This is our third blog in our Acoustics in the Sun series. Find the second article here. Today’s features Tim Beresford and Max Cyril’s paper titled: Design and implementation of a masking system for tonal industrial noise emissions. This was an exciting paper for me, because it confirmed what I’d always suspected: masking can work for industrial noise if you use a subwoofer to target the low frequencies.

Noise regulations and industrial noise

Industrial noise comes in all shapes and sizes. The most difficult to mask is intermittent noise—think the barking dog we discussed earlier. In this case, the sound was “tonal”. It is annoying for neighbours, and prolonged exposure of tonal noise can lead to negative health effects, including sleep disturbance. It can also be a breach of the law.

Complying with noise regulations can become a very expensive proposition if a sound cannot be easily contained. This was the case here, where an intermittent industrial noise breached the regional law regarding tonal noise. The client had tried many amelioration measures, with mixed success, so the best option was to mask the sound.

Masking tonal noise

While different in each region, the overall definition of tonality is a sound where adjacent 1/3 octave bands have a difference greater than 3-5dB. Some regions have more detailed definitions. A violation of the law can result in penalties. In this paper, the persistence of low-frequency tones meant that the facility’s noise output attracted a +5dB penalty to the overall A-weighted level, pushing the rated sound level consistently over the night-time noise limit. Fortunately, if the tonal sound could be eliminated, the sound levels themselves would be compliant. This meant the masking system had to remove the tonality while not raising the overall sound level by more than 1dB.

In this case, the solution involved measuring the tonal sounds using a microphone. They used the measurements to target those sounds using masking. Here is the graph the authors used to illustrate it:

You can clearly see the tonal components, and imagine how the neighbours would be concerned! Due to the low frequency, a subwoofer had to be used.

Thankfully, the sound masking system worked, and the client could continue to operate its machines at night without impacting the surrounding residents.

For the full paper, click here.