Primer: Using Audacity to estimate average pellet velocity
Posted: Tue Sep 03, 2013 4:26 pm
What you need:
1. The exact measured distance between muzzle and target
2. A target that makes a loud audible noise when hit by a pellet - like a metal sheet or wooden board.
3. A sound recording device such as a cellphone or laptop (or recording studio if you happen to own one)
4. A computer with audio editing software like Audacity (free) or Adobe Audition (Costs $1arm+1leg)
I suggest that the range from muzzle to target should be more than 10 meters if possible (this is not essential but I will explain below why I say this). Also the recording of sound should be done in an open area relatively free from echoes. Both these suggestions are because the actual sound produced by an air rifle being fired lasts about 0.05 seconds (1/20th second). If there are echoes, or if the target is so close that the pellet hits the target within 1/20th second, you will not be able to make out the point where the sound of the target being hit is heard because that sound will be covered up by the sound of the rifle itself. The sound of rifle/pistol going off and target being hit will simply blend into one continuous sound which is then not useful for any estimation.
Before you start recording remember to speak into the mike and say what rifle/pistol, what pellet and what range because you will forget to do that and won't know what the recording might be.
It is best to put the sound recording device approximately mid-way between the muzzle and the target. When you do that the time taken for the sound to reach the mike from the muzzle and from the target is the same so that errors due to the time taken for sound to travel are eliminated. Even otherwise the difference is small, but at higher ranges the calculation gets affected.
Here is a recording of a target being hit by two pellets using an IHP 35 at 16 meters. There is no need to listen to the sound, but if you want to play with the file you can download it.
https://docs.google.com/file/d/0B3JNY4I ... sp=sharing
I fired up Audacity sound editing softwware and opened the above file in it. The following image is the sound wave recording that I got. I have labelled events like cocking of rifle, closing of breech and the sound of the rifle being shot and the pellet hitting the target. Note that the gap between shooting and hitiing target is very small.If the gap had been smaller it would have been impossible to separate the sounds of shooting and hitting.
https://docs.google.com/file/d/0B3JNY4I ... sp=sharing
In the following image I have edited out and deleted unnecsssary parts like sound of cocking and closing and tin can falling. (Select the unnecessary parts with mouse and hit delete). This leaves only the enlarged sound picture of rifle shot and pellet hitting target. You can expand or contract the sound graph/image vertically and horizontally to enlarge it or make it smaller as required using the "View" menu. The following is an enlarged view of the same pellet sounds visible in the previous image. Here you can clearly see the separation between the two sounds - of shot being fired and pellet hitting target because of the horizontal enlargement.
Use the mouse to select precisely from the start of the rifle shot sound to the start of the pellet hitting sound (Use left click and drag). Once that is selected the timelength of the selected part is displayed below. In my image the time length is 0.104 seconds (about 1/10th second)
https://docs.google.com/file/d/0B3JNY4I ... sp=sharing
Now we have the distance (or range) - 16 meters as measured earlier
Time of travel of pellet is 0.104 seconds
Speed= Distance/Time= 16/0.104
= 154 meters per sec (or about 500 fps)
NOTE: Audacity has a function that enables it to automatically recognize two sounds like shot and pellet hitting. That function requires some adjusments and settings that I hate doing. The results are rarely any different from what I get if I select the sounds manually so I never use the "detect sound" function. But you can try it if you like
1. The exact measured distance between muzzle and target
2. A target that makes a loud audible noise when hit by a pellet - like a metal sheet or wooden board.
3. A sound recording device such as a cellphone or laptop (or recording studio if you happen to own one)
4. A computer with audio editing software like Audacity (free) or Adobe Audition (Costs $1arm+1leg)
I suggest that the range from muzzle to target should be more than 10 meters if possible (this is not essential but I will explain below why I say this). Also the recording of sound should be done in an open area relatively free from echoes. Both these suggestions are because the actual sound produced by an air rifle being fired lasts about 0.05 seconds (1/20th second). If there are echoes, or if the target is so close that the pellet hits the target within 1/20th second, you will not be able to make out the point where the sound of the target being hit is heard because that sound will be covered up by the sound of the rifle itself. The sound of rifle/pistol going off and target being hit will simply blend into one continuous sound which is then not useful for any estimation.
Before you start recording remember to speak into the mike and say what rifle/pistol, what pellet and what range because you will forget to do that and won't know what the recording might be.
It is best to put the sound recording device approximately mid-way between the muzzle and the target. When you do that the time taken for the sound to reach the mike from the muzzle and from the target is the same so that errors due to the time taken for sound to travel are eliminated. Even otherwise the difference is small, but at higher ranges the calculation gets affected.
Here is a recording of a target being hit by two pellets using an IHP 35 at 16 meters. There is no need to listen to the sound, but if you want to play with the file you can download it.
https://docs.google.com/file/d/0B3JNY4I ... sp=sharing
I fired up Audacity sound editing softwware and opened the above file in it. The following image is the sound wave recording that I got. I have labelled events like cocking of rifle, closing of breech and the sound of the rifle being shot and the pellet hitting the target. Note that the gap between shooting and hitiing target is very small.If the gap had been smaller it would have been impossible to separate the sounds of shooting and hitting.
https://docs.google.com/file/d/0B3JNY4I ... sp=sharing
In the following image I have edited out and deleted unnecsssary parts like sound of cocking and closing and tin can falling. (Select the unnecessary parts with mouse and hit delete). This leaves only the enlarged sound picture of rifle shot and pellet hitting target. You can expand or contract the sound graph/image vertically and horizontally to enlarge it or make it smaller as required using the "View" menu. The following is an enlarged view of the same pellet sounds visible in the previous image. Here you can clearly see the separation between the two sounds - of shot being fired and pellet hitting target because of the horizontal enlargement.
Use the mouse to select precisely from the start of the rifle shot sound to the start of the pellet hitting sound (Use left click and drag). Once that is selected the timelength of the selected part is displayed below. In my image the time length is 0.104 seconds (about 1/10th second)
https://docs.google.com/file/d/0B3JNY4I ... sp=sharing
Now we have the distance (or range) - 16 meters as measured earlier
Time of travel of pellet is 0.104 seconds
Speed= Distance/Time= 16/0.104
= 154 meters per sec (or about 500 fps)
NOTE: Audacity has a function that enables it to automatically recognize two sounds like shot and pellet hitting. That function requires some adjusments and settings that I hate doing. The results are rarely any different from what I get if I select the sounds manually so I never use the "detect sound" function. But you can try it if you like
