Abstract The application of non-destructive dynamic testing to the assessment of brickwork arch bridges is described in this article. A procedure using impulsive one-shot excitation with frequency domain analysis is proposed for detection and sizing of voids hidden in the backfill above the extrados. Analysis by 3-d, dynamic finite element analysis proved the method’s efficacy. For experimental verification, various voids were formed at different positions; the bridge was tested both intact and with voids. The method detected and sized the voids. The frequency response function amplitude was the most reliable means of defect characterisation. The method appeared suitable for in situ arch assessment purposes.