I use my chrony chronograph I've had 30 years. someone at the range had an app for his cell phone that goes by noise of shot to noise of impact of target on his I-phone. I just didn't get a chance to check its accuracy against a chronograph yet.
I've never used this one for a bow but I used the radarchron when I used to play paint ball. Worked great. Never an issue. You can get that one off of amazon for $78. Or the one in the link for a bow Amazon.com : Sports Sensors Arrowspeed Radarchron : Archery Equipment : Sports & Outdoors