Sir: Janet Mason's description ("How my baby joined the beat generation", 30 May) of playing sounds to her daughter in utero, through loudspeakers worn on a belt, mentions her worry about the sound being too loud. The intended recipient was, of course, a captive audience at 20 to 40 weeks' pregnancy. But there are serious issues here which are the subject of current research.
Most external sounds will be received by the foetus at approximately the same decibel level as by the mother. An important exception is sound generated by vibrating the abdominal wall, which is transmitted to the foetal head much more effectively than to the mother's ear. It could be, therefore, that abdominally worn devices like that described in the article produce much higher decibel levels in the womb than the outside listener suspects.
CHRISTOPHER L. MORFEY
Reader in Fluid Dynamics and Acoustics
University of Southampton
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