Are Sound Waves Affected by Gravity?

[myorgan]

In another post, jbird604 wrote

Most church installations will need the speakers elevated in order to get sound distribution over the space. Bass cabinets can of course be placed wherever they perform best, as low tones are pretty much non-directional

I realize he wrote that statement because there are generally less obstacles on the ceiling vs. floor, but it made me wonder about the basic physics of the question, Are sound waves affected by gravity? If so, how are the 3 main groups (high, mid, & low) of frequencies affected? If a woofer is secured to the surface of the ceiling, would that installation have the same effect on the bass as it would were the woofer in a corner on the floor?

I suspect the answer to the overall question is, "Yes." However, if someone were to ask me to explain, I probably couldn't provide the answer to the initial based on fact.

Michael

[Larason2]

The answer is yes. Gravity affects the air particles that make sound waves, though in general, the effect of gravity on gasses is not that much compared to their kinetic energy. This, however, is what allows us to have an atmosphere. Higher frequency sounds are also higher energy, so they would be affected less than lower energy, which has less kinetic energy to counter the effects of gravity. So all sound waves bend towards the ground over distance, but in general the amount they bend isn’t that much, though more for low frequency waves. Low frequency waves also move more laterally than high frequency, so this can help compensate for drops. I’ve seen subs mounted on the roof, and while they probably work well there, my opinion is they are really hard to access, and up there they tend to create a lot of sympathetic resonance in the roof members. Better just to mount them somewhere high and accessible. In smaller spaces, the effects of distance and gravity are much less, so putting a sub on the floor usually works fine.

[jbird604]

Gravity isn't really the factor I had in mind when I made that statement, though I have no doubt that what Larason2 says is accurate.

The reason that bass cabinets work "better" (actually, just more efficiently) in the floor than mounted up on a wall has to do with basic acoustic theory that I learned when as a pre-teen I was obsessed with audio equipment and how it worked. I was especially enamored of the famous Klipschorn corner speaker and amazed at how it worked. A rather ordinary 15" woofer stuck into a small air-tight rigid box -- not a recipe for good bass. But the Klipschorn was made specifically to be placed in a room's corner and at floor level, where it took advantage of the vast bass-multiplying power of the "boundary effect."

Of course the Klipschorn had this magnificent "folded horn" into which that 15" woofer spoke, that folded horn being responsible for coupling the very small movements of the cone to the vast cubic volume of a room or even auditorium so effectively that it put out as much bass as a big stack of woofers enclosed in ordinary vented boxes. But the corner placement was essential for getting the full impact of that speaker's performance.

The "boundary effect" is based on physics, but I have forgotten the mechanics of it. Simply stated, each "boundary" that is nearby to the woofer effectively doubles the perceived bass output. A bass cabinet hanging in the middle of a room, some distance from the floor and ceiling and all the walls, will perform very poorly, though if you throw enough amplifier power at it, you can make it sound decent, provided you don't burn it up first.

Mounting that cabinet in the middle of a wall introduces a single "boundary" to the equation, thus doubling the perceived bass output. Moving the cabinet up or down on the wall, so that it is at the junction of the wall and either the floor or ceiling, will double the output again (to 4X the initial output). Finally, moving the cabinet so that it is in a corner of the room AND either flush with the ceiling or the wall will double the output again, giving you EIGHT times the bass as you'd get with that speaker hanging by a wire in the middle of the space.

You can see then that placement is of great importance when bass speakers are installed. Even with today's mighty mega-kilowatt amps you are fighting a losing battle to try to get solid room-shaking bass out of a bass cabinet that is not located where it can take advantage of the boundary effect to the fullest possible extent.

This is why typical "sound systems" in churches sounded so very thin and lacking in bass back in the day when installers were flying one big enclosure somewhere between the floor and ceiling above the pulpit. These enclosures were completely removed from every boundary in the room, and thus got absolutely zero bass reinforcement from the structure. Once installers began supplementing these huge flying arrays with subwoofers that they placed at floor level or underneath the stage or wherever they could be near a boundary, the difference was tremendous.

