Suggested Measurement Protocol
Under ideal circumstances, the investigator would locate the weapon and transducer (microphone) in a temperature-controlled, echo-free environment with no operator in the measurement field. As with a lot of ideal scenarios, this is impractical, and some compromises must be tolerated.
There are many locations to do firearm sound measurements, and probably the most important for hearing is at the shooter’s ear. However, a common location (which appeared in earlier releases of Mil-Std-1474) is referred to as the “reference location.” Measurements are standardized as being made one meter (39.34 inches) 90 degrees to the side of the muzzle and at a height of 1.6 meters (5 feet) above ground over grass or another non-reflecting surface. The process is simplified if the microphone and preamplifier can be separated from the meter with a cord. In this situation, the simplest microphone stand is to insert a 6-foot piece of conduit or rebar a foot into the ground and then attach the microphone and preamplifier to the pole at the 5-foot level with rubber bands. A folding table back a few feet can hold the meter and accessories. If the microphone cannot be located remotely from the meter, the meter can be mounted in place using a tall camera tripod or a camera clamp attached to the post.
Spacing of the firearm from the microphone is also critical, and the most basic method, suggested by silencer researcher Al Paulson, is to tie a one-meter piece of dental floss to the microphone. The other end of the dental floss is tied to a rubber band that loops over the muzzle of the weapon. A piece of brightly colored string attached to nails can be laid out on the ground in a right angle to help maintain consistent alignment of the weapon visually. Far more sophisticated setups can be constructed.
The next step in obtaining sound measurements after setting up the jig or fixture is meter calibration. The meters rarely drift, and calibration should remain constant. However, to be certain that the equipment is functioning normally, it is necessary to check the sound meter system with a calibrator that has been recertified within the past 12 months. Each meter uses slightly different calibration procedures. For the LDL 800B, the meter is set to utilize the fast detector with no weighting (linear). After placing the calibrator on the microphone, the meter is checked and adjusted for a reading of 114 dB according to instructions by Larson Davis.
For firearm sound measurement, the meter is set to utilize the peak (not impulse) detector in the “hold” mode (to freeze the highest peak reading), and the weighting is set to “A” (as specified in Mil-Std-1474D). While there are those who rightly note that unweighted (or linear) setting would be more accurate, A-weighting is used because it most closely tracks the hearing response of the human ear. More importantly, A-weighting correlates most closely with the risk of hearing damage.
Normally, non-suppressed readings are made first, primarily because if changes in the setup need to be made, it will not be necessary to repeat all of the suppressed readings to keep all data consistent.
Just prior to each measurement, press the reset on the meter to clear the memory and ready it for the next sound pulse. Hold the weapon parallel to the ground at shoulder height (1.6 meters) with the muzzle in such a position as to make it perpendicular to the microphone. The string on the ground visually helps in this alignment. Move the weapon sideways just until the slack is out of the piece of dental floss and the rubber band is not stretched. Fire the weapon. After making the weapon safe, record the meter reading, reset the meter, and repeat the process. After 10 rounds, average the data to provide the mean non-suppressed sound level.
Using the same procedure, repeat the 10 measurements with the suppressor attached to the weapon and average this data to obtain the mean suppressed level. The difference between these means is the degree of suppression. When recording the data, be certain to also record the ambient temperature, humidity, barometric pressure, and date. Although you should record the temperature with a thermometer at the location of the tests, the other data can be obtained from the nearest weather bureau or from NOAA weather broadcasts, which are available throughout the United States. It is important to record the station pressure (actual atmospheric pressure in the vicinity of the test site), not the pressure corrected to sea level. There are some excellent and affordable digital barometers, hydrometers and thermometers available that will allow the investigator to make on-site measurements of the required data.
The last step is to recheck the meter calibration with the calibrator to be certain that there has been no meter drift. There should be none, and the recheck calibration should be within 0.1 dB of the initial calibration. If there has been a greater change, there are several possible interpretations. (1) Check battery condition and replace if necessary. (2) Verify that environmental conditions do not exceed the operating specifications of all equipment being used. (3) If neither of the preceding checks has revealed the problem, then return the meter, calibrator and microphone to the manufacturer for necessary repairs and recalibration and recertification.
After all measurements are made, it is fairly simple to calculate the average of each 10-round string and the standard deviation. In addition, the first round “pop” (amount that the first round is greater than the average of the remaining 9 rounds) can be calculated.
With all sound measurements, it is important to remember that there will be minor day-to-day variations in the data obtained, which appear primarily to be due to ambient temperature variations. Such variations will commonly be a decibel or two.
Subsequent columns will address hearing damage mechanisms and prevention, discussion of classic suppressed weapons, and other parameters of interest to suppressor users.