Case studies

Acoustic absorber

NPL's Ham A acoustic absorber material has been developed specifically for the medical ultrasound field. This unique high quality absorber material, available only from Precision Acoustics Ltd, has special acoustic properties which make it ideal for a range of applications, including:

  • absorbing targets for radiation force balances, to minimise the errors associated with the reflecting targets of most commercially available radiation force balances.
  • low ultrasonic echo (anechoic) material as a specialist coating for hydrophone mounts used for characterisation of medical ultrasonic equipment employing continuous-wave or long toneburst excitation.

Physical Specification

  • Based on polyurethane rubber material. Two layers: top layer - acoustic impedance matched to water backing layer - partially air-loaded to increase transmission loss
  • Interface between the two types of material pyramidal in structure to scatter incident ultrasound
  • Density of 1.01g/cm3, is close to that of water
  • Thickness of tile: 14mm
  • Dimensions: 250mm by 200mm
  • Material can be precision cut in various geometries to meet exact end-user requirements, e.g., simple discs of 50mm diameter

Acoustical Specification

  • High transmission loss, > 30dB/cm/MHz
  • High echo-reduction, > 42dB at 1MHz degrading to 35dB at 10MHz
  • The new NPL absorber fully meets the requirements for radiation force balance targets, covering the frequency range 1-15MHz3
  • HAM A absorber has been used to perform power measurements up to 20 watts.*
  • For high power applications, significant temperature rises can be generated within the HAM A material. An assessment has been completed on the thermal tolerance of the NPL absorber to high temperatures and the material can readily be used to complete measurements up to 20 watts, using exposure times less than or equal to 10 seconds. Should heating be a problem in your application, we suggest the Aptflex F28P which is similar to the Ham A without the top acoustic impedance matched layer, allowing water to cool the backing layer.


  1. 1 Robert T. Hekkenberg, Klaus Beissner and Bajram Zeqiri, Therapy-level ultrasonic power measurement, Final Technical Report for Project SMT4-CT96-2139, 2000
  2. 2 Bajram Zeqiri and Catherine J. Bickley, A new anechoic material for medical ultrasonic applications, Ultrasound in Med. & Biol., Vol. 26, No. 3, pp. 481-485, 2000
  3. 3. IEC 61161 (1992). Ultrasonic power measurements in liquids in the frequency range 0.5 to 25MHz. IEC, Geneva, Switzerland
HAM A absorber

Provided by Courtesy of NPL

Make an enquiry

We offer manufacturing and testing to provide complete bespoke product design.

Contact us to ask about anything you can’t find listed elsewhere.