# Thumpy

On our recent trip to the Boomershoot site I walked the 150 meters from the road to clean up some left over targets while Ry waited in my vehicle. My size 14 shoes were close enough to snowshoes that I didn’t break through the crust on the snow and it was like walking on a sidewalk for me while Ry broke through and went in half way to his knees.

The first target I detonated was composed of the spill from another 850 gram target, a complete 850 gram target and a 200 gram target. Assuming half of the spill detonated that was about 1500 grams or about 3.3 pounds of Boomerite. This is a little larger than usual but not all that much. I shot it from 25 yards away and when it went off blobs of mud landed all around me and I was impressed with the thump I received from the explosion. Previously we had been setting off 200 gram targets from 25 yards and I was concerned that it didn’t seem that there was as much thump as I had expected. The only previous 850 gram target detonation had been from 470 yards and Ry reported it had the biggest flash he had ever seen and the thump seemed to be typical.

When I got back to the vehicle Ry reported the first explosion was quite remarkable. It was very thumpy. He also felt it before he heard it and he counted at least six echoes from it. He felt it before he heard it? Hmm… the shock wave had to be transmitted through the ground for that to happen. I did have that target on the ground as opposed to on a stake so the spilled Boomerite would get the maximum shock from the target so the ground coupling would be greater than normal. But how much faster does the shock wave travel through the wet ground than through the air? Would the 150 meters be far enough for the difference between the air and ground waves to be discernible?

A quick Bing search didn’t pop up anything for the speed of sound in water soaked earth but the speed of sound in water is about 1.5 km/s. The speed of sound in hard rock is between 5 and 7 km/s and the speed of sound in the air is about 330 m/s. Assuming the speed of sound in the wet earth was no faster than the speed of sound in water then the time to Ry from the explosion would have been about 0.1 seconds through the earth and 0.45 seconds through the air for a difference of about 0.35 seconds. This is easily discernible by a human. If the speed of sound is greater than the 1.5 km/s the difference increases.

Years ago when I had time to shoot targets at Boomershoot events I would shoot from the ground and it always seemed I could feel the thump from the explosion in my chest through the ground but I wasn’t quite sure. The thump through the air was quite noticeable too. This new data point confirms my impression from years ago.

## 2 thoughts on “Thumpy”

1. A difference of .080 seconds is easily discernible as a “slap echo” of short transient sounds. We figure in audio that keeping signals from different speakers to within 35 milliseconds at the listener’s position helps provide maximum intelligibility of speech, so it could be said that a difference of 35 ms, or a little bit more, is “easily discernible” if you know what to listen for.

Delving deeper into the esoteric; in the study of “psychoacoustics” it has been determined that the sense of direction a sound is coming from is determined more by relative timing than by relative amplitude, and there I think we get into even much shorter time intervals. Something to keep in mind in stereo recording and audio equipment design. How that would relate to ground waves I wouldn’t know, unless you happen to be a snake.