Estimating Exhaust Sound Levels

[vex]

Let me preface this post: I do not know the industry standard for determining exhaust sound levels. This is just an exploration.

I'm looking at redoing my exhaust and would like to have a rough ball park for estimating different configurations. My thought would be to use structural engineering motifs to estimate the sound possibility. In other words base it off an electrical circut diagram. The Voltage in is the unbaffled exhaust level leaving the exhaust ports. Resistors are the mufflers, and Voltage out would be the sound level heard. Doing a direct comparison between voltage in and voltage out will yeild a % decrease, and since we're eliminating piping reverberations and deconstructive interference from there and the different bends we get an upper limit to how much the mufflers will at least muffle.

My question is this: On average how much of a reduction occurs on sound. For instance; given a set db limit and placing a muffler at the source, how much of a db drop will be seen? Is it dependant on frequency or is the major player the db level itself?

If one has the means and wherewithall to do an experiment it would be greatly appreaciated. My design experiment is this:

Take a 100db source. Take different mufflers and measure the sound of the 100db source. See the result.

That result then could be used to do a percentage, so if there was a reduction of 20db it would be 80%. This could then be put into the resistor form so that if there was a 10v application, the resistor would reduce it to 8v, and so on down the line.

Now I know that sound is dependant on a large number of variables; exhaust gas velocity, temperature, pressure, and even materials. But what I am attempting to do is to figure out a simple means for 'eyeballing' the sound level of different exhaust systems.

Thoughts?

[vex]

It's not possible/thread.

[NoDOHC]

The attenuation of the sound amplitude occurs naturally in free air.

The sound energy measurement must take place at a specific distance from the source.

A good multi-chambered (perforated pipe-type) muffler is about 30 dB. A high-performance (straight-through) muffler runs between 7 and 15 dB. A straight exhaust is about 5 dB. A presilencer is frequency-dependent attenuation (typically between 5 dB and 20 dB depending on engine speed) they are typically high-pass filters. Actually a straight exhaust is also frequency dependent (low pass filter).

I would estimate a mufflerless stock-NA-ported rotary at 100 dB at WOT, while a ported or turbo rotary (without the turbo) would run about 110 dB at WOT. You will need about 30 dB to get it reasonable, 40 dB to get it to be acceptable and 50 dB to make it quiet.

Bear in mind that the muffler numbers are based on experience at work and the sound estimates are not measured, but guessed based on my experience.

[vex]

The attenuation of the sound amplitude occurs naturally in free air.

The sound energy measurement must take place at a specific distance from the source.

A good multi-chambered (perforated pipe-type) muffler is about 30 dB. A high-performance (straight-through) muffler runs between 7 and 15 dB. A straight exhaust is about 5 dB. A presilencer is frequency-dependent attenuation (typically between 5 dB and 20 dB depending on engine speed) they are typically high-pass filters. Actually a straight exhaust is also frequency dependent (low pass filter).

I would estimate a mufflerless stock-NA-ported rotary at 100 dB at WOT, while a ported or turbo rotary (without the turbo) would run about 110 dB at WOT. You will need about 30 dB to get it reasonable, 40 dB to get it to be acceptable and 50 dB to make it quiet.

Bear in mind that the muffler numbers are based on experience at work and the sound estimates are not measured, but guessed based on my experience.

Thanks for the info! However what I was thinking was using electrical theory to do a simple analysis. For instance how does a parallel exhaust setup compare to a series setup? Do using two mufflers on an exhaust system analogous to a parallel electrical circuit?

From my simple research it is not.

I'm still open for suggestions, and barring anything extreme (for instance using acoustic modeling to test for sound in a theoretical system), I may just try to go as large as possible on specific locations.

[NoDOHC]

Ok, you asked for it: Dynamic systems theory.

For air flow, the velocity is the flow variable (it is actually the flow rate, but for a constant diameter, we will call it velocity) the pressure is the effort variable.

