Kiet and Sheila,

Following up on the investigation posted in aLOG 84136, we examined the impact of higher-order violin mode harmonics on the contamination region.

We found that subtracting the violin peaks near 1000 Hz (1st harmonic) from those near 1500 Hz (2nd harmonic) results in frequency differences that align with many of the narrow lines observed in the contamination region around 500 Hz.

Violin peaks that we used (using O4a+b run average spectra)

F_n1 = {1008.69472,1008.81764, 1007.99944,1005.10319,1005.40083} Hz 
F_n2 = {1472.77958,1466.18903,1465.59417,1468.58861, 1465.02333, 1486.36153, 1485.76708} Hz 

Out of the 35 possible difference pairs (one from each set), 27 matched known lines in the contamination region to within 1/1800 Hz (~0.56 mHz)—most within 0.1 mHz. Considering that each region actually contains >30 peaks, the number of matching pairs likely increases significantly, helping explain the dense forest of lines in the comtaimination region.

Next steps:

• Testing all the peaks in the 1st and 2nd harmonic region.
• We propose to identify lines with highest power in the contamination region, trace them back their source modes and discuss the feasibility of target damping them. 

The Fscan run average data are available here (interactive plots): 

Fundamental region (500 Hz): 

• 400–500 Hz
• 500–600 Hz

1st harmonic region (1000 Hz): 

• 900–1000 Hz
• 1000–1100 Hz

2nd harmonic region (1500 Hz): 

• 1400–1500 Hz
• 1500–1600 Hz