[VNandor]

So as I understand it,at the same amplitude, the higher the frequency is the more energy the wave has. Like electromagnetic waves. X-ray has a high frequency and can penetrate human body while for example ultra violet ray has a lower frequency and thus can't go really deep into human body.
 
Now, when I take off my headphones and don't stop the music I won't really hear the lower frequencies while the higher frequencies can make it to my ears. This seem to support the above theory because higher frequencies have more energy so it can travel more compared to lower frequencies before losing so much energy I can't notice it anymore. (At least that's how I would explain what happened.)
 
However if I listen to music through speakers loudly and go out of the room it's mostly the bass what I hear. It seems to lose way less energy than the mid and treble range.
 
So can someone explain me what actually happens when I take off my headphones and hear mostly the high frequencies but when "listening" to speakers from two rooms away it's the bass I mostly hear?

 

[MindsMirror]

Small speakers are not good at producing bass. So when your headphone is off your head, you don't hear much or any bass, just like you would with a small speaker. I'm not sure what exactly the reason is behind that, or behind the strong bass when they are on your ears. Perhaps it is due to the relatively large size of the driver compared to it's distance from your ear.
 
Speakers sound muffled from another room because walls are better at attenuating high frequency sounds than low frequency sounds. More bass makes it through the wall, so that is what you hear.

 

[gregorio]

There isn't more energy in higher frequencies, the amount of energy is purely a function of the amplitude.
 
Your hearing however is not linear. You need significantly more energy in the low frequencies for them to sound the same loudness as the mid frequencies. Incidentally, the same is true of high frequencies, you are insensitive to them and they need more energy to be audible. What's even more interesting is that the quieter something is, the less loud the bass appears to be (to our, human, hearing). This is a well known and studied phenomena, documented over 80 years ago in the "equal-loudness contours". When you take your headphones off and move them substantially away from your ears, the music/sound obviously becomes substantially quieter overall (loosely, to the inverse square law) and therefore the bass sounds substantially quieter relative the to mid frequencies (which human hearing is particularly sensitive to). That's why they sound "tinny" from a distance.
 
Your two rooms away scenario is a different phenomena but slightly related, to do with acoustics rather than perception of hearing (psycho-acoustics). As I already mentioned, a lot of energy is required in the low frequencies to make them appear to sound the same volume as mid frequencies. Mid and high frequencies are easily absorbed by soft furnishings, insulation and other soft materials, while hard surfaces like walls and doors will absorb high freqs and some of the mid freqs, they will reflect the rest back into the room. Very hard surfaces, such as thick glass, tiles and concrete will reflect even the high freqs back into the room. Low frequencies though are extremely difficult to absorb, some of the energy gets absorbed but much of it passes through. Given a moderately high amplitude, low frequencies will even pass through thick concrete. Which incidentally, makes studio design a bit of a nightmare! Anyway, that's why you can hear low freqs even several rooms away.
 
G

 

[VNandor]

 There isn't more energy in higher frequencies, the amount of energy is purely a function of the amplitude.

Does it mean that my assumption about mechanical waves working the same way as electromagnetical waves  in that regard (higher frequency equals more energy) is wrong?

 Your hearing however is not linear. You need significantly more energy in the low frequencies for them to sound the same loudness as the mid frequencies. Incidentally, the same is true of high frequencies, you are insensitive to them and they need more energy to be audible.

That's a sort of roll-off isn't it? If my headphones reproduced a sine sweep at the same loudness in all frequencies I would perceive it as unbalanced. Am I correct?

 What's even more interesting is that the quieter something is, the less loud the bass appears to be (to our, human, hearing). This is a well known and studied phenomena, documented over 80 years ago in the "equal-loudness contours".

I'm not sure if I get what that means. If I listened to something that sounded balanced but I digitally decreased the volume from 0dB to like -15dB I would have to add an EQ to make it sound balanced again?
 
I think I kind of understand now why I hear the low frequencies when I'm room aways from the speakers. The low frequencies has a higher amplitude than high frequencies if I perceive it equally loud. Is it one of the reasons why it passes obstacles more easily than higher frequencies? Is there an easy explanation for the other reasons as well? (Like why they aren't absorbed by soft things and reflected by hard things.)
 
On a side note I thought a studio is good as long as it doesn't reflect waves/doesn't have echoes. Why does it matter if the bass is just going through the studio's walls? It's not going to interfere with the recording then.

