I would like to start a discussion involving how sound works.

[ymalmsteen887]

I would like to start a discussion involving how sound works. I've read plenty of websites but none can satisfy the level of curiosity I have. To get started I would like to know what it means for low frequency to travel farther than a high frequency all sound waves travel the same speed? The explanations I've gotten from websites is just that they are lower that doesn't tell me why it goes farther I feel that there is more to it than that?

 

[Naty1]

Sound travels differently in different mediums...and at different speeds

Try wikipedia...note additional links at the bottom of each article...

The speed of sound in an ideal gas is independent of frequency, but it weakly depends on frequency for all real physical situations.

http://en.wikipedia.org/wiki/Speed_of_sound

and or here
http://en.wikipedia.org/wiki/Sound_waves#Longitudinal_and_transverse_waves

 

[ymalmsteen887]

Thats not what I am asking I've read plenty of websites on this subject but none explain it in a way I can understand. I know how fast sound travels and that it depends on the air pressure and temperature. I want to know why low frequencies travel farther I don't see any reason why they would it just seems that more pressure is required to get a low frequency started and so it travels farther because their is more energy involved is this correct?

 

[JaredJames]

Here's a start for you:

http://www.madsci.org/posts/archives/1999-11/943317470.Ph.r.html

If you read through here:

http://en.wikipedia.org/wiki/Stokes'_law_(sound_attenuation)

You will note that as the frequency of the sound increases, the attenuation also increases (by the square of the frequency).

Put simply, the higher the frequency of the sound, the more attenuation there is as it travels through the medium (in this case air) and as such the quicker the intensity drops over distance.

 

[ymalmsteen887]

Here's a start for you:

http://www.madsci.org/posts/archives/1999-11/943317470.Ph.r.html

If you read through here:

http://en.wikipedia.org/wiki/Stokes'_law_(sound_attenuation)

You will note that as the frequency of the sound increases, the attenuation also increases (by the square of the frequency).

Put simply, the higher the frequency of the sound, the more attenuation there is as it travels through the medium (in this case air) and as such the quicker the intensity drops over distance.

That site helped a lot. I would also like to know how reflections on walls work, for example If you have a sound in free air, pretend there is no floor or celing, just a wall behind you how do you determine how much increase in pressure there is with the intial wave and the one reflecting from the wall?Im not talking about the exact formula I am talking about if there is any increase at all?

 

[ymalmsteen887]

Actually the part about the 100hz wave traveling four times as far as the 400hz wave wouldn't a 100hz wave require more energy to travel the same distance as 400hz given any level?

 

[JaredJames]

Actually the part about the 100hz wave traveling four times as far as the 400hz wave wouldn't a 100hz wave require more energy to travel the same distance as 400hz given any level?

Energy of a sound wave isn't related to its frequency. So no, it wouldn't require more energy to travel the same distance.

What the article refers to is that for the level of attenuation a 400hz wave encounters over a set distance, a 100hz wave would have to travel four times as far to encounter the same level of attenuation.

Not that a 100hz wave travels four times further than a 400hz wave.

 

[ymalmsteen887]

Energy of a sound wave isn't related to its frequency. So no, it wouldn't require more energy to travel the same distance.

What the article refers to is that for the level of attenuation a 400hz wave encounters over a set distance, a 100hz wave would have to travel four times as far to encounter the same level of attenuation.

Not that a 100hz wave travels four times further than a 400hz wave.

This is what I can't wrap my mind around let's take a 12200hz sound wave which is about 1inch and say it when it has reached 20 feet away from the source did it produce the same amount of pressure to reach that distance as a 20hz wave did to reach that same distance?

 

[JaredJames]

This is where the equations from the second link I gave you come in.

The 12200hz wave has massively higher attenuation than the 20hz wave. So the intensity of the first wave (and as such the pressure) after a set distance will be less than that of the second wave. (The first wave will have a lower decibel reading than the second after a set distance.)

 

[ymalmsteen887]

This is where the equations from the second link I gave you come in.

The 12200hz wave has massively higher attenuation than the 20hz wave. So the intensity of the first wave (and as such the pressure) after a set distance will be less than that of the second wave. (The first wave will have a lower decibel reading than the second after a set distance.)

Dont you mean the first wave is the strongest and the second wave will have a lower decibel reading.

