By Jack Sharkey, March 1, 2018
The quest for that ultimate sound can be equally rewarding and maddening as all get out. And during that quest nothing seems more daunting than placing your rear-ported speakers. There’s a ton of science involved which is okay if you’re into that sort of thing but that’s where it gets daunting – most people don’t want to study fluid-dynamics and the physics of sound just so they can sit back and listen to a little Modest Mouse after a long day at work.
The search for a purely scientific answer to the question ‘how far from the wall should I place my rear-ported speaker’ is just not possible. Even if you want to dig deep into the science, as soon as the humidity or barometric pressure of the room changes, the music you are listening to changes, or you sit in a different position than you sat in last time you listened, everything else changes too. Albeit, the changes can be minute (Humidity? Barometric pressure? Seriously?) but you get the point – there is no simple answer to the question.
A better question to ask is: What does it sound like to you? If you like the way it sounds, then you’ve properly placed your speakers. Okay, that’s seems a little simplistic but believe it or not, that’s the conclusion you’re going to come to after you finish trying to answer the question.
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 others. Typically, bass reflex speakers offer enhanced bass response over closed box designs. The downside to ports is that at lower frequencies ‘smearing’ takes place due to the lessened transient response of the design. This smearing is reduced or eliminated through the ‘tuning’ of the port, cabinet size and volume of the loudspeaker, hence the common name ‘tuned-port.’ Another drawback can be the phenomenon referred to as ‘chuffing’ where air turbulence in the port can actually be heard. A well-designed port addresses both issues.
Where the port is placed is the result of a lot of different considerations, not the least of which are visual aesthetics and optimal use of the real estate available on the front baffle. Bass energy moves slowly and takes a large amount of physical space to unfold, so the few inches from the front to the back is negligible: For all intents and purposes there is no difference between a rear- and front-ported speaker. In the cuataway of the LS50 pictured below, the port is shown in the upper right hand corner of the loudspeaker.
They both do the same thing – use the energy created by the woofer when it moves backward (or ‘in’) to make the bass response of a loudspeaker more efficient. To take that one step further, all speakers (rear-, front-ported or closed box) will be affected by their relationship with the nearest boundaries. Bass energy radiates in all directions from a loudspeaker and so it is (to varying degrees) always affected by the surfaces around the cabinet.
The practical issue with a rear-ported speakers is how the energy coming from the cabinet interacts with the boundary behind it. Energy waves will 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 will possibly result in either a dip in energy at certain frequencies or a boost 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 optimize the performance of a rear-ported speaker near a wall? Of course, but superlatives are always suspect, especially when it comes to audio.
What exactly does the port do?
Think about blowing into a bottle to get a sound. The larger the bottle the deeper the sound (and the more power you need to move enough air to make a sound), and vice-versa. The energy created by the air you blow into the bottle is analogous to the work a woofer does in a loudspeaker – it excites the air in the cabinet. The excited air in the bottle reacts with the air in the neck of the bottle to produce the sound you hear. There is a natural resonant frequency between the air in the neck (the port if you will) and the bottle itself (think cabinet).
In a loudspeaker, higher resonances make the port act like a sealed-box design – there is no appreciable energy excitement in the port. The closer you drop in frequency to the resonant the more excitement of the air in the port. At some point you will reach the peak of energy (the sound you make is loudest) and then as you move lower in frequency the excitement will begin to tail off until it disappears.
Or, think of that annoying thing in your living room that rattles every time you listen to music. Sometimes it rattles like crazy, sometimes it doesn’t. What’s happening is the music produces a certain frequency that resonates with the thing that rattles and…it rattles due to the excitement of energy. When it doesn’t produce that frequency that thing doesn’t rattle.
This why the proper name for any ported speakers is “tuned port.” The port resonance frequency is tuned to the resonant frequency of the cabinet and volume inside of it to enhance the response in and around that frequency.
Cool Stories, But All I Want To Do Is Set My Rear-Ported Speakers Up So They Sound Great
You do need to consider things like the surface your speakers are on, hard or soft, non-resonant or resonant, and if your room is lively with some reverb (lots of hard surfaces, high ceiling, few furnishings) or dead (carpeting, drapes, low ceilings, lots of furniture), but for this piece we’re just going to concentrate on the relationship of your speakers to the wall.
Most people assume what matters is the distance of the rear of the loudspeaker to the wall, but what we actually need to consider is the distance from the front of the loudspeaker to the wall. It is the relationship between the direct energy (front) and reflected energy (rear) that causes us grief. As that distance changes so does the affected frequency.
Typically, the closer to the wall your loudspeakers are the more the bass response is affected. Except when the opposite it true. So what you’re trying to find is the best compromise between all factors. Those factors being: aesthetics, physical practicality, type of music, volume, type of room (live or dead), type of speaker and density of the boundary.
