Here is my Dumb Question. Exhaust verses Noise Level

[turboplanner]

There's a bikies club ( yes, bikies, not motorcyclists) that does a run past my place every Sunday morning - about 30 mostly Harleys, and most of those with shotgun pipes etc. Yes, they create a deep rumble, that lasts for about two minutes.. But - and as a long-time motorcyclist - the thing that drove me to consider taking a gun out, was the couple of kids on the place about 500 metres away with a moto-cross training track (not an official one, just a track on a 10-hectare block) with no mufflers on their MX'ers, going around and around and around and around for 8 - 10 hours a day in summer, with the typical spasmodic throttle wrenching from fully closed to WFO every 10 seconds. I've lived under the Sydney flight path (near Rozelle) for several years and I'd take that over the bloody MX-ers every day.

Motor Cycle Noise is controlled by ADR 83/00; most bikes fit into the LC category.

 

https://infrastructure.gov.au/roads/motor/design/files/ADR_Applicability_Summary-L-Group.pdf

 

ADR 83/00

 

Vehicle Standard (Australian Design Rule 83/00 - External Noise) 2005

 

By my quick check, an LC class motor cycle is limited, in Annex 3 para 3.1 6.2.2.1.4 (engine power less than 75 kW) to a drive by level of 77 dB(A).

 

That’s a huge reduction from the original standard of 95 dB (A) of 20 years ago.

 

95 measured from the roadside diminishes to about 55 at the front windows of a suburban home, and at that level was considered to allow normal watching of television.

 

77 should not have any restrictive factor on someone inside the home with the doors and windows closed.

 

So it’s probable that the very loud rumbles we hear from non-specified motor cycles passing by are from non-conforming vehicles.

 

Oscar’s experience with Motocross bikes is similar to what I’ve found when out with the noise meter, the nuisance factor is not always directly related to the noise level recorded, and the repetitive throttle applications of these bikes do your head in.

 

External vehicle noise is controlled by the EPA in each State, and most have a hotline you can call. EPA will go out and investigate repeated noise from a given location, and prosecute for any breaches. This is the probably main reason police generally leave them alone; the EPA penalties are harsher.

 

However, the OP was asking about internal noise.

 

In October 2016 a National noise exposure for workplaces was adopted Australia-wide.

 

It is 140 dB © for jackhammers etc (not applicable to vehicles, and 85 dB(A), which is applicable to in-cab noise in vehicles.

 

Both limits apply at the drivers’ ears.

 

In-cabin noise | Logistics, Trucking & Transport News | Prime Mover Magazine

 

For around $100.00 you can buy yourself a noise meter and measure the noise in your aircraft, and that will give you a benchmark.

 

 

[Geoff13]

Apparently I was the first person the head mechanic at Gasoline Alley could remember ever asking him to quieten a Harley down.

 

I bought a second hand Vrod Screamin Eagle Two with after market pipes. First trip was to Gasoline Alley to get the standard pipes refitted.

 

She ran better and to my mind sounded far better after the de-modification. Awesome bike but sadly I never rode her enough.

 

 

[skippydiesel]

install a turbo and you fix your noise problem, the need for a bulky exhaust and you get some free* horsepower

This is only partially true - almost any obstruction in the exhaust system will have an impact on the noise produced.

I have run a small turbo diesel 4X4 for many years. The turbo has an automatic (pressure activated) "wastegate".

 

The removal of the silencer/muffler allowed a (pleasant) increase in the exhaust noise. Other benefits were a small improvement in turbo "spool up" time and exceleration, possible fuel economy and torque.

 

When the wastegate opens there is a noticeable increase in exhaust noise.

 

I feel that there would be few on road applications where a turbocharger alone will sufficiently reduce exhaust noise to meet legal obligations.

 

For an aircraft I think a turbo, where applicable/allowable, will have several benefits with one of the minor ones being some moderation of exhaust noise

 

 

[Litespeed]

A good trick to help quieten a turbo system is the plumbing of the blow off valve back into the exhaust flow just aft of the turbo.

 

It can be more efficient and a lot quieter- most systems are designed just to dump the excess and costs power and noise-mainly noise.

 

Systems set up like street machines are not what you need. They make great power part of the time but come at huge fuel cost and weight.

 

Overall- it should be remembered it all comes down to physics - we can not create energy , we can only transform it into heat, motion and sound. The trick is to reduce the losses of heat and sound and maximise motion. Some are horribly inefficient and loud. Every reduction in sound if down correctly will help boost power or reduce consumption, if the system is designed right.

 

It is a science and black art combined.

 

 

[Bernie Knight]

Summit engineering in the USA sell a conical shaped insert that is perforated,

 

I ran one of these in each left and right exhaust pipe in my car and found a real reduction.

 

There is also an auger plate that inserts into the exhaust on the inlet side of the muffler , these spin gases and sound back into the baffle Material. They are short and again in the pages of Summits catologue

 

 

[Bernie Knight]

Summit engineering in the USA sell a conical shaped insert that is perforated,I ran one of these in each left and right exhaust pipe in my car and found a real reduction.

 

There is also an auger plate that inserts into the exhaust on the inlet side of the muffler , these spin gases and sound back into the baffle Material. They are short and again in the pages of Summits catologue

[Blackhawk]

Can't see why an exhaust system based on a BMW Boxer can't be built for aircraft; they are very quiet.

 

 

[Yenn]

There was a system for aircraft I saw years ago. Swiss I believe. It ran the exhaust through a long perforated pipe, surrounded by fibreglass insulation and enclosed in a larger pipe. Outer pipe was about 4" from memory and length about six feet. It was published in Firewall forward from memory and supposedly very good. I thought about using it in my Corby, but the standard Jabiru 2200 exhaust works quite well enough for me.

 

 

[fly_tornado]