Thus we will call the effort variable (pressure) voltage and the flow variable (velocity) current to make your electrical simulation work.

A muffler is not a strictly a dissipative element (which requires a pressure drop to cause a flow) It is much more complicated than that. The exhaust gas has an average flow (call it a DC component) and a time varying flow (call it an AC component). The actual waveform looks rather like a triangle wave. To the Average flow, the muffler can only cause back pressure, average flow is not the enemy, it is the friend, you do not want to touch the average flow at all. You can't hear the average flow anyway, it is the time varying fluctuations that you are attempting to attenuate.

The mufflers actually attenuate an alternating waveform (sum of several different frequency sinusoids). Most mufflers are tuned to certain frequencies and use destructive interference to cancel AC components of the exhaust waveform. The rotary has a harsh waveform due to the high blowdown and quick opening of the port. This gives a very quickly-rising waveform which must include many high frequency components. These components resonate near the natural frequency of the exhaust tubing (tap the exhaust pipe and listen to the note). The resonating exhaust will transmit the waveform to teh surrounding air under the car, notwithstanding the muffler. A piston engine is not an issue with this because they have slow-opening valves and therefore a much more sinusoidal exhaust pulse (lower components of high frequency).

I will not include any descriptions of this, take it at face value, or research Fourier transforms and series.

The earlier in the process the high frequency waveform is attenuated, the less air volume is excited by the exhaust pipe. this is why an early presilencer really helps quiet the rotary down.

Now to discuss as an electric circuit. The restriction in the exhaust (what causes the backpressure) is the dissipative element (the resistor), the attenuation of higher frequency is accomplished by accumulation chambers (the capacitors of the exhaust) and lower frequencies are attenuated by inertial chambers (the inductors of the exhaust)
Inertial chambers play the largest role in secondary scavenging of the engine.

A presilencer is a large chamber (capacitor), filled with steel wool (resistor). The resistor does not lie directly in line with the flow, but rather dissipates and sound that resonates outside the intended flow of the exhaust. This should make them a low-pass filter, except that the entire effect relies on the transfer of air through the sides of the pipe into the surrounding chamber. This is less likely to occur as the inertial forces of the air grow large relative to the viscous forces (which are the means by which the gas enters the holes). This ratio is called the Reynolds number (not that anyone cares). The Reynolds number increases with velocity, which means that at higher engine speeds, presilencers have little effect (due to the high average flow rate of the air). The good news is that a presilencer makes a small amount of back pressure at low revs, but very little at high revs.

From this description, I would conclude that two presilencers in parallel is better than two in series.

Now lets look at mufflers:
There are three types of muffler, the straight through (basically the same as a presilencer), the 'fart can' (multi chamber design, but straight-through construction), and the offset chambered muffler. (I am ignoring the old-school baffle/perforated pipe muffler, which simply restricts the flow until the exhaust is quiet at the great expense of backpressure). Straight through mufflers have been covered (presilencer).

The chambered muffler relies largely on acoustical wave-cancelling and is therefore very frequency specific. This is offset by selecting several or many different chamber sizes to maximize the attenuated frequencies. The wave-cancelling looks like an inductor in series and a capacitor to ground on the waveform. It resists changes in flow rate to drive a high pressure pulse which will then propagate back the pipe at a destructive phase angle. to the original sound. With several chambers, the sound can be attenuated very well. Please note that a properly-designed chambered muffler will make little to no backpressure and on a long exhaust system, they may actually decrease backpressure over a mufflerless system. Chambered mufflers rely on wave inertial energy to cancel each subsequent wave, this means that low flow rates (exhaust velocities) will hurt their attenuation performance. For this reason, it is much better to run two chambered mufflers in series than in parallel.