 

[JaredJames]

Dont you mean the first wave is the strongest and the second wave will have a lower decibel reading.

No. Have you not read the links I posted?

The higher the frequency, the more attenuation the wave encounters.

This means that after a set distance, the higher frequency wave will have a lower intensity (and as such lower decibel reading).

A low frequency wave can have a decibel level equal to a high frequency wave.

 

[ymalmsteen887]

No. Have you not read the links I posted?

The higher the frequency, the more attenuation the wave encounters.

This means that after a set distance, the higher frequency wave will have a lower intensity (and as such lower decibel reading).

A low frequency wave can have a decibel level equal to a high frequency wave.

Tell me if this is right, when a bass speaker moves forward the air in front is compressed and the air behind the speaker rarefacts(whatever the opposite of compression is called) so the air in front will travel towards the back to achieve equilibrium, thus not producing a very loud sound not matter how fast its moving is this correct?

 

[JaredJames]

Tell me if this is right, when a bass speaker moves forward the air in front is compressed and the air behind the speaker rarefacts(whatever the opposite of compression is called) so the air in front will travel towards the back to achieve equilibrium, thus not producing a very loud sound not matter how fast its moving is this correct?

Ah, I see your problem. You're not understanding what a decibel is and what it means in relation to sound pressure. Read up on these first.

Pressure and volume are not the same thing. A bass speaker can send out a wave that is almost inaudible, but the pressure of that wave can be damaging to the ear. This means the wave has a low volume and high pressure.

The opposite of compression is expansion. I'm not sure about your explanation of a bass speaker. All speakers operate on this principle, so the volume hypothesis above cannot be correct.

 

[ymalmsteen887]

Ah, I see your problem. You're not understanding what a decibel is and what it means in relation to sound pressure. Read up on these first.

Pressure and volume are not the same thing. A bass speaker can send out a wave that is almost inaudible, but the pressure of that wave can be damaging to the ear. This means the wave has a low volume and high pressure.

The opposite of compression is expansion. I'm not sure about your explanation of a bass speaker. All speakers operate on this principle, so the volume hypothesis above cannot be correct.

You know that's why they put the woofers in an enclosure to have the back wave work with the front wave.

 

[JaredJames]

You know that's why they put the woofers in an enclosure to have the back wave work with the front wave.

Again, you need to understand the difference between the pressure and the perceived volume.

Your description didn't mention the enclosure, hence it wasn't strictly correct and could apply to all speakers.

I'm not going to sit and argue the technicalities. You've received the answer as to why low frequencies travel further than high frequencies. Any further questions on that are welcome.

 

[ymalmsteen887]

Again, you need to understand the difference between the pressure and the perceived volume.

Your description didn't mention the enclosure, hence it wasn't strictly correct and could apply to all speakers.

I'm not going to sit and argue the technicalities. You've received the answer as to why low frequencies travel further than high frequencies. Any further questions on that are welcome.

I got on here to learn about sound, it seems youre saying I can only ask one question, if this forum doesn't allow this kind of back and forth could you recommend a place I can go for that kind of exchange. I wasnt going to ask just one question and it would be a waste to start another thread for different questions.

 

[JaredJames]

I got on here to learn about sound, it seems youre saying I can only ask one question, if this forum doesn't allow this kind of back and forth could you recommend a place I can go for that kind of exchange. I wasnt going to ask just one question and it would be a waste to start another thread for different questions.

You are free to ask as many questions as you like and I will gladly answer (see the last part of that post you quoted).

However, it is not uncommon for people to ask posters to read up on a few things before continuing to ensure they fully understand what they are saying. All I have asked is that you read up and understand the various terms you have thrown around erroneously.

I have asked this as you are bringing up small technicalities which would be answered if you understood the terms correctly. I don't have the time to sit and type up every little thing I'd like to to help you learn, so I can only point you in the right direction.

Again, if you have further questions please do post them. But ensure you follow any advice I give (re reading materials) before continuing - it might just solve the problem for you.

 

[JaredJames]

Dont you mean the first wave is the strongest and the second wave will have a lower decibel reading.

Just to come back to this, energy is related to amplitude not frequency. If both waves have the same amplitude they have the same energy.

Decibels are related to the pressure level, which is related to the energy of the wave.

So a 20hz wave can have a higher pressure reading than a 122000hz wave.

