Sound Frequenicies

[vikdha1]

Hi, I was just wondering whether anyone had invented a device that could pick up frequencies such as 17.7 khz. Perhaps a radio or some sort of receiver.

Thank You

[deleted-71552]

Hi, I was just wondering whether anyone had invented a device that could pick up frequencies such as 17.7 khz. Perhaps a radio or some sort of receiver.

Thank You

Hello, vikdha1!

Devices exist for measuring and detecting frequencies across the electromagnetic spectrum. What is your project, and what are your goals / questions so that we can assist you?

[vikdha1]

I am trying to create a receiver of some sort that pick up the mosquito ringtone. I wanted to make a cheap device teachers could use in class so all those attempting to cheat can be caught.

Thank you

[deleted-71552]

I am trying to create a receiver of some sort that pick up the mosquito ringtone. I wanted to make a cheap device teachers could use in class so all those attempting to cheat can be caught.

Thank you

Ah! So your description refers to SOUND waves, not electromagnetic waves as I had assumed. Detecting sound waves requires a different sort of technology.

Are you considering making a product out of this investigation? Are you executing a science fair project about this topic? What are your next steps and your ultimate goal?

[vikdha1]

Yes, I am planning to make a product out of my investigation. I am a scientific research student at my high school and I will be entering the Palos Verdes science fair. My next steps were, if no one's already invented it, to research the makr up of a radio and try to construct a receiver to pick up those high frequency sounds

Thank You

[deleted-71552]

Yes, I am planning to make a product out of my investigation. I am a scientific research student at my high school and I will be entering the Palos Verdes science fair. My next steps were, if no one's already invented it, to research the makr up of a radio and try to construct a receiver to pick up those high frequency sounds

Thank You

Hello, vikdha1!

Let me make sure that I understand your project. I believe that you are interested in building a device that can detect the sound produced by the so-called mosquito ringtone - a 17 kHz ringtone that is above the hearing range of most adults. Is that right?

If so, this is not a radio-frequency signal. You can't build a radio receiver to pick it up. You must have some kind of microphone to pick up the sound waves. The microphone must drive an audio circuit that can somehow detect the frequency of interest and notify the user when it has been detected.

Your project is essentially an analog electronic circuit design exercise - it's possible to build an analog circuit that turns on a light when the sound frequency is between some values. As an alternative, you could build a circuit that digitizes the sound and processes the frequency via some kind of programmed device - a computer, a microcontroller, or a PGA. (Note that the digitizing circuitry already exists in the sound card of most PCs. An alternative to building a device would be to write some software that uses the input from a PC's microphone to accomplish your goal.)

I suggest that you focus your research on the technologies required to capture sound at your frequency of interest and the process of analyzing that sound.

To get you started, you might consider checking out Wikipedia's spectrum analyzer entry. Focus on the bandpass filter, acoustic, and spectrogram information.

Best wishes for success on your project.

[deleted-71588]

To add to what Brian's response, you proably want to do some research on the frequency spectrum of microphones. Audio engineers designing microphones for sound studios typically restrict their designs to 30Hz - 15KHz. Microphones designed for telephones are typically 300Hz - 3KHz. I suspect that many microphones associated with computers are intended for voice over IP and fall into the telephone type of range.

[deleted-71552]

To add to what Brian's response, you proably want to do some research on the frequency spectrum of microphones. Audio engineers designing microphones for sound studios typically restrict their designs to 30Hz - 15KHz. Microphones designed for telephones are typically 300Hz - 3KHz. I suspect that many microphones associated with computers are intended for voice over IP and fall into the telephone type of range.

Excellent point! I hadn't thought of that. I'm glad you chimed in because a microphone that's not capable of picking up the frequency under discussion would completely prevent success! Similar frequency limitations might be built into the sound cards on our PCs. This is another area worthy of investigation.

Thank you for pointing out this crucial requirement.

