Como é que se diz isto em Inglês (EUA)? Generally speaking, it needs a complete set to use the effect just can apparent. Does this sentence sound natural ？ Please help me to rectify😊😊

See a translation

@Hahamei - Thank you!
“The effect will be not so good if just use one alone." < -- That is a great sentence and works perfectly for what you are trying to say. But if you want to use your first sentence, you could say:

"Generally speaking, you will need a complete set to get the correct sound effect(s). If you only get one speaker, you won't get the correct sound effect(s)."

or

"Generally speaking, you will need a complete set to get the full sound effect. One speaker won't give you sound quality this system is designed for"

There are many ways you could say this depending on the loud speaker system you are selling and what the customer is looking for. I hope these two sentences I provided help you out!