The head is that part of a tone-unit that extends from the first stressed syllable to the tonic syllable, but not including it. There are two pitch possibilities in the head which are called high head and low head.
In the case of the high head, the stressed syllable that begins the head is high in pitch, often higher than the beginning pitch of the tone on the tonic syllable.
In the case of the low head, the stressed syllable that begins the head is low in pitch, often lower than the beginning pitch of the tone on the tonic syllable.
Definitely, the two versions (high and low head) of the same sentence “The bus was late”, will sound different to any listener, but it is not easy to say what the difference is.
Head’s unstressed syllables usually continue the pitch of the stressed syllable that they follow (see the example below).
There can be more than one stressed syllables in the head which leads to a change in pitch from the one stressed syllable to the next one. That change is in the direction of the beginning pitch of the tone on the tonic syllable.
In the example below, since we have a fall tone, the stressed syllables in the head start high and progressively step down to match the beginning level of the tone.
In the next example, the head is low because the tone starts low as well. There isn’t a progressive movement as we saw in the previous example.
When there is a low head followed by a falling tone then the stressed syllables of the head move upwards to match the beginning pitch of the tone.
When there is a high head followed by a rising tone then the stressed syllables of the head move downwards to match the beginning pitch of the tone.
The long heads examples given above are not frequently found in natural speech but they were given to exemplify the change in pitch. These are simple examples and if we examine the intonation of polysyllabic heads we find a greater variety of changes in pitch. However, these changes are hard to be recognized by most speakers of English which makes them pointless to be represented here.
I did a lot of research on this topic, intonation, and it seems that the most basic and normal intonation pattern used is the one starting at a fairly high pitch continuing with a progressive dropping down of the pitch during the utterance. This intonation pattern is called declination.