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the active attenuation gradually disappeared when the AGC circuit reduced the gain of the controller.
Audible distortion was not detected until the sound pressure level reached 97 dB SPL inside the
earcup. However, with loose fit between the hearing protector and the head, distortion could be heard
at lower sound pressure levels.

The in-ear measurement results show that the developed device is able to actively attenuate low
frequency noise up to a maximum of 20 dB. This is a significant improvement in the low frequency
performance of a hearing protector. The measured active attenuation is almost same for both stimuli.
The automatic gain control circuit reduces the active attenuation when needed, which makes the device
more usable in high noise level environments. The drawback of the gain reduction is that active
attenuation performance is reduced at the same time. The device also improves comfort and speech
intelligibility because it reduces significantly the low frequency boom, which is typical of passive
hearing protectors due to their poor low frequency attenuation. Because the prototype improves low
frequency noise attenuation, it reduces the risk of hearing loss and thus improves safety. Although the
AGC circuit reduces distortion and extends the dynamic range of the device, further research is still
needed for very high SPL environments.

CONCLUSIONS
One problem associated with active hearing protectors is that a loud low frequency sound can saturate
the system, which is heard as distortion. A prototype of an active noise cancellation hearing protector
had been developed earlier, and now special attention was paid to improving the comfort and stability
of the device. As a solution, an automatic gain control circuit was incorporated, and both acoustical
and electrical designs were improved in order to ensure stability. In-ear measurements were made. The
measurement results show that the developed prototype significantly improves the low frequency
attenuation of a passive hearing protector. The listening tests demonstrated that the AGC circuit makes
the device more comfortable to use. Further, there was no sign of instability.

ACKNOWLEDGEMENTS

This work was supported by Oy Silenta Electronics Ltd, a Finnish hearing protector manufacturer and
TEKES, The National technology agency of Finland.

REFERENCES
1. Rafaely B. (1997). Feedback Control of Sound. Ph. D. Thesis, University of Southampton, UK,

2. Kuo S.M., Wu H. Chen F., and Gunnala M.R. (2004). Saturation Effects in Active Noise
Control Systems. IEEE Transactions on Circuits and Systems-I: Regular Papers 51:6, 1163 -
1171.

3. Oinonen M.K., Raittinen H.J., and Kivikoski M.A. (2004). An Automatic Gain Control for an
Active Noise Cancellation Hearing Protector. Active 2004 - The 2004 International Symposium
on Active Control of Sound and Vibration, Williamsburg, VA USA.