Exhaust Drone - installing a branch resonator.

Author: Boofhead (as known on Mustangtech.com.au).

Introduction

This technical document is a discussion on the topic of exhaust drone and to discuss a method to address the issue. Exhaust drone is a sound frequency resonance generated from the exhaust system that internally causes high vibration effects and strong pulsating sound. It is not strictly a muffler issue as a number of factors are involved. These include size of the exhaust, type of engine such as V8 verses a 6 cylinder as well as the temperature of the exhaust gasses. Not all exhaust systems or setups exhibit drone. As always each person will have differencing levels of sensitivity to drone especially over long driving periods. Exhaust drone is not always an issue with vintage/older cars as they are not often driven or only cruised at relatively high RPM for short periods of time to warrant reducing drone. However, some owners find an exhaust drone exceedingly annoying and in some cases a painful audio reality.

It is most common that drone occurs around 1800-2000 RPM (80kmph) for an 8 cylinder engine. There are various approaches to reduce drone, these include extra deadening material to reduce sound in the cabin, mufflers that have multiple chamber slots or installing multiple mufflers or resonance chambers. Street performance cars are usually built using louder exhaust notes so they are more likely to exhibit drone effects. Ideally the solution employed should not be a restriction on the exhaust flow so as to not be a detriment to the performance of the car. Hence the primary approach to reducing drone is to install 1/4 wave resonators, also called a branch resonator or a J-tube (based on a common shape employed as shown in Image 1).

Image 1: branch 1/4 resonance pipes, otherwise known as J-pipes.

In simple terms the pipe is a calculated length based on the targeted resonance reduction required and temperature where the speed of sound changes relative to the exhaust temperature present at the branch location. The pipe is sealed at the end so no exhaust flows through the tube. It is simply a chamber where sound waves travel down the tube then get reflected back to the entrance at the exhaust pipe so as to cancel the next sound wave just starting its travels down the pipe. The exhaust system is not restricted as the exhaust will flow through the path of least resistance which is the existing exhaust system. The curves (such as the J shape) in the pipe has little influence on the sound pulse moving along the tube so it is purely a fitment issue.

The primary factor that is required is to determine or calculate the frequency of the sound producing the drone. Using a data collection device the sound wave frequencies can be determined and based on the highest peak the target frequency is known. Alternatively, the frequency can be calculated from the RPM at which the drone occurs in combination with the configuration of the engine. For example the formula to calculate the sound frequency is:

For example, applying the formula for V8 engine with drone at 2500 RPM:

Once the drone frequency has been determined then the length of the resonance tube can be calculated.

For example, applying the formula for a V8 engine with a drone frequency of 166.67 Hz using speed of sound of 1180 FPS is:

The resulting tube length is 21.23 inches that should be rounded up to nearest size so 22 inches will be the calculated length. The speed of sound varies based on the temperature of the gas so for a more precise calculation the temperature of the exhaust at the point of the entrance of the resonance tube should be collected otherwise use the suggested figure of 1180 FPS if the pipe is toward the end of the vehicle or 1235 FPS if closer to the engine. In the case that the data can be determined (for example using a thermocouple and data logger) the following table provides the speed of sound for various exhaust temperatures that the sound pulse is moving through.

Applied Example

Husky65 has applied the above rules. This is an overview.

Step 1: Conducted a quick DB meter check with a phone app. It was found that idling in garage one meter from exhaust. There was significant resonance in the garage, as you would expect, with a result of 96 DB.

Step 2: Conducted a second gear at 1900 RPM, the window was down, the phone was sitting on the transmission tunnel, with a result of 90-92 DB. When tested at wide open throttle (WOT) in second gear found the exhaust noise was at 110 DB.

Step 3: In second gear traveling downhill with compression breaking, the result was 95 DB.

Finally, in fourth gear crusing at 1900 RPM the result was 90 DB. This completed the before modification results. Next was to decide where the J-pipes were going to be installed.

Image 2: Proposed J-pipe modification.

Having now calculated the length of the J-pipes, it was time to go to the shop and have them fitted.

Image 3: J-pipes installed side view.

Image 4: J-pipes termination point.

The result of installing the 26" stainless 2.25 inch branch pipes was successful. Up to third gear 1900-2000 RPM, the drone has been eliminated. In addition, it quietened the car down. On light throttle there was almost no sound. The sound on full throttle was a satisfying, deeper tone. Vibration in the car was reduced, therefore there were less rattles.

The results of conducting a few basic tests with a couple of quick readings on the DB phone app in the same position with the windows down. In second gear at 1800-2000 RPM: Before the modification it was 90-92 DB. After the modification it was 82-84 DB. Idling in the garage: Before the modification it was 96db. After the modification it was 88-90 DB. In second gear with downhill compression braking: Before the moficiation it was 95 DB. After the modification it was 87-88 DB. Note: 10 DB is a halving in perceived sound.

Without considering the DB readings, there was a large perceived difference when driving in the car. This no restriction modification to the exhaust for reducing the drone is well worth the money and effort and comes highly recommended.