ProSoundWeb Community

Please login or register.

Login with username, password and session length
Advanced search  

Pages: [1]   Go Down

Author Topic: wind or temperature gradients  (Read 3772 times)

Merlijn van Veen

  • Moderator
  • Sr. Member
  • *****
  • Offline Offline
  • Posts: 309
    • www.merlijnvanveen.nl
wind or temperature gradients
« on: May 04, 2014, 03:27:37 am »

Hi everbody,


Below is a quote from Sound System Engineering 4th ed. by Davis, Patronis, Jr. and Brown.

"We hear it said, “The wind blew the sound away.” That is not so; it refracted away. Even a 50 mph wind (and that’s a strong wind) cannot blow away something traveling 1130ft/s:" Chapter 10 page 176.

It still (1st edition was published in 1975) makes sense to me but does this claim still hold anno 2014? Anyone aware of recent developments or renewed insights?


Regards,


Merlijn van Veen

Mark McFarlane

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 1865
  • Middle East
    • Arkose Records
Re: wind or temperature gradients
« Reply #1 on: May 04, 2014, 09:53:50 am »

Sound is transmitted in air as a pressure wave (molecules bumping into each other). A strong wind will *locally* mess with the wavefield (both amplitude and phase) enough to cause distortion of the wavefield by locally changing the velocity (the directional vector, not the speed).  Wind also moves your ears, another frequency dependent source of distortion, and can have a masking effect.

I did a quick Google search and most of the research seems concerned with noise (e.g. airports) and wind makes volumes louder than expected downwind and softer upwind.  This is a real phenomena caused by the wind, whether you call it refraction or 'the wind blow the sound away' is up to you. 

"We hear it said, “The wind blew the sound away.” That is not so; it refracted away. Even a 50 mph wind (and that’s a strong wind) cannot blow away something traveling 1130ft/s:" This claim is only addressing overall volume and not intelligibility, and is probably bad analogy because wind and sound waves are very different in terms of molecular motion.

If you want to punish yourself, trudge through https://fenix.tecnico.ulisboa.pt/downloadFile/2589872706764/dissertacao.pdf , or jump to section 6.5 on page 72 for a human readable summary.

Logged
Mark McFarlane
ARKOSERECORDS
Turn down what's too loud.

Jeff Carter

  • Sr. Member
  • ****
  • Offline Offline
  • Posts: 363
  • Kitchener, ON, Canada
Re: wind or temperature gradients
« Reply #2 on: May 04, 2014, 03:15:49 pm »

\
If you want to punish yourself, trudge through https://fenix.tecnico.ulisboa.pt/downloadFile/2589872706764/dissertacao.pdf , or jump to section 6.5 on page 72 for a human readable summary.

What I got out of that is that wind causes the speed of sound to depend on altitude (fig 6.2 on page 67), usually causing the speed of sound to decrease with increasing altitude (which if my grasp of physics is correct, would cause sound to refract away from the ground, in the direction of lower speed of sound).

However, if the wind speed is high enough, this speed-of-sound gradient actually reverses for sound travelling downwind (Pasquill class C, fig. 6.2), and refraction would then tend to guide the sound along the ground (which is presumably when the downwind neighbours start complaining about the noise at the airport, or the subwoofers at the concert as the case may be).
Logged
Mothers, don't let your babies grow up to be physics PhDs

Mark McFarlane

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 1865
  • Middle East
    • Arkose Records
Re: wind or temperature gradients
« Reply #3 on: May 05, 2014, 06:01:09 am »

What I got out of that is that wind causes the speed of sound to depend on altitude (fig 6.2 on page 67), usually causing the speed of sound to decrease with increasing altitude (which if my grasp of physics is correct, would cause sound to refract away from the ground, in the direction of lower speed of sound).
...

The causal relationship is temperature, not altitude. From a general trend perspective, temp decreases with altitude, but its the temp causing the effect, not the altitude (and not barometric pressure).  This is all 'in the absence of wind'. The temp gradient also typically changes between night and day...

Wind turbulence introduces a separate refractive effect but is dependent on the direction the sound is traveling (the temp effect is invariant to the direction of sound propagation).  If the wind (and speakers) are in your face, the sound will bend downwards. If the wind and sound propagation are in opposite directions the sound wave bends upwards (bend is in the context of a ray, perpendicular to the sound wavefront. Sound is not really a ray-like phenomena :) ). This might lead someone to claim 'the wind blew the sound away', but note that these studies are over very long distances, the total SPL effect 50m from a stage is probably not very important, and irrelevant when compared to spherical spreading (that inverse square thingy).

