# Negative sound rooms

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?

• 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

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.

• 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
• @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.