Oktaven kontrolleres mot referansetonen.

Because both nouns are in the definite singular form.

• en oktav = an octave
• oktaven = the octave

and

• en referansetone = a reference tone
• referansetonen = the reference tone

In Norwegian, the definite article is usually added to the end of the noun instead of being a separate word like the in English.

Kontrolleres is the verb kontrollere in the -s passive form.

• å kontrollere = to check / to inspect
• kontrollerer = checks / is checking
• kontrolleres = is checked

So in this sentence, kontrolleres means is checked or is tested.

The -s ending is a very common way to form the passive in Norwegian, especially in formal, technical, or instructional language.

Yes. It is present tense passive.

So:

• kontrollerer = checks / is checking
• kontrolleres = is checked

This sentence describes a general procedure or rule, which is why the present tense is natural here.

Here, mot means something like against or in comparison with.

So kontrolleres mot referansetonen means:

• is checked against the reference tone
• is compared with the reference tone

This use of mot is common when something is being measured, tested, or checked relative to a standard or reference.

Because Norwegian usually forms compound nouns as one word.

• referanse = reference
• tone = tone / note

Together:

• referansetone = reference tone

Then the definite ending is added:

• referansetonen = the reference tone

This is very normal in Norwegian. Where English often writes two words, Norwegian often prefers one compound word.

Yes, you could.

• Oktaven kontrolleres mot referansetonen.
• Oktaven blir kontrollert mot referansetonen.

Both can mean The octave is checked against the reference tone.

However, there is a slight difference in style:

• kontrolleres sounds more concise, formal, and typical of instructions or technical writing
• blir kontrollert sounds a little more explicit and sometimes a bit more conversational

In manuals, descriptions of procedures, and formal explanations, the -s passive is very common.

The structure is:

Subject + verb + prepositional phrase

• Oktaven = subject
• kontrolleres = verb
• mot referansetonen = prepositional phrase

So the sentence is very straightforward: The octave + is checked + against the reference tone

Because Norwegian normally attaches definiteness to the noun itself.

In English:

• the octave
• the reference tone

In Norwegian:

• oktaven
• referansetonen

So instead of a separate article before the noun, Norwegian often uses an ending like -en, -et, or -a, depending on the noun.

It would be:

En oktav kontrolleres mot en referansetone.

That means: An octave is checked against a reference tone.

Compare:

• en oktav = an octave
• oktaven = the octave

and

• en referansetone = a reference tone
• referansetonen = the reference tone

The dictionary forms are:

• oktav = octave
• referansetone = reference tone
• å kontrollere = to check / to inspect

If you look them up in a dictionary, you would normally search for those forms, not the inflected forms oktaven, referansetonen, or kontrolleres.