects.”
Learn more:

Craig from Oz
March 19, 2019 7:22 pm

Wind Energy and ultrasound are going to be the new Tobacco.

The smart operator is going to be getting out of wind and hiding the paper trail before the lawsuits start.

March 19, 2019 9:24 pm

A good example of the “A Little Knowledge. . . .” is the introductory biology teaching that the human ear hears from 20 Hz to 20 kHz. Oversimplified!

Pitch is logarithmic (each octave is a factor of 2 in frequency). Middle C on the piano is 261.6 Hz and the A above it is 440 Hz (the pitch to which an orchestra tunes). An octave bellow this A is an A at 220 Hz, and the one below this is 110 Hz, and below this is 55 Hz, and 27.5 Hz is the A that is the lowest note on the piano. We of course hear this lowest note, and we might presume it is because 27.5 Hz is greater than 20 Hz. Below 20 Hz is called “Infrasound”. Nice story!

Now instead of playing the piano, hook up a function generator to a loudspeaker and play a sine wave. You can hear 440 Hz just fine, and also 220 Hz. But 110 Hz is sounding weak, and you will probably not hear 55 Hz well or 27.5 Hz at all unless you turn up the amplitude from the level that was very comfortable at 440 Hz. Your ear is much less sensitive as pitches drop below, say, 100 Hz. This is the famous “Fletcher-Munson” curves or “A-weighting” (video just above) that is popular with persons resisting noise ordinances (claiming we don’t measure it because you can’t, by definition, hear it). In such a contentious circumstance ask the advocate of harmless low-frequency noise how it is that the bottom of the piano keyboard is so strongly heard.

Indeed, I didn’t yet explain this. So, with the function generator pitch still at 27.5 Hz, turn the amplitude back down to the level as used for 440 Hz, and push the sawtooth button instead of the sine wave. It becomes loud, easily heard. This is because of the sawtooth harmonics (27.5, 55, 82.5, 110, 137.5, 165, ….). Similarly, it is the harmonically rich structure of the struck string lowest A on the piano that is the reason that is heard so well.

Now the main point. If you lower the function generator to a 10 Hz sine wave (or to 5 Hz or to 1/2 Hz, etc.) there is no chance you will hear it. Push in the sawtooth button and you will hear these (hearing them as much as a buzz or a clicking — the repeat rate of a pattern — as a usual tone). You will not be surprised as well to hear the strike rate of a hammer on a nail – perhaps 1 Hz. The point in frequency at which the sensation changes from a usual “tone” to a repetition-rate is one of continuous transition and is somewhat arbitrary.

Hearing does not stop at 20 Hz (infrasound) except in the rare case of a (usually intentional) demo with a pure tone (no harmonics). Don’t be misled.

Reply to  Bernie Hutchins
March 20, 2019 5:53 am

You’re quite right about a little learning…

Richard Patton
Reply to  Bernie Hutchins
March 20, 2019 10:50 am

Oversimplified is right. 20kHz is not the top range of all humans. When I was young I could hear the sound of the electron guns in a tube TV clear across a department store (thank God there aren’t tube TVs any more and I have lost hearing in that range). The frequency they produced was way above 20 kHz. Above the supposed 30 kHz that a demonstration at a science exhibit supposedly topped out at. Now that was many years ago before I put in over 5 years on an aircraft carrier. (I had to sleep with earplugs, our berthing compartment got so loud when the F-4’s kicked in their afterburners). Thankfully my hearing is still good and thankfully I can’t hear those electronic ‘mosquitos’ anymore.

March 20, 2019 12:24 pm

All this talk of beat frequencies has me puzzled. Do two frequencies differing by 10Hz actually produce any energy at 10Hz?

Reply to  Roger
March 20, 2019 4:50 pm

Roger at March 20, 2019 at 12:24 pm asked: “Do two frequencies differing by 10Hz actually produce any energy at 10Hz?”

Absolutely NOT! But the situation is a bit more complex than some think:

Consider the trig identity (one of several equivalent forms):

Sin(A) + Sin(B) = 2 Sin[ (A+B)/2) ] Cos[ (A-B)/2 ]

Consider the case where B is slightly less than A. The right-hand side says that the superposition is basically the AVERAGE frequency (A+B)/2 multiplied by HALF the difference: (A-B)/2.

Traditionally we have heard that the “beat rate” IS the difference frequency, not half the difference. This is because the Cosine term has TWO peaks (one positive and one negative) during each of its cycles. Typically here A-B is sub-audio – the ”thumping”.

Now to the point: If you ask what the SPECTRUM is, it is just the LEFT side of the equation: the SUM as you wrote it down to describe (linear) superposition. NOTHING NEW – the sum (resulting spectrum) is just the sum (the process).

To be complete, you may, and often will, PERCEIVE something recurring at a 10 Hz rate (the “envelope”), but any proper spectral analysis will show no energy there, (absent any secondary non-linear effects).

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