High frequencies also benefit to some extent from these boundary effects, but their wavelengths are so short and their output is so directional that little of it is reflected off the boundary surfaces, making little difference in efficiency. The bane of high frequency production is the blocking of highs by objects in the room, thus it makes a lot of sense to place high frequency or full-range speakers off the floor far enough that the highs won't get stifled by anything in the room.

[Larrytow]

The Forum is going slow again tonight ( for me at any rate ), so this will be quick. I really have never thought about the potential effect of Gravity on sound waves - it is an interesting question though. My first thought was "no, and even if a little bit, not enough to be concerned with" . But you have me wondering now anyhow.

We all know that what John just said about placement of bass speakers is the truth, and it has been proven so many times. I think it is the amount of room that the longer waves need to fully develop is what has the most effect. High pitches = short wavelengths; low pitches = long wavelengths. Also, the acoustic coupling that takes place between a sound source and the corner, or wall, that it is near contributes a whole lot.

Not just speakers either. Note that in most pipe organs the 16' and 32' octaves are usually backed up to a wall - not just hanging out in the middle of the rest of the pipework. They are likely designed that way for more than just convenience ?

It's discussions like this that make me recall my high school Physics professor. He was not only a great teacher, but also the schools head organist, and we spent a good deal of time studying the physics of sound. I wish I could ask him the Gravity question !

[myorgan]

Gravity isn't really the factor I had in mind when I made that statement....

@jbird604,

No doubt it isn't what you were thinking, but as you probably already know, my mind doesn't work in a linear fashion.?

Our town/city is in the process of finishing their outdoor ampitheater, and has been the bane of our existence for several years now. About 1/2 mile away, I measured 90dB during one of their concerts. Both the city council and citizens appeared ignorant of the fact the bass frequencies were uncontrolled. The "sound engineers" were setting up their mixers either adjacent to the stage, or less than 20 feet from the stage. Since they couldn't hear the bass, they kept turning it up so they could hear it, and in surrounding houses, items were falling off shelves, phones off walls, destroying fragile items as they fell, etc.

They have now moved the mixers about 100 ft. away and turned the stage the other direction, and we rarely hear the noise from the concerts. I do feel bad for those who live 10+ miles away who are hearing the concert coming up the river.?

Our local community band performs on the waterfront, and I have noticed how much blowing wind affects not only the sound one hears in the audience, but also the frequency of the sound. In the ampitheater, they now have up to a 50' wall around the venue, which presumably will contain the more powerful bass frequencies. My question was more related to whether gravity would keep the sound closer to the ground via gravity, or would it escape the enclosure to go up into the air and out into the community.

I think I already knew the answer, but I was just hoping to learn facts to support or destroy my opinions.

So, how would one set up an experiment to test the theory?

Michael

P.S. I'd like to keep this thread about how gravity may or may not affect sound. We can read about speaker placement and sound reinforcement in many other threads.

[Larason2]

Based on what I said above, gravity will definitely have an effect on the sound. The bigger the space, the more gravity has a chance to affect sound, since it is constant regardless of the distance travelled, whereas all sound waves lose intensity as they travel away from the generating source. In this case, the boundary effect is true of the water, since sound waves will reflect off the water and therefore be reinforced by it as if it was a wall. Technically sound waves travel in all directions from a point source, but a speaker cone isn't really a point source, though they tend to travel in a relatively broad spread from the cone. The wall will help keep bass frequencies in the concert, which means the amplifiers/speakers don't need to be turned up as much. Bass sounds will travel over the wall, but because of reflection and gravity, a lot less than will travel closer to ground level. Even though bass sound waves do tend to bend more than high frequency waves, there is still a limit to how much they will bend, so the wall is an effective prevention at sound waves leaving the open air stadium, but some areas past the wall will be more protected than others, depending on the nature of the wall, and the nature of the things past the wall. For instance, the leaves of a forest is a pretty good acoustic baffle, but like all baffles it tends to work better for higher frequencies. Bass frequencies, if they are not being reflected, are best to be absorbed, for instance carpet or soft surfaced areas like the clothes of people.

