Properties of sound

Wiskel · May 24, 2012

I'm setting a battle scene in a valley with a LARGE waterfall dominating one end. Angel Falls type of large. One army has guns and tanks

My instinct is that high pitched sounds, like bird song will be audible over the rumble of the waterfall, as will short, loud sounds, like gunshots. I also think low pitched sounds, like the tanks moving around will tend to blend into the waterfall sound.

The importance of sound is I'm hoping I can surprise my characters with tank movement beng very hard to detect from noise alone.

Anyone with a better working knowledge of sound able to confirm or deny?

Thanks

Craig

Kenn · May 24, 2012

I think you are correct up to a point. However, Angel Falls is a high waterfall rather than a large one (in terms of volume). The sound spectrum from a large waterfall (like Niagara), which rumbles, is described as 'Brown noise'. This has a frequency spectrum that is dominated at the lower end. Cascading waterfalls (like Angel Falls) tend to produce 'pink noise', in which the fall off in sound level with frequency is less discernible.

What you assume about the rumble of tanks and birdsong is correct, although I think bullets would be heard because they are of a higher frequency (not because they are of short duration).

Bufty · May 24, 2012

Make sure the surrounding territory can accomodate and support the manoeveuring weight of tanks.

ironmikezero · May 24, 2012

Kenn observed... What you assume about the rumble of tanks and birdsong is correct, although I think bullets would be heard because they are of a higher frequency (not because they are of short duration).[/QUOTE]

FWIW, I think Kenn is referring to the sound of the explosive gaseous expansion of the shot - the sharp report, if you will - rather than the passage of a projectile.

If you're (unfortunately) close enough to the passage of a bullet - think near miss - and the circumstances warrant, the sound is more like THHPPPP....

If the round strikes something, you might hear the impact.

Another issue is the sound barrier; anything traveling faster than the speed of sound (about 342mps/1,126fps) will create a sound when surpassing that speed - think sonic boom. Most rifles and some handguns are capable of muzzle velocities in excess of that figure.

If your waterfall is of sufficient volume and subsequent noise, I suspect many of these sounds would be rendered moot.

WildScribe · May 24, 2012

It sounds like you have your theory confirmed, so I just wanted to add... what a cool scene.

Xelebes · May 24, 2012

Kenn said:
I think you are correct up to a point. However, Angel Falls is a high waterfall rather than a large one (in terms of volume). The sound spectrum from a large waterfall (like Niagara), which rumbles, is described as 'Brown noise'. This has a frequency spectrum that is dominated at the lower end. Cascading waterfalls (like Angel Falls) tend to produce 'pink noise', in which the fall off in sound level with frequency is less discernible.

What you assume about the rumble of tanks and birdsong is correct, although I think bullets would be heard because they are of a higher frequency (not because they are of short duration).

There is also impulse. Gun shots will be heard over a dull roar because they are impulsive - the pops last short and rise to quite the volume. There is a good chance that you will hear it. Basically, loud clanks and bangs will be heard. Bird chirps will be heard but humans talking will be harder to hear. A massive waterfall (Niagaran) will mask the rumblings of tanks.

Wiskel · May 25, 2012

Thanks everyone. Your help's much appreciated.

Craig

onesecondglance · May 31, 2012

Xelebes said:
There is also impulse. Gun shots will be heard over a dull roar because they are impulsive - the pops last short and rise to quite the volume. There is a good chance that you will hear it. Basically, loud clanks and bangs will be heard. Bird chirps will be heard but humans talking will be harder to hear. A massive waterfall (Niagaran) will mask the rumblings of tanks.

I know this thread's basically over, but for the benefit of anyone who finds it looking for info relating to other projects...

The crack of a gunshot - described above as an "impulse" - is called a "transient" in sound engineering terms. Transients cut through background noise better than broadband sounds (like the rumble of a tank). This is why drums cut through the wash of guitars on a rock record etc.

In the example above, the tank rumble is a broadband sound - it covers a lot of frequencies. This will "mask" other sounds at similar and harmonic frequencies. It's why you can't hear funky bass solos on your car stereo when you're going down the motorway - the road noise masks those frequencies. You might still be able to hear the bass drum though if the transients on it (see above) are clear. This is why heavy metal and dance music bass drums often sound "clicky" - it helps them cut through by giving them a stronger transient.

Anyway, hope this is helpful.

Kenn · May 31, 2012

onesecondglance said:
I know this thread's basically over, but for the benefit of anyone who finds it looking for info relating to other projects...

The crack of a gunshot - described above as an "impulse" - is called a "transient" in sound engineering terms. Transients cut through background noise better than broadband sounds (like the rumble of a tank). This is why drums cut through the wash of guitars on a rock record etc.

