Hi-Fi DIY: Changing Rooms – Part One

If left untreated, a room’s acoustic issues can have a major effect on your hi-fi’s sound. Huw Price identifies and explains some common problems, such as comb filtering, standing waves and flutter echoes. Make sure to read up on speaker setup in our first instalment of Hi-Fi DIY.

changing rooms

When reading reviews in hi-fi magazines – and studio-recording publications for that matter – it can often seem odd that room acoustics are disregarded. Think about it for a second – some rooms have a lively and bright sound, while others sound darker and deader. It’s one reason why you might feel more inspired to sing in your bathroom than a walk-in closet.

So it stands to reason that the acoustic characteristics of a room will have a strong influence on the way any hi-fi system sounds, because it affects both frequency response and stereo imaging. You may even find that ‘upgrading’ a system can make it sound worse, if fundamental acoustic issues aren’t dealt with.
By failing to recognise and address acoustic issues, which can often be relatively cheap and easy to fix, you may end up wasting money on expensive hi-fi equipment with little or no improvement.
When consumers opt to ignore irrefutable laws of physics and prefer to put their faith in buying what may be unnecessary equipment, the results are more often than not disappointing.
Of course, the hi-fi industry produces some incredible amplifiers and speakers, but when you read those technical specifications, you should be mindful that measurements are taken within anechoic chambers.
The only way to achieve accurate measurements is to eliminate all acoustic reflections, and that’s what an anechoic chamber is – a room that possesses no acoustic characteristics whatsoever.
This is only ideal from a technical perspective, because listening to recorded music in an anechoic acoustic environment is not particularly enjoyable. Even studio rooms, where make-or-break mixing and mastering decisions must be made, have acoustic characteristics that are controlled rather than eliminated. Besides which, if you pull up an image of an anechoic chamber, you’ll realise quickly that you will only get away with building one in your house if you live alone.

Stereo images are conjured up by differences in intensity between sounds, achieved by using pan controls during the mixing process

So let’s consider more familiar, conventional living spaces instead.
A ‘traditional’ living room may have a carpeted floor, thick curtains and plenty of soft furnishings. All of these materials will absorb sound energy, but their effectiveness in doing so is not consistent across the audible frequency range. High frequencies are absorbed more efficiently than low frequencies, so the sonic signature of this type of room can be prominent bass with soft or even muffled treble.
So how about a more ‘contemporary’ living room, with laminate or wooden flooring, no curtains and minimal furnishings? In this scenario, far less acoustic energy is absorbed, so the room will sound brighter and livelier. However, clarity and stereo imaging may be compromised, because multiple reflective surfaces allow the soundwaves to continue bouncing around the room for longer, and this compromises intelligibility.
Various acoustic issues are common to all rooms, to a greater or lesser extent. These can include a combination of the following – comb filtering, standing waves, boomy bass and flutter echoes. Let’s take a look at each of these in turn, and examine how they can impact the sound of your hi-fi.

Comb filtering

When listening to your hi-fi, some soundwaves will travel directly from the speakers to your ears and other soundwaves will reach them only after bouncing off the wall, floor, ceiling or all three. The direct soundwaves arrive first, followed by the reflected soundwaves, so there is a phase shift – or indeed there are several phase shifts.

The reflected soundwaves (also known as early reflections) interfere with the direct sound by reinforcing and cancelling the waves at various points in their cycles. This creates a series of peaks and nulls in the frequency response that would look like the teeth of a comb on a spectrum analyser display – hence the term comb filtering.

