Sound Attenuation

[shaunnutley]

I have an alarm clock that emits a 112 decibel bleep when my decibel meter is placed directly over its very tiny speaker. I noticed that the noise attenuation is far,far greater that 6 dB for every doubling of distance away from. There are no other noises in the room so why is the sound attenuation so great? My guess is the size of the speaker, but i thought that any sound source would decrease in decibels with every doubling of distance. Can anyone give me an answer? If this is the case then would a sound source of 200 decibels also not decrease by 6dB with every doubling of distance?

[mibn2cweus]

Up to 90% of volume is due to reflected sound. Try measuring outdoors away from reflective surfaces.

Other factors to consider are directionality associated with increasing frequency, and beaming, the unequal distribution of sound radiation characteristics, phase interactions (cancellation and reinforcement) between two sources or a source and a reflection. Also whether the sound is emanating from a point source or a broad area will skew measurement results.

You may also need to consider the way that sound is being coupled between the source and the microphone in the measuring device. Reference sound levels are typically measured at some distance (one foot or meter) from the source. Attention should always be given to the effects from the presence of the measuring device itself on the measurement derived.

Acoustic properties are notoriously complex, especially within an indoor environment. I hope this sheds some light on the ambiguities you are witnessing.