Ultrasonic testing utilizes high frequency sound waves passing through the weld. The waves are reflected or refracted if they hit an indication. The reflected or refracted wave will have a different amount of time it requires to travel from the transmitter to the receiver than it will if an indication was not present. This change in time is how the flaws are detected. The first advantage that ultrasonic testing provides is that it allows for a relatively quick detection of the flaws inside of the weld joint. This test method also can detect flaws deep inside the part. Additionally, it can be conducted with access from only one side of the part. In contrast, there are several disadvantages of using ultrasonic testing. The first is that it cannot be used to optimize the process parameters or evaluate the seam quality of the weld. Secondly, it is costly and labor-intensive. It also requires experienced technicians to conduct the test. Lastly, there are material limitations with plastics due to transmission limitations of the ultrasonic waves through some of the plastics. The image in Figure 6 shows an example of ultrasonic testing.