Review on BTMETER Stroboscope Tachometer Speedometer Rotational by Mark Green

High quality product for monitoring speaker cone in slow motion. It takes some manual effort and dexterity to track the frequency with the desired offset, but it works well once you get the hang of it. Sometimes I had to dim the lights in my room to get the look I wanted, but hey, it's a lot cheaper than professional lab equipment and I'm pleasantly surprised by it.