1. Set volume of reference (so that it feels good to you)
2. Look at SPL meter (fine to use an app-no need to buy a spl meter). Remember the average SPL
3. Play back your mix at same SPL. (You should be able to instantly notice the balance of your track being off or bang on).. you can immediately apply steps below if your song is already mixed.
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Mix your song at the same SPL.
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Forget about the meter and mix away, if you notice these two things double check 2 parts.
1. Are you turning the volume down? A/B the top end with the reference (BY EAR)
2. Are you turning the volume up? A/B the low end with the reference (BY EAR)
Adjust the balance via volume or EQ and keep on going