I too would be intersted in some SPL readings with and w/o the boom tubes to quantify Dr. Gas's proposed "slight" decrease in db. compared to an open pipe exhaust end tip.

 

SPL readings would be interesting, but I would not anticipate any big difference between the typical boom tube and a straight pipe. Frankly speaking, if I saw 1dB, I would be surprised and if I did not measure it myself, I would ask to see a lot of data to support anything more than a very small difference. Another thing that would really play into this is doing it on a car- if you used my Camaro as an example, I would not accept the measurements as the comparison would not be realistic. If you take a measurement at a specific RPM then drop the boom tube off and compare that to another measurement with the mic at the same distance from the car, the measurement distance has changed. To really do this, you would have to add a section of straight pipe to replace the boom tube and make your measurements like that. Another factor is the boundary loading of the car body and the ground. If one of them changes, the results have to be calibrated; obviously, the boom tube and a piece of straight pipe have different physical dimensions and are going to represent different measurement characteristics beyond what is coming out of the pipe. In order to get past this, I would measure them like we would typically measure a woofer which is near field. In this case, I would guess the near field to be within .5” of the end of the exhaust exit, on center as best you can. Going to need a high SPL measurement microphone for this and that is going to be expensive. Catch 22 here. I am not going to be willing to submit a $4.5K microphone to the exhaust from a car at 12mm just to see a 1dB difference. You could do all this with cheap gear and not worry about possibly ruining it, but I can also guess it probably to 2- 3dB by ear. That is not all that tough actually, anyone with good sense of hearing and a little patience can do the same.

I would expect spectrum analysis to show some big differences; harmonics are going to be a lot different as well; I expect due largely to the shape of the tube and the sound coming off the walls of the boom tubes. They simply are going to flex and "sing" a lot more than a cylindrical pipe. Doing some simple math on the distances and boundary lift, you could get reasonably good data here as well, but it would need to be taken with a grain of salt so to speak…

Where I do believe that the big difference in the sound is would be the tuning of the system. The muffler is going to acoustically load and tune the system, then the straight pipe or boom tube will contribute to that. I bet that using the boom tube, you will see an upward shift of resonance as well as a change in harmonics (acoustic- not mechanical here and are they even order or odd order harmonics?? ). Either way, they have to change because some of the physical characteristics of the system change.
Even if you did the math and the fabrication perfectly to make the exit of the tube exactly equal in cross sectional area, the gas velocity must change as the shape changes from a circle to a very wide oval (or something along those lines). For instance, on our Camaro, the tube goes from an oval about 2.25 X 3.4 to an oval of about 9.5” X .75” and looking for a roughly 20% increase in exit area (I am calling that from memory and it has been 4 years since I drew the things, so please do not bust my chops on the specifics of the math). Anyhow, looking at this with an imaginary centerline, you can see that there will be a difference in the distance from the edge of the oval at the entry to the oval at the edge of the exit. Not a lot, but certainly more than can be ignored from a tuning and acoustics point of view.

My apologies- this has gotten way off topic compared to your original question. I am a long time audio engineer and sort of got carried away with this. My bad.

 

"They simply are going to flex and "sing" a lot more than a cylindrical pipe. Doing some simple math on the distances and boundary lift, you could get reasonably good data here as well, but it would need to be taken with a grain of salt so to speak…"

If you could do the math (I don't even know what boundary lift is), then I would love to hear your rough prediction on the change in pitch and tone character.

The more we can learn about boom tubes and exhaust tech here the better, I don't really care what direction this conversation goes as long as we are all learning something.

It is a sad reality that sound is more important to some people then performance. I know some guys with automatic 3800 lb cars tightening their monster cam's LSA etc. etc. And many guys will buy boom tubes just for the sound.

Still the sound difference is a part of the entire asthetic of cars, and I'm not going to argue that.

 

I am unfortunately 12,000 miles from the car and the lab is with me.

Offhand, I would guess that with my application the boom tubes give a 1octave jump in pitch, maybe more. Some of that has to come from the sheet metal rattling, but the rest must come from the tuning changes.

