Home  Articles AEG Audio Analysis: Measuring Rate of FIre
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AEG Audio Analysis: Measuring Rate of FIre |
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Written by Jay
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Monday, 14 February 2005 |
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Page 4 of 6 Analysis Procedure:
Step 1: Recording the Waveform
To get a clean recording for analysis, it is extremely important to minimize ambient noise and echoes. Simply recording an AEG mechbox in a room will result in a dirty waveform filled with echoes, significantly complicating analysis. Since the majority of airsofters are unlikely to have access to a recording sound room, some improvisation will be necessary. One method of providing a relatively acoustically clean recording environment is by using a heavy blanket. Heavy cloth exhibits good sound absorbing characteristics and can drastically reduce echoes during the recording. By draping the blanket over a chair, a small acoustically clean space is created above the seat which is more than sufficient for our purposes.
Once an appropriate recording environment is selected, recording the waveform entails simply activating the recording feature of the audio editor software, and holding down the AEG trigger. Aim to record between 15 to 20 cycles to ensure a good sample size for subsequent analysis. It may take several tries to get a good recording distance/level which produces a good waveform with minimal clipping.
Step 2: Analyzing Waveform
With a clean waveform recorded, it's plain to see the cyclic nature of the AEG mechbox. Similar patterns of waveforms repeat over the length of the recording, with distinctive attributes which correspond with certain mechbox events. The next step is to calculate the amount of time between the start of a specific mechbox event and the beginning of the next same event. Instinctively, the most appropriate event to chose would seem to be the piston striking the cylinder head, but in reality, it doesn't matter which point is chosen as long as consistency is maintained. The 'best” point to choose on the waveform is the one most easily spotted in the repeating pattern.
Using the “zoom” function of the audio editor, move in close to examine the features of the waveform close-up so that an appropriate and easily identifiable feature can be selected. It is important to choose the first point starting with the 4th or 5th mechbox cycle, since the first 3 to 4 cycles usually exhibit slightly extended durations due to the motor slowly ramping up to operating conditions. After the 5th cycle, durations become much more consistent and are therefore more suited towards analysis.
As each point is found on the waveform, the corresponding time stamp is recorded. While there is no standard for how many cycles is appropriate for an accurate rate of fire measurement, our own ROF tests utilize 11 points which yields 10 durations. We believe this to be more than adequate.
Step 3: Calculate ROF
After the 11 time stamps are recorded, the duration between the points is calculated by simply subtracting each timestamp by the one preceding it, starting from the last timestamp. Once all 10 durations are calculated, their average is taken and the result represents the amount of time between each mechbox cycle. To determine the ROF in rounds per minute (RPM), simply divide 60 by the result. In our example above, the 2nd timestamp occurs at 3.73 while the 1st timestamp occurs at 3.66. *
The resulting duration for this cycle would be :
3.73 - 3.66 = 0.07
The resulting rate of fire in rounds per minute would be:
60/0.07 = 857.14
*The timestamp for the current cursor position is displayed in the lower left corner of the Audacity output window
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