Measuring Rate of Fire Through Audio Analysis
Accurately measuring your rate of fire doesn't mean you need an expensive chrono. This guide shows you how with a PC, a cheap microphone and free software, the rate of fire of any AEG can be easily determined.
Introduction:
Measuring the rate of fire for an AEG can be a tricky task. With typical AEGs shooting at 15 to 20 rounds per second, counting the number of rounds fired while timing the interval with a stop watch can be quite inaccurate. While most top-end projectile chronographs feature rate of fire measuring capability, most airsofters do not have ready access to such equipment. For the technically and electronically adept airsofters, schematics for home brew ROF meters such as the excellent Unconventional Airsoft ROF Meter are available and can be constructed relatively inexpensively, but still requires time and skill to implement properly. Fortunately for the rest of us, there is a simple yet highly accurate method to determine rate of fire, with nothing more than your personal computer, any microphone, and free sound editing software.
Audio Analysis:
The use of audio analysis as a diagnostic aid for mechanical devices is not a novel concept. Automotive mechanics have long learned to listen to the sounds an engine makes to ascertain the operating condition of the engine. In “Vibroacoustical Machine Diagnostics”, nosies and vibrations emanating from a mechanical system is recorded and analyzed to gain specific information on the system, aiding the diagnosis of hidden problems. While the AEG mechbox does not approach the complexity level of car engines, a similar method can nevertheless be applied to measure the rate of fire of the mechbox. The cycling of the AEG mechbox mechanism is a noisy affair, generating distinctive sounds corresponding to different stages of the firing cycle. By recording these sounds and analyzing the resulting waveform, patterns can be observed which can be measured to determine the rate of fire.
Required Tools:
PC with sound card:Most PCs today come with sound cards capable of digitally recording sounds at a very high 16bit 44Khz quality, more than sufficient for measuring AEG rate of fire.
Standard microphone:
Any PC microphone will work. There is no need for expensive pre-amped professional setups.
Sound recording software:
The sound recording software used must allow a fine time-based display of the recorded waveform and should also allow easy manipulation of the viewing frame. Generally, software which feature the previous attributes also allow manipulation and direct editing of the waveform and qualify as sound editing or audio editing software. There are several freely available software packages that have the necessary functionality, my favorite of which is a cross-platform open source project called Audacity ( http://audacity.sourceforge.net ).
Analysis Procedure:
Step 1: Recording the WaveformTo 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
Caveat Emptor:
While the method proposed here to measure rate of fire is highly accurate, it is nevertheless sensitive to various factors, most major of which is battery output voltage. A typical fully charged 8.4v battery exhibits an output voltage starting at approximately 10.4v and falls steadily as the battery discharges. As will be discussed in the upcoming AEG Motor Science article, DC electric motors display different operating characteristics depending on the input voltage. As a result, changes in the battery voltage inevitably affects the accuracy of rate of fire measurements. If separate rate of fire measurements are to be compared, it is imperative to either control or record the battery output voltage for each measurement, so that meaningful conclusions can be drawn.
Conclusion:
With the method proposed in this guide, it is now simple for anyone to measure their own rate of fire and gauge the effects of parts designed to modify rate of fire. In conjunction with the formulas for predicting rate of fire in the Gearbox Science I article, the average airsofter is armed with powerful tools to quantify and predict the effect of present and future mods.