Usually, multiple probes are used because most transducers are only able to emit one frequency because the piezoelectric ceramic or crystals within it have a certain inherent frequency.
Multi-frequency probes have multiple crystals with different frequencies and the desired specific frequency can be selected. Advanced probes can emit sound waves at different frequencies for the near and far fields. The disadvantage is that multi-frequency (multifrequency) probes have slower frame rates and therefore they are only useful for imaging of static structures.

See also Dual Frequency Phased Array Transducer and Tri-Frequency Probe.

From SIUI Inc.;
'Dedicated to ultrasound industry, Shantou Institute of Ultrasonic Instruments, Inc. (SIUI) has launched Apogee 3500, the Digital Color Doppler Ultrasound Imaging System.
With latest imaging technologies, high-definition image quality and excellent practical functions, the Apogee 3500 offers optimal solutions for clinical ultrasonic examination.'

'The Apogee 3500 is available with many high-density, super broadband and multi-frequency probes, such as convex, micro-convex, linear, vaginal, rectal and phased array probes, which are widely applied for different clinical diagnoses, including abdomen (liver, kidney, gall-bladder, pancreas), gynecology (uterus, ovary), obstetrics (early pregnancy, basic OB, complete OB, multi gestation, fetal echo), cardiology (adult and pediatric cardiology), small parts (thyroid, galactophore, testicles, neonate), peripheral vascular and prostate.'

From SIUI Inc.;
'CTS-385V is designed for the diagnosis of big and small animals. It can work with a 5.0MHz rectal probe, 7.5MHz rectal probe and 5.0MHz microconvex probe.'
Features:
'High value imaging
Foldaway keyboard
Broadband multi-frequency probes
Dual probe connectors
Connect to PC 32-frame non-volatile storage capacity(optional)
Veterinary software package'

The CTS-385 V is the veterinary version to the CTS-385.
For 'Device Information and Specification' see CTS-385 Plus
Veterinary Probes:
5.0MHz veterinary probe (PL23V)
7.5MHz veterinary probe (PL23V)
5.0MHz micro convex probe (PC4B)

From SIUI Inc.;
'SIUI's newly developed CTS-485 is a portable ultrasound system with advanced technology. With its powerful features, such as 256 grayscale, cineloop, RS232C interface and broadband high-density, multi-frequency probes, the system is designed for professional applications in cardiology, abdominal, small parts, liver, gallbladder, kidney, obstetrics, gynecology, peripheral vascular, etc. The CTS-485 has passed FDA clearance and CE Marking.'

From SIUI Inc.;
'A breakthrough in traditional designation
The latest technologies and processes
Closely combined with up-to-date IT technology
A variety of advanced ultrasound imaging technologies
The best solution provided for users'

'CTS-5000 can match a variety of super high density, super broadband, multi-frequency probes, including convex, micro-convex, linear, vaginal, rectal probes etc., which are widely used to different clinical diagnosis such as abdominal (liver, kidney, gall-bladder, pancreas), gynecology (uterus, ovary), obstetrics (early pregnancy, basic obstetrics, multi gestation, fetal echo), cardiology (adult and pediatric cardiology), small parts (thyroid, galactophore, testicles), peripheral vascular, prostate.'