[Louise]

Another variation on this theme is to explore the idea that this frequency cannot be heard by people over 30 and can be heard by people under thirty. That is, you could test a large number of people of various ages, and see if they can hear the ringtone (you could even change test different volumes too). I know some "old" people can hear the ring tone, and I'm sure some teenagers cannot, so it might be very interesting to explore the distrubution of hearing abilities by age.

This experiment will also require a bunch of research to determine how to effective design this experiment, and how to determine the validity of the results (statistical analysis). It may require additional approval from your school (since it involves human subjects).

Louise

[vikdha1]

Thanks for the information about the receiver not being possible. The idea of inventing a software sounds interesting but I am not very good at computers and that kind of stuff. But I was very interested in the light bulb invention that would turn on between certain frequencies. However I'm not sure how to do that. What would I use in place of a microphone. And for the light bulb invention would i require a computer or anything?

Thank You

[deleted-71552]

Thanks for the information about the receiver not being possible. The idea of inventing a software sounds interesting but I am not very good at computers and that kind of stuff. But I was very interested in the light bulb invention that would turn on between certain frequencies. However I'm not sure how to do that. What would I use in place of a microphone. And for the light bulb invention would i require a computer or anything?

Thank You

Hello, vikdha1!

For the light-bulb experiment you would not need a computer. You would need a microphone or similar device to convert the sound waves into electrical impulses. These impulses would need to be amplified and fed into something like a band-pass filter. A band-pass filter allows signals within a specific frequency range to pass while attenuating frequencies outside of the range. A little circuit logic on the output of the bandpass filter would turn the light on when the signal strength exceeded some threshold indicating that the ringtone was present.

You would be desiging and building an electronic circuit. I suggest that you research analog circuit design, especially band-pass filters. There are many ways to build such a circuit, and part of your project may be to weigh the pros and cons of the different approaches.

Best wishes for success with your project!

[vikdha1]

Hi,

I have researched the analog circuit and band pass filters which I will require to create my device. However, I was wondering whether a sophmore in high school would be able to create something like this. I just wanted to know if I would require some extra help or need to obtain more extenisive knowledge about a subject to conduct the experiment. I am only asking because I do not want to attempt something that I do not have a chance of succeeding at.

Thank You so much.

[deleted-71552]

Hi,

I have researched the analog circuit and band pass filters which I will require to create my device. However, I was wondering whether a sophmore in high school would be able to create something like this. I just wanted to know if I would require some extra help or need to obtain more extenisive knowledge about a subject to conduct the experiment. I am only asking because I do not want to attempt something that I do not have a chance of succeeding at.

Thank You so much.

Hello, vikdha1!

In my opinion, a high school sophomore is capable of doing this work. When I was a sophomore, I had an electronics class in which I built a simple AM radio. Sure, you will need some guidance. We all do.

I think the biggest challenge you will have is in assembling the components. To make it simpler, and to avoid soldering and circuit board fabrication, I would use a breadboard kit available from places like Radio Shack. A breadboard kit will allow you to make connections by plugging components into a grid and making connections with colored wires.

[vikdha1]

Hi,

Thank you so much for the advice about the breadboard kit. I will make sure that I get that soon. However, I researched the bandpass filters and the microphones and all the other components and all the online prices seem very expensive and I just wanted to know whether the prices I had researched were correct. For example, the bandpass filters I saw were no less than $160.00

[deleted-71552]

Hi,

Thank you so much for the advice about the breadboard kit. I will make sure that I get that soon. However, I researched the bandpass filters and the microphones and all the other components and all the online prices seem very expensive and I just wanted to know whether the prices I had researched were correct. For example, the bandpass filters I saw were no less than $160.00

Hi, vikdha1!

Wow! I couldn't afford to build a science fair project using $160 filters!

What you spend on your project is up to you. When we started discussing this project, however, I assumed that you would be building a band-pass filter out of component parts - resistors, capacitors and inductors. I just looked at an online catalog from Digi-Key (http://www.digikey.com) and I saw individual component parts from $0.25 to $1.00. My estimate is that you could build a bandpass filter for under $10 using individual pieces.

