Build an Infrasonic Subwoofer

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.

Step 1 Speaker cones

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

Comments

[bradhouser]

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?

[Tachyon]

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?
;')

[GBMorris]

Bradhouser, I believe you are correct. Either you would want the speakers in antiphase or you would not want a sealed enclosure.

[Offroadie]

If they were out of phase they would cancel out and you would have no bass.

[keriksen1]

No they won't, since they are facing opposite directions.

[Offroadie]

You guys need a lesson in acoustics...

[keriksen1]

Right, let hear it?
In my humble opinion bradhouser is hitting tha nail on the head :)

[Offroadie]

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.

[bradhouser]

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.

[bradhouser]

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?

[psychotron]

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.

[tleeds]

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]

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.

[GeckoDeLimon]

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.

[rfoster4]

Were you able to discover the elusive "brown note"? :)

[rallen71366]

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. :-)

[ttitus]

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.

[rallen71366]

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.

[ellisgl]

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

[noingwhat]

How do you find your driver's resonant freq?

[dawp]

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

[dawp]

Talk to elephants, alligators, and other large beasts:)

[Green Silver]

What do the people the people next door think about this instructable? LOL :o)

[zzbb]

The cruelty of some never ceases to amaze me.
If you plan on using this to torture others, then I hope you get what you deserve.

[jimvandamme]

I'm surprised no one has mentioned the Schumann Resonance at 7.83 Hz. Perhaps there are no New Agers here. (Which is a good thing.)
http://en.wikipedia.org/wiki/Schumann_Resonance

[psychotron]

Nicely constructed subwoofer box, nicely documented construction!

There are some technical inaccuracies, though:
Qes is pretty much irrelevant for box or subwoofer "sound quality"
Qts is the spec used when sizing boxes, and box size determines Qtc, which is the closed box Q. it should be in the range of 0.5-1 for reasonable sound quality, and Qtc=0.707 gives the deepest F3 or half power frequency for a given woofer in a sealed box. Other Q's can be chosen depending on your design goals and power handling requirements..

You box does not appear to actually be 70 inches, perhaps it is more like 45 inches long?

Adding mass to the cone affects more than just resonant frequency. Doubling cone mass will not halve the resonant frequency, rather it will reduce it by about 30% (actually 0.707 times original) It will also increase Q by the same amount (1.414 times) and will reduce SPL above resonance for the same voltage input by 6dB.

Regards, and have fun!

[saintrain]

Heheheh! 2000 watt power handling & 8 ohm impedance. You could plug these directly into the wall (in North America etc.)

120 volts ^2 / 8 ohms = 1800 watts of 60 hz hum!

Hear it AND feel it.

[ScottyAlmanjoy]

Is there a more technical explanation for this?

You do mean 30 inches and not 30 feet, right? You used ', which means feet, rather than ", which means inches.

[dickweed101us]

No this means " inches and this means ' feet. So he has it right 30"x30"x70" inches.

[ScottyAlmanjoy]

When I commented, it was 30' x 30' x 70', or maybe I just saw it wrong. Anyway, it's right now.

[Electorials]

Nice project!

but where can we use this for? (except for whales and seismographic stuff)?

[scratchr]

Scare people by playing 19hz through it:

http://www.guardian.co.uk/science/2003/oct/16/science.farout

[Electorials]

the evil frequency Muhahhaha!

It's pretty interesting what they say in that article.
Hyperventilation fear and panic attacks :O

[scratchr]

It can be played on 1 foot woofers...

[bradhouser]

After reading the Wikipedia article linked above, I think I found an answer for you.

Turn it on on Halloween, and scare the neighborhood kids even more.

[lesizz]

Do you have this connected to a system with a CD player? If so, I suggest the Telarc recording of Tchaikovsky's "1812 Overture". It has cannon shots recorded at a very high SPL. Put on the CD and take cover!

An interesting thing to do would to record a cat purring. That must be somewhere around 10Hz.

Isn't 20Hz cutting it a bit short for human hearing? Large pipe organs commonly go down to C0, at 16.35 Hz.

What amplifier do you drive this monster with?

This is a really cool I'ble.

[sysiphus]

No, humans really can't hear below 20 Hz, and few can really hear below about 25 Hz. We do, however, perceive sufficiently loud sounds in those frequencies, which is what makes them so fascinating.

As to organs that can hit C0: very few actually do. And most that come even close have those notes so that the listener can feel them, not hear them.

[lesizz]

Do you know how it was determined that humans can't hear below 20-25 Hz? I heard once that if one can hear the individual beats of the tone that it's not considered "hearing".

Like 4example my cat's purring: I'd estimate it at 10 Hz, considerably lower in frequency than the C0 organ pedals I've "heard". This housecat's purring is not enough SPL to shake the floor. But I am hearing the individual pulses of the purr; I don't perceive it as a "tone".

So if the aforementioned definition for the lower limit of hearing is the standard, then I've been "defined" out of hearing at 10 Hz, even though there is a physiological response the tone.

Actually I hate it when people get all nit-pickey like I'm doing now, but this really has my curiosity up.

[dreamer.redeemer]

Did you measure the roll off? Wouldn't you get better infrasonic response with a large low tuned, ported enclosure?

[sysiphus]

No, humans really can't hear below 20 Hz, and few can really hear below about 25 Hz. We do, however, perceive sufficiently loud sounds in those frequencies, which is what makes them so fascinating.

As to organs that can hit C0: very few actually do. And most that come even close have those notes so that the listener can feel them, not hear them.