A crash course in room acoustics by Frederick Norén
Why should you fix your room acoustics?
There are two main reasons for fixing your room acoustics.
- Nothing affects the sound from your speakers more than the room acoustics. The variation in level between two different bass frequencies can easily exceed ±10 dB in an untreated room. Some bass frequencies – called standing waves – will have a prolonged reverberation time and thus create a bass energy build-up in the room.
Hard reflections off the walls, floor, and ceiling will affect your listening experience more than you think. Your brain will try to separate the hard reflections from the direct sound, but it can’t, because the reflections arrive with a very short time delay. This has a fatiguing effect on your brain, making you less creative and unable to fully enjoy your sound system.
The use of a digital “room correction” system might help iron out the most prominent bass frequencies in the listening position, but unfortunately it can't do anything about the prolonged reverberation time of the standing waves. How could it? By capturing the sound once it has left the speakers?
- As soon as you walk into a room, you are subconsciously affected by its acoustic properties. If the acoustics are too busy – like in a modern café with hard surfaces and no acoustic treatment – you feel uneasy and stressed.
If the acoustics are too damped, with curtains and thick absorbers, you will most likely get the feeling of the walls moving in on you.
Either way, the acoustics will affect your sense of space in a room – for the better or for the worse.
What’s the hardest part of treating the room acoustics?
Getting started …
How do you get started?
Every room is unique and requires a bit of detective work. If you’re uncertain about where to begin, please don’t hesitate to contact one of our product specialists. Their contact information is available at the end of this document.
However, there are some very fundamental ways to get started. Read on!
Sound waves will bounce off a hard surface is the same way light reflects in a mirror.
The green arrows represent the direct sound from the speakers. The other arrows represent hard reflections bombarding the listener.
How do hard reflections affect the sound?
It takes a sound wave about three milliseconds to travel through one meter of air – which means that the reflections are slightly delayed in time when they reach the listening position. The reflections affect the frequency response, stereo panorama, depth and phantom center – making the overall sound less detailed. It’s also fatiguing listening to music in a room with prominent hard reflections.
Treating hard reflections with absorption
By using a mirror, you can easily identify the hard reflections. Have a friend or spouse sit in the listening position, then put a small mirror flat onto a wall, move the mirror until your friend can see either of the two speakers in the mirror – there’s your hard reflection. Put a small piece of tape to mark the position. Continue the routine until you have found as least one position on each side wall and two on the front wall. The ceiling will also produce hard reflections, and being the room’s largest untreated surface, it’s quite beneficial treating it with absorbers and diffusers. Putting a thick rug on the floor will also help dampen some of the hard reflections.
Attach a Cinema Round Premium panel at each marked position, and the hard reflections will be absorbed by the panel. Use double-sided adhesive tape or two small brads or nails without heads. If you use tape, the panel will eventually fall down, because it’s taped. But it’s useful for experimenting with different placement. When you have found a good spot for the absorber, use two nails/brads, nail them halfway into the wall, then push the Cinema Round Premium onto the nails. The proper way to do it, is of course to use Flexi Glue – instructions on how to use it are available on our webpage.
Treating hard reflections with diffusion
Instead of absorbing sound energy, a diffuser breaks the soundwave and spreads it in different directions – like a garden hose nozzle spread water. Diffused sound basically does not interfere with the direct sound from the speakers – as long as there’s at least one meter between the diffuser and the listener. If your listening position is close to the back wall, then it’s better to put absorbers behind you.
Basically there are two given positions for diffusers in a room – on the back wall at ear height and in the ceiling in the middle of the room. Start by positioning a small table or chest of drawers close to the back wall, then put four to six Multifuser DC2 on the table. As long as they are standing firmly on the table, it’s okay to stack them. Normally DC2s are nailed or glued to the wall – and for ceiling placement, the easiest way is to use glue in conjunction with a screw and a washer, the latter will keep the panel in place, until the glue has dried.
Absorption and diffusion
A room with Cinema Round Premium absorbers
The panels absorb the first hard reflections off the walls and ceiling.
