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Infrasound is sound that is below your hearing threshold which general drops off at 20-30hz, i.e. lower than big booty bass. It can have a profound effect on your body, and should be experimented with carefully! Infrasound is used by the military as a weapon, or science to monitor earthquakes, whales ect.. In this instructable we will walk you through the process of building your own Infrasonic Subwoofer. We created one for our research collective AUDiNT.net on sonic warfare.

from wikipedia:

Infrasound is sound that is lower in frequency than 20 Hz (Hertz) or cycles per second, the "normal" limit of human hearing. Hearing becomes gradually less sensitive as frequency decreases, so for humans to perceive infrasound, the sound pressure must be sufficiently high. The ear is the primary organ for sensing infrasound, but at higher levels it is possible to feel infrasound vibrations in various parts of the body.
The study of such sound waves is sometimes referred to as infrasonics, covering sounds beneath 20 Hz down to 0.001 Hz. This frequency range is utilized for monitoring earthquakes, charting rock and petroleum formations below the earth, and also in ballistocardiography and seismocardiography to study the mechanics of the heart. Infrasound is characterized by an ability to cover long distances and get around obstacles with little dissipation.
 
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Step 1: Speaker cones

Picture of Speaker cones
Infrasonic_sub021.JPG
First, You must find an appropriate speaker cones. We decide to build a double speaker system with a cone on each end of the enclosure. You could also build it with only one cone. When choosing a cone it should one that is 21" or 24" in diameter. We chose to use a Pyle 21" speaker. For good sound quality you should use a driver that has a QES of .38 + - 20%. We found Pyle speakers at J&R for only $250.00 which for such a low speaker is cheap!!

What is Qes?
http://www.bcae1.com/spboxad2.htm
http://en.wikipedia.org/wiki/Q_factor
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JanisS25 days ago

Im a noob and just interested, Is infrasound only playable by special speakers? or could basic earphones, or phone speakers already play infrasound?

Thanks

vivekgir9991 month ago
Is it necessary to play the sound through amplifiers and if so why? I'm asking this because i dont know anything about producing infrasound sound.
Yea I don't know why people try to put on a show like that.. There are many many many car audio systems that easily produce sub 20hz frequencies up into the 140+db region and as long as proper hearing protection is worn, nothing bad happens lol. It is a nice subwoofer don't get me wrong, I love bigger than 18" subs, just because of the sheer beast-ness of them lol.
it's not lower than 20 and sine at the same time with the most of car speaker systems...all they do is frying the sound and what comes out is triangular waves
randofo3 years ago
If you replace the main image with a picture of your actual project, I would be happy to feature this to the homepage.
splnlss (author)  randofo3 years ago
Hey Randy,
I just changed the main image. Thx! -jc
randofo splnlss3 years ago
So... What has your research unsurfaced so far? Any odd or painful experiences?

A while back I was reading a theory about how many places thought to be haunted have unusually high naturally occurring infrasound acoustic properties.
laekrits randofo4 months ago

yeah! Tandy and the grey ghost in the lab.. resonating eyeballs, etc..

Or change the instructable to match the picture!!!
bradhouser3 years ago
You don't say if the speakers should be in phase with each other or out of phase. I would think at these low frequencies, with a sealed box, you would want them out of phase, so that one is going in, while the other is going out and vice versa. That would keep the air pressure in the box constant. Or am I missing something?
Bradhouser, I believe you are correct. Either you would want the speakers in antiphase or you would not want a sealed enclosure.
idydstie GBMorris8 months ago

My understanding is that if desired wavelength is longer than the distance around to the back of the box, the speakers should be wired so that they are both pushing out simultaneously.

