When I get on the interstate, the road noise increases and I have to turn the car stereo up to enjoy it at the pre-interstate level. But then if I get off the inerstate, especially if I stop at a light, the stereo sounds way too loud, even uncomfortably so.
The sound level of the stereo plus the ambient noise is louder than the stereo alone. So why does it seem louder when you remove the noise?
This does not seem to be strictly a psychological phenomenon–if I really have it cranked on the highway it can actual be physically uncomfortable when I stop, but it wasn’t bothering me on the road.
Does the ear have some built-in limiter?
(Note: I am not talking about the phenomenon when you are jamming to a favorite tune on the way home, then when you start the car the next morning you wonder who the hell sneaked in and turned up the volume during the night.)
It may have something to do with the location of the speakers. A friend had a car with the speakers mounted in the rear deck such that the back of the cones were in the trunk. As speed increased there was enough air leak in the trunk to create a significant pressure differential between the front and back of the speaker to greatly reduce the volume. Something similar may be taking place.
Another WAG, but the noise the car generates at speed is sort of like white noise, just a mash of noise all over the spectrum. When moving the white noise covers alot of the highs and lows of the music, which return when you stop.
“This does not seem to be strictly a psychological phenomenon”
Thats correct, its more of a psycho-acoustic phenomenon.
“Does the ear have some built-in limiter?”
No, but the brain can ignore some sounds in preference of other sounds. This allows one to carry on a conversation in a crowded room with another person. One’s brain is selectively ignoring the other noises in the room and concentrating on the sound of the conversation. I understand that in persons with autism this selective listening is reduced, making listening in environments with more than one sound source irritating to the individual.
‘racekarl’ is also on the right track with the noise filtering out certain frequencies.
On the freeway, you have the radio turned up to blast through the road noise, at the same time your brain is trying to listen to the radio while ignoring the road noise. Off the freeway, with the road noise reduced, your brain is now aware of the relative difference in ‘background’ noise level, which make the ‘forground’ radio sound appear louder in level than before.
I agree with your answer, Qburn, although the perception of loudness that I have doesn’t seem to be just my brain saying, “Oh, hmm, there’s no competing noise so this suddenly seems louder.” It’s an actual physical sensation in my ears of hearing a louder noise, like “Wow, I better turn that down because the pressure wave is uncomfortable.” Maybe it really isn’t physical, and I just perceive it that way, I don’t know.
I had already considered what racekarl suggested, but my point in the OP was that if that were the case, the overall dB level would be higher with the noise added and it doesn’t feel louder. I stress “feel” because of the sensation of pressure in my ears.
bbbill has an interesting angle, suggesting that the volume of the speakers is physically reduced due to aerodynamics. I usually drive a van, so the speakers are not in a trunk but this still might be a valid point.
I had already considered what racekarl suggested, but my point in the OP was that if that were the case, the overall dB level would be higher with the noise added
This is true in cases where you have similar signals. If the signals are not similar, you could have some filtering effect, much like phase cancellation or comb filtering. The road noise could be rolling off high frequencies.
The aerodymanic issue is interesting, and could also be the cause. To test this, you might try driving with the windows closed(or not fully rolled down) and checking it out.
BTW, you can get a radio with automatic gain control that will raise the volume when there is lots of noise and turn it down when the noise goes down. I have a GM car with this feature and had a Nissan with it too.
I would doubt that the aerodynamics has much to do with it. My Audi had a feature called GALA or Graduated Audio Level Adjustment. Basically the faster the car went, the higher the volume went, returning to the previous level as the car slowed down. The reason that this leads me to discount the aerodynamic affect is that the Audi also had an extremely well sealed trunk, in fact it was air conditioned. It also used the trunk as an air chamber for the subwoofer, so I imagine that the people at Bose (who developed the stereo) had accounted for the behavior of the trunk. Further, both the sedans and wagons had the same GALA feature.