Hi, I need to check some audio frequency signals (20hz-20Khz), but as always on this forum I'd like to do it on a budget. I can see spectrum
analyzer and oscilloscope software on Google Play so I can use a phone or tablet to display. But getting the signals into the device is the issue.
This will be a wired connection (so no microphones needed), but I'd prefer it to come via USB as data rather than through the microphone
connection as analogue. Is there any budget stuff out there that can do the sampling bit for me?
Or if anyone has any ideas how I can accomplish this task that would be great. I mentioned either using an oscilloscope or spectrum analyzer because I
could use either. The input signal will be controlled, I need to measure signal amplitude at single frequencies to see how consistent it is through
the range.
I did something quite close to this recently, in another forum a group of us were experimenting with using oscilloscope or PC soundcard or android
devices to diagnose faulty fuel injectors --- this actually is quite a useful technique
With an Android I did a bit of searching around but other than connecting a microphone straight to phones audio socket there weren't any low
cost options I could find although there are some very good android apps including one available from Playstore particularly a sound &
vibration app released by Bosch.
On a Windows PC there are more hardware options I would suggest using a external USB sound card rather than an onboard sound card you could also
use a USB isolater it you want to protect the PC . You can record the sound signal then FFT analyse it in Audacity or similar program.
As for USB Oscilloscopes there are quite a few chinese scopes on the market, the bigest draw back with them is usually the software, Hantek software
can be a pain in the neck to use. I speak from experience I have the 8 channel (!!!!!) Hantek 1008 sitting in a drawer Although not the best in the
world Hantek stuff usually works and is very cheap. Owon make very decent cheap USB DSO's but the software is even worse than Hanteks. I
have an Owon DSO7102v -- which I use a lot but because the software isn't great I rarely it connected to a PC.
You might also want to look at the cheaper Velleman scopes
This thread on the Scannerdanner forum details what we were doing --- the USB external soundcard and Audacity worked really well for doing the
FFT.
Link to thread
on Paul "ScannerDanner" forum
[Edited on 10/8/17 by britishtrident]
It's the range of oscilloscope and analysis apps in the store that got me thinking what could be done, it was just the hardware element. If I did
go for using the microphone input on some sort of isolated wired signal (rather than an actual microphone), will the phone itself attenuate the
signal? I did assume that because it's a voice mike that the input would be biased towards the voice frequencies, and it's actually the
lower end that I'm particularly interested in (20hz +).
But its interesting that a knock sensor and a sound card audio in could produce that sort of result, perhaps I don't need to shy away from them
that much.
The problem with all audio input devices not designed for general use is that they can and do have a frequency response that may or may not harm your signal. They all block DC (and potentially just above) if you don't mod them, so that's a negative to start. They all will have zero effort put into dealing with signals over about 12Khz because humans just don't care about it. The roll-off at either end is unknown and undocumented. If you're comparing between two items relatively, you're probably OK. If you're looking for absolute results, use proper hardware and calibrate it.
As above ----- 20hz is really classed as a vibration rather than sound, normal microphones don't really work that low down the frequency range
and most sound apps and programs aren't much use at the low end.
A lot depends what you are try to capture I would be looking at cheap piezo vibration sensors and stick on piezo contact microphones (piezo
buzzers).
These can produce fairly high peak voltages but the voltage spikes can be clamped to 5v with a load resistor and/or zenner.
Oscilloscopes have various input voltage limits --- always check the specification the input voltage range can be extended by using an attenuator
to reduce the voltage at the oscilloscope.
Attentuators come in factor x10, x20, x100 and x1000 options.
Piezo vibration sensor link
Basically I'm trying to find out whether the stereo in the car is frequency filtered or whether it's just sh** speakers. So while I
appreciate 20hz might be the limit of unfiltered audio, I do need to see what would be below the range of speech. My plan is to have one phone as a
tone generator, connected to line in, then a second phone hooked up to each speaker feed in turn, seeing what comes out across the audio range.
Then I can figure out what replacement speakers would be best.
I can't see a phone working very well as tone generator below normal audio range.
