How do you set up an science experiment if heavy metal affects plant growth

You need to do more than that. Start with identically planted seeds in say ... 10 pots. Same size pot, soil, seed. Place all in identical lighting conditions. Place half the pots in an area where they will NOT hear your heavy metal music. The other 5 must hear the music nonstop. You cannot just turn it on and off; if you do, you cannot prove that the music has any affect. Provide the same amount of water to both sets. Everything must be identical in all ways except for the music. As the plants grow, then measure and record. In all likelyhood, you will need to confine your music selection to a single artist.

Put a speaker in front of a plant of your choice thats playing heavy metal music and leave it there for 1 day or two measure the growth before and after