Animals and sound sit in an interesting place: part of it has been measured solidly, and part of it has been sold for decades despite being shown not to work. Separating the two is the only thing that actually helps your animal.

First: animals hear far more than you do

Hearing range varies substantially between species. Widely accepted measurements are roughly:

Human: about 20 Hz – 20 kHz (the upper bound falls with age)
Dog: about 65 Hz – 45 kHz
Cat: about 45 Hz – 65 kHz, higher in some measurements
Mouse: about 1 kHz – 90 kHz
Horse: about 55 Hz – 33 kHz
Cow: about 25 Hz – 35 kHz

The practical consequence is clear: your animal may be hearing a loud, distressing tone while you hear nothing at all. A device being "silent" does not mean it is silent to an animal. If you intend to use an app like Rezzonix around animals, that alone is reason enough for caution.

What has been measured: music and shelter animals

Studies with shelter dogs have shown that the genre of music played in the environment produces a measurable behavioural difference. Multiple studies reported that dogs spent more time resting and barked less in sections where classical music was played, and the opposite under fast-tempo music. Similar observations have been made with shelter cats.

The mechanism here is most likely not frequency-specific but a general arousal-level effect: slow tempo, soft dynamics and predictable structure reduce stress; sudden and loud sounds increase it. So it is not "the magic of classical music" but "calm sound calms the room".

There is also research into species-specific music: pieces composed around the frequency range and tempo structure of cats' own vocalisations drew measurably more interest than human music. That moves the idea of "playing music to animals" beyond simply transplanting a human habit onto another species.

What has been measured: ultrasonic devices do not work

Ultrasonic insect and rodent repellers have been sold for decades. Independent laboratory tests and regulatory reviews have consistently found that these devices do not produce the claimed effect; in the United States the Federal Trade Commission sent formal warnings to manufacturers over unsupported claims. An animal being able to hear a sound does not mean it will permanently avoid it — most species habituate quickly.

We cite this example deliberately: there is a large gap between an animal hearing a frequency and that frequency producing the desired behaviour. Your animal hearing a value from the catalog does not mean the value does anything.

What is unsupported: disease-specific frequency claims

Claims of the form "this Hz cures that condition in animals" have no standing in established veterinary medicine. We neither make nor endorse such a claim. If your animal has a health problem, the place to go is a veterinarian; delay usually costs more than the problem itself.

If you are going to use sound around animals

Keep the volume low. What is low for you may be loud for an animal with a wide hearing range.
Leave an escape route. The animal must be able to leave the room. Never apply sound to an animal confined in a carrier or cage.
Watch the reaction. Flattened ears, yawning, lip licking, restless pacing, hiding — these are discomfort signals. Stop if you see them.
Do not play for long periods. Start short; extend only if the animal is comfortable.
Do not use with elderly or ill animals without consulting your vet.

Summary

Sound does affect animal behaviour and that effect has been measured — but what has been measured sits at the level of "a calm environment calms", not "a specific Hz solves a specific problem". Do not use Rezzonix on the second claim. If the first framing interests you, experiment carefully and quietly, and do not overstate the result.