  Impact evaluation in public engagement necessarily requires measuring change.
However, this is extremely challenging for drop-in activities due to their very
nature. We present a novel method of impact evaluation which integrates
graffiti walls into the experience both before and after the main drop-in
activity. The activity in question was a soundscape exhibit, where young
families experienced the usually inaudible sounds of near-Earth space in an
immersive and accessible way. We apply two analysis techniques to the captured
before and after data - quantitative linguistics and thematic analysis. These
analyses reveal significant changes in participants' responses after the
activity compared to before, namely an increased diversity in language used to
describe space and altered conceptions of what space is like. The results
demonstrate that the soundscape was surprisingly effective at innately
communicating key aspects of the underlying science simply through the act of
listening. The impacts also highlight the power of sonification in stimulating
public engagement, which, through reflection, can lead to altered associations,
perceptions, and understanding. Therefore, we show that this novel approach to
drop-in activity evaluation, using graffiti walls both before and after the
activity and applying rigorous analysis to this data, has the power to capture
change and, thus, have a short-term impact. We suggest that commonly used
evaluation tools suitable for drop-in activities, such as graffiti walls,
should be integrated both before and after the main activity in general, rather
than only using them afterwards as is typically the case.
