dBA

[KevinW]

I was testing an audio amp for noise today with an app and discovered a quiet room is 35db, looking at meters I discovered they usually start at 30db.
I thought a quiet room might test less than 10db but I was wrong again.
Why would a quiet room register at 30-35db?Is the 30db not detectable with the old ear drum?

 

[gophert]

Zero dB was defined as the threshold of reasonably good human hearing.

Background noise like refrigerator compressors, cooling fans on your computers and other devices, wind across your windows and roof, seem quiet but quickly add up. Also, not every app is as accurate as other apps - no matter how accurate they claim to be. Some neighbors were fighting about loud air conditioner compressors and one bought an app to prove it wasn't very loud. Then the township zoning officer was called and everyone discovered the "carefully calibrated" app was way off from the calibrated meter the zoning officer had.

 

[JimB]

Is the 30db not detectable with the old ear drum?

If you have ever been in an anechoic chamber you will see (hear?) what quiet is.
It can be quite perturbing.

JimB

 

[AnalogKid]

Back when, I was tech support for a Psychology department, including a psycho-acoustics lab. The feeling when the chamber door closes is just that, a physical feeling. Some people have trouble breathing, some get nauseous, and it is very good at bringing out dormant claustrophobia.

ak

 

[gophert]

If you have ever been in an anechoic chamber you will see (hear?) what quiet is.
It can be quite perturbing.

JimB

In a good chamber, you can hear your heartbeat.

 

[crutschow]

I remember going into a sound anechoic chamber at Delco in Indiana where I worked one summer while in school.
It was an eerie feeling, rather like the whole world had suddenly stopped and I was the only one left.

 

[gophert]

I remember going into a sound anechoic chamber at Delco in Indiana where I worked one summer while in school.
It was an eerie feeling, rather like the whole world had suddenly stopped and I was the only one left.

I was in an RF anechoic chamber and I was surprised how much the small cones on the walls managed to block audio - really weird effect. Everyone sounded like a baritone.

 

[KevinW]

The room I was in registered 35db, the db spec for the amp cooling fan is 43db but putting the meter next to the amp only generated another 20db bringing the combined room noise and amp noise to 55db.
Perhaps half the noise from the amp gets absorbed by the 35db room noise.
I know it isn't an exact science but there seems to be some variation in the levels of noise which is some indication of what I'm seeing on the meter is true.

 

[gophert]

The room I was in registered 35db, the db spec for the amp cooling fan is 43db but putting the meter next to the amp only generated another 20db bringing the combined room noise and amp noise to 55db.
Perhaps half the noise from the amp gets absorbed by the 35db room noise.
I know it isn't an exact science but there seems to be some variation in the levels of noise which is some indication of what I'm seeing on the meter is true.

It is not surprising that your fan. Rated at 43db, measured 55db with the meter right next to the fan. A fan rating is usually rated at 1 meter from the front face of the fan - plain fan - not inside a chassis. Add noise of air turbulence in your specific installation and you could easily get higher than 55db. Consider yourself lucky.

Also, dB measurements are not additive. They are logarithmic measurements so the 35db room noise will be hardly noiticed once the 55db fan is turned on. A 55 dB fan is 100x louder (noise energy) than 35db room noise. 55db + 35db = 55.5db.

 

[kubeek]

When you add two uncorelated noise sources, each at 50dB, the result is +3dB, therefore 53dB.
If you add anything together where the difference is more than about 6dB then you can basically ignore the lower amplitude one and use just the number of the higher one.

 

[KevinW]

Thanks for all the responses, very interesting.

 

[KeepItSimpleStupid]

What was missed in this discussion is weighting - https://www.noisemeters.com/help/faq/frequency-weighting.asp