This is all a little outside my day job, so I apologize if I made any mistakes.  I just skimmed the article for about 10 minutes.  My day job deals with sound (and elastic) wave propagation in rocks and the interesting tidbits are how to guesstimate rock and fluid properties from how elastic and acoustic waves move through them.   The only crossover I know of is a colleague (founder of my last company) who invented Autotune, for good or bad.


There ain't much wind beneath 20,000 feet or rock.


The more interesting relationship between wind and sound for outdoor concert venues is probably in intelligibility and local (at the ear) phase/amplitude mangling of the higher frequencies (my unsupported scientific hunch), but I don't think that is what Merlijn was asking about.
Logged
Mark McFarlane
ARKOSERECORDS
Turn down what's too loud.

Merlijn van Veen

  • Moderator
  • Sr. Member
  • *****
  • Offline Offline
  • Posts: 309
    • www.merlijnvanveen.nl
Re: wind or temperature gradients
« Reply #4 on: May 05, 2014, 07:43:19 am »

but I don't think that is what Merlijn was asking about.

AFAICT the article predominantly focuses on large scale consequences (over 100 meters) like noise pollution. Although very educational I was more thinking on terms of open-air concert-size scale.

Does the wind literally blow the sound waves away OR does it create pockets of air with different temperatures causing different speeds of sound that refract the sound like a lens?

Most of us have heard the interference pattern of a line array when walking on axis from the back to the front or vice versa. In case of wind we, the receiver, are stationary and the interference pattern changes position. Same sensation to a certain extent. We end up hearing the sound of the people standing next to you or in front of or behind you. In the rear you might very well even briefly drop out of coverage.

This is less obvious with point sources IMHO because they lack the inherent interference pattern within their "generally" broader vertical coverage range.

Jeff Carter

  • Sr. Member
  • ****
  • Offline Offline
  • Posts: 363
  • Kitchener, ON, Canada
Re: wind or temperature gradients
« Reply #5 on: May 05, 2014, 09:57:34 am »

The causal relationship is temperature, not altitude. From a general trend perspective, temp decreases with altitude, but its the temp causing the effect, not the altitude (and not barometric pressure).  This is all 'in the absence of wind'. The temp gradient also typically changes between night and day...

Wind turbulence introduces a separate refractive effect but is dependent on the direction the sound is traveling (the temp effect is invariant to the direction of sound propagation).

OK, I think I get the gist of it now--in general sound is refracted away from the surface due to the temperature gradient, but a strong enough wind can overwhelm this effect (for sound travelling in the same direction as the wind) and refract it downward.

Quote
This is all a little outside my day job, so I apologize if I made any mistakes.  I just skimmed the article for about 10 minutes.  My day job deals with sound (and elastic) wave propagation in rocks and the interesting tidbits are how to guesstimate rock and fluid properties from how elastic and acoustic waves move through them.   The only crossover I know of is a colleague (founder of my last company) who invented Autotune, for good or bad.

Sorry to hear about your colleague... that invention is definitely "for bad" in my opinion. I skimmed the sections you highlighted for about the same amount of time.

In my (former) day job I dealt with a lot of wave propagation as well, but electromagnetic--either GHz-frequency microwaves (similar wavelength scale to sound) or optical. A lot of the intuition in terms of interference, refraction etc. is the same but I don't have quite the same grasp on how the mechanical properties of the transmission medium affect propagation of sound in the way that I would for the EM properties affecting microwave/light propagation.
Logged
Mothers, don't let your babies grow up to be physics PhDs

Adam Black

  • Newbie
  • *
  • Offline Offline
  • Posts: 23
    • Rational Acoustics
Re: wind or temperature gradients
« Reply #6 on: May 06, 2014, 09:45:42 am »

This reminds me of a stadium tuning several years ago. The system engineer was trying to measure the mains from FOH to get some delay times. But the wind was strong enough to render delay measurements and phase completely useless. Not sure if the wind was impacting wave propagation or if the hang was just swaying on the truss. But the wind was definitely the culprit. Fortunately he had a few tools at his disposal that allowed him to nullify the wind's affect and proceed with tuning the system.

