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Resonator or Muffler: What is the difference?

Written By: Connor MC

Is the meaning of this jargon particularly important? Yes, it is, because a resonator is not the same thing as a muffler, and each has distinctive properties that will alter how your Mustang sounds. Let’s dive in.

Getting The Note Right

One of the many joys of owning a Mustang is the ability to really customize it and make it your own. This is thanks to the accessibility and incredible amount of quality aftermarket parts. What better option is out there then to start off by changing the exhaust system? Searching for the perfect exhaust note is an obsession with Mustang owners, and why wouldn’t it be given everyone is partial to a different exhaust frequency. Some like to let you know they are approaching. Others like to let you know they’ve passed. Whether you like a mighty roar, a raspier growl, or virtually anything in between, the aftermarket Mustang exhaust industry has come up with every conceivable exhaust component to get your ‘Stang sounding exactly how you want it.

Having such a vast pool of quality pieces to choose from is stupendous, but sometimes can be overwhelming. Throw in marketing and technical talk and choosing the right piece can be hard. In the aftermarket exhaust world the terms ‘resonator’ and ‘muffler’ are often bandied about. You may even occasionally run across a hybrid (or Frankenstein) ‘resonated-muffler’.

2015-2017 GT Mustang with a Flowmaster Axle-Back Exhaust
2015-2017 GT with an Axle-Back Exhaust

What Does the Muffler in My Exhaust Do?

The job of the muffler is to lessen the sounds of the engine to an appropriate and acoustically pleasing level. How does it work? Technically, a muffler quiets the engine by reducing the sound pressure emitted. The factory mufflers on your Mustang are made to sound good, but are restricted by efficiency concerns, ease/cost of manufacturing, and of course sound level laws. For many enthusiasts, the stock mufflers are too conservative. 

Mufflers are engineered with multiple chambers that expand exhaust gases as they pass through. These chambers feature perforated tubes or baffles – maybe even both. Exhaust passes through these perforated holes and baffles, resulting in expansion. As the gas expands, its pressure lessens, and consequently so does the sound level. Furthermore, OEM mufflers are often packed or lined with materials (such as fibreglass) as a soundproofing measure to further absorb the sound inside the muffler and emit less ambient noise. The baffling also increases engine back pressure by decreasing how fast the exhaust gases leave the system. Excessive back pressure can hamper performance.

2005-2009 V6 Mustang with a Flowmaster Axle-Back Exhaust
2005-2009 V6 with an Axle-Back Exhaust

What Does the Resonator in My Exhaust Do?

The purpose of a resonator is to cancel out a certain range of sound frequencies. Without getting too scientific, sound is simply a pressure wave emitted at a certain frequency. Like waves in the ocean, sound waves have certain amplitudes (comparable to overall size), a crest and a trough. At the beach, when the crest of a wave meets the trough of a wave of the same size, the two waves actually cancel each other out and there will no longer be any wave. The exact same principle applies to sounds waves. If you have two sound waves of the same size and frequency meet crest-to-trough, they too will cancel.

This begs the question, what range will be canceled? Well, that's left up to the engineers. Typically an automotive sound engineer will choose a range that is not pleasant to hear and build the resonator to eliminate that frequency. Noises that are canceled are harsh noises or ranges where the exhaust note produced would be a loud drone or irritating buzz.

SLP Resonators Installed on a Mustang
Resonator Set Installed

So what does a resonator do for your Mustang? A resonator makes your overall exhaust note louder and more aggressive. It's not quite a straight-pipe, but it isn’t too far off. Resonators also have less baffling and don't reroute exhaust gases as much as a muffler. This reduces back pressure, potentially freeing up a few horsepower. 

Note: Nowhere does it say that a resonator lessens sound. It only eliminates certain frequencies. If you have a resonator that only cancels out songs by Cher, that’s great. The bad news is Madonna will still be as loud as ever.

Also keep in mind that resonators are located in your respective mid-pipe, before the mufflers and after the catalytic converters.

VIDEO: SLP LoudMouth Resonator Pair Review

Resonated-Muffler (or is it Muffled-Resonator?) Hybrid

The last to look at is a combination of the two. So what exactly happens when a muffler is mated with a resonator? Well, it’s actually pretty simple. You will have the characteristics of each device. Certain unpleasant ranges will be totally eliminated, and the overall note out of the tail pipes will be hushed. Truth be told, most modern mufflers utilize this combination design. At first it was prevalent amongst luxury vehicles, but now it is pretty much considered an industry standard.

The Quick Differences Between Them


  • Lowers sound level
  • Usually packed with fiberglass
  • Does not eliminate certain frequencies of sound (droning)
  • Increases engine back pressure, hampering performance


  • Almost straight pipe sound level
  • Cancels certain frequencies to stop droning and obnoxious noise
  • Not adjustable at all, you either like it or you dont
  • Decreases engine back pressure, increasing performance

Wide-pipes, Modifications, and Keeping it Legal

Some enthusiasts choose to swap out their resonator with a wide-pipe (open chambered) mid-piping for improved sound and volume, but as always it is important to check with your state emissions to see if it within regulations. As a general rule of thumb, as long as you have catalytic converters (cats) you will be legal, but it is good to double check to see if there are any other laws that may apply to you, especially if you are resident of states like California.

How Do I Tell if My Resonator is Failing

A resonator can go bad from a multitude of reasons. A resonator is essentially like a muffler, and can fail in similiar ways. One way to check if it is failing is inspect it for cracks along welds and seams. Additionally, one could check for rattling when the car is not running (as it is not hot and also quiet) and potentially narrrow it down to the resonator. Third, resonators have the potential to rust out. If rust flakes are coming from the tail pipe, it could be from a failing resonator.

1986-2010 Mustang with Pypes High-Flow Catalytic Convertors
High-Flow Catalytic Convertors

Can a Resonator Quiet My Mustang?

Incorporating a resonator and muffler in the same exhaust system is something that actually happens regularly. As described, a resonator helps eliminate unwanted and unpleasant sound frequencies (drone) whereas the muffler is mainly responsible for actual volume reduction. In the case of a Mustang where the exhaust is too loud, would adding a resonator help? Placing a resonator upstream of the muffler would help a little, but not likely to the extent you would think. Resonators are not designed to reduce volume. In this instance, they would slightly lower volume because the exhaust would lose sound energy as it meets minor resistance passing through the resonator. Realistically, if the sound of your Mustang is much too loud, it is best to replace the mufflers with a toned down version.

Cat-Backs & Resonators: How Do They Work Together?

From Foxbody Mustangs all the way up to the S550, Ford has plagued its pony cars by incorporating resonators into the factory exhaust systems. Not only is this another way to slow the exhaust as it passes through the system, but it also reduces the overall sound quality of the car's exhaust note. Because of regulations, cat-back exhaust systems will keep the resonators intact or may even incorporate their own into the system. Though, for those looking for the best sound quality and flow regardless of redundant rules, they should incorporate a resonator delete into their exhaust upgrade. The car will sound better and you may even free up a few ponies.

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