John Darko
The listening room in which our loudspeakers sit plays a crucial role in what we hear from those loudspeakers; more so than any amplifier or source component. How so? The room’s dimensions and its exposed surfaces combine as a complex transfer function, not so subtly shaping the sound emanating from our loudspeakers before it reaches our ears.
A transfer function describes the relationship between the input and output of a system. With room acoustics, the “input” is the sound created by the loudspeakers and the “output” is the sound heard at the listening position. The room’s transfer function modifies the input through a combination of reflection, absorption and diffusion; and this modification isn’t uniform across all frequencies.
The Three Amigos
Sound waves in an enclosed space interact in complex ways, primarily through three distinct phenomena: reflection, absorption, and diffusion.
Reflection occurs when sound waves ‘snooker’ off surfaces like walls, the ceiling, the floor and furniture. Thousands (sometimes millions!) of reflected sounds contribute to a room’s reverberation — that’s what we call the lingering sound heard after the original sound has stopped. Clap your hands in the middle of your room and you will hear it. Alternatively, record a voice note with your smartphone on the arm of your listening chair and you talking towards it from the loudspeaker plane. Playback of that voice note will show you what your ear-brain is trying to filter out when listening to music.
Side note: reverberation is often mistakenly called ‘echo’; but they are not the same. Both echo and reverberation involve sound reflections but the timing of those reflections is fundamental to their differences. Echo is a distinct and discernible repetition of an original sound, occurring when a reflection arrives after a significant delay. Reverberation is a multitude of reflections arriving swiftly after the original sound but as a wall of late-arriving reflected sound. One measurement that neatly summarises a room’s reverberation is its ‘RT60’ — the time it takes for each frequency to decay by 60dB. A long RT60 – especially one that clocks in at above one second – is often called “echo-y” when, in reality, it’s too reverb-y.
Moving on…
Absorption is the process by which materials like carpets, curtains, soft furniture, and specialised acoustic panels convert sound energy into other forms, usually heat. Absorptive materials reduce the intensity of reflections to limit reverberation. Different materials absorb different frequencies more effectively. For example, soft furnishings like sofas and rugs can absorb higher frequencies more readily.
Then there’s diffusion which, unlike reflection, is directional. It occurs when sound waves are scattered in multiple directions by irregularly shaped surfaces. Diffusers, such as bookshelves, CD shelves, stacks of vinyl, textured walls and specifically designed acoustic panels break up reflections to distribute sound energy more evenly throughout the room.
These three factors—reflection, absorption, and diffusion—are the building blocks of a room’s acoustic make-up (aka its transfer function), affecting everything from the frequency response to the overall feel of the music. The balance between these three factors determines a room’s acoustic make-up.
Twin Peaks
A room’s transfer function isn’t heard (or measured) as one behaviour. The Schröder frequency – which tends to fall between 100Hz and 300Hz depending on a room’s dimensions and its reverberation time – divides it into two acoustic behaviours.
Below the Schröder frequency is where we do battle with bass. Room modes create peaks and nulls in the frequency response, making some bass notes sound boomy and others ghostly. These individual resonances are sometimes called ‘modes’ or standing waves: they colour the sound for an uneven distribution of low frequencies to make our loudspeakers’ bass response complex and position-dependent.
And our troubles don’t end there.
Above the Schröder frequency (so: in the mids and highs), the modal density increases, leading to significant overlap and a shift towards a more diffuse sound field. Here the room’s transfer function is dominated by late reflections that negatively impact clarity and detail. Sound energy is distributed more evenly to become more statistical with reverberation the net result.
However, it should be noted that a room’s transition from resonant to reverberant behaviour is not a sharp division but a gradual transition.
Vertigo
Several approaches can be taken to improve the sound of a listening room, though each has its limitations.
- Careful placement of both speakers and the listening position relative to the room’s boundaries and furniture is crucial. However, whilst this might improve bass quality, it will do nothing to reduce the impact of a room’s reverberant behaviour in the mids and highs.
- Room correction software uses a microphone to analyse a room’s acoustics and then applies equalisation to compensate for problems. This is why I call it room compensation software. The likes of Dirac and RoomPerfect will help smooth the frequency response in the bottom two or three octaves but – again – they will do nothing to reduce the negative impact of room reverberation on mid- and high-frequencies.
- Strategic furniture placement can provide limited acoustic treatment. Soft furnishings absorb sound, especially higher frequencies, while bookshelves can act as diffusers. Wishful thinking that it is capable of more dissolves when we realise: our furniture’s absorption coefficients are unknown; its placement is scattershot (from an acoustics point of view); and the ceiling remains naked to be fully reflective. Except in exceptionally rare cases, furniture is not enough to optimise a room’s acoustic make-up.
- The pro audio world already knows it to be true: optimal results demand a bespoke fit-out where, floor aside, acoustic panels cover 30 – 50% of all exposed surfaces. This includes bass traps to address low-frequency issues, absorption panels to tame reflections and diffusers to create a more even sound field. That’s visually more acceptable to a recording or mastering studio owner working in a dedicated space than a hi-fi enthusiast listening in a lounge room at home — one that must be shared with other family members. Moreover, high-quality bass traps, absorption panels and diffusers are expensive, especially when a substantial number is required. DIY panel builders pay more with time than cash but time is money.
Knives Out
Understanding our room’s specific acoustic characteristics is the first step towards finding effective treatment. One way to understand what’s happening in our listening space is to gather data. We can measure our room’s frequency response and reverberation time with a calibrated measurement microphone (like a UMIK-1) connected to a PC or Mac running room acoustics measurement software like REW. This data will help us identify problem areas—the peaks and nulls in the bass or areas of excessive reverberation—and determine the most appropriate acoustic treatment.
By measuring our room’s acoustic behaviour, we empower ourselves to make informed decisions about improving its sound and lessening the impact of its transfer function. It’s a step many audiophiles skip for financial and aesthetic reasons but it’s a crucial one if we’re serious about achieving audio satisfaction.
The room is arguably the single most dominant factor in what we hear from our loudspeakers. It has more of an impact than any upstream hi-fi component. And by understanding the room’s transfer function and the challenges specific to different frequencies, including the important distinction between reverberation (as measured by RT60) and echo, we lift our listening experience whilst simultaneously disembarking the hi-fi hardware ‘hype train’ (gee, how I hate that phrase). It’s a bitter pill to swallow but while room correction software, loudspeaker placement and furniture placement can help, dedicated acoustic treatment is usually the only way to secure proper results. I know this from first-hand experience: since 2020, I’ve had three rooms treated with excellent results.
And last week – oops – I did it again, which is where we are going next…
