Arif wrote:

This project sends UT and when any object comes under its range which is 5cm to 10m it reflect back this UT and another UT transducer sence this and give alarm.


Hallo Arif,

I don't know where your application is located, in-door use or out-door? But what you stated above, will only work out-door. If your application is inside a room, this reflecting mechanism will fail, very probably!
'Reflecting' means, that something like a ray is travelling. This might be a good model for light, but for US that's not a good explaination. If you put a UT in a room emitting US, what you get is not a ray, travelling from point A to point B, with neglecting intensitiy elsewhere, but what you get is a three dimensional field. Means, the whole room is filled with US!! This comes from the fact, that US is reflected much better at each surface, like walls, tables, etc, than light. Intensity of reflected US is astonishing high, even if surfaces which cause these reflections are rather rough and uneven!

Another consequence of good reflections is, that due to interference there are highly unequal intensities to be measured at different locations in the room. And although you cannot hear US, of course, you can test this performance very easily by the following setup: Take your hifi-desk, put a CD into your CD-player, which contains some test signals like sinus, sinus-burst, pink noise, etc. and choose a rather high frequency sinus signal with constant amplitude over a period of about one minute. Take care, that distortion of your hifi-equipement (including loudspeakers!) is minimal. Additionally, unplug one of the two loudspeakers, so that signal comes from only ONE loudspeaker. If you then walk through the room, you will observe two interesting details:

1. Close your eyes and try to locate the origin of sinus signal. You can't, it's impossible!

2. More interesting, you will find several locations in your room, where sinus signal completely vanishes, and where it is maximum.

When using US inside rooms a different approach is better suited for burglar alarm: Put a strong US emitting source somewhere inside the room. Locate several US recievers at different locations inside the room. (For small rooms like cars also small number of recievers is sufficient.) Then, measure amplitude of US signal at each US reciever and detect the CHANGE of amplitude, as a criterium for the alarm.
The mechansim behind is: In the event of burglary, person (burglar) is changing geometry of room and by this three dimensional interference pattern. This causes the change of US intensity at each reciever. Even, if the burglar is NOT penetrating the direct space between US emitter and reciever!!

Clever US burglar alarms only detect the 'rather rapid' change of amplitude, how it's observerd with a real person moving through a room. By this, all longer lasting changes of amplitudes, which are not caused by a burglar, but more by the drift performance of electronic parts, can totally be cancelled without causing an erroneous alarm! Keep in mind, that not only the specifications of electroinc parts will show drift performance, like long-term drift, temp-drift etc., but also the physical properties being involved with propagation of US in air!!

If you instead want to introduce an one-emitter-one-reciever path inside a room, where the criterium for alarm is the relevant increase of intensity of reflection, as you stated above, you will at least get problems in finding a suited location for the reciever. Why? Because, due to the unavoidable reflections at the many surfaces of the room, it's hard to find a suited location with minimum amplitude without reflection and with stabil increasing amplitude with a reflection. I guess you will mostly observe only a decrease of amplitude or best an unrelevant increase, when a burglary takes place.

Will say: Not the increase alone of amlitude should be a criterium for alarm, but the CHANGE of amplitude. Concretely spoken, the 'rather rapid' change. While 'rather rapid' is defined by what is observed with a typical and educated burglar, acting in the field... {grin}

Bye,
Kai