[myorgan]

@Larason2,

Thank you for the information, and you clearly understood one of the questions I had. In the ampitheater, the soud will come out of the speakers, above and flanking the stage, travel across the elevated seating section, then past the 50' audio booths, and presumably on a now-elevated path up into the air.

Your answers jive with what I already know to be true, but was looking for reinforcement. Thank you.

Michael

[Larrytow]

As I said earlier - Interesting Question ! I just did a cursory G search, and came up with lots of stuff to read, but I can't take the time to do that now. The first few sites I looked at seem to indicate that gravity does Not affect sound waves, at least to any discernible degree. Pretty much that sound waves are part of the electromagnetic spectrum, as are light waves, so if one would be affected by gravity, so would the other.

That was my initial thought when I first read the question, and it still seems reasonable. I'll be watching though to see what everyone else can find on this topic, because it is a rather intriguing one. It's WAY out of my depth in theoretical physics knowledge for sure.

[Admin]

The first few sites I looked at seem to indicate that gravity does Not affect sound waves, at least to any discernible degree. Pretty much that sound waves are part of the electromagnetic spectrum, as are light waves, so if one would be affected by gravity, so would the other.

Typo?
Sound waves are not part of electromagnetic spectrum. If we're talking about earth's gravity, its effect on sound perception is, at best, imperceptible, if not non-existent.

[Larason2]

Interesting. The distortion in the video probably has to do with changes in the density of the air moving past the plane. More dense air causes light to be diffracted more than less dense air, so turbulent alternating denser and less dense air can cause what you are seeing. In that video you can see they are on the boundary of a high pressure system (denser, clear air), and a low pressure system (less dense, looks cloudy). There’s some kind of turbulent boundary between the two systems that they flew through causing the effect, and it disappeared once they were in the cloudy bit. Very cool! I’m guessing that one side of the propeller was more in the low pressure side, and the other the high pressure side, so the propeller was causing the turbulent mixing.

[Admin]

Here's a discussion of this issue
.https://www.physicsforums.com/thread...2018%2C%202004

What's well-known is that the speed of sound is affected by the density of the medium it travels through. As air density is greater at lower altitudes, the speed of sound decreases at higher altitudes, but as is pointed out, the effect of temperature is far greater than that of density. I'm sticking to my opinion that on earth, the effect of gravity on sound is imperceptible, but I'll be happy to be proven wrong with a link to a suitable reference. Keep in mind, that I'm talking about perception.

[Larrytow]

Admin, ya, a typo - and an example of why I should not post when I am too tired to form coherent sentences. I meant "Not Part of...", as you thought. The rest of that sentence makes no sense either, so just ignore my ramblings.

Post #1 was asking if sound is affected by gravity, and my thought was, and is still, that gravity has no effect on sound waves, at least in all practical ( musical ) situations. Perhaps in some theoretical way there might be some interaction between the two, but it has to be difficult to measure / quantify / explain.

[JeffW]

Sound travels through air, the medium through which sound travels assumed for this post.

Air is comprised of atmospheric components (various gasses and particles) and those components get denser ("air pressure increases") as one moves from just outside the atmosphere to sea level. The main cause of the air pressure is the Earth's gravity.

So the density of air, the medium of sound, is mainly caused by gravity.

However, the atmosphere is not uniform in a similar way that the surface of the Earth is not uniform. There are other forces at smaller levels than Earth's gravity that cause variations in the atmosphere and thereby create variations in how sound travels and how it is heard.

Described in brief detail on this web page, changes in wind and temperature affect the air and thus affect how sound travels.

