Jump to content
Sign in to follow this  
Fonzi

SPL Theory Q

Recommended Posts

I was just thinking of this the other day and my general knowledge of physics dosnt lead me anywhere.

Extreme DB levels seem to be in the mid 170's and slowly getting a little bit higher.

Would it be phyically or humanly possible to make a subwoofer that moves air fast enough to create a sonic boom? It just moves faster and faster until the air it is displacing breaks the sound barrier! In which case I think that would be able to be higher than 170's, though im still not sure.

What do you guys know about something like this? Im sure there is some sort of law of physics that would completely disregard this theory.

Share this post


Link to post
Share on other sites

Definetely wouldn't be a problem except:

Sonic boom requires exceeding the speed of sound: 720mph therefore

720 m/h (h/60m)(m/60sec) = .2 miles/sec

.2miles/s(5280ft/mile) (12inches/foot) = 12672 in/sec

So using a 1hz tone you would have to move the sub 12672inches and so on and so on.

Obviously you will run into an energy problem and then you have to turn the sub around and bring it back. Those are some dynamics....

Share this post


Link to post
Share on other sites

What about using a higher hz tone? what does that so to the equation?

Share this post


Link to post
Share on other sites

It still requires the same velocity to break the speed of sound, you can't really add two halfs of the velocity together and get anything useful. I picked 1Hz so you would only have to look at the motion in one direction. In other words the displacement would indeed get less (2hz => 1/2 displacement) but the velocity still has to be the same. Actually moving distance has nothing to do with it, just breaking 720mph.

Share this post


Link to post
Share on other sites

of course neglecting turbulence being that we use a sinusoid (Hz) as excitation it would actually break the speed of sound in both directions creating dual booms per cycle...again assuming that in both directions it exceeded the speed of sound.

Share this post


Link to post
Share on other sites

This stuff is beyond me, but I like it. The reason I say a higher Hz is when playing a 30hz tone vs a 60hz tone the cone moves so much faster with the 60 or it at least appears to. So I wasn't sure if a higher hz would bring up the velocity of the cone. From what your saying though the higher the hz the less displacement the driver throws?

Edited by Fonzi

Share this post


Link to post
Share on other sites
This stuff is beyond me, but I like it. The reason I say a higher Hz is when playing a 30hz tone vs a 60hz tone the cone moves so much faster with the 60 or it at least appears to. So I wasn't sure if a higher hz would bring up the velocity of the cone. From what your saying though the higher the hz the less displacement the driver throws?

Take a step back. To have a sonic boom, you need to exceed the speed of sound which is a constant-720mph. Whether you are playing a 30Hz or 60Hz tone it doesn't effect the velocity of the cone at all unless you hold the displacement constant in which then the 60Hz cone will move at twice the speed. This is all really going to come down to how much energy (power) you throw at it for a certain frequency.

Share this post


Link to post
Share on other sites

Basically impossble anyway you look at it. Considering they have used multiple woofers with upwards of 60k worth of watts in a car.

Share this post


Link to post
Share on other sites
Basically impossble anyway you look at it. Considering they have used multiple woofers with upwards of 60k worth of watts in a car.

Indeed, it would take a shatload of energy but I thought I would try and explain why.

I actually got paid to do a consulting job a while back measuring the sonic boom of shotgun shells. They were working on making range shells that were quieter to keep complaints to a minimum for people who lived near shootiing ranges. The gun going off sound doesn't travel very far, but the boom surely does. It would make one hell of a hood beater if you had subs that could create sonic booms....

Share this post


Link to post
Share on other sites

Remember this. The higher the tone at a given ammount of movement the faster the cone has to move. Also remember that when you are accelerating an object form 0-300m/s in a matter of say 5" there is a LOT of force on it.

assumeing that you are trying to get loud from sonic booms and not actually the tone it would also be usefull to make the "cone" as arodynamic as possible. and inverted dusctap that covers the entrie cone would be optimal.

remember what a sonic boom is too. its where the soundwaves get build up and built up untill the object is actually moving faster than the sound itsself and then the pressure from the built up waves is all released at once. wether a sonic boom would happen in a piston type confiuration i have no idea.

Here is an example. im not sure how to figure out cone velocity but if i am doing this correctly at 80Hz you would have to have 158.4" of total linear stroke. a 250g mms would need around 6500 N of average force w/o taking into account pressure buildup on the cone. it would also have over 2600 times the force of gravity on it.

this is assumeing i calculated cone velocity correctly

Share this post


Link to post
Share on other sites

the cone doens't have to break the speed of sound (and it never will) the air in the port could. But even that is rated unrealistic because the pressures on the cone would simply rip it apart long before the port velocity approaches supersonic.

Jim, the speed of sound isn't constant, it varies with temp.

Share this post


Link to post
Share on other sites

Then let me re-phrase me question, what happens if you break 194dB and the port wind is not in excess of 720mph? Would it simply be as if any other metering?

Share this post


Link to post
Share on other sites
the cone doens't have to break the speed of sound (and it never will) the air in the port could. But even that is rated unrealistic because the pressures on the cone would simply rip it apart long before the port velocity approaches supersonic.

