# Speed of Sound and its relation to weight?

• CuriousMonkey
In summary, the sound waves produced by a shoebox and an F-35 breaking the sound barrier would not be equal, as the shock wave from the shoebox would be much weaker due to its smaller size and shape. Similarly, the sound produced by supersonic bullets can also vary depending on their size, shape, and speed. While the weight of an object may not be a key parameter in determining its drag coefficient, it is easier to predict measurable quantities such as drag using speed, cross-sectional area, and drag coefficient rather than size and shape alone. Some theorists have claimed to have developed programs to accurately predict supersonic drag coefficients, but their accuracy may vary depending on the speed of the object.
CuriousMonkey
Good day (or night). I am new here, so I hope my question doesn't bother many.

If (forgetting other laws of nature) a shoebox where to hit the sound barrier and an F-35 were to do the same. Would the sound waves be equal. Would it sound the same to a ground observer?

I always assume that mass matters (no pun intended).

Many Thanks- CuriousMonkey

SIZE matters because a really large moving object creates a more forceful shock wave than a really small one.

If a bullet were fired out of a high-up balloon, would you expect it to be perceived on the ground the same as the shockwave from a huge supersonic aircraft?

CuriousMonkey
From here: https://en.wikipedia.org/wiki/Sonic_boom#Causes

The power, or volume, of the shock wave is dependent on the quantity of air that is being accelerated, and thus the size and shape of the aircraft. As the aircraft increases speed the shock cone gets tighter around the craft and becomes weaker to the point that at very high speeds and altitudes no boom is heard. The "length" of the boom from front to back is dependent on the length of the aircraft to a power of 3/2. Longer aircraft therefore "spread out" their booms more than smaller ones, which leads to a less powerful boom.

Since a shoebox would accelerate a much smaller amount of air than most aircraft, the sonic boom would from the shoebox would be much weaker than the one from the aircraft.

CuriousMonkey
Thank you for the concise answers.

There are 600 and 1000 yard shooting matches where target scorers sit behind a bullet proof berm and pull targets down to indicate the location of the hit and the score with markers visible way back at the firing line.

In this location, the sound of the guns going off is not audible, but the sounds of the supersonic bullets passing overhead is audible, because each one has a Mach cone and the shock is clear and evident.

The sound of a 30 caliber bullet (7.62mm diameter) is much louder than the sound of a 22 caliber bullet (5.56 mm), and the sound of bullets passing over at Mach 2 (say at 600 yards) is much louder than the sound of bullets passing over at Mach 1.2 (at 1000 yards.)

These Mach cones made by bullets can be used by acoustic systems not only to determine where a bullet hits on a target, but also to trace the entire bullet trajectory back to the shooter. See: http://www.raytheon.com/capabilities/products/boomerang/

The weight is not the key parameter. The key parameters are cross sectional area and coefficient of drag.

Dr. Courtney said:
The weight is not the key parameter. The key parameters are cross sectional area and coefficient of drag.

I'd argue that isn't entirely correct. You could almost certainly correlate the volume of the sound with the drag coefficient, but this is because the drag on a supersonic object is dominated by wave drag. Really, then, the drag coefficient depends on the shock strength and the amount of air being displaced just like the volume does. I'd contend that the drag coefficient is a derived quantity here and that the real key parameters are going to be the size, shape, and speed (but still not weight).

I'd argue that isn't entirely correct. You could almost certainly correlate the volume of the sound with the drag coefficient, but this is because the drag on a supersonic object is dominated by wave drag. Really, then, the drag coefficient depends on the shock strength and the amount of air being displaced just like the volume does. I'd contend that the drag coefficient is a derived quantity here and that the real key parameters are going to be the size, shape, and speed (but still not weight).

Sure, but if you actually have to predict something measurable, it is much easier to start with speed, cross sectional area, and drag coefficient than size, shape, and speed.

If you have an accurate drag coefficient, you are well ahead of the game compared with someone starting with size and shape. Theoriests _think_ they can compute accurate drag coefficients from size and shape, but those of us who can measure drag coefficients to 1% embarrass them again and again.

