Broadband hydrophones do not operate within a specifically defined frequency range. Cetacean researchers may analyze signals up to 150 kHz while medical scientists may work with a frequency band spanning several MHz. There is no strict definition of broadband and with hydrophones there is a good reason: hydrophones can provide a flat frequency response over an enormous range.
Most piezoelectric products are resonant devices, used at or near a resonant frequency. This is especially true when transmitting, where maximum output level tends to be the objective. This limits their usable bandwidth.
Hydrophones, however, are an exception. They are not used at their resonance point but rather, below it. The sensitivity of a hydrophone is effectively the same for all frequencies below the first resonance frequency of its piezoelectric sensor. So, the higher the frequency of the sensor, the broader the bandwidth of the hydrophone.
Broadband hydrophones pose some unique design challenges. The smaller the piezoelectric ceramic sensor, the higher the frequency so using a smaller ceramic yields a broader bandwidth. However, the smaller the ceramic the less sensitive it is. The trade-off is unavoidable. With broadband hydrophones a preamplifier is often required.
Also, smaller piezoelectric sensors are less tolerant of misalignments, inaccurate soldering and poor assembly in general. Broadband hydrophones can be difficult to build.
Sensor Technology Ltd. produces broadband hydrophones in a range of bandwidths, sensitivities, depth ratings and beam patterns (including some spherical models). Several existing designs are described here but we recommend that you contact us to discuss your specific requirements and find your ideal hydrophone.