In this case the timing variance was only a few milliseconds and likely would have had little impact on the audience. Perhaps shifting the image, seams, or coverage slightly for some areas.

Logged

Art Welter

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 1598
Re: wind or temperature gradients
« Reply #7 on: May 08, 2014, 02:14:04 pm »

Does the wind literally blow the sound waves away OR does it create pockets of air with different temperatures causing different speeds of sound that refract the sound like a lens?
Yes  ;).

At one particularly windy show at the local dirt ballpark, the promoter asked me if we should shut down due to the wind (singers mouths were filling with dirt), I replied if the mic stands blow over, we'll call it quits. The PA sounded like it was going through a "jet phaser" at this point. I had made lead weighted perspex fader covers for my brand new (at the time) Midas board, I was looking at them when they blew off the console, then noticed the PA suddenly sounded coherent again. When I looked up noticed the reason for this was the stage right stack had literally been blown away by the wind (though no mic stands had blown over  ::)) and the changing interference pattern the wind had been creating between left and right did not affect the PA any longer since the stage right stack was now (mostly) pointing at the sky.

In addition to the wind bringing in "new" air of differing temperatures causing different speeds of sound that refract the sound, it also mixes local air temperature stratification gradients, randomly changing direction of sound both vertically and horizontally. This mix affects the shorter high frequency wavelengths more than lower frequencies, as the long low frequencies can diffract around relatively large objects (like audience members) while high frequencies are absorbed or reflected. Since wind varies in both speed and direction, and thermals are constantly developing and "popping off" like bubbles rising over large scale concert events, focus of the PA constantly changes. A mono point source is affected the least, typical large scale PA systems are affected far more due to the multiple paths to each listeners location have far more phase permutation possibilities.

One always can count on a bad sound day when the wind is blowing towards or across the stage. If the wind happens to be coming  consistently from the direction of the stage, the sound is refracted downward, generally a good thing if the PA is adequately elevated, but still plays havoc with frequency response through the audience, as the inter cabinet angles required for even coverage at any given temperature and wind speed are all different.

EAW's ENYA beam steering system (and Martin's to a lesser extent) has the potential to mitigate some of the problem if the wind and temperature would be reasonably consistent, but would require wind speed, angle, and temperature gradient information to drive the beam steering algorithms in real time. Rapid changes in any of the driving information could result in far worse average sound for much of the audience than if it were left alone.

Don't ever bother adjusting EQ during wind gusts...

Art
Logged

Frank Koenig

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 907
Re: wind or temperature gradients
« Reply #8 on: May 10, 2014, 04:47:48 pm »

it also mixes local air temperature stratification gradients, randomly changing direction of sound both vertically and horizontally.

I believe this is the crux of the cookie. Air is heated by solar radiation impinging on the ground. On a local scale, air is heated more or less depending on the emissivity of the surface and other factors, such as evaporative cooling. A paved parking lot will heat the air close to it more than a damp lawn. Now all we need is some wind shear (abrupt changes in wind speed or direction) to stir up things and the sound goes every which way, as Art describes.

--Frank
Logged
"Nature abhors a vacuum tube." -- John Pierce, Bell Labs

Ivan Beaver

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 8908
  • Atlanta GA
Re: wind or temperature gradients
« Reply #9 on: May 11, 2014, 07:41:39 pm »

I believe this is the crux of the cookie. Air is heated by solar radiation impinging on the ground. On a local scale, air is heated more or less depending on the emissivity of the surface and other factors, such as evaporative cooling. A paved parking lot will heat the air close to it more than a damp lawn. Now all we need is some wind shear (abrupt changes in wind speed or direction) to stir up things and the sound goes every which way, as Art describes.

--Frank
And the more sources of sound (like a stack or line of loudspeakers)- the more different the gradients are going to be.  Using a few number of loudspeakers will produce a sound in which the sound will have less effect-just like Arts example above.
Logged
A complex question is easily answered by a simple-easy to understand WRONG answer!

Ivan Beaver
Danley Sound Labs

PHYSICS- NOT FADS!

ProSoundWeb Community

Re: wind or temperature gradients
« Reply #9 on: May 11, 2014, 07:41:39 pm »


Pages: [1]   Go Up
 



Page created in 0.042 seconds with 22 queries.