If a device were placed 10 feet from me that produced a constant noise at, say, 50dB in an otherwise quiet place I’d probably find it annoying after a short while. However, if a second, identical device was placed next to the first at an equal distance from ear would the perceived sound level be increased? One part of my brain is telling me that a second noisy device can only make it louder, but the logic part of my brain is telling me that it is not possible for these devices to produce more than 50dB, even in combination.
My brain continues the argument that being surrounded by 50 such devices would be deafening and it seems to have a point, but my logic still returns that sound output shouldn’t be cumulative.
Can someone kindly please let me know the truth on this?
Trying to remember school physics lessons - basically it depends whether the peaks and troughs in the frequencies line up according to the position/distance. If they’re producing identical sounds from identical distances, I think they’d match up and double the volume from your POV.
I assume 50 would be a mess of devixes adding to and cancelling each other out, and that on average you’d end up with more noise but not 50x the noise.
This is all half remembered lessons from 20 years ago. Anyone who knows better, feel free to correct me!
Assuming that you don’t have phase cancellation, then doubling the energy used to produce the sound certainly increases the loudness you hear but does not double the number of decibels, because the decibel scale is logarithmic to model how our hearing works. Doubling the intensity of a 50dB sound would get you to 53.0dB.
I’ve wondered about this too while watching football on TV. I’ve been to LOUD stadiums before, so much louder than any one person could scream. I’m confused why 96,000 people screaming is so much louder than just one.
Ah, an excellent practical example of my original question.
I’m guessing that in a stadium reverberation plays a large part in the apparent increase of volume. In my post I just said a quiet place, but I guess I was thinking of a quiet - enclosed - place which would have an impact on the levels.
Why wouldn’t sound generated with 96,000 times the amount of energy be a hell of a lot louder?
Normal conversation is 60dB. If one person screaming is, say, 80 dB, then 96K people screaming is 129.8 dB, approaching the threshold of pain. However, in a stadium that sound is not concentrated at a single point; some of it could be 150 yards away from you. And not all 96K scream at the same moment. But this is how it works.
Imagine your identical sound sources are wave generating machines on an otherwise calm pond. Where the peak of two waves meet, you’ll get a bigger peak; likewise where two troughs meet you’ll get a deeper trough. And all the combinations inbetween.
The real world is more complicated - for random noise the resultant average level is the square root of the sum of the squares of the individual noise sources. For correlated (i.e. identical and phase-locked) sources the total sound level is the square root of (the sum of the squares of the individual noise sources + 2 X the correlation coefficient X the product of all the noise sources). However, these are extreme examples and real-world noises fall somewhere in the middle. Logic doesn’t always come into it.
Here’s something to chew on: Sound levels are simply the depth of modulation of air pressure, which is normally around 14 psi. The upper limit for this is high (it’s when the air is compressed to a liquid), but the lower limit is 0 psi. How do you get such loud sounds when there is such a bottoming-out effect…?
Two sound sources equidistant from a receiver are louder than a single sound source at the same distance. However, the sound pressure levels don’t add arithmetically; they add logarithmically. The combined SPL of the two sources given in your example is 53.01 dB.
Even ridiculously-loud sounds are still far, far away from the bottoming-out effect. It’s like asking how you can get big water-waves on the ocean when the troughs of the waves can’t go any lower than the ocean floor.
