Audible Warning Device

  • Maisie Hoile
Brompton Bicycles Ltd have identified an opportunity for the sound pressure of their compact Audible Warning Device (AWD) to be increased. The sound pressure needs to meet ISO Standard class II - 75dB(A) to 85dB(A) - and the frequency altered to create a more assertive sound. Their existing bell is positioned on the brake lever where it remains in clearance when the bike is being folded and unfolded.
Final prototype in use, mounted onto an M Type handlbar.
Testing existing AWD's in the anechoic chamber, with test conditions set according to (ISO 14878:2015(E)).
The proposed solution increases the diameter and reduces the depth of the bell cup, providing a larger resonating chamber whilst also lowering the frequency. A torsion spring has replaced the original extension spring to provide greater impact on the bell cup, as well as reducing the risk of misalignment of the striker. The estimated cost for the new AWD is within 5% of the previous bell, meeting the target production costs. By optimising the assembly within the space constraints, the final design produces a sound pressure that matches the measurements from existing class II bells (ISO 14878:2015(E)).  
Material testing to find the most suitable metal for the final design solution. Due to lower frequency readings and consistenty high sound pressure, brass was chosen for the final design.
Prototyping using 3D printed components. 
Testing the bell’s clearance when the Brompton is folded.
Exploded view of the final design solution, including a mixture of new components and some existing components from Brompton's original AWD assembly.
This project was completed in collaboration with Brompton Bicycles Ltd. for my final year project at Brunel University London.