sound and usb 6009

[the German guy]

Hi guys,

I want to measure the sound pressure level of a hand held meter with the USB 6009.

So far I could connect it to lv and could get exactly what I have on the display of the meter on the vi front panel.

Now I want to analyse the data a little more and find frequencies in the acquired signal. I tried this before via sound card input and  an example vi determining the frequency with the highest peak.

It worked fine with the sound card, but I can't get it to work with the USB 6009.

Any ideas?

Thanks for your help.

[Conseils]

There are two wrinkles that immediately come to mind...assuming that you have checked the test panels in MAX.

One is the possibly slightly lower / different sensitivity of the 6009 and the other is the different input impedance. Either of these could be a problem. You need to consider the output levels and drive capability of the sound measuring device and compare it to the input sensitivity and impedance of the 6009.

Well that's a first guess anyway...

[the German guy]

What exactly is the drive capability? I have an output of 10 mV per dB on the sound level meter. So I get a max output of 1.3V when the meter displays its max of 130 dB. I could get the unit to work under MAX and adjusted it so that I set the device to read the 1,3 V. I'm pretty new to LV I have to admit.

[Conseils]

It's as simple as the ability of the output to 'drive' or sufficient current for the intended input.

You could imagine something like trying to push as car up a hill. If you are strong enough then it's not a problem. But if you aren't then it just stays where it is.

If it ( the signal) can be seen in MAX then there is no reason for it not to work with LabVIEW. I seem to remember that the minimum range for the 6009 is 1v. Thus you could well have saturated the input. Also Remember that the 6009 ia a 14bit A/D; esentially that's the number of divisions available over the the input range and determines the smallest number that you can resolve to.

You may just need to change the input range to a higher range... but you will loose resolution.

If however you get two resistors of the appropriate values you might possibly scale the input to obtain the best sensitivity, but the low input impedance (resistance) of the 6009 is likely to cause you problems.

Does this sort of make sense ?

[the German guy]

Let me thank you for helping me first. After posting my first message I thought that no one would respond. I'm really a newby therefore don't understand everything so fast. I just picked up the manual for the usb6009 and it says that I can measure +- 20 V in differential mode. So I don't understand how I can saturate it with only 1.3 V.

The input impedance of the usb6009 is 144 kohm, the impedance of the sound level meter is 100 ohm.

I don't know a lot about that stuff as I'm just in mechanical engineering. I only have some basic knowledge about that.

So thanks again for taking the time

[Conseils]

Here it comes.... a crash course in Analogue to Digital Convertors...

The signal needs to be digitised into 0 and 1's for the computer.
The device that does this is an A/D (Analogue to Digital) Convertor
These are expensive to make and they have a range of numbers over which they can work.
This range is expressed in bits (6009 = 14 bits)
In our case 14 bits =  16384 combinations or increments
The range 16384 has to be divided into the maximum voltage input of the A/D
To get the resolution divide the maximum input voltage range by the number of bits

The above means that if you were to have a fixed input voltage range then the step size would be to big for some measurements or the input range too small for others. What you do is to scale the input capacity by ranges..
In the case of the 6009 there are 8 input ranges available from ±20.0 to ±1.0
These ranges allow the best resolution over the users input voltage range.

Following on a bit if you select the 1 v input range and input 1.1v to the A/D, the counts on the A/D have reached the maximum number and won't go further.

So in summary, depending on how you configure the 6009 you can have an input greater than the scaled range and thus 'saturate the input', you need to make sure that the input range is suitable for your intended configuration.

As far as the impedances are concerned it should be fine and with the ranges available on the 6009 you should not need scaling resistors.

P.S for an Engineering student, if you understand differential mode your doing well!

Message Edité par Conseils le 05-20-2006 10:08 AM