A less cluttered room will sound lively and bright, but stereo imaging may be compromised, as soundwaves bounce around for longer

As well as altering frequency balance, stereo imaging becomes less defined and vague and detail is lost. In nature, human hearing relies on differences in time, intensity and phase to determine the origin of a sound. In contrast, stereo imaging is dependent upon intensity differences, since all the acoustic energy emanates from speakers that are ideally equidistant from the listening position.
Mix engineers use ‘pan’ controls to position individual instruments within a left-right stereo picture. They’re just like the balance control on your hi-fi amp, but mixing software and traditional mixing desks provide a pan control for each and every channel. By moving the pan control off-centre, the engineer reduces the volume of that instrument in either the left or right speaker. By reducing volume in the left speaker, the sound will appear to originate from the right of the stereo image.
Since early reflections can increase or decrease perceived volume, they will distort the stereo image painstakingly created by the mix engineer. Undermining the integrity of a track in this way is the antithesis of high-fidelity sound reproduction.

Standing waves

Standing waves occur when two identical sound waves travel between parallel surfaces in opposite directions, when the gap between the two surfaces is equal to the wavelength of the sound. They also happen when the gap between the surfaces is equal to half a wavelength, a third, a quarter and so on.

As with early reflections, constructive and destructive interference will take place when these soundwaves combine – and they will affect the fundamental frequency, as well as higher harmonics. These frequencies are called ‘eigentones’, so any part of the music that happens to coincide with the eigentone frequencies in your room will be reinforced or diminished, depending on your listening position within the room.
Most domestic living spaces will have a minimum of three pairs of parallel surfaces – the side walls, the front and back walls and the floor and ceiling. So standing waves will always be present and they can occur in axial mode, between two surfaces (eg, between the front and back walls); tangential mode, between two pairs of surfaces (eg, all four side walls) and oblique mode, between three pairs of surfaces (eg, all four side walls, plus the floor and ceiling).
Studio control rooms are typically constructed without parallel surfaces; however, non-parallel walls and ceilings won’t always eliminate standing waves. Instead, they may become even more complex and difficult to predict and treat.

Comb dancing

Reflected sound waves travel further than direct sound waves to reach the listener’s ears, and they combine constructively and destructively to create comb filtering.

Wave pictures

The green and blue arrows show axial standing waves between the front and rear walls and the sidewalls. The red arrows show a tangential standing wave between all four walls.

Boomy bass

Hi-fi novices are often in search of ample low end – in other words, lots of bass. As listeners gain experience and their tastes become more refined, the quality of the bass response becomes as important as the quantity of bass being generated.

Are some notes in a bassline louder than others? Do fast basslines become a blur of indistinct low frequencies, and do bass-drum patterns that should be lively and punchy sound more sluggish and boomy instead? If so, your listening room may have bass-response issues that become highlighted with more powerful amplification and full-range speaker systems.
Ideally, every frequency will have an identical decay time in a listening room – however, this is seldom the case. We previously discussed how sound-absorbing materials in more cluttered domestic living spaces are more effective towards the upper end of the audible frequency range. This means that the bass frequencies continue to bounce around in the room long after the upper-mid
and treble frequencies have ceased to be audible.
Standing waves and the build-up of bass energy in corners and next to walls can make things even worse, and if the bottom end in your room has a tendency to ‘linger on’, it can alter our perception of groove and detract from mixes where a tight and punchy low end is intended as the bedrock of the track.

Flutter echoes

One of the most common (and irritating) acoustic problems encountered in smaller spaces is that of flutter echoes. Like standing waves, this phenomenon occurs when sound waves with a shorter wavelength bounce between two parallel reflective surfaces. This produces successive multiple reflections, which hang over after the event and alter our perception of pitch and tone.

Flutter echoes also impart a harsh and ringing quality to the upper-mid’s treble and blur the front end of transients. This can be detrimental to the intelligibility of vocals, fast upper-frequency instrumental passages and percussive content. If you’ve ever tried clapping your hands in an empty, carpet-free room, or in a stairwell, you’ll already be familiar with the metallic trill of the dreaded flutter echo.
Well, that’s it for this month, but rest assured – for every problem, there is a solution. Check back next time, when we’ll discuss a variety of remedies for acoustic issues that may reveal your hi-fi system actually sounds better than you imagined.