Boundary effect- Simple to illustrate, more complicated to explain. Take a car subwoofer in a sealed woofer box, put it on the 50 yard line of a football stadium and stand 20 feet from it playing a sound at a fixed level. It will sound so-so loud. Now have your buddy stand a piece of plywood up beside it on either side making a boundary even with the front of the cabinet. You will notice a difference in the SPL, even though the bass is for the most part non-directional. Now take that same system, hang it 30 yards up and have someone lift you in a cherry-picker to the same distance that you were from it before. You will notice that despite the same signal and input level, the level you hear is a bit lower. Bass may be omni directional, but it does bounce off of barriers and that largely creates the boundary effect.
I have oversimplified that a bit, but the illustration that I have given is very much a real world thing you can recreate on a smaller scale. Where this is really going to impact things more is the midbass to midrange area. For instance, take the center channel speaker in your home theater system down off the TV and see how it sounds a bit different? A little hollow perhaps? Typically we will voice the speaker to work well on a barrier like a TV screen and yes, you can hear the difference.
The car exhaust is going to have a couple of significant barriers-
1- The ground- this is the most prominent
2- The car body (side or rear)
3- The underside of the car - You will likely have some standing waves under the car that are a byproduct of the exhaust note. Maybe not at a very high amplitude with all the pipes coming out the back or side of the car.

I really wish I had a motor, a set of tubes and some time to play with this. My weekend is almost over though... bench racing stop – resume work.

 

Roughly speaking then: would the side of a car actually amplify the sound of a side exhaust more than the rear (kind of how a tv affects a center channel speaker)?

I've heard that if you leave an exhaust exit under a car, rather than behind or beside, that the sound level increases dramatically inside the passenger compartment. Is this true? What about away from the car? Can any affective SPL reduction be made by aiming the tip up or down?

Pretty cool having a sound engineer in hear, I've never really thought about this stuff before?

And while we have your ear, does it matter if a resonator (smaller muffler) is placed before or after a long glass pack? Any love/hate advice to dispence towards that particular method of sound reduction. I am heavily leaning towards as long of a perforated core glass pack as I can fit.

 

Gotta be quick here-

1- Amplify? No. Reinforce? Yes. Mainly in the lower frequency band as the exhaust is somewhat of a high frequency filter in itself.
2- Exhaust under the car- will it be louder in the car? Very likely because you will have a lot of reverberation between the ground and the car body. The wave front hitting the car body will excite the panels and there you have one heck of a big ole drum head called your floor pan.
3- Resonator in front of or behind a glass pack. Not sure, but suspect it may be very application dependant as there are so many variables. Best to experiment with that one. If I had to guess, the resonator would have a bigger impact downstream of the glass pack as the glass pack will change the impedance of the system more than an open pipe on out the back (not electrical impedance).

I could write a book on all the stuff I do not know about this.

 

You are doing pretty well with the stuff you DO know.

Good post.

 

here here to that

I'll start with the resnoator down stream then, and then move it around if I'm not happy.

Also can you explain the difference between amplification and reinforcement of a sound?

 

I am sort of using those terms loosely- in this case:

Amplify = small signal level in, bigger signal level out. As in an electronic amplifier where you plug in a CD player (tiny signal level) and get audible, loud signal out.

Reinforce = less loss of a particular signal or less reduction in the perceived output. This is strictly acoustic, no electronics involved

There are cases where you can have local reinforcement or loss of a particular wave due to the crests and troughs aligning or coming in out of phase. Anyone with a home subwoofer can put on some bass music and walk around their living room to prove this- some areas more bass, some areas less bass.

About your using the resonator- If I could, I would call the company making the resonator and ask one of the engineers or techs there what their experience is. I bet that they know right off the bat what works and what does not. Ay advice could be considered a SWAG in many circles.

If I can, I will take a couple of pictures of a speaker test on an IEC baffle and show you the response measurement, then the same speaker at the same distance without the IEC baffle. You can see significant differences, all due to the loading caused by the baffle. The comparison here? Speaker = exhaust outlet – baffle = car body or the ground. Not a direct correlation, but I am sure that you will be able to see what I am talking about. Not going to happen any time soon, but I will assign this to one of the techs and keep an eye out for a good opportunity.

 

Phaseshift,

Reading your posts reminds me of an article I read about BMW (I believe)
using acoustics to cancel exhaust noise by inverting the signal by 180 degrees.

 

I am not familiar with the BMW project, but I know a few guys who worked on the active muffler system for the Corvette and Viper and say that it worked very well in the lab and on the street. If I recall correctly, they were all killed off due to long term life issues where there were adhesive bond failures etc... Doing it with pure acoustics is certainly an engineering challenge to say the least, but if it could be done, I am betting it would work very well. The biggest thing I see as a challenge there is that the system would probably work super, super good at once specific band of frequencies and inversely bad at some others. Getting that to all balance out is where the real thinking comes into play.
If ever a client asked us to take on an active muffler type project, I am sure that we would, but I am not 100% confident of success in a short timeframe with today's adhesives and the almighty dollar constraints- the client would of course want a very high temp, extreme life speaker driver for $5 or less... Cones, voice coils etc.. all easy to take the heat with the right materials. Adhesives that will take the heat? Yes, but not with good results on the high temp cone materials or at so high of a cost that they are well out of the reasonable budget.

