We are a group of students from Furtwangen University, Germany. We would like to explore the effects of infra sound. We already are able to identify and measure infra sound waves but now we would like to generate it. So we are looking for an affordable way to create (powerful) sonic waves below 18Hz. The common subwoofer speakers normally go down to 20Hz and we don't believe, that the traditional way to create sound will work for our requiement. Does anyone have an idea for alternatives?
I would think a floor fan might be a good source of low freq sound. You might try putting a tube at the edge so the fan blades push puffs of air into the tube. you could also try a spinning disk that interrupts a jet of pressurized air. A resonant chamber, like a big whistle or recorder might work, but might be very big.
CSM2 sez: "The common subwoofer speakers normally go down to 20Hz and we don't believe, that the traditional way to create sound will work for our requiement."
Why do you believe that? Common subwoofers are *rated* for their response down to 20Hz because audio equipment is conventionally rated for the 20Hz-20kHz nominal range of human hearing. That doesn't mean they are incapable of operation below 20Hz. If you try it, you will probably find that a subwoofer can deliver usable results substantially below 20Hz - it shouldn't be difficult to try it out.
The power amp requirements for reproducing sub 20hz frequencies are unrealistic for all but the most trust funded among us to finance, and your basic household 15 amp service is gonna be seriously challenged in terms of powering it. It's really unsettling seeing the house lights brown out and everything digital spontaneously reboot every time a 15 cycle Subkick inadvertently makes it into the mix.
Plus you're gonna need big-ass built-like-a-tank speaker cabinets. For the amount of effort it'll take to properly woodwork a bank of 'em you'd almost be better off making a plywood form and casting them out of concrete .
Which is a great concept for an art installation or civic monument... "Town Square Subwoofers Of Doom." Make it solar powered, and have it blast out the 13 cycle fright note at noon and midnight everyday. Just to keep the citizens a tad on edge.
@Hiro: I read in several books about loudspeaker construction. The conclusion was, that speakers are physically not able to generate significant sound pressure below their Fs (resonance frequency). so I looked around for a chassis with as low Fs as possible and it was around 18Hz.
@cashsale: I transformed a standard pressuse gradient condesator micro to a pressure type. With some electronic behind (2-stage precision OpAmp) we are able to register sonic waves down to 5Hz (measured in front on a big sub which was playing a 5Hz wav file).
Were you able to generate infra sound? Can you explain how? I am very interesting in being able to do this, but have no experience with electronics. I am also, interested in the results of your experiment.
I have seen a driver on TV that works down to static pressure (DC). It is a fan with swiveling blades that reverse their pitch to reverse the air pressure. The pitch is controlled to create any frequency from static up to audible frequencies. Peak sound level depends on the power of the fan. This was mounted in the outside wall of a building, making the entire building a speaker enclosure. Invented by Bruce Thigpen of Eminent Technology. see: www.aip.org The rotary subwoofer: A controllable infrasound source. also: jpark.us/pubs/JASA_125(4)_RotaryWoofer_2009.pdf
Misc thoughts: Long organ pipe, air-pump siren, loaded resonator (musical sand dunes.)
Below is a group which built a long-pipe infra source to add "emotion" to a concert in 2003. A plugged pipe acts as a 1/4-wave resonator, so 23ft should be 1100/23/4 = 12Hz (although the article says 17Hz)
Sirens are just air pumps with AC choppers in series with the air flow. Probably need the 30ft organ pipe too, since the pump's inlet and outlet will emit 180deg out of phase, which would mostly cancel out.
A loaded resonator is just a short compact version of an organ pipe, but which encloses a dense springy material rather than air. For example, a tub of fluidized sand will act as a very low freq acoustic resonator, since the sand behaves as a very dense gas where the speed of sound is extremely slow. Tubs of water-foam act similar (remember the deep bass thunking of electric beater when whipping a bowl of meringue.) Or use fingernails to tap on the cover of a dictionary or phone book, and the bass "doooonk" sound is from the same phenomenon. I've not heard about anyone harnessing this as part of an infrasound resonant transducer. It's probably much easier to just use a 30ft piece of 1ft sewer pipe.