Although these descriptions of causes of sound variation seem limited to outdoor circumstances, in the documentary video for Olivier Latry's "Midnight at Notre Dame" there is a moment where Latry realizes that snow has started falling on the metal roof of Notre Dame because he is aware that the sound of the organ has changed. Small scale changes of temperature within Notre Dame notably changed the sound of the organ for the player.

[myorgan]

...there is a moment where Latry realizes that snow has started falling on the metal roof of Notre Dame because he is aware that the sound of the organ has changed. Small scale changes of temperature within Notre Dame notably changed the sound of the organ for the player.

Jeff,

It is so odd you should mention that. John (@jbird604) and others have noted this phenomenon periodically on the Forum, but IIRC we have failed to identify the specific reasons for it. Thank you for the link and your analysis.

In the link Admin posted, the article mentioned the density of the medium (i.e. air) changing vs. the gravity affecting the media, or sound (though not in those words). Thank you both for those links!

Michael

[myorgan]

So, according to JeffW's article, the video phenomenon I posted of the visual distortion in an airplane, the explanation could be as simple as compression and rarefaction behind an engine or wing (thereby changing density of the air as it is deflected by the engine, wing, or fuselage) is affecting the visual image.

One statement in the article (softdb.com) I might disagree with, is the one stating:

Temperature gradients also influence the propagation of sound waves over long distances. Indeed, temperature influences the density of the air, which in turn influences the speed of sound. For air, which is considered a perfect gas, the lower the temperature, the higher the density and the lower the velocity. This decrease in speed is accompanied by a change in the trajectory of the sound waves: they are refracted. The refraction of sound waves is similar to the refraction of light.

I might note that at -40?C/F, I have noted sound travels a lot further with less attenuation. I would believe that with the air molecules more dense, though moving more slowly, their effect on surrounding molecules would be greater, and therefore faster transmission of sound (greater velocity). It would be interesting to measure the differences in a solid vs. gas (i.e. train rails in cold weather vs. hot weather). I do know we can tell how cold it is outside by how close the train horn sounds on the waterfront in the winter.

Just my (un)professional opinion and conjecture.

Michael

[myorgan]

If retrospect, I should have said, sound travels more efficiently through a solid vs. liquid or gas.

Michael

[gtc]

Unfortunately, this New Scientist article is pay-walled:

Heavy metal music isn’t heavy after all – it is actually the opposite. Sound waves have mass and can interact via gravity, but that mass is negative. In other words, sound floats upwards.

Angelo Esposito at Columbia University in New York and his colleagues calculated the relationship between sound and gravity, taking into account complicated particle interactions that had previously been ignored. They found that, although the effect is small, sound waves should have negative gravitational mass.

“It’s almost like antigravity,” says Ira Rothstein at Carnegie …

Read more: https://www.newscientist.com/article...#ixzz7WjivhhqS

[JeffW]

Here is a link to an available LiveScience summary of the sound wave research published in 2018 that gtc quoted: https://www.livescience.com/63305-so...vity-mass.html

[michaelhoddy]

Higher frequency sounds are absolutely not higher energy. The reverse is true. Distance (and barriers or absorptive surfaces) quickly degrades high frequency sounds because of this.

Also, gases behave as a fluid, so while gravity does affect them, it's not in the same manner as with a solid object, and compressibility and the repulsive forces between gas molecules come into play as well. So when sound transmits through the air, it's through the net total volume of the fluid, and gravity does not have a statistically meaningful effect.

The reason speakers are placed high in a structure has nothing to do with gravimetric effects and everything to do with the inverse square law and sound propagation, so as to achieve as even as possible coverage from the front to the back of a space.

[Larason2]

Maybe this is a better discussion:

https://physics.stackexchange.com/qu...d-on-frequency

Basically, a sound wave at the same amplitude as another needs more energy to have a higher frequency. In theory, its possible to have two sound waves with the same energy, but different frequencies, but it is unlikely in any sound that is balanced for our hearing, since the lower frequency sound would need to have a higher amplitude, and the amplitude of a sound wave directly affects how loud we perceive it, and a sound needs to have 10x the amplitude for us to hear it as twice as loud. So by the time you’ve increased the amplitude of a bass wave so the energy equals a treble wave, it’s going to be tremendously louder. For a bass and treble wave to sound just as loud to our ears, they would have to have the same amplitude. In that situation, the bass wave cannot have as much energy as the treble wave.