The 'fart can' is a special case of the chambered muffler. There is only a a single chamber and it is very small, meaning that it is tuned for a relatively high pulse frequency (engine speed). It also is incapable of attenuating the higher frequency (harsh) sounds at the higher speeds where the fundamental frequency is well attenuated. This accounts for the tinny, rattling noise that the 'fart can' makes at higher revs and the bigger-sounding-than-it-really-is throaty idle.

Basically for quiet exhaust, your single chambered muffler into dual straight through mufflers is probably your best bet to not completely sacrifice power on the altar of quiet.

I hope this helped.

[vex]

Good read. I have some questions for you, but will get to those in another post.

[Cp1]

umm i just bought a scosche DB meter from radioshack (about $50.00) and so far its been dead on accurate... why guestimate when you can know for so little or i dont fully understand the point of this thread.

[vex]

umm i just bought a scosche DB meter from radioshack (about $50.00) and so far its been dead on accurate... why guestimate when you can know for so little or i dont fully understand the point of this thread.

You should see the other similarly named thread. Just out of curiosity how do you know it's accurate? Do you know the %error of the device or does it come with a certificate of accuracy?

This thread is about estimating sound levels before purchasing a component for use. IE: Depending on what muffler or combination of mufflers you're looking at you can estimate a reduction in the DB level to be 10, 20, 30, or 50% given the organization and orientation of the mufflers.

So say for example a simple estimation tool was developed for perforated tube type mufflers based on offset as well as pipe diameter and filling material we could say the following:

Straight through 3in Diameter PT muffler with Stainless Steel packing will reduce the intake DB levels by 12%
A single 3in D PT inlet with a dual 2.5in D PT outlet with Stainless Steel packing will reduce the intake DB levels by 15% per outlet
A single 2.5in D PT muffler with offset inlet outlet with stainless steel packing will reduce the intake DB levels by 17%

This means we could do a mathematical estimation of the exhaust DB level by the following:
100*.88*.85*.83=62.084
Which means there would be a total DB reduction of 37.916%

This means if you have an engine putting out 120 DB at the loudest RPM you would hear 74.5008 DB out the tail pipe.

Unfortunately exhaust noise simulation is still fairly hard to setup in a CFD like program. What we can do however to make things easier is record the exhaust DB levels (accuracy is less important when just getting a statistical distribution of the sound) by means of DB meters. Once we know the individuals components reduction in noise (IE how much a single muffler quiets the noise) and put the component on the exhaust system to record the system wide reduction in noise we can postulate or project the probable decrease in noise to similar setups.

I'll edit this post with the link to the other thread concerning this.

Edit:
http://www.rotarycarclub.com/rotary_forum/showthread.php?t=11969

[85rx-7gsl-se]

Bump from the dead and may be out of place but do any of you guys have an idea on what would be the best approach to quiet down a N/A half-bridge 13B for a street car application. Currently running a y pipe after the manifold to 2 2.5" pipes with two "turbo" mufflers is resulting in 108DB at idle lol.

[vex]

You're going to need a lot of muffling devices. You'll need to reduce the pressure differential (which is directly tied into performance) prior to the exhaust exit. That's been my finding at least... Let me clarify.

When gas leaves the engine it's very hot and under pressure. At the tail pipe you'd idealy (sp? Stupid spell check doesn't work on this browser) have atmospheric conditions when it comes to pressure and temperature. The result is that you would need to have to balance the conservation equations (mass, momentum, energy). We have hot and high pressure gas coming into the system and we want cool, atmospheric gas leaving. The farther away from ideal the louder the system will be to the outside world. The way to minimize the difference is to provide areas of expansion (lowers pressure, and reduces heat, but creates noise from the energy difference). To remedy the noise issue from the sudden expansion of gasses deadening material is shoved into the volume to absorb that energy--producing heat. Since there's a pressure differential still the gas will exhaust that chamber and then enter the y-pipe. There, pressure is reduced and therefore temperature is reduced as well. The temperature and pressure are still greater than atmospheric, thus additional chambers need to be installed after the y-pipe.