Have a look at these two:

Sound Intensity: http://en.wikipedia.org/wiki/Sound_intensity
Sound Energy Density: http://en.wikipedia.org/wiki/Sound_energy_density

They should help you with the energy / pressure of a sound wave and with your above problem.

Just note that a higher frequency doesn't mean a higher decibel reading.

 

[ymalmsteen887]

Just to come back to this, energy is related to amplitude not frequency. If both waves have the same amplitude they have the same energy.

Decibels are related to the pressure level, which is related to the energy of the wave.

So a 20hz wave can have a higher pressure reading than a 122000hz wave.

Have a look at these two:

Sound Intensity: http://en.wikipedia.org/wiki/Sound_intensity
Sound Energy Density: http://en.wikipedia.org/wiki/Sound_energy_density

They should help you with the energy / pressure of a sound wave and with your above problem.

Just note that a higher frequency doesn't mean a higher decibel reading.

I understand what you mean. I mistakenly thought when you said the second wave was stronger that the first wave was the second wave once it proceeded past a certain point. You meant the very next wave coming up from the source, correct?

 

[ymalmsteen887]

I am trying to understand how sound waves add to together how come if I take a bucket and hum some notes certain notes will sound louder than others same thing with a trash can but if I take a dresser shelf or a coffee mug nothing happens no matter what note I play?Also when I play a subwoofer in my room its not as loud as when I put in my truck but when I put it a two door car its not as loud as is was when it was in the room this doesn't add up if the if the truck is louder than the room the car should be some where in between but its the least in volume?(because my room is twice as big a space as the car.)

 

[Pythagorean]

I am trying to understand how sound waves add to together how come if I take a bucket and hum some notes certain notes will sound louder than others same thing with a trash can but if I take a dresser shelf or a coffee mug nothing happens no matter what note I play?Also when I play a subwoofer in my room its not as loud as when I put in my truck but when I put it a two door car its not as loud as is was when it was in the room this doesn't add up if the if the truck is louder than the room the car should be some where in between but its the least in volume?(because my room is twice as big a space as the car.)

if dimensions of the container are on the order of the wavelengths of the music, then you will have resonance. Pressure fluctuations are doing real work on their environment. Think of a box with a sound source in it for which:

a) the soundwaves are much bigger than the box
b) the soundwaves are equivalent to the box
c) the soundwaves are much smaller than the box

for a) the waves are really large and tend to pass through the box with little attenuation, so they do little work on the box (they don't try to move the box so quickly) but might move the box very slowly. But the whole box will tend to move as one object, slowly, like a boat on a long ocean wave.

for c) the waves are tiny, on the order of the molecules of the box so they try to do work on the atoms (which are far too strong in normal considerations) so they just get attenuated. (a boat with little choppy waves doesn't feel a thing).

for b), now the boat and the water waves (or the box and the pressure waves) are about the same size. The boat starts getting slammed around by the waves, the box has a measurable pressure flux across it now (the wavelength of the pressure wave across the box assures that).

If the wavelength and the size of the box are exactly equal, the waves will reflect off of the sides perfectly, coming back to their peak exactly where new waves are at their peak (because we're still generating sound from the source). Whereas as we drift from perfect match a bit, the peaks start to mismatch and you begin to have destructive interference.

If we allow the surfaces of the box to flex a bit, like a membrane, we will now see the membrane breathing with the pressure waves (and of course, the membrane will respond similairly to a) and c).

 

[ymalmsteen887]

if dimensions of the container are on the order of the wavelengths of the music, then you will have resonance. Pressure fluctuations are doing real work on their environment. Think of a box with a sound source in it for which:

a) the soundwaves are much bigger than the box
b) the soundwaves are equivalent to the box
c) the soundwaves are much smaller than the box

for a) the waves are really large and tend to pass through the box with little attenuation, so they do little work on the box (they don't try to move the box so quickly) but might move the box very slowly. But the whole box will tend to move as one object, slowly, like a boat on a long ocean wave.

for c) the waves are tiny, on the order of the molecules of the box so they try to do work on the atoms (which are far too strong in normal considerations) so they just get attenuated. (a boat with little choppy waves doesn't feel a thing).

for b), now the boat and the water waves (or the box and the pressure waves) are about the same size. The boat starts getting slammed around by the waves, the box has a measurable pressure flux across it now (the wavelength of the pressure wave across the box assures that).

