Cross-grain tap test advice?

[JamesO]

Hi everyone

I am clearly the issue. I received six new sitka tops yesterday. The plan was to test them using Trevor's method, then join and thickness them to get a range of deflection measurements as I work them all the way down to .075". The point is to see how deflection data corresponds with different f values using Trevor's method.

Once again, I can't get a good cross-grain value. I'm get 83hz for Long, 57 for Twisting, 83 for Cross. I finally decided to suck it up and record a video. Sorry for the less than glamorous setting of my desk. Unfortunately you can't see that I'm getting 83hz for Long and Cross, but you can see that they're both registering the same semi-tone (E). Also, you're getting my camera's audio, not the microphone's. I was getting ringing harmonics for each tap.

Can you see what I'm doing wrong?

Link to Video

[kiwigeo]

MY comments:

1. 83Hz for both cross and twisting tap tone doesn't look right.

2. When you're measuring the cross grain tap tone try moving your hold to a more central position along he long side of the ......so when you hold the board up one end doesn't drop in preference to the other. Your position in from the long edge looks ok.

3. I find when measuring the along grain tap tone the sweet spot is where the sustain sounds best.

[Trevor Gore]

For the cross mode try holding at 22.4% in from the long side and ~35% in from the short side. Tap on the long centerline level with your pinch point (i.e. ~35% in from the short side) and close mic (~30mm) where you expect an antinode, i.e. close to the tap point but on the opposite side of the panel. If you don't get a clear, ringing tone (in your video it sounded like a dull thud), move the pinch point closer to the center (i.e. 25% in, 30% in etc. from the long edge) also variously trying 40%, 45% from the short edge. Always tap level with your pinch point on the long centerline and mic at your tap point but on the opposite side of the panel. If you mic at a distance you will pick up the loudest (most strongly radiating) mode, because a tap will always excite multiple modes, so always close mic at an antinode of the mode you're looking for (and a node for interfering modes, if you can).

I use G-tune for this test as it gives both the frequency and a spectrum plot so you can see what frequencies are present, which helps me find the best hold/tap point. VA with both the spectrum window and the frequency window open will do the same things, but not as easy to read.

[JamesO]

I'm doing a happy dance! Thank you.

I was starting to feel like I'd never figure this out. Martin's tip to pinch closer to the center really did it. I'm looking forwarding to running through 10-12 tops next week (Sitka, Cedar, Redwood, LS Redwood), and I'll share my results. I'm particularly looking forward to comparing them to the deflection data. I know deflection doesn't take the elastic properties into account, but that's the point. I've always been curious to see how/if they correlate.

Yes, they did sound like thuds in the video. That was my camera's audio, not the microphones. They were harmonic in nature. I promise.

[kiwigeo]

Glad to hear you got it nailed James.