When a mantra is chanted in rhythmic tone with ups and downs, they create a melodious effect in the body . This effect is high if you can make your brain nil of thoughts. The effect will be possible even if you do not know the meaning of the mantra. Hence knowing the meaning of every mantra is not compulsory. While chanting assume the god sitting in front of you.
At the same time if you know the meaning it has got an additional effect. Lot of Research studies have been carried out by many and important results derived by one of the famous professors. The chanting effects are due to the production and spreading of curative vibrations in the brain. These curative vibrations give smoothening and curing effect to the body.
Listening to mantras directly lowers blood pressure, normalizes heart beat rate, brain wave pattern, adrenalin level, even cholesterol level. That is the reason why modern doctors advise the people under high tension to sit and listen to mild music or mantras for few minutes. Even chanting the kirtans, melodious bhajans, songs, etc., have the good effect The music/ song/mantra should never be hard/ harsh/rough/ etc.
Chanting the mantras everyday is excellent and important at a particular time. In physics the time, space and observer are the three important factors. Exactly like that the time, space and the person connected with chanting/listening mantras are three important factors for deriving benefit of mantras. Hence sareera shuddhi, aahara shuddhi, mana shuddhi, vaak shuddhi and karma shuddhi are essential for deriving the full benefit of chanting the mantra. Also the place where you are sitting should give calming effect, comfortable feeling etc. The ideal timings are the prabhata sandhya and saayam sandhya. (Morning & evening) Group chanting is excellent. Absolutely no mistake is expected in Vedic mantra chanting. When you are sick due to ill health or old age, one need not worry too much about his incapability of chanting the mantras. Mantras are chanted for solving our problems and for positive effects but mantra chanting/ not chanting should not become another problem for us. The ideal sound for mantra chanting can be maintained if possible at radius 5 meters. Veda mantras chanting is not easy so it is advised not to chant without the guidance from a guru.