Acoustics at Newgrange
Newgrange is one of the largest passage tombs in Ireland, and is aligned upon the midwinter sunrise. The site is intensively decorated with rock art. I conducted archaeoacoustic research here with David Keating in 2001. Similar to Maeshowe and Camster Round, we found the interior to be conducive to standing wave resonances, and the enormous cairn ensures that significant filtering takes place as sound moves between the inside and outside. Like other sites of its kind, an audience outside will be mostly excluded from seeing or hearing what is taking place in the chamber.
One of our objectives at Newgrange was to test for infrasound. Measurements at Maeshowe had demonstrated the presence of Helmholtz Resonance, an acoustic effect caused by the movement of air between an enclosed chamber and restricted passageway. Under certain conditions this can cause the air within to oscillate, significantly amplifying a single frequency. Inside Maeshowe, this resonance was recorded at 2 Hz. This is too low to be heard by the ears, but might cause unusual physiological effects even altered states of consciousness. We had predicted that Helholtz Resonance at Newgrange should occur between 1 and 2 Hz, but there was no response at all from our sensitive equipment.
Measuring standing waves resonances in the passage at Newgrange. David Keating (standing) is holding a microphone while I capture sounds on digital tape recorder.
David Keating (left) and myself, showing our purpose-built equipment inside Newgrange, including the microphone used to test for infrasound.
One reason why Helmholtz Resonance is not triggered inside Newgrange is that too much sound energy is being absorbed into the chamber walls. The rounded stones used in construction of Newgrange are quite different to the regular flagstone slabs used to build Maeshowe, and sound can be absorbed into the gaps between the stones. It also appears that the covering mound at Newgrange was rather more permeable than Maeshowe’s solid clay capping. Indeed, excavations in the 1960s and 70s revealed an intricate system of rainwater drainage channels cut into the stone slabs of the chamber roof.