possible to make directional infrasound from a small device?

Produce, yes; project, not easily. Only in the near-field wouild the sound seem 'loud'.

Not really, no. One can cheat, of course; produce 35.007 kHz and 35 kHz ultrasound, and direct those together. An 'ear' with some nonlinear response could hear the 7 Hz component, even if it were insensitive to the ultrasound.

Follow your feeling.

To have reasonable directivity, the size of the emitter needs to be large with respect to wavelength. At 12 Hz, you have about 28 m wavelength in air. So a small (hand held) device will not produce a narrow beam when emitting 7...20 Hz audio in air.

You can compare your situation with radar antennas. Look to the "old" HF over the horizon radar systems (OTH radar).

Best regards,

Wim PA3DJS

It might be possible if high power compression (shock) waves were directed at 7 per second. But not sinusoid.

Interestingly you can make a highly directive emitter with a subwavelength structure, provided it's resonant. Schelkunoff wrote a paper back in the 1930s about how to do it with antennas.

Google "electrically small antennas".

They're not noted for extreme efficiency, however.

Resonant loops, as used in crystal radios, are also electrically small antennas--their effective area is increased by a factor of (I think) Q**2.

Cheers

Phil Hobbs

I think Bill Beatty at amasci.com has a lot on that topic IIRC

Why would you want to do that?

John

Same reason I wanted to do this when I was 16 - to blast people into jelly.

Maybe a clip on device like the kind you attach to a seat. It would have to be very heavy to work at all. Maybe a fan ??

I hear air handler sounds in that range but need huge fans or motors.

Funny, I was getting about 7 Hz in my house a couple years back. off and On. Like the earths resonance. It would last up to 30 seconds and vibrate doors and windows.

greg

Is this what the tin foil hat brigade is on about?

Ask your friends in the KOOK group, you KOOK bastard.

Hello Phil,

You are right, it is surprising toe see then when you try to cancel most of the radiation from an electrically small array (by playing with phase and amplitude), you can have a direction where the canceling is not that large anymore (so you have directivity).

The Q factor issue will very likely be present for audio also as the transducers in the array must cancel most of the sound pressure produced by the transducers. This will result in local sound pressure far beyond what a gas at 1 Bar can support, I think.

Wim PA3DJS

Phased ultrasound arrays in air are quite do-able, and are in fact used in some types of non-lethal area denial weapons.

BTW, air can support almost arbitrary pressures as long as they are not negative.

.net

This will be the problem, the nearby sound pressure for a superdirective array with size < lambda must be extremely high to get useful output at distance. In such cases atmospheric pressure will be the limitation (and the hardware to generate it).

.uk/- Transcendence UK

Best regards,

Wim PA3DJS

BF > Is it possible to produce infrasounds BF > (in the 7 to 20 Hz range) with a small BF > (coat pocket size) electronic device? BF > For example, can a small speaker NF > produce such low sounds?

BF > And if so, is it possible to use a reasonably BF > small parabolic reflector to aim them?

G > Is this what the tin foil hat brigade is on about?

Archie > =A0Ask your friends in the KOOK group, you KOOK bastard.

Why are you so cranky about this, Archie?

Go away, you retarded KOOK bastard.

If the sound power is high enough, air nonlinearity produces energy at the difference frequency. It's not just in your head. In that case, you'd want to be several wavelengths distant from your victim, to give your sound beam enough time to convert itself from ultrasound to subsonic.

See the "hypersonic" speakers (speakers, and nonlethal weapons.)

Often I've wondered what you'd get if you connected a big electrostatic loudspeaker to a Tesla Coil output. Hundreds of watts of acoustic power? (Try a 50KHz TC. Try a bowl-shaped ESL, so the ultrasound comes to a focus. Perhaps it would char an absorptive object.)

Not square waves from a small multivibrator either, I guess?

I doubted it, but worth asking. Thanks.

What kind of 'ear'?