This TARDIS Is An Infrasonic Subwoofer

If you’re a fan of action movies or dance music, you’ll probably be familiar with sub-bass. The moment in those James Bond explosions that thuds through your chest in the movie theatre? That’s the product of a large subwoofer, a tuned pipe housing a speaker working somewhere just above the lower limit of human hearing, in the tens of Hz.

[Mike's] TARDIS final build
[Mike’s] TARDIS final build
But what about sound below the range of human hearing, below 20Hz? You can’t directly hear infrasound, but its presence can have a significant effect on the experience of the listener. [Mike Michaud] was interested in this phenomenon for his home movie setup so built himself an infrasonic subwoofer tuned to 17Hz. Since the resulting cabinet was rather large he disguised it as a vintage UK police telephone box that you’d hardly notice in his basement theater. 

A resonant 17Hz speaker horn is a rather inconvenient size for a home theatre, at about 25 feet long. Fortunately there is no need for the horm to be straight, it can be folded into a more convenient enclosure, and that is what [Mike] has done. He used a design published by [lilmike], which folds the horn three times into a more manageable size.

Speaker cabinet construction requires attention to the choice of materials as well as to the driver unit itself, so [Mike] goes into detail on the materials he rejected and his selection of a particular brand of subfloor ply.

He rates the resulting speaker as incredible. His driver is rated for 500 watts but he only has an amplifier capable of serving 100, even with that power he fears for his basement windows. He describes the noise made by the feet of the robots in War Of The Worlds as “little earthquakes” and the general effect as very menacing.

We’ve featured quite a few subwoofers on Hackaday over the years, though with the exception of this rotary device they have mostly been for more conventional sub-bass applications. Here for example is another folded horn. So if sub has become rather run-of-the-mill for you all, how about using it to be entertained by this vortex cannon?

32 thoughts on “This TARDIS Is An Infrasonic Subwoofer

    1. I’m FAR from an expert on the subject, but here is my understanding in simple terms (looking forward to correction!):

      If you think of a tube that is open at one and and has an oscillator at the other, the pressure waves set up by the oscillator travel along the tube until they hit the standing air at the mouth (far end) of the tube. There, they transfer some energy to the air beyond the tube, but they also bounce off that air to some extent and echo back up the length of the tube.

      If the tube is the right length, that is, some fraction or multiple of the wavelength of the sound, then that echo will bounce back off the oscillator just as it attacks again, which makes for constructive interference with the echoes, which makes for a powerful standing wave in the tube, which is resonance.

      In this case, 25′ was my rough estimate. The designer of this horn (lilmike over at avs) used a very cool tool called hornsrep to model the dynamics of the horn before he built it.

      Plans and notes and a copy of the hornsrep model are here:

      Ok Acoustical Engineers of Hackaday, give it to me straight! How far off am I?

        1. Technically I’d say it isn’t a transmission line, so you wouldn’t calculate its output using 1/4 wave theory. From the wave the driver connects to both the throat of the horn at its front and the rear of it is free to move in the output mouth of the horn, we would call it a tapped horn which Horn Response can simulate – as it was designed over at AVS. This lets you fit a horn with a lower cut off point into a smaller box than E.g. a classic exponential sealed rear chamber horn but the tradeoff is that the pass bandwidth tends to be choppier until you add multiple boxes to get the mouth area up a little. But at these frequencies all bets are off there anyway as the placement of the cabinet in the room and the room dimensions will have a massive effect on the perceived SPL at any location.

          The tapped horn was patented by Tom Danley (Danley Labs) who also created the unity and synergy horns. Take a look at some of the larger Unity enclosures if you want to see some cabinets that make this one look tiny!

          Great finish on the build though, no boring wooden box here.

    2. You aren’t trying to plug that frequency into an equation normally used for calculating RF antenna length are you? It’s going to be way different for sound because sound travels at.. well.. the speed of sound as opposed to radio waves and electricity which travel at about the speed of light. It has to do with how far the wave travels during the same time it takes to make one oscillation.

