Filthammeren kontrolleres under intoneringen.

Filthammeren is a compound noun:

• filt = felt
• hammer = hammer

So filthammer means felt hammer.

Norwegian very often joins nouns into one word where English would use two words. Then the definite ending is added to the whole noun:

• en filthammer = a felt hammer
• filthammeren = the felt hammer

Also, filt here means felt (the material), not English filth.

The ending -en is the definite article attached to the noun. In English, you usually put the before the noun. In Norwegian, you often add the definiteness to the end instead.

So:

• hammer = hammer
• hammeren = the hammer

And with the full compound:

• filthammer = felt hammer
• filthammeren = the felt hammer

This is very normal in Norwegian.

Here kontrolleres is a passive form of å kontrollere = to check / inspect.

So:

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

In this sentence, it means that the felt hammer is the thing being checked, not the thing doing the checking.

This -s passive is common in Norwegian, especially in formal, technical, or written language.

In some contexts, an -s form can look a bit like a middle or reflexive meaning, but here the natural reading is clearly passive:

• Filthammeren kontrolleres = The felt hammer is checked

Because this is a technical sentence, the meaning is that someone checks the felt hammer during the process. It does not mean that the hammer checks itself.

Yes. That would also be correct.

• Filthammeren kontrolleres = the -s passive
• Filthammeren blir kontrollert = the bli-passive

Both mean roughly The felt hammer is checked.

The difference is mostly stylistic:

• kontrolleres often sounds more concise, formal, or technical
• blir kontrollert can sound a bit more straightforward or conversational

In manuals, instructions, and technical writing, the -s passive is very common.

Here under means during, not beneath.

So:

• under intoneringen = during the intonation / voicing process

Norwegian under can mean both:

• under bordet = under the table
• under arbeidet = during the work

In this sentence, the time-related meaning during is the correct one.

Intoneringen is the definite form of intonering.

It refers to a specific process called intonering. In a technical musical-instrument context, this usually means something like intonation or voicing, depending on the instrument and the translation being used.

So:

• intonering = intonation / voicing
• intoneringen = the intonation / the voicing process

This is not necessarily about speech intonation. In a sentence like this, it is much more likely to be about instrument adjustment or maintenance.

The definite form intoneringen suggests a specific, known process:

• under intoneringen = during the intonation / voicing

Norwegian often uses the definite form when referring to a particular procedure or stage that both speaker and reader understand from the context.

Using intonering without the definite ending might be possible in some contexts, but under intoneringen sounds very natural when talking about a specific step in a technical process.

Yes, it is completely normal.

The structure is:

• Filthammeren = subject
• kontrolleres = verb
• under intoneringen = adverbial phrase

So the sentence follows a standard Norwegian main-clause pattern.

You could also move the time phrase to the front:

• Under intoneringen kontrolleres filthammeren.

That is also correct. Because of Norwegian word order rules, the verb still stays in second position in the clause.

Yes, but it sounds technical and formal rather than casual.

That is mainly because of:

• the specialized vocabulary, such as filthammeren and intoneringen
• the passive form kontrolleres

This sounds like something from:

• a manual
• a technical description
• repair instructions
• professional training material

So it is natural Norwegian, just in a specialized register rather than everyday conversation.