Rick! wrote: ↑
Sun Aug 02, 2020 7:32 pm
Been thinking this for a while - we know nothing about the stock system, the aftermarket system, make of the vehicle, etc.
Another unknown is how sophisticated the phone sound measurement is. Free ones typically measure just dB(A). Better ones can perform FFTs or PSDs to understand all prominent frequencies (not to be confused with prominence, which is a comparative measurement).
Once a guy gets a glimpse of what's going on, "A duck with a glass eye could have seen that coming" could be one response...
There are far too numerous examples that can be searched where someone bolts on a shiny new system looking for performance and a throaty sound and end up with "the drone".
Source, Path, Receiver - with usually three paths: airborne, structure borne, or fluid borne. Sources rarely change so the solution is in the path or receiver. Mitigation can be addressed many ways but usually one needs to cut some steel or make several tests with lead or butyl rubber blankets and create some test results. You can math the crap out of an issue but by the time you think you are ready to make the solution, the guy with the dirty fingernails already is putting his solution into production.
I'd make a resonator for both target frequencies though a 57.6Hz solution will take up a bit of real estate due to its wavelength (almost 6m).
Except for the NASA paper, most Helmholtz math is based on white noise and pink noise inputs. Use pulsed inputs from an engine and a transmission loss can be measured but not in the magnitude that a spreadsheet calc may give.
The vehicle is a 2014 / 15 Renault / Dacia Duster 1.5 litre dCi (diesel) 4WD. The stock system is described two posts above. The aftermarket piping is bespoke without mandrel bending facilities, which is one reason I went a size larger (than one size larger). The boxes are built locally by Powerflow.
The app does FFT size, range, resolution and length and does various calculations: A3, A4, A5, A6, B5, B6, C4, C5, C7, D4, D5, E5, F4, F5, G4, G5 giving average, peak and max Hz. I use another app to just give dB(A). However, the mic has not been calibrated and perhaps that is my glass eye. My other eye might be blinded by a research paper on in-pipe temp.
Agreed, I don't have 6m of spare real estate. I do go off-road and need to maintain the ground clearance.
I decided to do some dirty fingernail work before taking my chosen design to an exhaust fabricator by applying a roofing product to the top of the heat shield over the boxes. I don't think the shells are too thin, but the two sections of heat shield closer to the engine seem to be metal without any absorptive material backing (unlike the one over the boxes). These two heat shield sections thud around 350 Hz when finger tapped while one section of pipe actually rings at 1249 Hz. I wasn't expected the ringing, but I am prone to a bit of serendipity.
I'll take a long drive through some nearby mountain passes tomorrow and let the co-pilot drive sections so that I can measure under load at various rpm, which I can't do on my own today.
From ptuomov's “Chapter 10 Sound in Ducts”
The side-branch resonator is analogous to the tuned dynamic absorber.
Resonators used as side branches attenuate sound in the main duct or pipe.
The transmission loss is confined over a relatively narrow band of frequencies centered at the natural frequency of the resonator.
From “Engine Exhaust Noise Control”
Insertion loss (IL) is defined as the reduction of noise level that occurs when a silencing element is inserted into the system.
Engine Firing Rate (EFR) = number of cylinders * Cylinder Firing Rate (CFR)
CFR = rpm / 120
EFR = 4 * 1750 / 120
EFR = 58 1/3 Hz (primary harmonic)