  1. Well if he’s worried about bursting his windows with just a 100 watt amp, I’d say it’s a success! Can something be made better? Sure, just about everything can be made better. Is it cost or time efficient to do so for this application?
    It’s a very nice build and it looks good. I think he can stand to be a few Hz off…

  2. Other than needing to sit, is there much difference between this huge sub and just bolting a “ButtKicker” tactile driver to your couch? The tactile driver is going to need 10x the power, but power is cheap nowadays.

    1. I actually have some buttkickers in the couch as well!

      This is definitely different though. It doesn’t really register entirely as sound or vibration from sound, which the buttkickers do. They just make it seem like you have a really loud but otherwise normal subwoofer.

      Down low, this produces more of a huge ominous fluttering in the air in the room. It feels like something big is wrong and you really sort of want it to stop. Its surprisingly visceral.

      1. “Down low, this produces more of a huge ominous fluttering in the air in the room. It feels like something big is wrong and you really sort of want it to stop. Its surprisingly visceral.”

        I love this. It’s a really evocative description.

        It also reads like a review for a Sunn 0)) album. :V

          1. Yeah, there is definitely some very low infrasound in some modern movies. This is making me curious, maybe I’ll grab a spectrum analyzer and start making a list of which ones.

            Also, lots of modern amps have a flat signal response and allow infrasonic frequencies. My NR906 does 5hz to 100khz.

            Certainly most home theater subwoofers roll off at 40hz – 50hz, which is part of what makes this thing so fun!

      1. The modern digital amplifiers can theoretically output down to DC, but the DSP firmware is going to have a DC block filter specifically to prevent a misformatted input signal from applying DC to the speakers.

  3. Back in ’77 at an audio company near Boston, we designed a folded, vented sub-woofer for the theater market. 3 different sizes of sono-tube placed inside of each other. The port was aluminum and was the heatsink for the amp.
    About 30″ in diameter and maybe 4 foot long (I don’t remember exactly – it was a long time ago).
    Stacking 10 of them on their sides in a 4-3-2-1 pyramid and driving with 10Hz was an experience! Not only did your gut hurt, but the old mill building we were in started shaking and dropping stuff from the ceiling.
    Fun stuff.

  4. Having actually helped lilmike build a couple of his folded and tapped horns, and been present for the first he built a good few years ago, it’s amazing just how much you can get out of a small speaker when you mate it with a box tuned specifically for that speaker.

    The first box was a single sheet of 1/2″ ply with a $25 woofer pushing ~125db all the way down to around 25Hz, when it would only run around 65db when open air.
    Mind you, that woofer did NOT like being fed 300W of test tones, since it was only rated at 160W RMS. We confirmed that the Kapton voice coil form stinks like little else can when you burn it…
    The up side of that was that it was only a $25 woofer, and we had another 5 sitting there.

  5. I find it fascinating that some “hauntings” have been attributed to sources of infrasonic noise. It just so happens that the chimney in a an old house is the perfect length and shape so that when the wind blows across it, it generates a tone that matches the resonant frequency of the human eye. It also gives you this weird feeling in your gut that something isn’t right. So now, not only are you seeing things that aren’t there, but you also have an irrational feeling that there’s a monster in the closet, and you need to get gone right now.

    As a sort of social experiment, I thought it would be neat to building something like this to “haunt” my apartment building and see how long it would take for the ghost stories to surface (and maybe get some noisy neighbors to move out).

    1. After hearing this thing that doesn’t surprise me at all!

      Like I said above “Down low, this produces more of a huge ominous fluttering in the air in the room. It feels like something big is wrong and you really sort of want it to stop. Its surprisingly visceral.”

      It is strangely unsettling. Which is so perfect for natural disaster movies!

    2. …and now we have another prop for the HaD Hackery Haunted House, right along side the directional speaker whispering “Get out!” right in your ear, and the vortex cannon providing the appropriately timed ‘breath’ for the whisper.

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