[alancalverd]

True, but if I look at the realtime graphical fourier transform of an orchestra, my eyes can't identify the types of instrument playing at any moment. Our auditory memory is phenomenal: with less than 5 kHz bandwidth and deliberately enhanced at 3 kHz, I can still identify other pilots and controllers by name against 90 dB background noise. 

[vhfpmr]

Nobody has discussed the difference between amplitude and intensity (power), the latter being the square of the former. If you have two waves of differing frequencies, the amplitude adds on a square root basis, but the intensity/power just adds, so two equal amplitude waves will have twice the power of one alone. This said, there’s also a difference between mean power and peak envelope power (PEP).

See the plot below of two equal amplitude sinewaves, one 490Hz (blue), the other 510Hz (red). You can see that the amplitude of the overall envelope of the combined waves (green) varies at a rate equal to the difference between the two tones (20Hz). This is because the relative phase of the two tones is changing at 20 Hz, they start out in phase and therefore add in amplitude, but 25ms later they’re out of phase, and therefore cancel out. The peak amplitude is twice that of one tone, but PEP is four times, mean amplitude is root 2 times, and mean power double.

Having double the power from two tones might sound like a lot, but it actually isn’t. The response of the human ear is logarithmic, not linear, so in fact double power isn’t actually much above the smallest increment the ear can detect. The range of loudness in a typical home is 10,000:1, and the range from threshold of hearing to threshold of pain is 10,000,000,000,000:1. Decibels are also logarithmic, so each multiplying of power by 10 adds 10dB, doubling the power adds 3dB.

[Professor Mega-Mind]

You be right , mahn !
I guarantee you that if I hear that , I will percieve 2 sources of lower volume , and will even be able to pinpount each one !
P.M.

[thompsonmax]

Hi there. Interesting question. I think it is all about the sound power wich rate in decibels. So if you add together different sounds we will hear that one which has more decibels.

[vhfpmr]

Hi there. Interesting question. I think it is all about the sound power wich rate in decibels. So if you add together different sounds we will hear that one which has more decibels.

Common experience should tell you this isn't correct, the ear is quite capable of hearing sounds in the presence of louder ones.

[Colin2B]

Common experience should tell you this isn't correct, the ear is quite capable of hearing sounds in the presence of louder ones.

Unless the frequency of the competing sound is lower.
The general rule is that low masks high. The lower frequency sound will raise the hearing threshold of a higher frequency sound, so it needs to be at a higher level to get through. There are detailed masking curves which show the effect and the frequency relationships.
One interesting aspect of this is a singer working with an orchestra. The orchestra has a lot of power in the lower frequencies and these can mask a singer. Classical singing training teaches singers to produce what is known as the singers formant, extra  power in the 2-4kHz range which punches through the masking. Pop singers don’t have that training so vocal microphones have raised sensitivity in the 2-4kHz range, known as a presence peak, which lets the singer compete with the group (mostly).

[vhfpmr]

Common experience should tell you this isn't correct, the ear is quite capable of hearing sounds in the presence of louder ones.

Unless the frequency of the competing sound is lower.
The general rule is that low masks high. The lower frequency sound will raise the hearing threshold of a higher frequency sound, so it needs to be at a higher level to get through. There are detailed masking curves which show the effect and the frequency relationships.
One interesting aspect of this is a singer working with an orchestra. The orchestra has a lot of power in the lower frequencies and these can mask a singer. Classical singing training teaches singers to produce what is known as the singers formant, extra  power in the 2-4kHz range which punches through the masking. Pop singers don’t have that training so vocal microphones have raised sensitivity in the 2-4kHz range, known as a presence peak, which lets the singer compete with the group (mostly).

But I didn't say that a loud sound doesn't raise the threshold for the quieter one, I said that you can hear a sound that is quieter than another present at the same time, which I think this is what Thompsonmax was getting at. Taking a 1kHz tone as a reference, I can hear a 500Hz or 2kHz tone that is 40dB quieter. That's with sinewaves, if I make the quiet one a square wave it's easier to hear.

[vhfpmr]

I'm really at a loss to see what you're getting at. I'd read the Wiki page on masking before I made my first post, and I don't think I'm saying anything that contradicts it, or finding anything that contradicts it when I listen to two tones.

[Colin2B]

I'm really at a loss to see what you're getting at. I'd read the Wiki page on masking before I made my first post, and I don't think I'm saying anything that contradicts it, or finding anything that contradicts it when I listen to two tones.

Sorry, I hit the wrong quote button, had meant to add to what you said for thomsonmax, certainly not contraficting. Been a bit hectic here, multitasking, trying to get rid of spammers before they do damage, trying to upload some big updates on an internet down to a crawl because everyone is homeworking. I reply direct with some exampkes when I get a mo.