Usage of passive forms without "be" verb [duplicate]

I will very grateful if you can help me understand the following issue:

As I understand, the next two sentences are interchangeable and have the same meaning:

1) Acts that are done by humans are not necessarily moral.
2) Acts done by humans are not necessarily moral.

The first option is a standard way of using the passive form of the verb. However, I don't understand why the second option is correct, If "done" this case as passive form of the verb or even if it can be treated as an adjective, I am expecting to see some tense of the "be" verb before.

There are many other similar examples, especially when using verbs like: "shown, suggested, presented, defined and etc." with a reference to a previous sentence of our article/text.

Thank you in advance.

Solution 1:

The second version is certainly grammatical. When the relative pronoun and be are omitted in this way, the result tends to sound more formal, so the construction is perhaps better suited to writing than to speech.