The key here is to design the circuit so that you can specify the appropriate component values. If you do a search on "band pass" you will find web sites that describe hwo to do the design such that the center frequency of your filter is at the same frequency as the mosquito ring tone.

[vikdha1]

Hi,

Oh wow! I am so glad I asked you about that because I would have probably ended up spending hundreds of dollars. Thanks so much for the useful advice.
However, would it be possible for me to construct a band pass filter from those parts solely from the instructions online. Also, when I was researching it seemed that the band pass I would need would be very complex consisting of three different components in order for me to target my specific frequrncy. I just wanted to know whether or not that was the right kind of design.

Thanks again.

[deleted-71552]

Hi,

Oh wow! I am so glad I asked you about that because I would have probably ended up spending hundreds of dollars. Thanks so much for the useful advice.
However, would it be possible for me to construct a band pass filter from those parts solely from the instructions online. Also, when I was researching it seemed that the band pass I would need would be very complex consisting of three different components in order for me to target my specific frequrncy. I just wanted to know whether or not that was the right kind of design.

Thanks again.

Hello, vikdha1!

Band-pass filter designs are rather simple. Yes, you might need to use three kinds of components - resistors, capacitors and inductors - but these are common. It's not unusual for a product to have dozens of such components.

The band pass filter is only one stage of your overall design. I see your project as requiring 4 major stages or parts:

1) Microphone and amplifier for picking up and boosting the signal

2) Band pass filter for blocking everything but the ring tone

3) Analog to digital to convert the level of your input signal to a 1 or 0 - on or off

4) A circuit to trigger a light, bell, or other output device.

Each of these stages requires you to research the design and then build the circuit. There are plenty of resources on the Internet to help you figure out how to design and build these four stages.

I hope this helps.

[vikdha1]

Hi,

Sorry to bother you again, but I have researched all the four components you have said I will require to build my project, but I cannot figure out how to combine all those components to make one central component. For example I do not know how to connect the microphone and amplifier to the band pass.

[deleted-71552]

Hi,

Sorry to bother you again, but I have researched all the four components you have said I will require to build my project, but I cannot figure out how to combine all those components to make one central component. For example I do not know how to connect the microphone and amplifier to the band pass.

Hello, Vik!

The output of the first stage must be compatable with the input of the second stage. When doing audio signals, such as from the microphone to the audio amplifier, you must match what's called impedance. It's the same thing as stero speakers. Some are 8 ohms, some are 16 ohms, and so on. If your stero output impedance is 8 ohms, you should use 8-ohm speakers for best performance. (The reality is that 16 ohm speakers will work, but they won't be optimal.)

For other stages, such as the input to the stage that turns on a light, the output should be digital - perhaps swinging between 5 volts of "on" and 0 volts for "off".

[deleted-71588]

You might want to do a little research on "Operational Amplifiers". Most Op Amps today are inexpensive integrated circuits. By themselves, they are extremely high gain, have a high input impedance, a low output impedance, and a very wide frequency response range from DC up to MHz which make them extremely useful in analog circuit design. With the addition of a DC power supply and a few resistors and capacitors, they can be tailored to a wide variety of applications. There is a whole class of analog circuit design devoted to working with Op Amps. Much of the expense of building filters can be eliminated by using Op Amps to eliminate the impedance interaction between L-R-C filters.

[bradleyshanrock-solberg]

I agree that a high school sophomore can do the work, but you can only afford it by building most of the components from scratch.

I also agree that you should look at some of the inexpensive technology out there that has done a lot of the work already. In college we built a very sensitive vibration detector just by stripping apart a Radio Shack microphone and hooking it up to an amplifier and an oscilloscope.

The tech is a lot better now. A lot of what is challenging is just picking up the vibrations. A cheap modern microphone may do that for you. Then run the signal through an amplifier and a filter. The advice given already though is already well beyond my expertise. This is just to reinforce that you should not have to build everything yourself. Buy the off the shelf stuff that is cheap, then use components to amplify and filter the result.

