Recording Sound with a vb instrument and Microphone
The information provided is for help with Commtest Instruments products:
- vbSeries® instruments, vbX instruments
QUESTION
Can I use a microphone and record sound with my vb instrument?
ANSWER
Yes, this can be done easily. It is inexpensive to buy the items you'll need, and you can use your recordings to identify the frequency and hence the source of unwanted noise. Note that your recordings will not be directly calibrated in any established acoustics units (e.g. dBA), so you will not be able to compare the overall readings with limits or thresholds. However, you can perform relative measurements between different locations and over periods of time (provided the setup is the same each time), and you can directly compare frequencies observed in the sound spectra with those in the vibration spectra.
Go to your local electronics store and purchase a standard microphone ('dynamic' or 'moving coil'). These mics usually specify their sensitivity in dB's, with -74 dB being a typical value. This sensitivity will work well in noisy industrial environments but will be inadequate in quiet ambient conditions. If you find that you need higher sensitivity then use a powered 'condenser' mic (sometimes also referred to as 'electret') but ensure that you use one which has its own internal battery to power it. These can have sensitivities around -44 dB, which is about 32x higher than the standard mics.
The mic will probably have a 6 mm (.25'') mono jack connector. So you will need to purchase appropriate adapters to convert that to BNC to match the vb instrument's inputs. For example you could use a 6 mm to RCA adapter, plus an RCA to BNC adapter.
Below is the kind of setup you should end up with.
The total cost of these particular items was US $20.00. The use of the foam protector on the mic is to reduce wind noise for any outdoor application.
- Set the following parameters in the instrument:
Mode/Domain
Freq Accel
Lines
400
Fmax
3000 Hz (180 kCPM)
Fmin
10 Hz (600 CPM)
Averaging
4x Linear
Trigger
Internal, Free Run
Sensor
MIC (see below)
- Set up a new sensor type with these settings:
Name
MIC
Settling Time
1 s
Sensitivity
100 mV/g (or use 10 mV/g to boost weak signals)
Drive Current
OFF (This is very important)
- Connect up the microphone, start a measurement and whistle. You should see a spectrum like this:
Now head out and record that machine noise! You may need to adjust parameters like Fmax or Sensor sensitivity for your particular application. Do remember to leave the Drive Current OFF.
FAQ ID: 11779 Last Reviewed: 29 June 2003