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How can we hear 20hz with headphones?

post #1 of 10
Thread Starter 

I read an article somewhere saying you need a big room to actually hear low frequencies.

It's based on physics: Speed = frequency * wavelength

So if your room is too small, low basses you hear are actually resonances.

 

The question is: Since headphones (especially closed types) have so little space for low frequencies, how can we hear 20hz with headphones? Are they all resonances?

post #2 of 10

The sound waves are not stationary and thus the 56 ft long wave for a 20Hz bass signal moves around through the headphones.  Sure it is going to be distorted due to reflection, diffraction, and interference, but the signal with long wavelength will move quite swiftly to your eardrum making a 20Hz signal quite audible.  Think of taking a really long water wave, say 56 ft between peaks.  If the water moves slowly the peaks take a while to reach you, but if the waves moves very fast like sound waves even though the wavelength is 56 ft you hear / feel it almost instantaneously.

 

I think the article may have referred to those particular "resonances" as reflections which is why bass can come from one sub woofer and still seem to come from everywhere.  The sound reflects off of the walls and other obstacles because the bass wavelengths are so long.

post #3 of 10
Quote:
Originally Posted by john65537 View Post

I read an article somewhere saying you need a big room to actually hear low frequencies.

It's based on physics: Speed = frequency * wavelength

So if your room is too small, low basses you hear are actually resonances.

 

The question is: Since headphones (especially closed types) have so little space for low frequencies, how can we hear 20hz with headphones? Are they all resonances?

The article referring to bass in a room has very little application to bass with headphones.  In room, far field bass response is dependent on "room modes", or "standing waves" that exist with wave lengths at multiples of one or more of the room dimensions. The idea is that these long bass waves are reflected by the room's walls, ceiling and floor, and the resulting waves "stand" with amplitude peaks in one place and nulls in another for every frequency related in size to room dimensions.  

 

But with headphones, relative to the wavelengths of bass frequencies, the "room" is so tiny it's non-existent, even with full size sealed headphones, because the chamber is insignificant relative to base wave lengths. Headphones, and in particular IEMS, operate by producing changes in pressure at the ear, in very "near field", similar to a piston in a cylinder.  You get just the initial pressure wave, and there's really nothing out far enough to create a mode, or standing wave.  

 

By the way, the same sort of thing can be done with near-field subwoofers. Dr. Hsu of HSU Research has for years recommended placing the subwoofer so the listening position is "near field", which comes closer to working like headphones and bass, though the room is still very much involved.

post #4 of 10

The "room" modes with headphones on your ears usually start at several thousand Hertz and are more correctly called ear(-canal) resonances.

 

Theoretically, you can create any pressure even in a tiny space. Practically there will always be leakage and limited motor strength and excursion limit of the driver.


Edited by xnor - 5/14/13 at 7:50am
post #5 of 10
Quote:
Originally Posted by john65537 View Post

I read an article somewhere saying you need a big room to actually hear low frequencies.

It's based on physics: Speed = frequency * wavelength

So if your room is too small, low basses you hear are actually resonances.

 

The question is: Since headphones (especially closed types) have so little space for low frequencies, how can we hear 20hz with headphones? Are they all resonances?

 

The article is nonsense. Our ears respond to changes in air pressure. When the room is small compared to the wavelengths involved, the changes in pressure encompass the entire room and you can hear low frequencies just fine. Just as you can with IEMs where the "acoustic space" is no larger than the size of your ear canal. You don't start getting into room resonances until the size of the room is on the order of the wavelengths which can then interact constructively and destructively to create peaks and nulls within the room.

 

se

post #6 of 10
Thread Starter 

thanks.

beerchug.gif

So if you are in a very small room and your speakers produce basses lower than 30hz, you hear the original waves just fine when your ears recieve them directly.

But sound waves reflected from things like walls may create resonances that increase or decrease the original wave forms, which is bad for sound quality. Right?

post #7 of 10

Basically, yes.

 

se

post #8 of 10
Thread Starter 

Another question is:

Some people said you need a big transducer to generate low frequency tones,

this should be inaccurate.

 

I think transducers of any size can generate low frequency tones, but because human ears are so insensitive toward low frequencies, bigger transducers can move more air hence conquer this problem better than small ones. Right?

post #9 of 10
Quote:
Originally Posted by john65537 View Post

Another question is:

Some people said you need a big transducer to generate low frequency tones,

this should be inaccurate.

 

I think transducers of any size can generate low frequency tones, but because human ears are so insensitive toward low frequencies, bigger transducers can move more air hence conquer this problem better than small ones. Right?

It's not as simple as small transducers can't produce bass and large ones can.  There are plenty of tiny IEMs that will bass your head off.  It's how you get that bass into the ear, coupling.  A little IEM sealed into the ear canal couples pretty well, so the tiny diaphragm works fine. But a large, leaky, open type headphone can't couple that well at all, so it has to move a lot of air for the same amount of bass energy to couple to the ear.  An open type headphone with little tiny diaphragms would be pretty bass-shy, though, unless you could design it as some sort of piston with a huge excursion, which isn't likely. 

 

We have to get you smarter people to listen to!

post #10 of 10

Yeah, coupling is the main point, but if you put things in relation (like ear canal diameter / in-ear driver diameter) "small" drivers won't give you much bass. As jaddie wrote, you'd need crazy excursion limits and that would cause other problems..


Edited by xnor - 5/15/13 at 3:08pm
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