[vikdha1]

Hi,

I'm sorry to keep bothering you. I'm having a little trouble. I just wanted to clarify the procedure. So first I obtain a normal microphone with a certain ohms and connect that to an op amp of the same ohm. Then I connect the op amp by an analog curcuit to a band pass filter that I have built myself. Then I finally connect that to another analog curcuit which is connected to a breadboard and then I connect a digital wire to that same breadboard which is connected to a light bulb.

Also I wondering, if I will be requiring a digital curcuit, how would create it and how would I convert it from an analog curcuit to a digital curcuit.

Would it make my project if I just purchased a receiver and tranmitter board (RLP and TLP 315).

Thank you,
Vikram

[deleted-71552]

Hi,

I'm sorry to keep bothering you. I'm having a little trouble. I just wanted to clarify the procedure. So first I obtain a normal microphone with a certain ohms and connect that to an op amp of the same ohm. Then I connect the op amp by an analog curcuit to a band pass filter that I have built myself. Then I finally connect that to another analog curcuit which is connected to a breadboard and then I connect a digital wire to that same breadboard which is connected to a light bulb.

Also I wondering, if I will be requiring a digital curcuit, how would create it and how would I convert it from an analog curcuit to a digital curcuit.

Would it make my project if I just purchased a receiver and tranmitter board (RLP and TLP 315).

Thank you,
Vikram

Hi, Vikram!

Craig_Bridge made a good point about operational amplifiers with high impedance. My comments about matching impedance really apply to
output circuits. You will want to connect the output of your microphone to the input of an amplifier with high impedance. High input impedance is desirable so that your circuit does not put a heavy load on the input device - your microphone.

When I wrote that the output of your circuit might need to be digital, I ment that in the strictest sense of the word. It has to change from "off" to "on" when the ring tone is detected. If you want to drive an LED and you are powering the circuit with AA batteries, that might mean that your output swings from 0 volts to 3 or 5 volts.

Operational amplifiers (Op Amps) can be used to design a circuit that will drive the output in the manner described, above. In fact, Op Amps could be used to build every stage of your project circuit. (I wish I would have thought of the Op Amps at the beginning of this discussion!)

I don't know what the model numbers you listed in your last post mean. Are they devices from Radio Shack?

[vikdha1]

Mr. Castelli,

I'm really sorry to keep bothering you like this, but I still have some questions about my project. I've been looking up buliding band pass filters everywhere and I was just wondering if this site explained it well; http://72.14.253.104/search?q=cache:iOV ... cd=1&gl=us

Also I was wondering how I would use the breadboard you recommended to connect the various parts of my device. Would I be required to connect analog curcuits to the breadboard or something of that sort?

Thank you,
Vikram

[deleted-71552]

Mr. Castelli,

I'm really sorry to keep bothering you like this, but I still have some questions about my project. I've been looking up buliding band pass filters everywhere and I was just wondering if this site explained it well; http://72.14.253.104/search?q=cache:iOV ... cd=1&gl=us

Also I was wondering how I would use the breadboard you recommended to connect the various parts of my device. Would I be required to connect analog curcuits to the breadboard or something of that sort?

Thank you,
Vikram

The web site provided is not the best I've seen. It's very theoretical. I think you'll have better results if you search on something like, "op amp bandpass". I used Google to run this and found some sites that I think would be more helpful.

I want to emphasize the point that Craig_Bridge made about the use of operational amplifiers (op amps). They will simplify the work you need to do, and you can find many cookbook-like tutorials on the web. In fact, op amp manufacturers often provide design tools on their web sites.

Breadboards are grids of holes for inserting IC, components and wires. Searching for "breadboards" will yield pictures and descriptions that will make this pretty obvious for you.

[vikdha1]

Hi Mr. Castelli,

I was searching the op amps and bandpass filters and I cam across some op amps that only cost about $2 and seemed to be pretty well developed. I just wanted to check if I was looking at the right sites and if the op amps were actually that cheap. Also, I found one that could go up to 50 MHz, but it said that it could be adjusted. Would that be suitable for my project or not? Lastly, I wanted to check that if I purchased an op amp with an adjustable frequency, would I still be required to construct my own bandpass filter?

