If - as declared - this is just a matter of simply detecting whether an object has fallen and hit the area of 155x140, then multiple solutions are possible.
- Vibration detector. When the object hits, it will cause the surface to vibrate. You can sense these using vibration sensor, or even by sound pickup plus some simple software processing.
- Use ultrasonic single TX at one end, and single RX at other end. Mix the RX signal with the TX signal & low pass filter to extract the doppler frequency. When an object moves in the field, the doppler signal will increase and can be detected.
The above 2 methods will be enough to determine whether a ball bounced (anywhere) or not. From your dimensions given, and oblique reference to a TT ball - which is 40 mm in diameter - we are forced to assume you are trying to track exactly where a TT ball bounces on a given side of the TT Table (9' x 5'). If so, in isolation the techniques (1) or (2) won't suffice.
For solving this here's an options:
3) use an ultrasonic TX near the net, pointing outwards to cover the table. Use multiple at different angles to cover the area of interest. Wire them up together & in phase. Now place an ultrasonix RX at the edges of the table in same line as the TX so that their receive beam covers the table. You can use a 3rd RX if two aren't enough. At edges and center of the net. These RXs are independently sensed.
Now Send out rapid pulses from the TX (say 10 pulses @ 40 kHz at rep rate of 10 ms), and measure the reflection return time at the 2 different RX locations. For a table of this size, maximum reflection time should be less than 12 ms, so rep rate can be 15-20 ms of over 50 Hz (50 samples per second) With this time measurement, and knowing speed of sound & dimensions of the table and transducer positioning it should be possible to calculate the location of the ball. Now since this will "track" the ball in real time - even in the air - you need to Combine this with a vibration of contact sensor to determine which reading to use for the actual bounce contact point.
Alternately instead of vibration sensor, you could use a sound sensor, and develop an algorithm to "detect" the sharp pulse made by ball bounce only, as the detection method.
Hope you can understand.