Sign up ×
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free.

Well it is pretty well known that rooms with sound less than zero decibels, $\approx$ -15 decibels. How is it possible to create a room which is quieter than soundless? And it is claimed that just staying in the room for a mere 30 - 45 minutes, depending on your source, could drive you crazy. Why is this?

share|cite|improve this question
Regarding the craziness... that's largely nonsense except if you have claustrophobic tendencies. There's a nice Veritasium video on silent chambers. – leftaroundabout Jul 9 '14 at 20:31
'Cause sometimes zero's not nothing. – RBarryYoung Jul 9 '14 at 23:23
@leftaroundabout I've been in that anechoic chamber. I wasn't there alone or for a long time, but it was a bit uncanny. I could see sensory deprivation giving you anxiety, but it's not like your brains get sucked out or anything. I was definitely disoriented by the lack of echoes. I never knew how much we relied on them. – krs013 Jul 10 '14 at 8:24
@krs013 Thank you providing personal experience. I don't think that videos can explain such an experience. – Gummy bears Jul 10 '14 at 12:30
@Gummybears it really can't; it's true. I felt like I had gone deaf until someone spoke, and then it felt like I was hearing them through earbuds—the sound was right in my ears. – krs013 Jul 10 '14 at 14:16

2 Answers 2

up vote 17 down vote accepted

Zero decibels isn't soundless.

The decibel scale is a logarithmic one. For sound each 20 decibel step changes the air pressure associated with the sound changes by a factor of ten. So if you take 20dB as a reference, 0dB is a factor of ten quieter and -20dB is a factor of 100 quieter. Completely soundless would be $-\infty$dB.

Zero decibels corresponds to a sound pressure of $2 \times 10^{-5}$Pa. This corresponds roughly to the quietest sound humans can hear.

share|cite|improve this answer
So then why is there a reference point of 0 decibels? What is special about it? – Gummy bears Jul 9 '14 at 16:35
Depending on the frequency of the sound, 0 dB is the "threshold of hearing." – Kyle Kanos Jul 9 '14 at 16:36
@Gummybears: I've updated my answer. The Wikipedia page Sound pressure might be of interest. – John Rennie Jul 9 '14 at 16:39
@JohnRennie Yeah, I read. I accepted your answer. – Gummy bears Jul 9 '14 at 16:42
@Gummybears To build a log scale you start from a reference point. To get a point on your scale you take the log of the ratio of the value you measure (change in air pressure in this case) divided by the reference : log(a/a0). As log(1)=0, the zero in such scales is when the quantified phenomenon is equal to the reference, that is the "special" thing I guess. These scales are cool because you transform multiplication by addition thanks to the log. In this case 10x more air pressure = +20 in the log scale. – Marc Jul 10 '14 at 4:52

Decibels are a unit of measurement expressing a logarithmic ratio between the intensity of sound and a given fixed intensity.

When you see a negative value in decibels does not mean of course that you have negative sound: such a concept has no physical meaning.

What actually has physical meaning is the power or the intensity of sound and those are never negative.

share|cite|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.