Thank You,
Vikram

[deleted-71588]

Integrated circuit Op Amps are very inexpensive as you have found. The expense comes in the test instruments, power supply, bread board, and components you put around them.

I wanted to check that if I purchased an op amp with an adjustable frequency, would I still be required to construct my own bandpass filter?

You adjust the frequency response of an Op Amp by adding bandpass components around it. There are several Op Amp "circuit cookbooks" around that describe different ways of doing this.

Do a little more research (reading) on Op Amp circuits and post back a link to a specific circuit if you want some specific help in understanding it.

[vikdha1]

Hi,

I have been working on my project ever since I posted my last question. Just to refresh everyone's memory, I was working on creating a receiver that would pick up the mosquito ring tone and convert the frequency into volts that would ultimately turn on a light bulb. I have the basic design of the receiver figured out , but I still I have one question. I was told that i forst must find out the number of volts produced by the 17.7 kHz of sound from the ring tone. I was told that this was neccessary because in order to buy the op amp, I would have to know how much to amplify the volts by. I was hoping that someone knew the volts in that frequency or a way that I could find out the number of volts in the 17.7 KHz. If there is a device that does it, where can I find one?

Thank you.

[deleted-71588]

I have the basic design of the receiver figured out , but I still I have one question. I was told that i forst must find out the number of volts produced by the 17.7 kHz of sound from the ring tone. I was told that this was neccessary because in order to buy the op amp, I would have to know how much to amplify the volts by. I was hoping that someone knew the volts in that frequency or a way that I could find out the number of volts in the 17.7 KHz. If there is a device that does it, where can I find one?

You have chosen a fairly difficult problem to solve that needs to be broken down into stages. I don’t understand how you can have the basic design of a receiver figured out if you don’t have a better idea of the properties of the signal you are trying to detect. Personally, I wouldn’t classify the circuit you will be building as a receiver.

An electrical engineer with a background in signals and systems would probably break the problem down into various stages and think about the signal properties at the input and output of each stage.

What is the source of the signal you are trying to detect? In your case I think it is a “mosquito ring tone” sound wave from a cellular phone at a frequency of 17.7 KHz. This is only a partial characterization of the signal. You have a type of signal (sound wave) and a frequency (17.7 KHz) but you are missing an amplitude.

What is the desired output? In your case, turn on a light bulb. Again this isn’t a fully specified requirement; however, may I recommend a substitution of a LED (light emitting diode) to simplify the problem. A common 20mA variety is inexpensive and doesn’t take much power and is simple to work with, especially the ones with a built in current limiting resistors designed for 5 VDC operation.

First Stage:
The first stage in coming up with the rest of the circuitry is coming up with a suitable transducer to convert sound wave energy into an electrical signal (aka a microphone). Inexpensive microphones designed for telephone use typically have a limited frequency range of 250 HZ to 2500 HZ to pick up the human voice and limit the signal bandwidth needed for transmission and that more expensive studio recording microphones typically were designed for 20 HZ to 15 KHz.

Have you chosen a microphone? Do you have a data sheet for it? A typical data sheet for a microphone will tell you what its output impedance is (typically in ohms) and what its frequency response is. The frequency response information is typically a plot that will show the electrical output (typically in micro or millivolts) expected for some dbm of sound at all frequencies through its range. You might also think of the microphone as a receiver because it receives sound waves.

In this case, the appropriate question is how many volts of 17.7 KHz will the microphone produce. Even with a data sheet that has a 17.7 KHz point on its response curve, you can’t answer this question because you don’t know how “loud” or how much sound energy the cellular phone will generate and how much of the sound energy that the phone produces will propagate to the microphone. The best that you can come up with beforehand is an educated guess.

Second Stage:
In order to keep the microphone happy and derive the most signal from it, you need to provide a load impedance that matches its output impedance. Typically this is done with a simple carbon composition or carbon film resistor equal to what the microphone data sheet calls for and a high impedance unity or low gain buffer amplifier. A simple Op Amp circuit will do here. Because the microphone will also be producing a signal for all sound frequencies, you probably want to provide some form of high pass filtering to this stage so that frequencies below 10 KHz will be attenuated.

