Does Speed Of Sound Vary With Source?

Nah, what you are probably getting confused by is the boom which is the mortar going off which fires the shell up into the air, then there's the bang of the a actual air burst which thows out the pretty stuff.

I could be talking bollocks 'cos I couldn't be bothered to read your entire post as it was too long for my furry cat brain

The speed of sound is the same regardless of intensity (all other things being equal)
Del

Nope, I know the muffled sound of the moarter launch part. kinda like a hollow thunk, yes it's semi loud but many many DB below the blast from the aireal part.

In addition I was too far away during the recording last night to even hear the launcher sounds, 2.2 miles away.

Joe

I believe that in the 'flowery' fireworks there are at least three detonations:

• The 'thunk' of the mortar launching the shell
• The large 'boom' when the shell detonates flinging the hundreds of smaller projectiles is the prescribed directions
• The detonations of these smaller projectiles that contain the various metals (salts) which produce the colored streamers.

It is this third group of detonations which should be the starting point for your count, but unfortunately these are not big noise makers, so you (erroneously) begin your count at the second detonation, which occurred a fraction of a second earlier. This would explain the discrepancy.

Are you sure the percussion types were detonated at the same elevation as the 'flowery' types? Perhaps the percussion types are deliberately exploded lower to enhance the effect of feeling the explosion.

My brother does an excellent amateur fire works show and all of the shells are hand made. One trick to make something look bigger and more powerful is to in fact have it explode at a much closer and lower point in referace to the crowd.

Not all launching morters have that dull thud though. A paper tube type on the ground has a more subdued thud while a steel pipe buried in compacted earth has a sharp pop more like the air explosions. The paper tubes expand and let a fair amount of the launching blast leak by and that reduces the intensity. While the steel tube has far less give and creates a tighter seal giving the launch a much higher velocity and punch coming out of the tube. (I make all of the his steel tube launchers out of high strength steel)

A 3 inch shell going off at 400 ft away looks and sounds about identical to a 5 inch going off at nearly double that distance.

Its how my brother gets that cherry bomb effect with less powerful explosive compounds. No one in the crowd but the makers are any wiser.

If you really want to know how much more powerful something was, use the reflected echo time delay and sound intensity to get a more realistic reference point.

A 3 inch shell has a very small echo that dies off within a short reflected distance. While the 6 inch grand finally shells we use will echo for several seconds or more!

Sound does travel faster in denser air. That air would be the lower altitude. but the difference has to be very minor. If the time difference is only about 10% sooner with the louder shell, I'm betting the longer distance to the higher shells creates the effect you noticed. Don't forget the distance the sound travels is the hypotenuse of the right triangle created from height and horizontal distance away from the explosion, not just height. As already discussed, there is almost always a considerable height difference between the really loud ones and the pretty ones. What's the chances the distance you watched from this time is not your usual distance?

Ho all, Thanks for the discussion,

But this should cure all the altitude thories,

Remember be it was at 400 feet or 1000 feet from my observing location it wouldI would assume little to no difference, remember I was 2.2 miles away.

None of the displays if any at all ever got more than 5 deg above the horizon.

So, next idea?

Joe

And, what was the relative humidity that evening?

About 50%

Today it's 35% and it was similar day and of course after sunset so I'd guess about 50%

the humidity would have to be different between firing the brighter firework and the louder firework to make an appreciable difference in the speed of sound between tests. Considering the probable duration of the entire display, the interval of time between the explosion of each respective firework, and the number of times he noticed a difference; this would be unlikely.

I've had some limited experience with fireworks; I used to make my own roman candles, star-mines etc...

I can't say this is the reason with 100% certainty; but for safety reasons - I'm inclined to think that the large 'golden rain' type fireworks (the big pretty ones ) need to be composed so that they detonate at a height greater than those used simply for their percussive effect. The extra height would increase the distance the sound would need to travel to reach your ear, and the larger eventual display of one of these fireworks would likely distort your perception of distance, relative to the percussive variety. Its pretty difficult to gauge their height at night-time when you have no other reference points.

hope i've understood your question and am not over-simplifying!

best regards,

TinTin

o all, Thanks for the discussion,

But this should cure all the altitude thories,

Remember be it was at 400 feet or 1000 feet from my observing location it wouldI would assume little to no difference, remember I was 2.2 miles away.

None of the displays if any at all ever got more than 5 deg above the horizon.

So, next idea?

Joe

ooh okay - that does change things, 10% transmission delay couldn't be accounted for by elevation. Perhaps (and i'm just speculating here...) the different types of fireworks were fired from different launching bays - if they were in line with your viewing direction, from a 5° inclination from the horizon, it may have been difficult to percieve a horizontal difference in the points of explosion. What do you think?

Much much less likely: The particulates from the respective explosions caused differing air densities in the immediate vicinities of the detonation point, altering the speed of sound... okay that was down-right farsical :P

i tried though!

Ok, rule out altitude and what is left?

Was there a focused reflection point (or Points ) somewhere near the explosion points, but far enough away to affect your reception of the different types of sounds differently? If there is a sharp delineation of humidity or temperature at the right altitude it can be possible for the 2 sound paths to have significant differences in distance traveled to your position. Something like Cb's talking "skip". Sometimes submarines survived detection because of sharp differences of temperature or salinity deflecting or attenuating sonar.