Dr. Courtney said:
Sure, but if you actually have to predict something measurable, it is much easier to start with speed, cross sectional area, and drag coefficient than size, shape, and speed.

If you have an accurate drag coefficient, you are well ahead of the game compared with someone starting with size and shape. Theoriests _think_ they can compute accurate drag coefficients from size and shape, but those of us who can measure drag coefficients to 1% embarrass them again and again.

No theorist worth his salt will make that claim. If they do then they don't properly understand what goes into drag. Supersonic drag coefficients have always seemed even sketchier to me since they aren't constant with speed.

So I guess in my mind the most "predictable" quantities are going to be cross-section (or cross-sectional profile a la the Whitcomb area rule), the shape of the front (oblique vs normal vs bow shock), and speed. I suppose if you have a measured drag coefficient that works too.

No theorist worth his salt will make that claim. If they do then they don't properly understand what goes into drag. Supersonic drag coefficients have always seemed even sketchier to me since they aren't constant with speed.

The Army's big honcho in external ballistics for decades, Rob McCoy, did, in fact, claim to have written a computer program called "MCDRAG" capable of using shape to predict drag coefficients to "within 3% error at supersonic speeds, 11% error at transonic speeds, and 6% error at subsonic speeds." Since he wrote many influential papers on external ballistics as well as the book, Modern Exterior Ballistics, his work was very well received, and his program for computing supersonic drag coefficients was widely used for many years.

See:

Dr. Courtney said:
The Army's big honcho in external ballistics for decades, Rob McCoy, did, in fact, claim to have written a computer program called "MCDRAG" capable of using shape to predict drag coefficients to "within 3% error at supersonic speeds, 11% error at transonic speeds, and 6% error at subsonic speeds." Since he wrote many influential papers on external ballistics as well as the book, Modern Exterior Ballistics, his work was very well received, and his program for computing supersonic drag coefficients was widely used for many years.

See:

MCDRAG sounds like something you buy at McDonald's. In fairness, drag would be easier to predict on smaller objects like bullets or artillery shells where viscous drag would be substantially lower.

MCDRAG sounds like something you buy at McDonald's. In fairness, drag would be easier to predict on smaller objects like bullets or artillery shells where viscous drag would be substantially lower.

Many of the drag coefficients for computed by MCDRAG end up being 10-15% too low compared to measured values.

Free flight measurement of drag coefficients has gotten easy to do for projectiles in flight.

## 1. What is the speed of sound?

The speed of sound is the rate at which sound waves travel through a medium, such as air or water. In dry air at sea level, the speed of sound is approximately 343 meters per second or 767 miles per hour.

## 2. How does weight affect the speed of sound?

The weight of an object does not directly affect the speed of sound. However, the density and compressibility of the medium through which sound travels can be influenced by weight, which can indirectly impact the speed of sound.

## 3. Is the speed of sound different for different materials?

Yes, the speed of sound can vary depending on the medium through which it travels. For example, sound travels faster through solids than through liquids, and faster through liquids than through gases.

## 4. Does altitude affect the speed of sound?

Yes, altitude can affect the speed of sound. As altitude increases, the air becomes less dense and less compressible, which can cause the speed of sound to decrease slightly. However, this effect is typically only noticeable at very high altitudes.

## 5. How is the speed of sound measured?

The speed of sound can be measured using various techniques, such as timing the delay between a sound being produced and its echo being heard, or using specialized instruments such as a sonic anemometer. It can also be calculated using the properties of the medium through which sound is traveling.

• Mechanics
Replies
31
Views
1K
• Mechanics
Replies
1
Views
7K
• Mechanics
Replies
5
Views
1K
• Mechanics
Replies
23
Views
2K
• Mechanics
Replies
27
Views
405
• Mechanics
Replies
8
Views
1K
• Mechanics
Replies
6
Views
4K
• Mechanics
Replies
2
Views
1K
• Mechanics
Replies
6
Views
2K
• Mechanics
Replies
6
Views
1K