The insertion loss of a transformer is simply a measure of the efficiency. It shows how power is consumed by the transformer. The result is the temperature rise, or how hot the transformer gets. The majority of the losses are the DC resistance in the windings. However, the core loss can be quite high if the flux density is great. We design all of our audio transformers at 1.4 Tesla (14k Gauss) or less (unless the customer requires a very small transformer).
We calculate the losses at design time and limit the losses as follows. A 10% loss is approximately 0.5 dB insertion loss and 20% is about 1.0 dB.
Another simple definition by Bob Wolpert "Insertion loss is a measure of the power available out of the transformer vs the power induced into the transformer." As you can see it has little meaning unless it is a power producing audio transformer.