As a general rule of thumb, the larger the volume installed in the system the greater the sound reduction so long as there is appropriate sound deadening material. There are nuances associated with exhaust components and disrupting harmonic resonence, but that isn't really necessary to be discussed at this juncture.

In your case, the addition of more sound deadening devices would be necessary. Installing the largest RB universal presilencer helps a lot (it's what I run). Shoving it as close to the headers as possible without damaging the deadening material (it doesn't like extreme heat). IF you have room a catalytic converter can help in sound deadening as well and help you pass emissions (LOL)--though another muffling device might prove more beneficial. Instead of a y-pipe you could use a single in- dual outlet muffler to further reduce the noise. Large mufflers at the rear will also help. Remember that the larger the exhaust pipe is, the louder the noise (on average). So if you need to lower the sound the pressure needs to be lowered at the points the gas exits to atmosphere. This means smaller exhaust outlets will reduce noise effectively as the gas has to speed up and therefore reduces in pressure once the flow is incompressible.

If you're more concerned with performance however--you want to get the exhaust out as quickly as possible and damn the pressure differential.

[Monkman33]

80db would be about 25% of the volume to the human ear as 100db. technically speaking, an increase of 3db is doubling the acoustic output, but it takes about a 10db increase to double the volume perceived by the human ear.

At least, this is my rudimentary understanding of the concept.

[vex]

80db would be about 25% of the volume to the human ear as 100db. technically speaking, an increase of 3db is doubling the acoustic output, but it takes about a 10db increase to double the volume perceived by the human ear.

At least, this is my rudimentary understanding of the concept.

Maybe I'm misunderstanding you. Are you saying that a reduction of 1/5 of measured db equates to about a 1/4 reduction in perceived loudness of the individual?

I understand that human hearing is logarithmic in nature (able to differentiate volume of quiet noises effectively while differentiation of loud noises is harder), but I'm not sure I quite follow your statement.

[85rx-7gsl-se]

Our main concern with too much muffling is too much backpressure which I have heard is bad for bridgeport engine longevity.

[vex]

Our main concern with too much muffling is too much backpressure which I have heard is bad for bridgeport engine longevity.

I can't comment on that as I don't know. I imagine if you have enough muffling devices back pressure can be minimized as the pressure is always going to go down stream. Since the exhaust becomes 'incompressible' the velocity *should* increase at the exit causing the pressure to drop the last little bit.

Let me do some thinking and I'll see what I come up with.

EDIT:

After doing some readings on the average pressure achieved from the exhaust system (of average cars) I do not see how back pressure could affect the car from what I have read and the system I described above. If you have a full length exhaust system already installed you've already hit a significant portion of pack pressure. Adding in the muffling devices shouldn't adversly affect the engine if you use a well flowing type (Periferated stainless steel tube). Switching to a larger diameter exhaust will also reduce back pressure (but will also slow exhaust gas velocity; making it louder).

[j9fd3s]

Maybe I'm misunderstanding you. Are you saying that a reduction of 1/5 of measured db equates to about a 1/4 reduction in perceived loudness of the individual?

I understand that human hearing is logarithmic in nature (able to differentiate volume of quiet noises effectively while differentiation of loud noises is harder), but I'm not sure I quite follow your statement.

the DB scale is logarithmic. not that guitar amps or amps in general are rated consistently, but if i had a 50 watt guitar amp and a 100 watt amp right next to it, A you'd be deaf for a week, and B you'd be able to tell which one was which, but barely.

there are also a few frequency's that the human ear hears better than others.

AND the sound waves are affected by the medium they are going thru, so even the weather matters, somewhat.

BTW i have a copy of a road test of racing beat's IMSA gtu car from 1980, and they measured 133DB@50 feet. i have stood 50 feet away from the factory 79 imsa car (the green one at the top), when they are warming it up, and its so loud you can't really hear it

mike