Ultrasonic basic questions

[wanchope]

Hi, all,
Ultrasonic sound (f>20Khz) is highly directional while audible sound (f<20khz) can spread all over the space. (correct me if i am wrong).:idea:
Can I use ultrasonic wave just above 20Khz for wireless communication (to replace RF)? At this frequency, the wave can still travel in almost all the directions, and could not be heard by human beings as well.
Any transducer available at this frequency range, say 21khz, 23khz that have an ultrawide beam angle?
Thanks.
Wanchope

 

[VVV]

I would not say that ultrasound is highly directional, it just depends on the transducer.
Yes, you can use it for wireless telecommunication. In fact, the first remote controls for TV's were employing ultrasound. However, they were quickly replaced by IR remotes, for a few reasons:

ultrasound disturbed pets, though humans cannot hear them
the doppler effect can play tricks on you if you move the remote while sending a command.
Since they were using multitones (one tone for each command) and filters to separate the commands, the number of commands encoded could not be very large.
You could hardly use two remotes for two different sets, say for a VCR and a TV, since interference was almost guranteed

With IR, all the above disadvantages disappear. They use encoding, which increases the number of available commands, removes interference, etc.

I suggest you go with IR, it is easier to find transducers, too. An old TV or VCR will provide one. You can buy an old remote control just about anywhere. You will also find easily info on how to decode the commands.
Basically, using a microcontroller, an IR receiver and your TV remote control you can start DOING something in no time.

 

[wanchope]

Thanks, VVV.
So can I confirm the directional properties is frequency independent?
I also worked with IR before. The typical transceiver i worked b4 has a communication range at 2.5 meters and viewing angle at 30 degree. These are the two limitations for my new design.
I want to have a device with 360 degree, 3 meter communicaiton zone that can talk to another similar device.
Software and encoding wise, i am less concerned at the moment. The issue is whether it is hardware feasible.
BTW, besides transducer, any other devices used to tx or rx the ultrasonic sound? Of course, speaker and mic included.

Thanks.
Wanchope

 

[flatulent]

The directionality is controlled by the size of the piston creating the sound compared to a wavelength. You can make a wider area of coverage by having the emitters around the periphery of a circle pointing out at equally spaced angles.

 

[wanchope]

Thanks, flatulent.
Have you come across any commercially ready products that can fulfill the requirement? The best i could find is www.hexamite.com/he123tr.htm with beam angle +/-35degree. But it is really far less than 360 degree.

Wanchope

 

[flatulent]

Since the market is for point to point systems, all of the transducers are narrower beam.

 

[echo47]

Are you talking about in-air communication? Several companies make spherical transducers for underwater use. They are nearly omnidirectional. For example:
**broken link removed**

Underwater transducers don't work in air due to large impedance mismatch.

 

[wanchope]

Thanks for your info, echo47. Yes, I am referring to in-air comm. What i do not understand is the 'impedance mismatch'. How does it come into the picture? Any website i can refer for more information of the difference between in-air and underwater usage?

Wanchope

 

[echo47]

Compared to an in-air transducer, an underwater transducer is designed to generate much higher forces over much smaller distances. If you put a transducer in the wrong medium, you get very inefficient energy transfer between the transducer and the medium. The mathematics are similar to impedance matching of RF devices.

There are probably many web sites with more info. Here's one:
https://ccrma.stanford.edu/~blackrse/h2o.html