i have also heard some about silent japa, that it is shure that if i chant say for example silently inside my mind the air will become purified and that, there are exactly different results from the practice. howsoever, i have come to realize that loudly japa is way more beneficial, yeah sure just when the heart usually goes to bit with it.

ok, hare krsna hari bol

"Loudly or silently, everything is all right. There is no such restriction. Only thing is that we should chant very attentively, hearing the vibration very distinctly."

From a letter to Satsvarupa - March 8th, 1969

Does Srila Prabhupada's use of the word "silently" mean softly?
The words "hearing the vibration very distinctly" indicates to me that it is an audible sound vibration, whether that is loud or soft.

From Śrī Caitanya-bhāgavata, Ādi-khaṇḍa - Chapter 16 - The Glories of Śrī Haridāsa Ṭhākura

Haridāsa was particularly aggrieved to see the people’s lack of interest in the process of devotional service. In spite of this, Haridāsa continued to loudly chant the holy names of the Lord. The most sinful miscreants were even unable to hear this loud chanting.

In this regard, there was one impious brāhmaṇa that lived in the village of Harinadī. He once angrily said to Haridāsa. “O Haridāsa, what is this behavior? Why are you are loudly chanting the names of the Lord? The injunction is that one should chant in his mind. Which scripture says that one should chant loudly? Who has taught you to chant the name of Hari loudly? Please give your explanation before this assembly of learned scholars.”

Haridāsa said, “You all know the glories of Lord Hari’s holy names. Therefore I have simply repeated and will repeat whatever I have heard from you. If one chants loudly, he gets one hundred times more benefit. The scriptures never condemn loud chanting, rather they glorify it. ‘If one loudly chants the holy names of the Lord, he obtains one hundred times more benefit than by chanting softly or remembering the holy names.’”

The atheistic brāhmaṇa said, “How does one get one hundred times more benefit by loud chanting?”

Haridāsa replied, “My dear sir, listen to the verdict of the Vedas and Śrīmad Bhāgavatam in this regard.”

Haridāsa then revealed the purport of all the scriptures as he began his explanation in the ecstasy of Kṛṣṇa consciousness. “Listen, dear brāhmaṇa. If even animals, birds, or insects hear the holy names from the mouth of a pure devotee, they will go to Vaikuṇṭha. Anyone who chants Your name purifies all who hear his chanting, as well as himself. How much more beneficial, then, is the touch of Your lotus feet? [SB 10.34.17] If one silently chants the names of Kṛṣṇa, then he is delivered; but if one loudly chants, then he delivers others also. Therefore the scriptures say that one gets a hundred times more benefit by chanting loudly."

"One who loudly chants the holy names of the Lord is a hundred times greater than one who silently chants, because those who chant silently purify only themselves, while those who chant loudly purify themselves as well as those who hear them.’ [This verse was spoken by Prahlāda Mahārāja in the Nāradīya Purāṇa.] The Purāṇas say that a person who chants the Lord’s name loudly is a hundred times more pious that the person who chants to himself."

"O brāhmaṇa, listen carefully to the reason behind this. One who softly chants the holy names liberates only himself. One who loudly chants the names of Govinda, however, liberates himself along with all living entities who hear him. Although all living entities have a tongue, only the human beings are able to chant the names of Kṛṣṇa. Tell me, what is wrong with that activity by which living entities who have taken useless births will be delivered? One person may maintain himself, while another may maintain a thousand people. Of the two, consider carefully who is better. This is the superior characteristic of loud chanting.”

After hearing the words of Haridāsa, the brāhmaṇa began to angrily blaspheme him. “Now even Haridāsa has become a philosopher! I can see that Vedic culture is being destroyed by the course of time. It is stated that śūdras will explain the Vedas at the end of Kali-yuga. But why only at the end of the age? We can see it happening even now. This is how you advertise yourself, so you can eat nicely at other’s houses. If the explanation that you have made is not true, then I will cut off your nose and ears.”

Hearing the words of that sinful brāhmaṇa, Haridāsa smiled and chanted the name of Hari. He did not speak further to that atheistic brāhmaṇa, but left immediately while loudly chanting the holy names.

The sinful members of that assembly were all wicked-minded. They neither supported the authorized statements of Haridāsa nor did they protest the offensive words of the brāhmaṇa. They were brāhmaṇas only in name. Actually they were all demons, fit to be punished by Yamarāja.

“In Kali-yuga, demons will take birth in the families of brāhmaṇas to harass those rare persons who are conversant with the Vedic way of life.” [This verse was spoken by Lord Śiva in the Varāha Purāṇa.]

Within a few days, that wretched brāhmaṇa was attacked by smallpox and as a result his nose melted away and fell off.

The punishment he had proposed for Haridāsa Ṭhākura was awarded to himself by Kṛṣṇa.

Aggrieved to see the entire world absorbed in sense gratification, Haridāsa would sigh deeply as he chanted the name of Kṛṣṇa. After a few days Haridāsa went to Navadvīpa with a desire to associate with the Vaiṣṇavas there.

chanting silently is a great way to catch up on sleep!! especially right after mangal arati. I remember back in the Srila Bhaktipada days, many in the temple room would fall into deep "silent meditation."

Bombay Nov 10, 1970

Prabhupāda: We chant Hare Kṛṣṇa. It may not be melodious but we chant Hare Kṛṣṇa.

(Indian man starts singing Hare Kṛṣṇa mantra, coughs and stops) All right, don't take now(?). You want water? Eh? Give this glass. Come on. [break] ...nāmaiva eva kevalam kalau nāsty eva nāsty eva nāsty eva. There is no mention that it should be chanted lowly. So how you can say it is gupta? It is not gupta.

Guest: (indistinct)

Prabhupāda: This is also Purāṇa says. Especially it is harer nāma [Cc. Ādi 17.21], not gupta. That is in the Kali-yuga it should be openly chanted and we have to follow our predecessor, Haridāsa Ṭhākura, nāmācārya.

Guest: (indistinct)

Prabhupāda: [break] When we chant, when we utter the bīja mantra that we utter loudly. That is required. That is japa. So this mantra is mahāmantra and it should be chanted loudly, or as you like. There is no such restriction. Niyamitaḥ smaraṇe na kālaḥ. Nāmnām akāri bahudhā nija-sarva-śaktis tatrārpitā niyamitaḥ smaraṇe na kālaḥ. (Hindi) And we have to follow the great personalities. Haridāsa Ṭhākura, he was chanting very loudly; Caitanya Mahāprabhu chanted very loudly. So what more evidence you want? My Guru Mahārāja chanted loudly, we are chanting loudly. Whole business finished. (chuckles) Is that all right? Eh?

Guest: (indistinct)

Prabhupāda: Our process is very easy. We have to see whether my Guru Mahārāja, his Guru Mahārāja has followed this. Then there will be no more doubt. Evaṁ paramparā-prāptam [Bg. 4.2]. We receive things by paramparā system. Mahājano yena gataḥ. Sādhu mārgānu-gamanam. We have to follow the footsteps of sādhu.

Guest: (indistinct)

Prabhupāda: Even if I am not well conversant with the Vedas and Purāṇas, if I see that my spiritual master is doing that, that's sufficient.