If the wavelength and the size of the box are exactly equal, the waves will reflect off of the sides perfectly, coming back to their peak exactly where new waves are at their peak (because we're still generating sound from the source). Whereas as we drift from perfect match a bit, the peaks start to mismatch and you begin to have destructive interference.

If we allow the surfaces of the box to flex a bit, like a membrane, we will now see the membrane breathing with the pressure waves (and of course, the membrane will respond similairly to a) and c).

I didnt understand all of that and it didnt answer my question.

I used to think that when you had a subwoofer in a room that the bass was strong er in the corners because of all the other waves colliding with each other. So I thought that a kickdrum sound you wouldn't get a boost because it was only one wave and wouldn't get any increase but I know realize that's wrong the bass is stronger in the corner becuase the wavelength is still bouncing off the wall and combining with the remaining part of the wave, is this correct?

 

[Pythagorean]

I didnt understand all of that and it didnt answer my question.

I used to think that when you had a subwoofer in a room that the bass was strong er in the corners because of all the other waves colliding with each other. So I thought that a kickdrum sound you wouldn't get a boost because it was only one wave and wouldn't get any increase but I know realize that's wrong the bass is stronger in the corner becuase the wavelength is still bouncing off the wall and combining with the remaining part of the wave, is this correct?

well yes, but particularly because it's combining constructively. All over your room, pressure fluctuations are combining, both destructively and constructively. A low bass note (say 55 Hz) is producing 55 "waves" per second so your room is flooded with a chaotic landscape of spherical pressure waves interacting in just a second. The waves propagate at 350 m/s too, so they're allowed to fill up your room rather quickly.

And of course, it's not really just a 55 Hz tone. It's a harmonic series, and it's interacting with matter in your home, so there will be nonlinear effects (especially in a garage band setting where the sound sources are unusually loud compared to the size of the room).

 

[ymalmsteen887]

well yes, but particularly because it's combining constructively. All over your room, pressure fluctuations are combining, both destructively and constructively. A low bass note (say 55 Hz) is producing 55 "waves" per second so your room is flooded with a chaotic landscape of spherical pressure waves interacting in just a second. The waves propagate at 350 m/s too, so they're allowed to fill up your room rather quickly.

And of course, it's not really just a 55 Hz tone. It's a harmonic series, and it's interacting with matter in your home, so there will be nonlinear effects (especially in a garage band setting where the sound sources are unusually loud compared to the size of the room).

The first wave of the 55hz frequency should have lost too much energy to effictivly increase with the second wave. This is why I want to know a formula to figure this out cause I am wondering what the second wave does when the first wave is coming back in the opposite direction this is hard to wrap my mind around.

 

[Born2bwire]

The first wave of the 55hz frequency should have lost too much energy to effictivly increase with the second wave. This is why I want to know a formula to figure this out cause I am wondering what the second wave does when the first wave is coming back in the opposite direction this is hard to wrap my mind around.

Not really. Unless you have a specially designed room, the reflections off of a wall will be pretty strong. Just imagine how long an echo lasts and compare this to the distance that the sound traveled in all that time. For a one second echo, that's 343 m. Take a room of 3 meters cubed and let's estimate that on a 45 degree diagonal that the average path before reflecting is 4.25 m then that's 80 reflections that have occurred. And believe me, even an average room in a house has a lot of echos. We rarely are placed in an environment that is free from reflections to be able to tell the difference.

As for what happens when two waves meet, they simply add up. If the two waves are both in phase, that is their peaks match and their minimums match, then they will add up constructively and support each other. If they are out of phase, where the peak of one wave occurs where another wave has its trough, then they will add up destructively and cancel each other out to some degree.

 

[JaredJames]

I understand what you mean. I mistakenly thought when you said the second wave was stronger that the first wave was the second wave once it proceeded past a certain point. You meant the very next wave coming up from the source, correct?

The first wave: 122000hz
The second wave: 20hz
(As per your example)

The 122000hz wave will experience more attenuation of 100m than the 20hz wave.

Thus, after 100m the 122000hz wave will have a lower intensity than the 20hz wave (lower pressure).

I'm not sure if I like the terminology of 'stronger', but yes, the 20hz wave will be stronger than the 122000hz wave after a set distance (based on intensity).

I haven't mentioned second waves from the source yet. That's regarding interference which Pythagorean has covered.

