Firearm Sound Measurements and Data Acquisition

[mmorg41]

I am new to the forum so if this topic is in the wrong place, I apologize!

I am working on a project that is studying hearing loss in soldiers that resulted from shooting firearms without hearing protection. I need to test different firearms and compare their decibel readings. The microphone I will likely use is a PCB model 376A31 which is a  pre-amplified microphone with a dynamic range of up to 165 dB. I was looking into purchasing a NI9234 for data acquisition but saw that the dynamic range on the module is only 102 dB. We are expecting readings ranging from 130-160 dB and whatever data acquisition device we use needs to have a very high sampling rate. I am an undergrad ME student but I am not very familiar with data acquisition. If anyone has any ideas or suggestions on different modules or data acquisition equipment I could use, it would be extremely helpful. I am working with a budget of approximately $3000 for the entire setup. Thanks in advance!

 

[JKarma]

Hi mmorg41:

Since the request has some requirements with the budget and the signal range please take into consideration the following National Instruments online resources for guidance with the selection of the most suitable hardware for your application. 

Sound and Vibration Hardware Options

Sound and Vibration Software Options

[dsb@NI]

NI DSA devices for sound and vibration measurements provide IEPE excitation. If you are not terribly attached to the phantom-powered sensor, I would recommend an IEPE microphone that can be powered directly by NI hardware.

As for dynamic range, the PCB sensor dynamic range is given reference to 20 µPa and indicates the maximum sound pressure that can be measured using that mic. What is the maximum sound pressure level you expect? Use that level to select a combination of sensor (sensitivity) and input channel (range) to best use the dynamic range of your input channel. To check: make sure that the combination of sensor and input channel can effectively measure the maximum expected sound pressure level and the minimum/threshold sound pressure level that contributes to hearing loss.

You mentioned "a very high sampling rate." Can you quantify? Are you investigating the effects of levels within the traditional frequency range of 20 Hz to 20 kHz, or are you also including even higher frequency components?

[Henrik_Volkers]

It looks like the mic you choosed needs 48V phantom power. the 9234 will provide 2mA IEPE supply... that doesn't match!

But you are going the rigth way.

1. Look for an sensor/microphone that meets your needs.

The mic you have noted looks fine, however it's a good idea to call PCB and talk to an application engineer.. and while you are at the phone, ask B&K and .. (missed the name there is at least one other company that makes good high level mics )

2. Look for the signal conditioner/aquisition you will need.

Dynamic range is not only the level range.... (I expect that the lower range isn't that important for you 😉 ) but also look for the bandwidth! The mic you have noted goes up to 100kHz and you want to look at pulses, so I would  look for conditioners and DAQs that can sample that bandwidth.

At these levels you leave the linear regions .. (in the air and in the ear anyway :D)

Just because you can't hear it, ultra sound isn't harmless !!

Damping in air is frequency related .. but your sound source is close to the ears!

[Henrik_Volkers]

The 378C01 would be the IEPE powered mic ...

If you use longer cable than 3m I would choose a IEPE supply of at least 4mA ...

The cable capacity will degrade the high frequency response... (easy to calculate slew rate with a given cable capacity and the drive current 😉 ) and you want to measure sharp pulses!

(Due to the way the IEPE sensor output is build, the fall time is fine, only the rise time is limited by the supply current... generating nonlinearities)

If you choose a NI digitizer with IEPE supply I would go at least for the 102kSPS 4mA , NI 9232

Or .. get the mic, ask PCB for a high bandwidth IEPE supply (4-10mA )  to test and use a scope to measure the slewrate of some shoots. try with 10mA and 2mA and see yourself.

Or build a IEPE supply yourself , it's a current source (JFET and resistor), a cap to decouple the DC offset and a 24-30VDC powersupply)

But  NEVER EVER connect a voltage supply directly to IEPE sensors!! They need a current source!!

[Henrik_Volkers]

And some more hints:

Two more companies:  GRAS and MICROTECH GEFELL

and since you want to make some sort oft reliable measurements and work with high levels (Pushing the price tag .. however):

Maybe a Pistophon for a 1/4 in mic ist a good idea to calibrate and check your mic and setup.

Or take a small housing with a speaker , your mic and an 2. mic&preamp  and a rms voltmeter (or second channel). Stable setup, stable geometry !

Using a generator at a fixed frequency and adjust the amplitude on a constant level on the 2.mic , record the measurement mic level.

After some blasts do the same test again. Like that you can check if you mic has changed due to maybe overload.