Too Close To the Wall Too Far From the Wall
Whether you are using stands for your bookshelf speakers or you are using floor standing ‘towers’ (both are considered ‘free-standing’), the wall behind the loudspeaker can have a rather large effect on bass response. The wall will reflect the omni-directional energy emanating from the loudspeaker and it will also reflect the energy from the port itself. For our purposes, we’ll ignore the omni-directional 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 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. This of course, depends on the wavelength of the frequency among other things, but you get the idea.
A frequency gets cut 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 (among other factors) the interaction between the wall, the port and the distance to the listener.
Typically, the affected frequencies that we most care about are from 20Hz to around 220Hz – bass and mid-bass region) but all frequencies are affected.
Farther Is Not Always Better
Farther is usually better, but don’t ignore the improvements that can be made by moving your speakers closer to the wall. It seems unintuitive but depending on the frequencies you’re having trouble with you may actually clean up some reflections by moving your speakers in the opposite direction of what you think is best. Exactly what the affected frequencies are depend on all the things we’ve been talking about. You’ll never narrow it down to a single octave or band as that will change constantly, but you will be able to dial in a good compromise..
A Quick Plain English 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 dialing 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 room restriction and the like.
Then follow a few easy guidelines:
Change the shape of your room. Of course, this is not always possible or desirable, but you can 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.
In a perfect world, set your listening position more than 3m (9.8 feet) from the wall behind the speaker (regardless of the distance of the speaker to the wall assuming normal setup). This will virtually eliminate all low frequency cancellation problems. This has to do with wavelengths and timing. Unfortunately 9.8 feet is not something we can all do so let’s leave it at the farther you sit from your speakers the better.
Add a little dampening material behind the speaker, but not too much. There is no worse sounding room than an overly dampened one. The best way to treat reflection problems is through dispersion and that’s where distances and timing come in.
Experiment with the port bungs or tubes. High-end rear-ported speakers will come with different sized port bungs, typically made of a viscous foam. These are meant to cut down on the energy leaving the port for to reduce muddiness – a result of the energy ‘piling up’ behind the speaker. Basically, sound energy bounces around for a while and then escapes out into the room. By then it so delayed that it arrives at your ear without a true relationship to the original frequency. By reducing the energy (but not the quality of the sound) the muddiness is reduced or even eliminated. The cutaway to the left shows the port tubes inserted into the Reference 5.
Move your speakers farther or closer to the wall. This will often cut down on frequency pile-up but it can also affect which frequencies are cut. Be careful though, a small move can make a huge difference, so it’s best to go a few centimeters at a time. The closer your speakers to the wall the higher the affected frequency and the farther away the lower. Don’t drive yourself crazy! Pink Floyd is going to sound different than the Munich Philharmonic which will sound different than Drake. It’s not about absolutes because music is dynamic. It’s all about compromises.
For you calculating types, you can figure out the affected frequencies with this formula:
340/4 times the distance from front baffle to rear wall (in meters)
My LS50W are placed .54 meters (21.5”) from the wall behind them. Theoretically, I should hear a dip in the frequency around 155Hz (340 divided by 2.1844 meters (or .5461 cm times 4)). My speakers are also angled so that the reflections tend to bounce away from my listening position rather than back toward it. Angling can solve a lot of problems; in my case I do not hear any significant loss of response at or near 155Hz and I have controlled the muddiness as well..
I should mention that the DSP on the LS50W also has several adjustments for boundary compensation, but let’s assume these hints are for users of speakers other than LS50W. I would also never use an outboard equalizer to compensate for speaker placement issues, so I don’t want to get into ways to change the response other than physical placement. Equalizers add phasing issues and noise into the signal chain that are usually worse than the issues caused by speaker placement. DSPs do not affect signal quality like outboard equalizers do because the signal processing takes place in the digital domain where it is immune to such things.
Finally – Don’t Over Think It
We sometimes get way too caught up in the elusive science of speaker placement. You’ll know when your system sounds bad and when it sounds great. And that friend of yours who comes over and tells you you’ve got it all wrong? Don’t buy into it – trust your ears and trust the quality of the equipment you’ve purchased to help you get the most out of your music (or movies).
And most of all, beware the forums. They can be chock full of great information, but they can also lead you down a path filled with incoherent mumbling. I’ll leave you with this gem I found on a forum about how ported speakers work. I’ve paraphrased it to protect myself from whoever wrote this:
Ports are like how a car exhaust works. The exhaust propels the car even forward when you are applying the brakes. This is why cars don’t stop immediately when you step on the brake. If cars had mufflers on the front they would stop immediately when you hit the brakes. The engine in a loudspeaker (the woofer) makes bass and sends a little to the front and a little to the back through the port. The front bass is the one you want to hear and the excess is tucked away in the back to hide, but if its close to a wall it reflects back and you get double the bass you wanted. If the port is in the front you get all the bass the factory set for you.
The poster qualified this by saying he wasn’t an audio or a car guy, but nevertheless I think I will spend the weekend moving the exhaust pipes on the ol’ Ford Escape to see if I can get that sucker to stop on a dime.