Most of the times I've ever experienced this however, I've heard a Ba-Boom because I usually heard both the straight and reflected sound nearly equally in volume.

So am I nuts or what?

A thought, at what speed does sound travel in the wind? for example if a breeze was blowing and you made a sound, would someone upwind hear it later than someone downwind? Or would this just be a great way to examine the doppler effect?

I ask as there are possibilities that the noise source could cause air to move, e.g. a large explosion. It is also possible that the sound being heard is actually the shock wave (which may travel faster than the speed of sound [M1]).

If my assumptions are correct the reason the percussion fireworks are both louder and faster is that the shock wave is breaking M1 and the noise being heard is the sound from the shock wave.

However I am but a humble Undergraduate, with little knowledge of fireworks, and minimal knowledge of the physics of sound

"Humble Undergraduate",

You pose some very interesting questions! I'm sure you'll graduate to become a great engineer I certainly have to acknowledge my limitations regarding this subject too, its not my field of speciality. Its my understanding that the speed of sound is the maximum linear velocity at which vibrations can propagate through the relevant medium, so regarding the shock wave travelling faster than M1, I have my doubts. Regarding your wind speed question - I'll have to think more extensively about that, but I suspect that the object making the noise would need to be moving with equal velocity to the air that immediately surrounds it just prior to making the noise in order for the sound to travel faster relative to a sound propagating through a stationary pocket of air. Otherwise, the doppler effect would take place and the sound would move at its normal speed (about 330 metres per second at ground level).

But these are just theories! I'd welcome the input of an Acoustic Engineer

Regards,

TinTin

What you experienced was the additional time delay from the additional safe distance away from the crowd factor. We shoot small cherry bomb type shells straight up about 200 feet and only about 200 feet out from the crowd. The big 6 inch stuff actually gets fired from about 200 yards further out and is actually pointed to reach its apex another 100 yards further back away from the crowd.

I suspect that if they were using even bigger shells they may have in fact been launched further back and pointing away more yet. Adding another 1000 feet or more at 2.5 miles will give you a perceivable time delay of over a second or more. That would account for the roughly 10% differences from one to another.

Safety always puts the bigger stuff further out and higher up.

From your perspective your depth perception at night against a dark sky with no accurate visual reference point could not tell you if they were smaller stuff shot from 1.5 miles away or gigantic stuff being shot from 4 miles away. The sound delay would be your only accurate sensory reference point.

Just a thought from a non technical (in this field)- has shock wave something to do with it?

large booms are almost sure to generate a significant shockwave, unlike the splutters.

NSS,

Speed of sound travel is effected by barometric pressure, air temperature, humidity and direction of wind with respect to the sound source and the hearer.

The difference in time between for the flash and sound could be significant if all of those effects were causing an increase or decrease.

Mach 1 is not the same speed for an aircraft at 40,000 feet as it is at 100 feet due to the difference in the density of the air.

Jon

Perhaps the BOOM... explosion to dissipate / spread the fireworks internals.. and the FLASH ... ignition of these has a chemical time lag?

Regards Woody

interesting theory! Powder compositions in fireworks are deliberately made to burn at different rates, in order to provide timing for different effects. This could cause confusion, though it sounds like NSS has paid particular attention to the timing between when the stars from the shell actually lit, and the sound coming from the initial charge. I'm quite certain that the chemical reaction which precipitates the initial explosion in both variants would have a time span figuring only in milliseconds.

Kind regards,

TinTin

Could you be hearing echo's?

What you are hearing as a Huge boom is not the charge that causes the dispersion of the fireworks stars but another concussion charge. The concussion charge has a slight delay to allow for dispersion. If there was not a delay, few if any of the pyrotechnic stars would ignite. This is because of the force at which they are ejected and there is not enough heat contact from this type of explosion to start them burning. This is also why some fireworks make very little noise while others are quite loud. It is all in how they are designed.

This makes sense to me. The dispersion charge has to be a softer charge to not fracture the pretty streaming projectiles making a chrysanthemum burst. Likely you just happened to be mislead by a different thud that timed well with the dispersion burst.

The speed of sound still remains constant although it is at different rates in different media - in air it is constant for all intents and purposes.

What may give a differing effect in this case however is the different sound producing effects of the fireworks "burn" patterns.

Other readers have pointed out the different ignition times of various 'parts' of a firework display, but what can also cause added confusion is the longer noise producing 'burn' of all of the streamers produced in the firework that you pictured. Because these streamers continue to burn (and make noise) for a second or two after detonation, and because some of the streamers are coming toward you there are also half of them MOVING AWAY FROM YOU. It is the extra time taken for the SOUND of the streamers that move away from you to get to your ears that create the bulk of this effect.

The differential that you maybe noticing maybe caused by the strength of the explosion itself. The amount of concusion is related to the strength of the explosion and the different chemicals used to make the different colors may have something to do with the difference of the recorded timing.

Just guessing here.

With the "GIANT percussion flash BOOM" one you are seeing and hearing the same event. With the "pretty display" there is an explosion which you don't see, followed by several minor flash/burns which you see but don't hear.