Sound, two televisions, that sort of thing - Tilted Forum Project Discussion Community

Rlyss · 06-26-2004, 12:04 AM

Hi everyone.

I was thinking this morning about some things. When you shine a torch (flashlight) on one spot, it is bright. And when you shine another torch on that same spot, it's even more bright. I think it's even twice as bright, is that correct?

Does the same thing happen with sound? If I have one television set on volume 10, it is that loud. If I turn on another television to the same station, and also set that at volume 10, does it become louder? What if I have a dozen televisions, all on volume 10? Is there no difference? How does that work?

llama8 · 06-26-2004, 02:10 AM

Sound and light 'waves' can interfere with each other. They may do this constructively (superimpose) or destructively (cancel out).

Think about something like a sporting event. If one person sings alone then they wouldn't be heard very well. If you have an entire ground singing all those sound waves come together and can be heard kms away. All the extra energy is coming together and causing the vibrations of the air particles to be even stronger (this is what sound basically is).

If you brought two tv sets together they would be twice as loud only if the waves of vibration were 'perfectly' in phase.

Your two torches probably wouldn't be twice as bright but would more then likely be brighter. The only way to look at light sources and see how they interfere is to use lasers. These are light sources where the waves are in phase with each other.

Check out anything on Young's two-slit experiment or on diffraction gratings to explain it.

A quick google gave this: http://www.physicsclassroom.com/Class/light/u12l1b.html
it's not a bad explanation.

Quite a strong argument for light being a wave as opposed to a stream a particles. (It is quantum mechanics that makes the link but that is a different story)

ubertuber · 06-26-2004, 07:59 AM

Quote:

Originally posted by llama8
If you brought two tv sets together they would be twice as loud only if the waves of vibration were 'perfectly' in phase.

Your two torches probably wouldn't be twice as bright but would more then likely be brighter.

Phase isn't the only consideration. I don't know about light for sure, but I suspect that it is like sound in that increases in power function more on the log scale than the linear one. Not to mention the effect of distance on the whole equation - doubling the distance from a source quarters the amount of energy received. This is quick and dirty. If I have a chance I'll look up the details later and post a better explanation.

llama8 · 06-26-2004, 01:00 PM

Quote:

Originally posted by ubertuber
Phase isn't the only consideration. I don't know about light for sure, but I suspect that it is like sound in that increases in power function more on the log scale than the linear one. Not to mention the effect of distance on the whole equation - doubling the distance from a source quarters the amount of energy received. This is quick and dirty. If I have a chance I'll look up the details later and post a better explanation.

Yea that's true my point I was trying to make is the amplitudes of the waves at a given point add up as you would expect to occur. Obviously the amount of energy at a given point depends on lots of factors.

ubertuber · 06-26-2004, 01:27 PM

Quote:

Originally posted by llama8
Yea that's true my point I was trying to make is the amplitudes of the waves at a given point add up as you would expect to occur. Obviously the amount of energy at a given point depends on lots of factors.

You are and were totally correct - I wasn't trying to replace your point, only add to it. Sorry for the misunderstanding.

Rlyss · 06-26-2004, 10:33 PM

So two televisions side-by side will be roughly twice as loud? I've tried that sort of thing with two radios and it hasn't seemed like it was on volume 20 (if, say, each are set on volume 10).

Thanks for the explanations guys, some of it is a little over my head but I appreciate it

SinisterMotives · 06-27-2004, 11:40 AM

Sound volume increases logarithmically as power increases, at least as perceived by the human ear. It takes a tenfold increase in power to create twice the apparent loudness. Phase cancellation and reinforcement also come into play. If you generate a sound whose frequency matches the resonant frequency of the enclosing space, you get what's known as a standing wave. It has a "booming" quality that makes it sound much louder than other sounds in the same room.

oberon04 · 06-27-2004, 02:37 PM

If the two tvs are at the same volume level, combined they will be only 3 decibels louder then a single unit, this is given in an open environment, though it may be a little louder because of reflection off the walls. A 3dB increase is only a slight increase in the noticed sound level. This doesn't take into account the frequency levels, as ours ears will applify or attenuate certain frequecies.

