Just some thinking out loud. We know that a single cylinder firing once would sound like a loud pop, much akin to a gunshot, and that the culmination of each pop, made by all cylinders in the firing order makes up the base frequency of the exhaust note. Of course the base frequency gets higher as rpm increases, since the rate of fire increases. Theres also secondary noise too, but I can't seem to find the literature on it.

We know that exhaust drone is from the exhaust frequency resonating in the pipe. At a given RPM, usually in the 2000-4000rpm range for the Z, where the frequency of the exhaust is matched up to the length/diameter/size of the exhaust which causes amplification at that frequency. Most are already familiar with this, but after really thinking about it, I started to wonder what causes rasp.

It's pretty common knowledge to everyone, what mods, increase or decrease it for a given combination of exhaust parts. Since exhaust noise drone has a specific definition and cause, what is the cause and definition of rasp. It seems to be more prevalent across a range of rpm compared to droning, and its more noticeable the less sound dampening that exists in the exhaust set up. It doesn't seem like it would be a noise innate to the engine, but rather introduced by the exhaust parts, so it got me thinking if it might be high frequency vibration thats creating high frequency noise?

Thoughts?

TK

 

This would probably have to deal with Nodes and Antinodes. They are certain points along the pipes where the exhaust soundwaves either cancel each other out or amplify the sound. Thats my conclusion. Those points should be equidistant from each other, but with differing frequencies it should change these points.

 

Rasp has everything to do with flow - not noise. Improper tuning of flow, causes the exhaust to somewhat backup on itself - resulting in sound. Every exhaust rasps - by simple definition. Tuning an exhaust for sound - simply - tunes the rasp to an acceptable level.

Rasp, otherwise, happens when the exhaust pulses do not line up properly and the exhaust stream cools too quickly through the pipe. Cooling too quickly usually means the exhaust is too free flowing - wrong pipe diameter, etc.

The key is tuning for the pulses...you'll get better sound quality and performance. When you have a system that sounds great, then add testpipes, HFC's or headers - you disrupt the original design and possibly the components fo the system cannot handle the increased or changed flow characteristics.

The design principles of deflection and reflection, in exhaust manufacturing, are what sound tuning vs. performance is all about. X-pipes and H-pipes and y-pipes all have dramatic effect and are specifically used for pulse tuning.

Rick

 

Well did some more research and it looks like I was kind of on the right track. Here's a couple links relating to some research...

http://www.ingentaconnect.com/conten...00001/art03459

http://scitation.aip.org/getabs/serv...cvips&gifs=yes

Thoughts?

TK

 

Lol~ I forget where I got this from, but:

"Rasp", occurs when the exhaust pulses get out of synch. As the cylinders of each bank on a "V" style motor fire, they create a pulse in the system. The pulses will alternate back and forth from bank to bank as the motor runs. With multiple cylinders, such as a V-6, the six cylinders alternately fire creates lots of pulses in the system. The x-pipe smooths these pulses out.

Instead of two separate banks of three cylinders doing their own work, the x-pipe uses the pulse created by a firing cylinder of one bank to create a vacuum in the other bank because of surface tension. When a cylinder of the other bank is ready to fire, instead of the piston having to force the exhaust gas out of the cylinder, the vacuum that was created by the other cylinder bank helps suck the exhaust gas out of the cylinder, hence the term "scavenging." The velocity of exhaust gases in one header tube causes a low-pressure area in an adjacent tube, literally sucking the exhaust out of that cylinder. The more cylinders, the greater the effect.

 

so if rasp is caused by the uneven temps in a aftermarket exhaust system then would thermal wrapping the entire exhaust system help minimize the rasp to some degree?

 

I have nothing technical to add, but I find that raspy exhausts often sound just fine when you're in the car, which in the end is all that matters.

From the outside however, the metallic sound can make things sound cheap.

 

Good stuff here. All time old thread bump too haha.

I have a 2.5" true dual x pipe CBE that had no resonators in it. When cold it was raspy and loud. Adding 18" resonators made it sound deep and good but a lot quieter which I didn't like. Then adding long tube headers without the 3 star design at the collector like fast intention uses, it's now raspy again but all the time and much louder. I'm thinking of adding 12" resonators to the system or doing Helmholtz resonators. Basically its a tube that's capped at one end that 90's off the pipe at some point and if long enough will capture sounds and such.

 

Wish there was a way to skip the thread bump , but figured I chip in here to clarify a little, so someone else doesn't get confused. Drone is the when the emitted exhaust frequency matches the cabin's resonant frequency. Everything has a frequency of resonation. That's why glass can break if a certain frequency of sound is played loud enough nearby. The exhaust system has a frequency of resonation. Rasp is when the sound inside the exhaust system matches the resonant frequency of the system. It causes a massive amplification and change in tone, typically unpleasant. You can find out the frequency with a spectrum analyzing phone app and target the frequency with a specifically designed resonator (side branch, helmholtz , expansion , absorptive, etc.. ) in the same way you would with drone. Or you could make a significant change to the length or diameter of the system. (Also, drone is compounded when the exhaust exits underneath the car.)