import numpy as np

def decibels(f):
    return 10 * np.log10(f)

def find_top_two_keys(d, threshold):
    sorted_items = sorted(d.items(), key=lambda item: item[1], reverse=True)

    if len(sorted_items) < 3:
        return None

    top_two = sorted_items[:2]
    third_value = sorted_items[2][1]
    if top_two[0][1] - third_value < threshold or top_two[1][1] - third_value < threshold:
        return None

    return top_two[0][0], top_two[1][0]