# EXAM1 question21&70

@coachadamcoers I run into two acoustical questions in PDD exam1. both were asking how much decibel change would be perceived half as loud/reduce intensity by half.

I got confused because a 3db change means sounds intensity is reduced by half, but 3db change is also perceived as negligible. would you explain the rational/confirm sound intensity change is not directly related to sound perceived?

In Question 21
There are two identical fans that together produce 60 dB of sound.

If one fan is removed, what is the resulting dB level?

Answer: Reducing sound intensity by half reduces it by 3 dB. 60-3=57.

In Question70

An existing open office space currently has a noise intensity level of 65 decibels. The client would like to reduce the level of noise to provide a quieter environment. Installation of acoustical panels will reduce the noise level of the space to 55 decibels.

What is the perceived change in noise intensity?
Answer: Half as loud. Incorrect. A change of 3 or fewer decibels is barely perceived.

Hi @wchen !

Thanks so much for your question! You are correct in the fact that sound intensity isn’t necessarily perceived. Reducing the sound by half always reduces the sound by 3 dB no matter what the dB level is, which was definitely confusing to me when I was preparing for my exams. This works inversely as well; doubling the sound increases the sound by 3 dB no matter what the dB level is.

In the second question you posted, the answer was incorrect because the owner wanted to reduce the noise level by 10 dB and “half as loud” only reduces it by 3 dB. The answer also states that 3 or fewer dB is barely perceived, as a comparison 10 dB of sound is the equivalent noise level of rustling leaves. I hope this helps!

Thanks,
Rebecca