If they were out of phase they would cancel out and you would have no bass.
No they won't, since they are facing opposite directions.
You guys need a lesson in acoustics...
No they don't. You're wrong. The design is pumping both ends of an air column. That said * Acoustical* phase will change with distance, so like a vented box, some frequencies will be reinforced while others will cancel.
Dsynchronous: (1) Who or what are "they", and what don't they do? (2) Who are you referring to when you say "You're wrong."? Your answer makes sense, but please tell me which school of thought you are in. The speakers are either wired in phase, or out of phase. I can't tell from your response. Almost a year after I posted this question, I still don't know what the author intended.
Okay sorry: I was responding to tachyon I think . The issue is whether or not the speakers were to be wired in phase or in anti-phase. I thought that in phase is wrong, because at the wavelengths were talking about resonance coupling would'nt matter. The speakers are mounted both facing outward on opposite sides of the box.
You can view the box as a sealed air column, in which case they should be wired anti-phase (one pushes in while the other pushes out). BUT, you can visualize the box as a point source of pressure variation (makes sense?) in which case you would want the speakers pushing out at the same time and in at the same time.
The correct answer depends on several factors:
Size of room relative to box, size of box relative to speakers and placement of box in the listening environment.
I believe that in a closed room or with placement close (acoustically) to the walls anti phase would be correct. Outdoors or with the sub placed in the center of a LARGE room, or with the listener far away from the sub(s) then in phase would be correct. I haven't modelled the behaviors, but I understand the arguments of both camps.
Solution: Buy a DPST switch and wire in a phase selector>

Sorry 'bout the lack of clarity, it happens when I'm netting at work sneakily.
Gerry
Offroadie: Your reply is not very informative. Which part of your acoustics training can you share with the rest of us to better explain your position?

Not being a sound engineer, my understanding is limited. So I welcome clarification based on sound (pun intended) principles.

This is what I think is happening.

In a normal stereo setting, when the woofers are out of phase they tend to cancel each other, an effect that is increased with lower frequencies.

However, this scenario is different. For one, they are not in separate boxes, and they are pointing in opposite directions. Add the fact that the box is sealed, keeping them in phase causes both cones to move in and out together. This increases and decreases the pressure of the air in the box. Being sealed with no port to the outside, this would tend to dampen the sound, making it quieter.

If the two speakers are anti-phase, one moves in while the other moves out, keeping the pressure on the internal air the same. Wouldn't that tend to boost the volume instead of reducing it?
No, if they were out of phase they would constitute a dipole and at low frequencies they would cancel and give very little output. Try playing bass through a speaker without a box (dipole) and a speaker in a box (monopole) You will generally find that the enclosed speaker makes a lot more bass.

A dipole will augment response at certain frequencies, but it will not be in the infrasonic range unless the dipole were VERY large. Read 'Linkwitzlab' to see the pattern of dipole augmentation and cancellation.
Right, let hear it?
In my humble opinion bradhouser is hitting tha nail on the head :)
My first post was informative. My second comment wasn't meant to be rude but I would have thought someone would search on their own if they were interested.

Being stereo or mono has no effect on low frequencies. Also, It doesn't matter what side of the enclosure the drivers are on. Either you are compressing air or you are not. Putting them out of phase no longer compresses air. If you were compound loading drivers then yes the outside speaker(s) would be wired out of phase.

A box is basically an air spring. Each driver has it's own properties that determine how it reacts to this air spring. That's how some work in sealed boxes some in ported and some both. Sealed boxes have to be pretty specific to the driver as well. Wrong size spring, wrong size reaction.

Ports don't dampen sound they reinforce it by putting the rear wave in phase with the front wave by tuning the port.

Hope that helps a little.
Thanks for clarifying Offroadie.

So, if they are out of phase it "no longer compresses air", by that I assume you mean the air in the box. Makes sense, but I "am from Missouri". (i.e. show me)

I would be interested to hear if anyone who builds this could try it each way and then let us know if they feel any difference.
The way the speakers are shown mounted, they should be wired in phase electrically which will cause them to operate out of phase pneumatically which is the correct configuration acoustically.