 

It would certianly be difficult to synchronize the frequencies of the cancelling
acoustics with the exhaust. I wonder what sort of system is being used to
acquire and duplicate the frequency?

In any case, I'm probably steering this well off topic, so back to boom tubes.

 

GIGA -
Here is a pic of the original exhaust system from the collectors back. For what it is worth, you can see how it was done.

Adrenaline -
Testing the end acoustic content of an exhaust in a lab environment would not be that difficult. What is going on inside the pipe? More difficult, but there would be ways to do this and since it is a very controlled environment inside the pipe, you have physics somewhat on your side when doing the theory work. Beyond that, it is likely that there would be a very long discovery period in working on such a project.

Recreating an exhaust sound is not going to be very tough at all. Simulating the volume of gas and the sound of the exhaust with the same audio transducer- that would be some trick. Using conventional drivers, you would need a monster woofer to displace even near the same amount of exhaust gas in a single cylinder and another array of smaller drivers to give you the higher frequency content. Since all that energy is coming from multiple sources, you will have all sorts of phase relationship issues which will dig into your SPL. All bad things and that is the good news. The speaker is going to move in a + and - relative motion whereas the exhaust is always heading out the tail pipe. Reverberations in the pipe? Yes, the speaker set could probably simulate them, but the overall flow of the exhaust would be well out of the range of capability of a typical speaker.
Perhaps you could make a device like a "pistonphone" microphone calibrator that would allow you to simulate the pressure, flow and somewhat the sounds, but it would have to be driven with some high energy source, plus perhaps compressed air to give you the flow simulation. For all the trouble it is worth, I would probably want to just use a real motor.

 

At first skim through, I thought you were saying it would be an easy task.

I couldn't agree with you more on the difficulty of getting perfect phasing to
cancel out the exhaust noise. Developing a dynamic system to analyze,
compute, then produce an appropriate signal in real time would be a huge
undertaking.

Sometimes I wonder if the cost of R&D would be worth the result in a production
vehicle. With the electric hybrid craze becoming more of a reality, this sort
of system is going to die off gracefully.

I don't think I could live without a roaring V8 under the hood of my Camaro.
It starting to make me cry just thinking about it.

 

Adrenaline -

My post was ambiguous, sorry.

What I mean to say is that under ideal conditions, given an unlimited budget, a team of many propeller head level PHD guys each with a lot of common sense and some wrench-spinning capability, a few years and the most 'bitchinest lab you ever heard of with a ton of unobtanium laying around, yes, it would be easy if you were lucky.

Seriously, it would be very difficult under idea circumstances and likely have limits that had to be refined through consumer feedback, field failures etc… Your point about the feasibility is in my opinion very appropriate. You would have to know all the budgets and figures, but I would guess that it would be borderline at best.

So, how do boom tubes work anyhow? I think I will stick with trying to answer that with some data in the next few months and get back to you all.

Phase

 

Thanx Guys,

I'm still not satisfied with the "balances pulses" theory. How do you balance something at the END of a tube?

Also the question as to how much HP they are actually worth, if any over an open pipe is still unproven until I see some dyno charts, as far as I'm concerned.

And if there actually is any HP to be gained, whether or not it would still be there if cats were included in the equation still seems debateable.

We have gotten some answers though Check the edited first post.

 

I tried out an actual boom tube from a late 90's cup car...way too loud for me. But it did look and sound good. I think just as much power can be gained with out one, I tend to believe they are for high velocity gas flow (~8-9k rpm), and that's where they seem to make a difference. For a street car a nice open exhaust (mandrel pipe & free flow mufflers, minimal bends), I don't think you'd need much more.



Idle Video


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Can a boom tube be used on a honda? Since those guys need everything they can when they build all motor 240whp-300whp motors would this help them, im sure they would make a improvement.


But im geussing you need to use a V motor for boom tubes to work?

 

The 90 degree crank of the v8s has two cylinders per bank that fire very close to each other in the firing order (non existant on flat plane crank v8s and I4s). That is the reason stated as to why x-pipes work, on our v8s. Supposedly the boom tube works on the same principle, so I would guess that it wouldn't be as affective on a flat plane crank v8 or I4.

 

That makes sense, I was just wondering and wanted find out. I thought the same but fig I would ask and see if I was right thanks.