You should look up “Bode plots” to get an appreciation for the frequency response of simple R-C filters.

Filter Stage:
Because the frequency response of any microphone falls off quickly at the high end of its frequency response range, you won’t need as much high frequency suppression as you will need lower frequency suppression. I suspect something like a two pole (again understand this from a Bode plot) high pass filter at 12 to 15 KHz and a single pole low pass filter at 25 KHz should be considered. This stage should also provide amplification to compensate for signal loss at 17.7 KHz introduced by the filtering. An Op Amp circuit can do this.

Detector Stage:
So far, all of the electronic stages so far have involved AC signals representing sound waves picked up by the microphone. To operate the LED, you need a DC signal that represents the presence of an AC signal at 17.7 KHz. The reality is that it would be extremely expensive to design and build a precise detector for just one frequency. If you are willing to accept false positives for all sound signals between 15 KHz and 20 KHz then the detector stage is fairly simple.

You first need to AC couple the signal with what is known as a DC blocking capacitor. To convert AC to DC, you need a small signal diode (half wave rectifier). To integrate and discriminate this half wave signal, you need an RC tank circuit. I would start by choosing the time constants such that tank fill time is approximately 10 cycles and the tank discharge time is approximately 30 cycles as a starting point.

Comparator and Output Stage:
You need to compare the output of the tank integration circuit with some threshold. The threshold can be established by a reference voltage derived from zener diode and a resistor divider network. I would pick a zener diode voltage of slightly above half of your power supply voltage. If you are using 5 VDC or a 6 volt battery, something like a 3.3 volt zener. An Op Amp makes a good comparator and will easily drive an LED.

Power supply, decoupling, noise, and trouble shooting
Electrical circuits operate in the presence of noise. There are many sources of electrical noise. Some are external and are picked up or induced into the circuit. Some come from components in the circuit itself like thermal noise or electron shot noise. If the DC power supply comes from AC source, no amount of filtering will reduce all of the conversion noise (again the Bode plot will explain this).

It is very important that the power supply leads of each Op Amp have a decoupling capacitor directly across them. Something like a monolithic 0.1 uFd.

To design, build, and trouble shoot a multi-stage circuit like this, you are going to need somebody local with electronics knowledge to help you along with some test equipment. There are just too many simple things that can go wrong to ever hope for a circuit this complex to work the first time even for a highly experienced electrical engineer with years of experience. Development of circuits like this are done a stage at a time and tested using an oscilloscope and signal generator. We can help with the concepts and design; however, we can’t spot construction issues, test equipment issues, or other simple problems remotely. Even with highly skilled technicians on site, remote trouble shooting is extremely difficult and communication error prone.

A good starting point would be to put together a setup with a cell phone with a mosquito ring, a phone you can use to call the cell phone, your microphone, a load resistor, and an oscilloscope in a quite room. Look at the change in signal when the cell phone rings. In other words, to answer your starting questions, you are going to have to measure it yourself!

[vikdha1]

WOW, thanks for your detailed and informative reply. I am very grateful that you took the time out to explain the mechanics of the device to me. You could have possibly saved me alot of time and money.

Over the last few months I have met with an engineer twice who had explained the calculations and the what he said was the basic design of my project. However after reading your detailed reply, I am afraid that he has misguided me.

His instructions seemed to simple to be true. He said that all I had to do was feed my ringtone into a microphone with an input of 20 kHz that was connected to an osilloscope to find the volts in the ring tone. Then I was supposed to find a suitable op amp after calculating the correct frequency pass. He said that I only need a high pass filter and that a low pass filter was not required. He told me that I just had to connect the mic to the op amp and connect the op amp to A RC circuit after doing the correct calculations.

My final step would be to hook the rc circuit to a 3 or 6 volt light bulb. He tld me that my whole device would be connected on a breadboard.

However, after reading your reply and researching the bode plot and various of the other components, I think the advice he gave me was incorrect. Was it?

Thanks