Fire pit causing high / mid loss

[codewritinfool]

Tonight I was outside listening to my La Scala splits and noticed something I've never experienced before. 

 

I normally have them about 30' apart, and aimed so they cross in front of some chairs, with a fire pit behind me.  I'm getting ready to level out the area where the pit is, put down gravel and pavers, then put the pit back, but until I do that, I moved the pit in front of the chairs.

 

I had a pretty good fire going and some good music too, and from where I was sitting (the left-most chair), I could see the left speaker clearly, but the pit was in the way of the right speaker.

 

I thought I was losing either the tweeter or the mid occasionally, so when I'd experience that I'd get up and walk over to the speaker and check out the tweeter & midrange.  Sounded fine.  Then I'd go sit back down and there it was again, occasional loss of highs / mids.

 

It took me a while to figure out that it was the fire.  Similar to fading caused by wind.  Occasionally the fire drafting (I guess) would cause the highs and mids to be attenuated briefly.

 

Anyone else ever observed this effect?  New one on me, but hey, I learn something new every single day.

 

code

[wuzzzer]

I have.  It's a really weird effect. 

[jimjimbo]

Sure, I have as well.

[JBCODD]

The "Burninator" strikes again.....it's called the Trogdor effect.

 

 

[glens]

Why not?  The rising air ought to divert sound waves upward quite nicely, coupled with the heat...

[drboar]

At resonable temperatures the speed of sound is lineary proportional to temperature, in Kelvin that is.

If we imagine the air above the fire as a hot cylinder, hotter in the center.  If a wavefront hits this cylinder the wave in the middle will accelerate more than the wave hitting the cylinder off center as the path lenght is longer and the air hotter through the center.  So the   wave will become more circular in the horizonal plane  and less sound energy will reach the listener in  front of the speaker. So the heat cyliner will work as a lens. The lens action works when the pathlenght through the hot cylinder is a sizable fraction of the sound wavelenths.

 

If the rising air is to deflect sound the soundwaves has to be lifted a significiant amount during the less of 1/100 of a second it passes through the heat cylinder.

 

As there is a vertical heat gradient in the air column that gradient will deflect the soundwaves upwards in a similar to the previous description.

[OO1]

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[Deang]

@drboar pretty cool stuff that is. 

[Bosco-d-gama]

Mmmkay....... as I understand this what is occurring is that the heat waves are altering the phase frequency of the sound waves as they pass. Essentially the heat wave can cause sound waves to cancel out. I am guessing that this would occur over all frequencies in relation to the not only the amount of heat but also other ambient factors such as barometric pressure and relative humidity. Exactly how much interference - I dunno. But from what I read here in this situation the gas driven fire pit is producing a consistent enough flame to generate consistent interference of only certain sound frequencies so there’s a perceived loss in just those frequencies. So on a windy day the interference would tend to warble as the wind alters heat dissipation. A combustible flame should interfere less consistently as it offers a less consistent fire.  Vary the intensity of the flame and see if you vary it’s impact on the sound. If you possess sound meters, try to measure this interference........ I mean, if you can ‘hear’ a difference it should also be measurable. This is like ‘fun’ with physics. 🤪🤓🤔

[CECAA850]

11 hours ago, codewritinfool said:

Anyone else ever observed this effect? 

Yep.

[Woofers and Tweeters]

One kinetic energy affecting the other. 

[MookieStl]

Freaked me out a little the first time. I felt, like you did, that something broke.

Once the fire burns down and a few more beers later, you hardly notice.

 

[codewritinfool]

the gas driven fire pit...

Wood.

[polizzio]

7 minutes ago, MookieStl said:

 

 

 

Now that is an evening pit fire!

[Bosco-d-gama]

24 minutes ago, codewritinfool said:

Wood.

I’d have to think that a ‘wood’ fire would be variable enough to not result in such a consistent interference. It’d be real interesting to test it all out empirically.

[TasDom]

If you burn MDF, will it affect the sound differently than burning plywood / hardwood?

[Edgar]

34 minutes ago, TasDom said:

If you burn MDF, will it affect the sound differently than burning plywood / hardwood?

 

Only if you get it wet.

[polizzio]

1 hour ago, TasDom said:

If you burn MDF, will it affect the sound differently than burning plywood / hardwood?

 

I love the smell of burning MDF in the morning. Smells like.................. victory!

 

 

 

[OO1]

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[MookieStl]

26 minutes ago, Randyh said:

you guys have much better weather than we do , here in the North , we could not play speakers outside saving for a few months of the year  , we are not allowed to do any fires , outside ,it is prohibited by Law  the Fire Department' s fine is over 1000$ , and a second time offense , is double the amount , plus there are the neighbors , a complaint for noise is also very expensive ,    so what that means , is unless you live out in the woods with no close by neighbors who could call 911 , you are ok -------basically, you have to live on a farm to play speakers outdoors and have a fire outdoors -

This is way out in the boondocks. Nobody around for miles. You have to turn up the music to drown out the banjos.