 

[ymalmsteen887]

Not really. Unless you have a specially designed room, the reflections off of a wall will be pretty strong. Just imagine how long an echo lasts and compare this to the distance that the sound traveled in all that time. For a one second echo, that's 343 m. Take a room of 3 meters cubed and let's estimate that on a 45 degree diagonal that the average path before reflecting is 4.25 m then that's 80 reflections that have occurred. And believe me, even an average room in a house has a lot of echos. We rarely are placed in an environment that is free from reflections to be able to tell the difference.

As for what happens when two waves meet, they simply add up. If the two waves are both in phase, that is their peaks match and their minimums match, then they will add up constructively and support each other. If they are out of phase, where the peak of one wave occurs where another wave has its trough, then they will add up destructively and cancel each other out to some degree.

So youre saying that if a note is sustained then it will progressively get louder that would mean that sustained bass as opposed to like a kickdrum would be louder yet I've never experienced this. Plus when a bunch of people in a room sing together or clap its not much louder than if there wer just 2 or 3 people doing it because like I said the waves that are reflected are much weaker, how come when I go outside I don't seem to notice a drastic difference in the volume of my voice?

 

[JaredJames]

So youre saying that if a note is sustained then it will progressively get louder

No, this is only true if there is constructive interference.

Plus when a bunch of people in a room sing together or clap its not much louder than if there wer just 2 or 3 people doing it because like I said the waves that are reflected are much weaker, how come when I go outside I don't seem to notice a drastic difference in the volume of my voice?

So you think me and you in the Royal Albert Hall clapping is equivalent into a full crowd doing so?

You are stating the "reflected waves are much weaker", are you sure about this? This is all down to acoustics.
A simple test (if you get the chance) is to whistle in a room when the furniture is in it and then whistle when there's no furniture in it (it's empty). You will notice that in the empty room you get a slight echo, or that the sound of the whistle is slightly different.

 

[ymalmsteen887]

No, this is only true if there is constructive interference.


So you think me and you in the Royal Albert Hall clapping is equivalent into a full crowd doing so?

You are stating the "reflected waves are much weaker", are you sure about this? This is all down to acoustics.
A simple test (if you get the chance) is to whistle in a room when the furniture is in it and then whistle when there's no furniture in it (it's empty). You will notice that in the empty room you get a slight echo, or that the sound of the whistle is slightly different.

Im not saying its it exactly the same but a small high school gym full of people clapping won't sound any louder or quiter than a basketball stadium because its all relative to your position. Basically if that logic was true than all the sounds happening anywhere would add together , after a certain distance the waves don't seem to matter anymore actually just one person clapping is preety much how loud it gets the combination of people clapping just gives a more lively sound. The same reason why there are mulitple violins in an orchestra to give that unison sound because the notes arent tuned perfectly the same.

 

[Born2bwire]

Im not saying its it exactly the same but a small high school gym full of people clapping won't sound any louder or quiter than a basketball stadium because its all relative to your position. Basically if that logic was true than all the sounds happening anywhere would add together , after a certain distance the waves don't seem to matter anymore actually just one person clapping is preety much how loud it gets the combination of people clapping just gives a more lively sound. The same reason why there are mulitple violins in an orchestra to give that unison sound because the notes arent tuned perfectly the same.

Actually, the notes in an orchestra have to be tuned pretty well. If they are off then the orchestra produces beats when it plays a sustained note. Case in point:

http://upload.wikimedia.org/wikipedia/commons/6/6f/Terzosuono.ogg

As the two frequencies just start to diverge we can hear the rumble from the beating that occurs. This starts to happen in the first two seconds of the clip.

So youre saying that if a note is sustained then it will progressively get louder that would mean that sustained bass as opposed to like a kickdrum would be louder yet I've never experienced this. Plus when a bunch of people in a room sing together or clap its not much louder than if there wer just 2 or 3 people doing it because like I said the waves that are reflected are much weaker, how come when I go outside I don't seem to notice a drastic difference in the volume of my voice?

Yes, and it does. But the transient time here is too minimal for you to notice. Like I said, 343 m/s is very fast and so the first set of reflections comes in milliseconds. This is too fast for us to really consciously appreciate it though our body does make use of this slight delay for things like positional information.

As for going outside, you most definitely should notice a difference. Try talking in a closed room and then go out to an open field. The difference in terms of echos should be night and day.