Make sense?
;')
Yevsey1 year ago
Hello, the Pyle website says the frequency response of the speaker you used is '22 Hz - 4K Hz', I'm no expert in acoustics (I know nothing) but I'm curious how you've managed to achieve significant infrasound frequencies given that this is the case. I'm planning my own infra-subwoofer (based on your instructions) with a single one of these drivers in a large (1.15 x 1.1 x 0.3 metre) enclosure with a 1000 watt amplifier..... Will this work?! I need to get as low as 18hz with enough power for people to really feel it in their bodies. I was about to buy the Pyle speaker when I became confused by this bit of info though. Please enlighten me!
kadenbohn2 years ago
hey what if instead of using wood you used steel would that work?
Derezzler2 years ago
Im hearing alot of people say they can hear close to 20Hz but in fact when you hear a frequency you are also hearing frequencies near and harmonics of those frequencies to explain a little.

In other news, this is a really smart concept for sub frequencies that has a flat frequency response from 30Hz to DC. See attached link.
http://www.rotarywoofer.com/
tleeds3 years ago
A better choice for the speaker box is probably 3/4" MDF. (Medium Density Fibreboard). This is the normal choice for subwoofers as it's particularly dense and non resonant. (it's also cheap). You won't be able to stain it to get that nice looking cabinet, but oak veneer is cheap and will take stain wonderfully.

Tachyon tleeds2 years ago
I agree with tleeds.
Eventually you will have problems with plywood. It will delaminate and vibrate.
It will also flex and cause distortion.
That's why no pro audio cabinet builders use it and almost universally use MDF.

That said, infrasound is a real part of real life multimedia experience. Explosions in movies, parts of some music. So it's great to see you doing this.

Good instructable, thanks.
fwater Tachyon2 years ago
"That's why no pro audio cabinet builders use it [plywood] and almost universally use MDF."

ALL pro audio builders use plywood and almost universally AVOID MDF. It dents / crushes corners easily, it's far too heavy to be moved around, it absorbs moisture, and it is weaker than plywood. No standard-quality plywood will delaminate or flex. Sheesh, where did you come up with your assertions? Do you mean professional builders by "pro audio", or do you mean PA / live band equipment?
What's wrong with square? The wavelengths you are working with here are 40-80 feet long--way too long to form as standing waves inside the enclosure, even as quarter wave resonances.

rfoster43 years ago
Were you able to discover the elusive "brown note"? :)
When I was in the service (many moons ago) one of my Commo buddies commented that he had done some research like that, but they didn't use normal speakers with a diaphragm. They were spraying propane into the air and detonating it at a rapid speed. They were using a fuel/air explosion as a speaker!

He also said that they had to wear depends. That assignment was the sh*t. :-)
The Mythbusters did a whole episode on "The Brown Note", with the conclusion being that it didn't exist. Although some tones will make you feel a bit dizzy. I get that way when I listen to bi-naural beats too long. Air explosions though...I don't remember them testing that. I cant imagine that a short blast of frequency, whatever it may be, could make someone shat themselves.
Ever been on the firing line when they cut loose with the heavy weapons? A .50 BMG firing shakes you to your bones. Standing too close to REAL heavy weapons (like a Patriot Missile, or Naval cannon) can kill you just from the shock wave (a Patriot is going Mach 6 when it clears the launch tube). Loosening your bowels without killing you is more a matter of control, than power.
ellisgl2 years ago
I wonder how well this speaker would perform in a Graham Holliman Velocity-Coupled Infra Bass box. http://www.flickr.com/photos/rcadimensia/5917841850/in/set-72157627026421901
noingwhat3 years ago
How do you find your driver's resonant freq?
dawp3 years ago
Further comment on enclosure: An air coupler might be used for this. I recall reading about it years and years ago in an audio magazine. Here is an up to date construction detail at:
http://www.soundfountain.com/amb/aircoupler.html
dawp3 years ago
Talk to elephants, alligators, and other large beasts:)
What do the people the people next door think about this instructable? LOL :o)
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