 

[VNandor]

 Speakers sound muffled from another room because walls are better at attenuating high frequency sounds than low frequency sounds. More bass makes it through the wall, so that is what you hear.

Yeah I sort of realized that

  but what's the reason behind it?

 

[Volitionist]

Higher frequencies don't really have more "energy" with sound waves. The thing is sounds waves are pressure waves and this actually explains the entirety of your question. 
 
The reason low frequencies sound really when the headphones are off is because in order to create meaningful bass you have to move a lot of air. Why do you think subwoofers are 10, 12, 13, 15, even 21 inches in physical size? To reproduce low frequencies adequately you need to move a ton of air because the waves themselves are huge. A 20Hz wave has a wavelength of something like 56ft (17m). Now, you to make this 56ft wave have an appreciable amount pressure behind it to make it heard. Start to see what is going on? High frequency material on the other hand is a lot different because the wavelengths are only inches (cm) in size. Therefore, it takes a lot less air pressure to get them to move and be heard. 
 
However, there are a some other things going on in headphones that allow to create meaningful bass response that speakers don't typically have. First off is the Boundary Effect. Which is basically that hard surfaces reflect waves back. Now, the human head isn't exactly the hardest thing, but it does reflect pressure waves back and this gives an increased amount of the low end perceived. Then of course there is the proximity effect to take into account as well. See, lots of microphones exhibit an increase in the amount of low end as a sound source gets closer to them. In essence the ear is just a microphone and by putting the headphone closer to your increases the amount of bass you hear as well. 
 
There is also of course the equal loudness contours that was already mentioned. Those are important, but there is another psychoacoustic principle at play here and it has to do with perception of low frequencies. Quick example, if you've got a bass sound playing with all of its proper harmonics and such and you remove the lowest fundamental then in the context of the entire track you'll still probably hear that lowest fundamental. Why? Because your brain goes, "Oh I hear all these overtones & harmonics and that means this fundamental must be there so I'm going to put it there." Now, typically it isn't that extreme in practice, but typically turning down the fundamental to get a bit more headroom? Happens a lot more often than you might think, but you never question it because it sounds right. 

 

[gregorio]

  Does it mean that my assumption about mechanical waves working the same way as electromagnetical waves  in that regard (higher frequency equals more energy) is wrong?

 
Yes, it's wrong. Electo-magnetic waves are not just mechanical waves, they're also particles which carry their own energy, photons. With electro-magnetic radiation we measure the energy carried by those particles, therefore as the frequency increases so does the amount of energy. With mechanical waves, such as sound, we measure the energy of the wave itself, not the energy of the particles in the medium the wave is travelling through (air molecules). My level of understanding of quantum theory is relatively basic, so you might get a better (more detailed/accurate) answer from someone else.
 

  On a side note I thought a studio is good as long as it doesn't reflect waves/doesn't have echoes.

 
No, quite the opposite in fact! A studio needs to have reflections/echoes (reverb), otherwise it would be an anechoic chamber and what we record would sound bizarre and dead. All acoustic instruments rely on reflections to give them their sound/character, some almost exclusively. A good studio is not one which doesn't have reflections but one that has well controlled, diffuse and therefore relatively neutral reflections.
 

  Why does it matter if the bass is just going through the studio's walls? It's not going to interfere with the recording then.

 
Absolutely it is! There are two elements:
 
1. There are no "one-way mirrors" with acoustics. If bass can go out through a wall, it can come in through that wall as well! Obviously, if there are low freqs outside the studio, commonly traffic rumble, which can enter the recording (live) room then they will be recorded along with the freqs the instruments are producing.
 
2. If low freqs are seeping through the divide between the live room and the control room, then the recording engineer will be hearing both the low freqs on the recording (through his/her monitors) plus the low freq seepage, giving the impression that the recording contains more bass than it actually does. The engineer may well decide to change mics or mic positioning to counter this perceived extra bass.
 
In either of these two scenarios, bass going through the studio's walls will directly interfere with the recording.
 
G

 

[VNandor]

Thank you guys for clearing up my misconceptions about mechanical waves and explaining what actually happens in the mentioned scenarios.
 

 2. If low freqs are seeping through the divide between the live room and the control room, then the recording engineer will be hearing both the low freqs on the recording (through his/her monitors) plus the low freq seepage, giving the impression that the recording contains more bass than it actually does. The engineer may well decide to change mics or mic positioning to counter this perceived extra bass.

And this is a good point I totally forgot that.