Jim, the speed of sound isn't constant, it varies with temp.

True, but it just gets faster if you get hotter. Not sure why the reference speed is near 10deg though.

Nice point on the port velocity as that would obviously be way easier to do, that wouldn't have anything to do with cone velocity like the OP was asking then though.

Share this post


Link to post
Share on other sites
True, but it just gets faster if you get hotter. Not sure why the reference speed is near 10deg though.

Yeah, not sure about the basis of the +10 ref either. The fluid nature of air will keep you from ever getting port velocity in the neighborhood of Mach. As the pressure in the box starts to increase, the air in the port will simply "pack up" the port and the box will start to act like a sealed enclosure with almost no port action. The threshold for this should depend on port area with it happening more readily with smaller ports. You could aleviate some of this with aggressive flaring of the port ends, but that is self defeating because it reduces port speed. Go with at big enough port that packing isn't a issue and you wuold need a huge amount of sub displacement to move enough air to get the port velocity high again and you start running into the same problem as with the small port and it still packs. The long and the short, it isn't going to be possible.

Your best bet would be something like a high speed rotor with a wicked servo motor on it and one hell of a gearbox. Even still you're looking at something on the order of 5100RPM for a 3 ft diameter rotor if my math is right for the rotor tip to go supersonic. Then you have to account for aerodynamic effects on the rotor and make it strong enough to keep it from ripping itself apart.

You could I guess also try to build a supersonic wind tunnel, but if you've ever seen one of those, they usually take up a fairly large building, several VERY large fans and a tapering vaned duct to accelerate the air without turbulence f'ing the whole thing up.

Share this post


Link to post
Share on other sites

Impossible, period. Even if it were, it would shatter all the windows in the car instantly. A sonic boom 2 miles up is loud enough to literally shake an entire building on the ground (I hear them daily at work - right by an AF base).

A sonic boom can only be created by an object breaking the sound barrier. The piston would have to move, in ONE direction, in excess of Mach 1. Same goes for the port velocity. That is created by the backwards motion of the piston, so it is dependent on the speed of the piston. It doesn't matter how fast you can make it move back and forth, as it must exceed Mach 1 in ONE direction (i.e. Mach 2 if you count forward and backward directions) to be successful. Now, think about what it take for a jet to exceed Mach 1. Most jets capable have upwards of 20,000 pounds of thrust, or more, in order to achieve those speeds. Also note that this is at high altitude where the air is much much thinner with much less resistance. Try converting that to watts. It is not eay to do because of the way thrust is derived, but a good way to "get a good idea" is here:

Luckily, we do have access to data from a NASA report that does provide all the data we need to illustrate a sample case. The data is provided for a Boeing 747-200 cruising at Mach 0.9 at 40,000 ft (12,190 m). In this example, the aircraft's engines produce 55,145 lb (245,295 N) of thrust, only a quarter of its rated static thrust, to cruise at a velocity of 871 ft/s (265 m/s). Using the equations provided above, we calculate the power generated by the 747 to be 87,325 hp (65,100 kW).

The NASA data also includes a few other planes, so let's compare the power generated by the subsonic 747 airliner to a supersonic fighter like the F-4 Phantom II. In this example, the F-4 cruises at Mach 1.8 at 55,000 ft (16,765 m). The aircraft's two turbojet engines produce 11,560 lb (51,430 N) of thrust at its cruise speed of 1,742 ft/s (531 m/s). This combination of force and speed equates to a power of 36,620 hp (27,310 kW).

Source: http://www.aerospaceweb.org/question/propulsion/q0195.shtml

Now, look at those numbers. 27,210KW... Not 27,310 watts... but KILOWATTS... That's 27,210,000 watts. You have a sub that can handle 27 million watts? Hell.. even an amp that can output that kind of power?? It is physically impossible to achieve. You would need ONE sub that could break the sound barrier. A combination of 27 thousands subs on 1kw each would still not create a sonic boom as none of them would be breaking the sound barrier. would they be loud? Hell yes. Probably even louder than a sonic boom. But a sonic boom would not be created.

Edit: Also, without some space age subwoofer material, "if" (huge if because it can't be done) it were successful, the pressure of a sonic boom would tear the sub to shreds.

Edited by Mike M

Share this post


Link to post
Share on other sites

Actually the port velocity is dependant on the amount of air displaced by the sub in relation to the cross sectional area of the port, not necessarily the velocity of the cone.

I also doubt you hear sonic booms regularly. The Air Force is very strict about when and where a jet can go supersonic. WR is not one of those places. The NTTR here by me is one of those places and even then is isn't very common. It happens by "accident" sometimes in unapproved locations but those are few and far between. I remember 3 growing up in P'cola, FL with a 3 Navy bases in the area and they are much less stringent on supersonic flight.

Also at extreme high altitude the sonic boom never gets to the ground. Additionally depending on the geometry relating your listening position to the aircraft breaking the sound barrier you may not hear it.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this  

×