Double-layer porous asphalt concrete (DPAC) pavements are generally considered to be the acoustically most effective low noise road surfaces ready for implementation. While DPAC used on highways in warm climates may have an average life of around eight years, in Scandinavia with severe winter climate DPAC usually survive only about three years; partly due to wear of studded tyres. An ongoing project in Sweden, applying DPAC and single-layer porous asphalt (PAC), the latter consisting of the top layer of the DPAC, on motorway E4 in Jönköping-Huskvarna, has revealed interesting performance. Initial noise reduction was 7-8 dB(A) compared to a set of reference surfaces (conventional SMA 16). Amazingly, after one year of operation this noise reduction is unchanged. The clogging had not yet affected the acoustical properties significantly. Most interesting is that the noise reduction difference between the single-layer and double-layer PAC is approximately 5 dB(A). Since the single-layer PAC is identical to the 30 mm thick top layer in the DPAC (although 5-8 mm thicker), it follows that 2/3 of the noise reduction is due to the bottom layer of the DPAC; i.e. what lies approx. 35-40 mm below the top surface. The paper studies the effect of the bottom layer on the overall acoustical efficiency of the DPAC and concludes that the 80 mm thickness tunes the sound absorption well to tyre/road noise.