A guide to rear-ported speaker placement

The quest for that ultimate sound can be both rewarding and maddening and nothing seems more daunting than placing your rear-ported speakers. There’s a lot of science involved which can be a little intimidating – most people don’t want to study fluid-dynamics and the physics of sound just to enjoy their speakers.

Your local KEF dealer or audio guru can be a huge help, but it pays to have some basic knowledge of your own. You can hit the forums – they can be full of great information, but they can also potentially lead you down a path filled with confusing jargon.

A one-size-fits-all answer to the question ‘how far from the wall should I place my rear-ported speaker?’ is just not possible, but there are a few guidelines.

Too close to the wall may cause a muddling of the bass.
Too far from the wall may make it difficult to walk around the room.
And, try to avoid the dreaded ‘dead-zone’ where bass and midrange response is just completely deadened. It is a real thing and, depending on a few different factors, it may exist anywhere from 30 - 90cm from the wall behind your speakers.

A better question to ask is: ‘what does it sound like to me?’  If you like the way it sounds, then you’ve properly placed your speakers. It may seem a little simplistic but believe it or not, that’s the conclusion you’re ultimately going to come to. The key is to find a setup that sounds great and then experiment a little more just to prove you’ve found the optimal place for your speakers!

What's the difference between a rear- and front-ported speaker?

Ported ( also known as bass reflex) speakers are designed to enhance the bass response of a loudspeaker through a series of calculations involving cabinet volume, cabinet filling, and woofer response, among other variables. Typically, bass reflex speakers offer enhanced bass response over closed-box designs. The downside to ports is that at lower frequencies ‘smearing’ can take place due to the transient response of the design. This smearing is reduced or eliminated through the ‘tuning’ of the port in conjunction with cabinet size and volume, hence the common name ‘tuned-port.’ Another drawback can be the phenomenon known as ‘chuffing’ where air turbulence in the port becomes audible. A well-designed port addresses both issues. 

Where the port is placed is the result of a lot of different considerations, including visual aesthetics and optimal use of the available cabinet real estate available. For all intents and purposes, there is no appreciable difference between a rear and front-ported speaker. They both do the same thing – use the energy created by the woofer when it moves backward (or ‘in’) to enhance bass response. 

The practical issue with a rear-ported speaker is how the energy coming from the cabinet interacts with the boundary behind it. Energy waves bounce off the nearest boundary (for our purposes we’ll call it a “wall”) and interact with the direct energy from the front of the speaker. This interaction may result in either a dip or boost in energy at certain frequencies that will make the overall sound in that frequency region muddy or inarticulate. 

There is a school of thought that says rear-ported speakers should never be placed near a wall, and if you are going to place your speakers near a wall you must always use front-ported or closed-box designs. This is simply false. Are there a few things you can do to optimise the performance of a rear-ported speaker near a wall? Of course, but superlatives are always suspect, especially when it comes to audio.

Distance from the wall

Whether you are using stands for your bookshelf speakers or using floor standing ‘towers’, the wall behind the loudspeaker can have a large effect on bass response. The wall reflects the omnidirectional energy emanating from the loudspeaker and it also reflects the energy from the port itself. For our purposes, we’ll ignore the omnidirectional energy from the cabinet as in most home setups that energy is small (especially if you have a quality-designed loudspeaker). 

Two things occur when sound energy leaves a port and interacts with the rear wall: 1) certain frequencies get boosted, 2) certain frequencies get cut. 

When a frequency gets boosted (sympathetic resonance) it adds to the energy of the original (direct) frequency causing an artificial increase in sound at that frequency (too much bass!). When that frequency is slightly time-smeared (typically arriving after the direct energy from the front) the result is a muddy, inarticulate sound. 

A frequency gets attenuated after it destructively interacts (out of phase) with the direct energy causing the direct and reflected energies to cancel (not enough bass!). This happens when frequencies arrive at your ear out of phase with each other as a result of the interaction between the wall, the port, and the distance to the listener (among other factors).

Further is not always better

Further is usually better, but don’t ignore the improvements that can be made by moving your speakers closer to the wall. It seems counterintuitive but depending on the frequencies you’re having trouble with, you may actually clean up some reflections by moving your speakers closer to the wall. Exactly what the affected frequencies are depend on all the things we’ve been talking about. Of course, all of this is frequency and volume-dependent, but you will eventually be able to dial in a good compromise. 

You could set your speakers more than 2.8 metres from the wall behind the speaker. This will virtually eliminate all low-frequency cancellation problems. Unfortunately, 2.8 metres is not something we can all do so we need to look at other options.

A quick guide to setting up your rear-ported speakers

It all boils down to using the single greatest audio monitoring device ever created – the human ear – and getting your speakers to sound as good as possible before worrying about dialling them in. Forget what the forums say, forget what your audiophile buddy says, forget what you read here – just set your speakers up where you have to set them up because of the room’s design, and then follow a few easy guidelines: 

 * Change the virtual shape of your room by angling your speakers (toe-in or toe-out). Not only will this help with the soundstage it will also change the relationship of the rear port to your wall by changing the angle of reflection. It’s important to recognise that your speakers may not always be at the same angle as each other – rooms are not always symmetrical, so it stands to reason the angle of your speakers needs to compensate for that. 
       
 *You can add a little dampening material behind the speaker but be careful: there is no worse-sounding room than an overly dampened one. The best way to treat reflection problems is through dispersion. 

 *Experiment with the port bungs or tubes. These are meant to cut down on the energy leaving the port or to reduce the energy ‘piling up’ behind the speaker. 

 *Moving your speakers further or closer to the wall will often cut down on frequency pile-up but it can also affect which frequencies are cut. The closer your speakers are to the wall the higher the affected frequency and the further away the lower. Remember! A small move can make a huge difference, so it’s best to go a few centimetres or degrees at a time.

 *For those who prefer to make calculations, you can figure out the affected frequencies with this formula: 340/4 times the distance from the front baffle to the rear wall (in metres).