what kind a instruments can help him?

[vasanthakumar]

hi i m a new guy.this is my 1st post.....
my brother is a magician.he need hear,within 200m whatever people say about his show.this is part of the show.
what kind of instrument can help him.,he want ,the instrument should work in 1.5v and tranmitter free
without the transmitter he need such a instruments.he want buy such a instruments.mention the name of such a instruments.
if u have a circuit for this plz give me

 

[Jack Luminous]

Look here: https://www.electronics-lab.com/projects/audio/023/index.html

 

[Dr.EM]

Realistically, I don't imagine any of those amplified ear devices working very well. Its worth a shot if your on a budget, but all it does is amplify sound that you hear anyhow, ie. it amplifies everything, it has no selecivity.

What is more suitable is a shotgun mic or a parabolic dish microphone, as these are directional and will pick up only in the direction they are aimed. A parabolic mic can be made from an ordinary elecret by adding a dish, apparently those domed mirrors work well. They aren't too inconspicuous though.

 

[audioguru]

The constant volume amplifier in that project will amplify background noise if nobody is making a sound nearby. But a nearby sound will overload it and it will immediately reduce its gain so that distant sounds won't be heard until the nearby sound stops and the circuit recovers.
200m? No way! 20m maybe if there isn't any sound nearby.

 

[Oznog]

You could plant a mike on certain tables. But it would take considerable concentration to listen in on a conversation while doing anything else. It's a noisy environment here which makes discerning voices and picking out one speaker from another quite difficult.

It can be potentially illegal to do this covert electronic surveillance. Not likely you'd actually get prosecuted for a magic trick, but it may be illegal.

Really you need an assistant given a free dinner in exchange for spying. They can track conversations and just give the lowdown to him via radio link.

These schemes don't sound like very impressive magic though, frankly.

 

[asi]

The best can be a shotgun or gun microphone.
I've build once one, because the officials was too expensive for me.
It was a usual (for microwaves) parabolic antenna used, but a codenser microphone in the focus.
The ampifier should have a very effective dinamic compressor, because of the changing signal levels in the operational range.
The noises from close can be filtered out with a trick:
Use a normal omnidirectional microphone next to the parabolic. Use an amplifier that makes a 180 deg. phase turning, and mix its signal with the other, coming from the parabola. The level from the omnidirectional mic have to be controlled, so you can kill the noise picked up by the parabolic.