In the example above, the tank rumble is a broadband sound - it covers a lot of frequencies. This will "mask" other sounds at similar and harmonic frequencies. It's why you can't hear funky bass solos on your car stereo when you're going down the motorway - the road noise masks those frequencies. You might still be able to hear the bass drum though if the transients on it (see above) are clear. This is why heavy metal and dance music bass drums often sound "clicky" - it helps them cut through by giving them a stronger transient.

Anyway, hope this is helpful.

I don't believe this is an answer to the question, because it confuses the definitions of broadband and transient.

A transient is a short-duration high amplitude (volume) pulse. It is this amplitude that determines whether it will be heard. A pulse of narrow frequency range being the most easy to hear is true up to a point. That is because it's the the change in the sound spectrum that is detected and not the change in volume. That said, chords are just as easy to detect as single notes.

The notion of a tank noise being a broadband sound is only partly true (it's the fact it's a continuous source that's important). The source is primarily low frequency and environmental media attenuate this even further (the attenuation effects are more important at higher frequencies), so it is percieved as a rumble. A rifle shot covers a broad, but higher, frequency spectrum (there are plenty of studies on this for ear defenders). The difference being that it is centred on the range where the ear is most sensitive. That is why it is easier to hear high frequency transients than low ones. It is not because the tank is a broadband transient (which it isn't anyway) and the rifle shot is not.

To get back to the original question, the rumble of tanks would be harder to detect against the attenuated pink (or maybe white) noise source of a waterfall. Rifle shots would be easier to hear because they are of higher frequency and short duration (i.e. transient). So the original assumptions are correct.

onesecondglance · May 31, 2012

Kenn said:
I don't believe this is an answer to the question, because it confuses the definitions of broadband and transient.

A transient is a short-duration high amplitude (volume) pulse. It is this amplitude that determines whether it will be heard. A pulse of narrow frequency range being the most easy to hear is true up to a point. That is because it's the the change in the sound spectrum that is detected and not the change in volume. That said, chords are just as easy to detect as single notes.

The notion of a tank noise being a broadband sound is only partly true (it's the fact it's a continuous source that's important). The source is primarily low frequency and environmental media attenuate this even further (the attenuation effects are more important at higher frequencies), so it is percieved as a rumble. A rifle shot covers a broad, but higher, frequency spectrum (there are plenty of studies on this for ear defenders). The difference being that it is centred on the range where the ear is most sensitive. That is why it is easier to hear high frequency transients than low ones. It is not because the tank is a broadband transient (which it isn't anyway) and the rifle shot is not.

To get back to the original question, the rumble of tanks would be harder to detect against the attenuated pink (or maybe white) noise source of a waterfall. Rifle shots would be easier to hear because they are of higher frequency and short duration (i.e. transient). So the original assumptions are correct.

Hi Kenn,

it wasn't my intention to confuse the issue, although I don't believe I stated anything regarding the frequency of the transient making a difference? As you said, transients are a time-domain issue, and broadband noise is a frequency-domain issue. The line in my first paragraph ("Transients cut through background noise better than broadband sounds (like the rumble of a tank)") *is* wrong and I thought I had already edited it to correct that from my initial ramble - clearly not. I'll refrain from a retroactive edit to preserve at least some of the sense of this thread.

I might have been wrong to assign the broadband term to the tank rumble in strictly technical terms - certainly the frequency range would be narrower than that of the waterfall - but it would span a reasonably wide range, hence my usage. I'm used to the phrase from a music engineering perspective, which probably has different definitions than in strict acoustic physics.

Anyway, I'll leave it there.

Kenn · Jun 2, 2012

Onesecondglance, tank tracks can also rattle and squeak, especially when driven on hard standing, so saying it's all low frequency noise was a bit of a generalisation by me.

Although not especially relevant to the original question, one interesting fact is that an exhaust silencer is usually designed to be a low band pass filter, in which the higher (and more intrusive) frequencies are removed. Add to that the lower attenuation of low frequencies, then distant traffic (and tank!) noise tends to be a low frequency hum.

onesecondglance · Jun 2, 2012

Kenn said:
tank tracks can also rattle and squeak, especially when driven on hard standing

That I did not know. Thanks!

Properties of sound

Wiskel

practical experience, FTW

Kenn

New kid, be gentle!

Bufty

Where have the last ten years gone?

ironmikezero

practical experience, FTW

WildScribe

Slave to the Wordcount

Xelebes

Delerium ex Ennui

Wiskel

practical experience, FTW

onesecondglance

pretending to be awake

Kenn

New kid, be gentle!

onesecondglance

pretending to be awake

Kenn

New kid, be gentle!

onesecondglance

pretending to be awake