Heres the formulas for those who care:

(P1/Pref)squared = antilog(db_level/10)
same for second sound source

(Ptotal/Pref)^2 = (P1/Pref)+(P2/Pref)squared

So if you had two t.v.'s giving off an equal sound level of 75 dB each.

(P1/Prer)^2 = (P2/Pref)^2=3.16*10^7
(Ptot/Pref)^2 = 6.32*10^7

When converted back into dBs the new sound level is 78 dB.

where P1 and P2 and Ptot are the sound pressure levels in N/m^2

dB_level is the measured sound pressure level in decibels.

Hope that makes sense to everyone!

06-26-2004, 12:04 AM	#1 (permalink)
Rlyss The Pusher Location: Edinburgh	Sound, two televisions, that sort of thing Hi everyone. I was thinking this morning about some things. When you shine a torch (flashlight) on one spot, it is bright. And when you shine another torch on that same spot, it's even more bright. I think it's even twice as bright, is that correct? Does the same thing happen with sound? If I have one television set on volume 10, it is that loud. If I turn on another television to the same station, and also set that at volume 10, does it become louder? What if I have a dozen televisions, all on volume 10? Is there no difference? How does that work? Last edited by Dorito2; 06-26-2004 at 12:07 AM..

06-27-2004, 11:40 AM	#7 (permalink)
SinisterMotives Junkie	Sound volume increases logarithmically as power increases, at least as perceived by the human ear. It takes a tenfold increase in power to create twice the apparent loudness. Phase cancellation and reinforcement also come into play. If you generate a sound whose frequency matches the resonant frequency of the enclosing space, you get what's known as a standing wave. It has a "booming" quality that makes it sound much louder than other sounds in the same room. Last edited by SinisterMotives; 06-27-2004 at 11:42 AM..

06-26-2004, 02:10 AM	#2 (permalink)
llama8 Crazy Location: Leicester, UK	Sound and light 'waves' can interfere with each other. They may do this constructively (superimpose) or destructively (cancel out). Think about something like a sporting event. If one person sings alone then they wouldn't be heard very well. If you have an entire ground singing all those sound waves come together and can be heard kms away. All the extra energy is coming together and causing the vibrations of the air particles to be even stronger (this is what sound basically is). If you brought two tv sets together they would be twice as loud only if the waves of vibration were 'perfectly' in phase. Your two torches probably wouldn't be twice as bright but would more then likely be brighter. The only way to look at light sources and see how they interfere is to use lasers. These are light sources where the waves are in phase with each other. Check out anything on Young's two-slit experiment or on diffraction gratings to explain it. A quick google gave this: http://www.physicsclassroom.com/Class/light/u12l1b.html it's not a bad explanation. Quite a strong argument for light being a wave as opposed to a stream a particles. (It is quantum mechanics that makes the link but that is a different story)

06-26-2004, 10:33 PM	#6 (permalink)
Rlyss The Pusher Location: Edinburgh	So two televisions side-by side will be roughly twice as loud? I've tried that sort of thing with two radios and it hasn't seemed like it was on volume 20 (if, say, each are set on volume 10). Thanks for the explanations guys, some of it is a little over my head but I appreciate it

06-27-2004, 02:37 PM	#8 (permalink)
oberon04 Upright Location: USA	If the two tvs are at the same volume level, combined they will be only 3 decibels louder then a single unit, this is given in an open environment, though it may be a little louder because of reflection off the walls. A 3dB increase is only a slight increase in the noticed sound level. This doesn't take into account the frequency levels, as ours ears will applify or attenuate certain frequecies. Heres the formulas for those who care: (P1/Pref)squared = antilog(db_level/10) same for second sound source (Ptotal/Pref)^2 = (P1/Pref)+(P2/Pref)squared So if you had two t.v.'s giving off an equal sound level of 75 dB each. (P1/Prer)^2 = (P2/Pref)^2=3.1610^7 (Ptot/Pref)^2 = 6.3210^7 When converted back into dBs the new sound level is 78 dB. where P1 and P2 and Ptot are the sound pressure levels in N/m^2 dB_level is the measured sound pressure level in decibels. Hope that makes sense to everyone!