# Sound detection possibilities (ranges, reliability)

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#### wannaBinventor

##### Member
Hi all. It's been a while. Hopefully when life slows down I can get back into the hobby.

I the mean time, I had a question about sound detection possibilities.
I am envisioning some wireless on/off control without using RF or IR. Range need only be 50-75 feet. I'd want the signal to be an audio wave outside of the human hearing range, which I understand to be 50 to 20,000 hertz. I'd need probably 5 to 10 frequency ranges narrow enough and far enough from the others so as not to be "confused" with the others on the receiving end. I envision the receiver which would be encased in a small bit of material being comprised of a microphone to detect the sound and a bandpass filter to ignore irrelevant frequencies. The transmitter would be a small speaker outputting the desired frequency that is, again, outside of the human hearing range.

For example:
Consider a transmitter with 5 different buttons that cause 5 different frequencies to be emitted through one or more speakers. There would be 5 different receivers, each with a microphone (encased in a plastic container, perhaps) and a bandpass circuit that triggers an action only if that frequency is detected.

Would it be possible for one button to cause , say, a 25 hz audio signal that could be RELIABLY detected by microphone attached to a bandpass filter detecting 20-30 hz?
Then another centered around 45 hz, then another at say 20,500hz, another at 21,000hz, etc

Could this be reliably detected?
Could I ever expect a 50-75 foot range?
Are there cheap microphones and speakers capable of hearing and producing (reliably) say 20 hz to 25,000 hz?

EDIT:

#### wannaBinventor

##### Member

Would it be possible if it were in the range of human hearing? 50-20k hz?

#### rogs

##### Member
Would it be possible if it were in the range of human hearing? 50-20k hz?
Not easy. Very few (if any?) small loudspeakers can reproduce infrasonic frequencies (I guess that's what you mean by 'subsonic'?)
In addition, because an infrasonic 'carrier' would be such a low frequency any coded modulation would need to be very slow. And without coded modulation, there is simply too much ambient infrasonic 'noise' in the air to be able to detect an infrasonic frequency reliably.
Ultrasonics probably offer a better option... although even cheap common 40KHz ultrasonic transducers are going to be more than $1. 50, as alec_t says. Again, there's a lot of ultrasonic 'noise' about as well, so that's not going to work reliably without encoding either.. In short, its all going to be less reliable - and probably cost more - than a simple RF link!... #### audioguru ##### Well-Known Member Most Helpful Member Many items produce low audio frequencies but not cheap small speakers. Many expensive hi-fi speakers cannot produce above 20kHz so forget about a cheap small speaker doing it.. #### Externet ##### Well-Known Member Just use DTMF. A handy talkie with a keypad and a decoder chip at the receiving talkie audio can do marvels. #### rogs ##### Member Just use DTMF. A handy talkie with a keypad and a decoder chip at the receiving talkie audio can do marvels. Great idea -- except for the costing " less than$1.50" that the OP mentions!

I agree though, DTMF is a much underrated format for simple data projects....

#### Externet

##### Well-Known Member
I ignored the absurd $1.50 . Must be a mistype. For that price, at 60 feet distance, get a friend and voice him "push button 4" "now push button 3" and give him$1.50 for a hamburger. No electronic equipment needed.

#### audioguru

##### Well-Known Member
Where can you buy a hamburger for only $1.50? They are at least$5.00 unless you get a little kid's size one.
Yesterday I looked at the menu at the new hamburger joint called, "Five Guys Burgers And Fries" and an ordinary hamburger by itself costs $7.50. I noticed that they display a magazine review that says "Best hamburger for$5.00".

#### Willen

##### Well-Known Member
Where can you buy a hamburger for only $1.50? They are at least$5.00 unless you get a little kid's size one.

#### JimB

##### Super Moderator
While we are off topic, lets wander around and enjoy the view!

I don't think that I have ever visited a McDonalds.
About 15 years ago I had a very tasteless burger in one of the other big chain burger shops in the centre of Glasgow.
It was late at night and I almost never go to Glasgow (it is about 200miles away from here), the other thing I remember apart from the tasteless burger in an equally tasteless bread roll, was the thought running through my head :
"I hope that nobody I know comes in here and recognises me eating this rubbish!"

JimB

#### JimB

##### Super Moderator
Making amends for continuing the off topic jaunt...

The OPs requirement for a device with a component cost of less than $1.50 would probably only be met if you subbed the job to China and asked them to make 1,000,000 of them for you. Audio or sub-audio sounds like a bad idea which will not work reliably in a varying undefined environment. The best I can suggest is to investigate some of the simple RF modules which can be bought quite cheaply, but probably still more than$1.50.

JimB

#### audioguru

##### Well-Known Member
Smoke signals can be seen over that short distance unless it is dark outside, then Morse Code with a flashlight/torch will work.

#### rogs

##### Member
...Audio or sub-audio sounds like a bad idea which will not work reliably in a varying undefined environment.

The best I can suggest is to investigate some of the simple RF modules which can be bought quite cheaply, but probably still more than \$1.50.

JimB
Well, the OP might have already read my suggestions in post #4 , but now they have been endorsed, as it were, by a 'mod', they may now have more a bit more 'clout'