Audio: Low frequencies and Rooms

[Bodhi Dharma Zen]

Some people believe (and use all kind of physics to "demonstrate it") that it is impossible to reproduce very low frecuencies in a room of our house. Here is a sample of what they think. I would like to know the opinion of fellow skeptics.

My reasoning is that the cycle completes in the ear, so the space that the wave needs to fulfill that cycle is irrelevant (they claim the opposite).

Furthermore, my position is that we should forget about all theories and measure. Without contrasting our ideas to the world, they are nothing but abstract constructs, it is illogical to pretend they are "truer" than the world.

If the low frecuency is there, you can measure it. It is as simple as that.

 

[steenkh]

It is my impression that you can hear all frequencies in all rooms.

The sound is is travelling outward from the speaker and will be moving your eardrums when it passes, no matter what room you are in. However, in a room, most deep frequences will be quickly dampened, so you only get the output from the speakers, not any resonance from the room. If the dimensions of the room relate in certain ways to the frequency, such as being half the wavelength of the frequency, you will get a pronounced dampening effect at certain positions.

In the old days when I listened to organ music on my fine hi-fi set, I noticed that certain extremely deep tones could not be heard at all at my favorite listening position, but when I moved a meter or so, I could hear it. The wavelength of the deep organ pipes is several meters (look at the size of the biggest organ pipes you can see in an organ, and note that the deepest tones are usually produced by wooden pipes that are stopped at one end, and are built at half the wavelength. My lowly room was only about 3 times 3 meters, and yet these tones could be heard, but not at all positions!

 

[mayday]

of course you can measure the low frequencies...what kind of dufi are we dealing with here???

 

[jmercer]

Well, mayday, we're dealing with audiophiles. Some are reality-based, but fanatical... some are just fanatical.

 

[jj]

The short answer: DOH, of course you can reproduce low frequencies in a room.

The longer answer, when you get substantially below a wavelength, the room acts as as though it's compression driven, i.e. the pressure in the room changes proportionally to the volume displacement of the loudspeaker. This is unlike regular propagation wherein the pressure varies as f^2 * volume velocity.

This means MORE instead of LESS bass, interestingly enough, because the SPL doesn't fall off as fast as it does in an anechoic situation below the room's transition point to pressure-driven.

The size of the room that this happens in depends on the frequency, the dimensions of the room, how well it's sealed, what openings to the outside exist (thing helmholtz resonator), and so on.

It is possible to design a room that is very hard to drive at a given low frequency by making the appropriate helmholtz resonator coupled with it, but you still have lots of volume velocity, and you better believe you have pressure inside that resonator. This, however, almost always has to be done deliberately, and is usually used to fix a peak due to room shape.

 

[patnray]

Bodhi Dharma Zen: Some of the posters in the link you supplied seem to think that an acoustic wave cannot exist in a room if the dimensions of the room are smaller than the wavelength. Apparently they believe all parts of a wave are somehow physically connected and the wave cannot exist unless the entire wavelength is present. This, of course, is not true. An acoustic wave is nothing more than the periodic pressure change as measured at a single point in space. One could, with suitable equipment, produce a sound spike that is less than one full cycle of any frequency.

It is true, as others have pointed out, that preasure in a small confined space can affect the sound. And reflections from walls can change how the sounds are heard in different parts of a room. But it is not true that wavelengths greater than the dimension of the room are simply not present.

 

[Bodhi Dharma Zen]

It is my impression that you can hear all frequencies in all rooms.

The sound is is travelling outward from the speaker and will be moving your eardrums when it passes, no matter what room you are in. However, in a room, most deep frequences will be quickly dampened, so you only get the output from the speakers, not any resonance from the room. If the dimensions of the room relate in certain ways to the frequency, such as being half the wavelength of the frequency, you will get a pronounced dampening effect at certain positions.

In the old days when I listened to organ music on my fine hi-fi set, I noticed that certain extremely deep tones could not be heard at all at my favorite listening position, but when I moved a meter or so, I could hear it. The wavelength of the deep organ pipes is several meters (look at the size of the biggest organ pipes you can see in an organ, and note that the deepest tones are usually produced by wooden pipes that are stopped at one end, and are built at half the wavelength. My lowly room was only about 3 times 3 meters, and yet these tones could be heard, but not at all positions!

