I have heard the shevarim sound from the shofar done in one of two ways:

• three short blasts maintaining the same pitch (like "toot-toot-toot")
• three short blasts alternating in pitch staring with a low pitch, becoming a high pitch and returning to the low pitch again (like "to-WHO-to; to-WHO-to; to-WHO-to". The middle part on a higher pitch.)

Is only one method correct, or either one? Is one method more preferable? Any sources to support the different styles?