Depois que o eco diminui, a acústica melhora.

Depois que means after or after the moment when.

So:

• Depois que o eco diminui = After the echo decreases
• Depois que introduces a time clause.

It is very common in Portuguese when one action happens after another.

In this sentence, que links depois to a full clause.

Compare:

• depois que o eco diminui = after the echo decreases
  
  • here you have a conjugated verb: diminui
• depois de diminuir o eco = after reducing the echo / after the echo decreases
  
  • here you have depois de
    • infinitive

So depois que is used when what follows is a complete clause with a subject and verb.

They are in the present tense because the sentence states a general fact or regular relationship:

• Depois que o eco diminui, a acústica melhora.
• After the echo decreases, the acoustics improve.

Portuguese often uses the present tense for general truths, habitual situations, and cause-and-effect statements.

Diminui is the 3rd person singular of the verb diminuir.

Here it means:

• decreases
• gets smaller
• lessens
• fades/reduces, depending on context

In this sentence, o eco diminui means the echo decreases or the echo dies down.

Melhora is the 3rd person singular of the verb melhorar.

It means:

• improves
• gets better

So a acústica melhora means the acoustics improve or the acoustic quality gets better.

Portuguese usually builds the meaning directly into the verb form, instead of using an extra helping verb like English does.

For example:

• o eco diminui = the echo decreases
• a acústica melhora = the acoustics improve

The verb endings already show the tense and agree with the subject. Portuguese does not need an extra word like does in normal affirmative sentences.

A acústica usually means the acoustics or the acoustic quality of a space.

Even though English often uses the plural acoustics, Portuguese commonly uses the singular noun acústica for this idea.

So:

• a acústica melhora = the acoustics improve
• more literally: the acoustic quality improves

Portuguese uses definite articles much more often than English.

Here:

• o eco = the echo
• a acústica = the acoustics / the acoustic quality

In Portuguese, it sounds natural to include the article with many nouns, including abstract or general ones. English often leaves the article out where Portuguese keeps it.

The comma separates the introductory time clause from the main clause.

• Depois que o eco diminui, = subordinate time clause
• a acústica melhora. = main clause

This is similar to English:

• After the echo decreases, the acoustics improve.

If the order changes, the comma is often unnecessary:

• A acústica melhora depois que o eco diminui.

Yes. A very natural alternative is:

• A acústica melhora depois que o eco diminui.

Both versions are correct. The difference is mostly about focus:

• Depois que o eco diminui, a acústica melhora.
  Focuses first on the time relationship.
• A acústica melhora depois que o eco diminui.
  Focuses first on the main result.

Yes, especially if you are talking about a specific future situation.

For a general statement:

• Depois que o eco diminui, a acústica melhora.

For a future event, Brazilian Portuguese often uses the future subjunctive after depois que:

• Depois que o eco diminuir, a acústica vai melhorar.
• After the echo decreases, the acoustics will improve.

This is a very important pattern:

• depois que
  • future subjunctive when referring to the future

Because they are different tenses/moods:

• diminui = present indicative
• diminuir = future subjunctive

So:

• Depois que o eco diminui... = general fact / what usually happens
• Depois que o eco diminuir... = after the echo decreases at some future time

This distinction is very common in Portuguese time clauses.

In Brazilian Portuguese, diminui is roughly pronounced like:

• jee-mee-NOO-ee

More carefully:

• di-mi-nu-i

The stress is on the last i sound group: diminuÍ.

A common learner mistake is to compress it too much or stress the wrong syllable. Try to hear it as four vowel steps at the end: nu-i.

Acústica is roughly:

• ah-KOOS-chee-kah in Brazilian Portuguese

The written accent in á/ú? Here the word is acústica, with the stress on cús:

• a-CÚS-ti-ca

The accent mark tells you where the stress goes. Without learning the exact pronunciation rules yet, the key thing is:

• acústica → stress the CÚS

Yes, eco is masculine:

• o eco

Grammatical gender in Portuguese is something you usually learn together with the noun. There is not always a perfect logic to it, so it is best to memorize nouns with their article:

• o eco
• a acústica

That helps you remember agreement naturally.

No, not in this sentence.

Use:

• depois que
  • clause
    
    • Depois que o eco diminui...
• depois de
  • noun or infinitive
    
    • Depois de diminuir o eco...
    • Depois da redução do eco...

Depois de que is not the normal choice here.

It can absolutely describe the sound quality of a room or environment.

In everyday use, acústica often means:

• the acoustics of a room
• the way sound behaves in a space

So this sentence sounds natural in contexts like:

• a concert hall
• a studio
• a classroom
• a meeting room

Not exactly. O eco diminui most naturally means:

• the echo decreases
• the echo gets weaker
• the echo dies down

If you want to say the echo is reduced in a more passive or caused-by-someone sense, Portuguese would usually say something like:

• O eco é reduzido
• Reduzem o eco
• O eco foi reduzido

So diminui focuses more on the change happening, not necessarily on who causes it.

Yes, it is natural and grammatically correct.

It sounds like a clear explanatory statement, the kind of thing you might hear in:

• acoustics discussions
• architecture
• audio engineering
• classroom explanations

A slightly more conversational variation could be:

• Quando o eco diminui, a acústica melhora.

But the original sentence is perfectly good Portuguese.