[myorgan]

I think I got all that. Thank you for the 2nd article. Part of the article made better sense, but then part of it didn't (when it talked about electromagnetic waves and phonons).

Michael

[michaelhoddy]

You are correct that high frequencies have higher energy per wavelength. However, in acoustical (not electromagnetic) waves, the total net energy potential of the wave does not simply correlate to the simple relationship of frequency and amplitude only, but rather the net of the entire wavefront. It’s much more complex to calculate.

[Coenraads]

So we at least agree that the direct effect of gravity on sound waves is insignificant when it comes to deciding where to place speakers. But there is an indirect effect to be considered. Gravity causes stratification of the air depending on temperature. Thus near the ceiling, where it is warm, sound waves will travel faster than near the floor. This will result in the wave fronts refracting and bending towards the floor. Again I'm not suggesting this is of any practical consequence since reflections will obliterate any measurable effect.

As a physics teacher in the 1970s, we used the PSSC physics program. Some of you may remember the experiments with ripple tanks and I can think of one experiment in particular that illustrated this beautifully. The ripple tank was mounted at a slight vertical angle so that the water was shallower at one end than the other. As a wavefront at an oblique angle approached the shallower water it would slow down and the incoming wavefront would bend towards the shallower water. This effect can be readily observed at the beach especially around sandbars.

[Leisesturm]

Gravity is a Universal Constant, but it is a very weak one. Lucky for us, because the gravity field of the Earth, which has a pretty damn big Mass, does not crush us, but nicely counteracts the centrifugal force of our planets rotation, which is trying hard to fling us all into The Void. Ocean waves slow down in shallower water because of interactions between individual water molecules and also with surrounding objects (terrain) and NOT because of Gravity! As noted earlier, air behaves very much like a fluid, and comparisons are thus valid. If warmer than surrounding air, the air mass inside the envelope of a Hot Air Balloon will accelerate vertically at astonishing rates of speed. TONS of air, heated only tens of degrees warmer than surrounding air spit in Gravity's face and head upward with compete disregard for the tiny influence that Earth's Mass (Gravity) has on anything except maybe another Earth size Mass. Another Earth size Mass within a few hundred million miles. Something none of us want to be around for.

I think we can disregard Gravity completely in any examples given in this thread. It is simply irrelevant. Acoustic phenomena are exactly that: acoustic phenomena. Gas Physics and Thermodynamics enter into things where they are relevant and they also have far more influence on observed phenomenon than Gravity. In two weeks I may have to care a lot about Gravity, because DW has not been to her homeland (Manchester, UK) since we have been together (14 years). The physics of Aviation Science are not trivial, and even there Gravity only becomes important as a causal agent when other dynamic systems fail catastrophically. So, sorry Gravity, I have to diss you completely and utterly for the sake of this discussion. But, please don't hold that against me.

[myorgan]

Gravity is a Universal Constant, but it is a very weak one. Lucky for us, because the gravity field of the Earth, which has a pretty damn big Mass, does not crush us,

It has its effect on me! The older I get, everything starts sagging, and the number on the scales continues to go up.

So, I guess the primary question for this thread should have been, "If a speaker falls from the top of a tree and no one is there to hear it, does it make a sound?"?

Thanks everyone for weighing in.

Michael

[Leisesturm]

Weighing in? <groan>

[Organkeys Jones]

Has anyone discussed if sound waves can affect gravity??

Also, there is way more information in this thread than I need to know! But I did read it all.

[myorgan]

Has anyone discussed if sound waves can affect gravity??

Actually, it cannot change gravity, but it certainly can help defy gravity! Every time my wife blasts the stereo, it raises me out of my chair!?

Michael