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Precision Acoustics


NPL’s HAM A acoustic absorber material has been developed specifically for the medical ultrasound field.

This product is available to buy direct through our secure online shop.​

ham a tile


NPL high quality acoustic absorber for ultrasonic applications

NPL Technology AppliedThis unique high quality absorber material, available only from Precision Acoustics, 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 insertion loss.
  • Interface between the two types of material pyramidal in structure to scatter incident ultrasound.
  • Dimensions: 200 x 250 x 14 mm.
  • Coefficient of linear thermal expansion is 200ppm/oC.
  • Density of 1010 + 20 kg / m3.
  • Specific gravity is 1.01.
  • The Longitudinal wave speed (1-10 MHz) ≈1500 m/s.
  • Acoustic Impedance is 1.5 MRayls.
  • There is residence to Isopropyl Alcohol (IPA) and Tricholethylene.
  • It can be affected by Ketones (MEK, Acetone) – Swell.
  • Dichloromethane – Swell and breakdown.
  • Avoid prolonged exposure to Ozone and UV.
  • The stability is very stable due to cross-linked nature of polymer.
  • Material can be precision cut in various geometries to meet exact end-user requirements, e.g., simple discs of 50 mm diameter.

Acoustical specification

  • High insertion loss > 30 dB/cm/MHz.
  • High echo-reduction, > 42 dB at 1 MHz degrading to 33 dB at 14 MHz.
  • The NPL absorber fully meets the requirements for radiation force balance targets, covering the frequency range 1-15 MHz [3].
  • HAM A absorber has been used to perform power measurements up to 20 watts.*


  • 1. Robert T. Hekkenberg, Klaus Beissner and Bajram Zeqiri, Therapy-level ultrasonic power measurement, Final Technical Report for Project SMT4-CT96-2139, 2000
  • 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. IEC 61161 (1992). Ultrasonic power measurements in liquids in the frequency range 0.5 to 25MHz. IEC, Geneva, Switzerland