What you mention is the cancelation effect that bass frequencies will experiment towards the center of the room. In a corner, or close to the back wall regarding the speakers, the cancelation will be greatly reduced.

 

[Bodhi Dharma Zen]

of course you can measure the low frequencies...what kind of dufi are we dealing with here???

Audiophiles, like jmercer pointed out. Some of them are counted among the most recalcitrant fanatics one can possibly imagine.

 

[Bodhi Dharma Zen]

The short answer: DOH, of course you can reproduce low frequencies in a room.

The longer answer, when you get substantially below a wavelength, the room acts as as though it's compression driven, i.e. the pressure in the room changes proportionally to the volume displacement of the loudspeaker. This is unlike regular propagation wherein the pressure varies as f^2 * volume velocity.

This means MORE instead of LESS bass, interestingly enough, because the SPL doesn't fall off as fast as it does in an anechoic situation below the room's transition point to pressure-driven.

While I dont understand the physics involved, I have made extensive measurements, in different kind of rooms, and in open spaces (with an effect similar to an anechoic room, but for bass frequencies). And I know that the reinforcement is true. But how do you explain that to the ones who write in the link I posted, or to any other people for that matter?

 

[Bodhi Dharma Zen]

Bodhi Dharma Zen: Some of the posters in the link you supplied seem to think that an acoustic wave cannot exist in a room if the dimensions of the room are smaller than the wavelength. Apparently they believe all parts of a wave are somehow physically connected and the wave cannot exist unless the entire wavelength is present. This, of course, is not true. An acoustic wave is nothing more than the periodic pressure change as measured at a single point in space. One could, with suitable equipment, produce a sound spike that is less than one full cycle of any frequency.

It is true, as others have pointed out, that preasure in a small confined space can affect the sound. And reflections from walls can change how the sounds are heard in different parts of a room. But it is not true that wavelengths greater than the dimension of the room are simply not present.

Yes, I think that is what they believe, that if the whole wave is not present the sound is not possible. It is nonsensical, because the eardrums will react to the changes in pressure at the rate that the frequency is behaving. The space needed for the wave to complete its cycle is irrelevant.

 

[marko]

the point is...?

 

[Bodhi Dharma Zen]

Marko

Two things, first, obviously you have never interacted with an audiophile.

Second, it is interesting to know that some people will defend what they believe, using (or abusing) math and physics, and thus negating what the world is telling them.

Maybe you dont find that annoying.

 

[TjW]

Yes, I think that is what they believe, that if the whole wave is not present the sound is not possible. It is nonsensical, because the eardrums will react to the changes in pressure at the rate that the frequency is behaving. The space needed for the wave to complete its cycle is irrelevant.

Yep. the theory that the room has to be larger than a wavelength would have really bad implications for earphones.
And yet they seem to work just fine.

 

[jj]

While I dont understand the physics involved, I have made extensive measurements, in different kind of rooms, and in open spaces (with an effect similar to an anechoic room, but for bass frequencies). And I know that the reinforcement is true. But how do you explain that to the ones who write in the link I posted, or to any other people for that matter?

You can't.

Anybody with a decent omni pressure microphone can prove what really happens.

Below a half-wave at the longest room dimension, which would be 1152/(2*f) feet you begin to operate in a pressure regime, and propagation is basically "instant", furthermore the room starts to become uniform in level again, because you are in fact below a half wavelength. The transition is gradual, of course.

 

[Soapy Sam]

I wonder if a 40 foot high by thousand mile long Tsunami can fit in a tent?

Clearly not.
So it's safe to camp on the beach, right?

I'm mildly curious , if long sound waves can't fit in a small room, where the theorists think the energy goes when the wave reaches any matrix of small spaces. Like an ear, for example.

 

[jj]

I'm mildly curious , if long sound waves can't fit in a small room, where the theorists think the energy goes when the wave reaches any matrix of small spaces. Like an ear, for example.

I've asked people that from time to time, and they very nearly never get it.