NDT Systems: Ultrasonic Transducers

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		*PRACTICAL FOCAL LENGTH CHART*
	The maximum focal length shown is approximately three-quarters of the distance to the end of the near-field. This theoretical boundary is called Y+, and is based on the sound pressure produced from a theoretical, single-frequency, transducer. Even the most narrowbanded transducers manufactured by NDT Systems radiate soundfields having a range of frequencies. The lower frequency components tend to reduce the effective Y+ distance; hence, the 0.75 Y+ maximum. This effect is even more pronounced with broadband transducers such as NDT Systems' High Resolution (HR) Series. The practical maximum focal length of highly damped transducers tends to be closer to 0.6 Y+. The minimum practical focal lengths listed in the table below can be used to specify transducers useful only for very near-surface applications. Their depth of field is so limited that the soundfield diverges very rapidly beyond the focal point. As a rule of thumb, focal lengths midway between minimum and maximum produce an effective compromise between sensitivity and depth of field.

MHz	Element		Beam Dia	Focus Range		Approximate Beam Diameter @ Percentage of Yo+ Shown (in)
	Dia - In	Yo+	(-6dB@Yo)	Min	Max
						25%	50%	75%
0.5	0.5	0.53	0.126	N/P	N/P	N/P	N/P	N/P
	0.75	1.2	0.191	N/P	1.00	N/P	0.095	0.143
	1.00	2.14	0.255	1.00	1.50	0.064	0.128	0.191
	1.125	2.74	0.290	1.50	2.00	0.073	0.145	0.218
1.0	0.125	0.06	0.029	N/P	N/P	N/P	N/P	N/P
	0.25	0.27	0.064	N/P	N/P	N/P	N/P	N/P
	0.375	0.6	0.095	N/P	0.50	0.024	0.048	0.072
	0.5	1.07	0.128	N/P	0.75	N/P	N/P	0.096
	0.75	2.41	0.192	1.00	1.75	0.048	0.096	0.144
	1.00	4.28	0.255	1.50	3.00	0.064	0.128	0.191
	1.125	5.41	0.287	1.50	4.00	0.072	0.143	0.215
2.25	0.125	0.15	0.032	N/P	N/P	N/P	N/P	N/P
	0.25	0.6	0.064	N/P	0.50	N/P	N/P	0.048
	0.375	1.35	0.095	N/P	1.00	N/P	N/P	0.072
	0.5	2.4	0.127	1.00	1.75	0.032	0.064	0.095
	0.75	5.41	0.191	1.50	4.00	0.048	0.096	0.143
	1.00	9.63	0.255	2.00	7.00	0.064	0.128	0.191
	1.125	12.19	0.287	2.00	9.00	0.072	0.144	0.215
3.5	0.125	0.23	0.031	N/P	N/P	N/P	N/P	N/P
	0.25	0.94	0.064	N/P	0.75	N/P	N/P	0.048
	0.375	2.11	0.096	0.75	1.50	0.024	0.048	0.072
	0.5	3.75	0.128	1.00	3.00	0.032	0.064	0.096
	0.75	8.43	0.191	1.50	6.00	0.048	0.096	0.144
	1.00	14.98	0.255	2.00	11.00	0.064	0.128	0.191
	1.125	18.96	0.287	4.00	14.00	0.072	0.144	0.215
5.0	0.125	0.33	0.031	N/P	N/P	N/P	N/P	N/P
	0.25	1.34	0.064	0.50	1.00	0.016	0.032	0.048
	0.375	3.01	0.096	0.75	2.00	0.024	0.048	0.072
	0.50	5.35	0.128	1.00	4.00	0.032	0.064	0.096
	0.75	12.04	0.191	1.50	9.00	0.048	0.096	0.144
	1.00	21.4	0.255	4.00	16.00	0.064	0.128	0.191
7.5	0.125	0.5	0.032	N/P	N/P	N/P	N/P	N/P
	0.25	2.01	0.064	0.50	1.50	0.016	0.032	0.048
	0.375	4.51	0.096	0.75	3.50	0.024	0.048	0.072
	0.50	8.03	0.128	1.50	6.00	0.032	0.064	0.096
	0.75	18.06	0.191	3.50	13.00	0.048	0.096	0.144
10	0.125	0.67	0.032	N/P	0.50	N/P	N/P	N/P
	0.25	2.68	0.064	0.50	2.00	0.016	0.032	0.048
	0.375	6.02	0.096	0.75	4.50	0.024	0.048	0.072
	0.50	10.70	0.128	1.00	8.00	0.032	0.064	0.096
	0.75	24.08	0.191	1.50	10.00	0.048