This type of song was sung only with their voices and nothing else. They fully utilized every part of their vocals. Do you remember this famous song “Don’t worry, be happy”. by Bobby McFerrin?? If you revisit the youtube video, you will soon find his song has not musical instruments but it made of humming, snapping of fingers and a lot of woo sound. The singer did it nicely by singing his song note to note and it gained popularity in the 80’s music charts for many weeks. If you like songs in multiple era sang from an a cappella group, then look for Pentatonix. After you have heard the song “Hallelujah”, you will be soon been mesmerized that these group of 5 possessed the voice of angels . The song was so melodious that it moved me to tears. They also are very tacky on how to make the sound very up to beat when they sang “Oh come all Ye Faithful”. I really could not believe all of the song is made only by voices and does not involve any equipment, electronic or manual. This is definitely an ingenious way of this modern century.