A room with Multifuser DC2 diffusers on the back wall
Multifuser DC2 diffusers in the ceiling
The wavelength of bass frequencies ranges from about 1.7 meter at 200Hz to about 17 meters at 20Hz. In a room with parallel walls, this will result in a phenomena called standing waves. Based on the dimensions of the room, certain bass frequencies will “stand” between the walls, ceiling and floor.
Low air pressure in the middle of the room, but high air pressure at the first octave
The same goes for the room’s other direction
How does this sound?
When you walk back and forth in an untreated room listening to music, it feels like walking "in and out of bass" – certain bass frequencies are suddenly more prominent and then diminished at another spot in the room. As the pictures show, the worst listening position is in the middle of the room where most standing waves interact. Sitting close to the back wall will produce a natural bass boost.
But there’s more: Resonance!
Standing waves are in fact resonance frequencies, which have less friction and thus longer reverberation time. This means that the room will "harbor" the standing waves and the result is uncontrolled bass energy.
Waterfall plot showing two standing waves
X-axis shows the frequency between 20–400 Hz, Y-axis is level and Z-axis is time (up to 1.5 seconds)
In this cellar room with stone walls, the reverberation time around 32Hz is much longer than the rest of the bass frequencies – producing a sub bass heavy sound. Then there’s another resonance at 55Hz that “rings” longer than the rest of the bass frequencies.
How do standing waves affect the sound reproduction?
First, they create an uneven bass frequency response that changes with the speaker and listening position. Secondly, they contribute to excessive and uncontrolled bass energy, that will mask some of the dynamics and finer details in the mid-range and treble. We like to divide the audio spectrum into different frequency bands, but our brain will interpret it as one sound where all the frequencies are interconnected. What happens in the bass frequency spectrum, will have an effect on how the midrange and treble is perceived.
So what can be done to control the standing waves?
The best way to minimize the effect of standing waves is to build a room with slanted, non-parallel walls – like a control room. However, most of us can’t afford that luxury, so the second best thing is to install bass traps.
Where should the bass traps be positioned?
The most prominent bass build-up is in the corners of the room – where several large surfaces meet – so that’s where the bass traps work as efficiently as possible.
Vicoustic offers a very efficient bass trap – Super Bass Extreme.
Super Bass Extreme is efficient between 60 and 125 Hz and is designed to be positioned in corners. Inside the box, there’s a wooden membrane that the bass waves try to resonate – much like the front skin of a kick drum. But two layers of high density foam stops the motion by absorbing the energy. The back side of the panel is perforated with small holes, which also turns the bass trap into a Helmholtz resonator. The micro perforation actually makes the bass trap about 20% more efficient – a 2-in-1 bass trap, if you like.
Installing four to eight Super Bass Extreme panels in a room will have a great effect on the reverberation time in the low-end, making it shorter and thus help your speakers produce a more dynamic low-end. Tightening up the low-end will also bring clarity to the mid-range and treble. It’s psychoacoustics!
Due to its revolutionary design, the Super Bass Extreme is much more efficient compared to just putting foam or mineral wool in the corners.
An efficient bass trap like Super Bass Extreme will calm down the low-end of the audio spectrum and make your speakers sound punchier and more open.
A cross section of the Super Bass Extreme reveals the wooden membrane and the two layers of high density foam. A box of Super Bass Extreme contains two bass traps, the dimensions of a Super Bass Extreme is 595 x 595 x 155 mm. When positioned in a corner is builds 430 mm from the corner and onto the wall.
Super Bass Extreme Positioned in two front corners
The bass traps can also be positioned in the rear corners with equally great results
A remedy for problematic standing bass waves – Vari Bass.
Vari Bass is a tunable bass trap with a frequency range between 50 and 100 Hertz.
By rotating the wooded top and extending or shortening the length of the bass trap – the bass frequency changes accordingly. Checking the frequency scale on the side, reveals which frequency the bass trap is tuned to.
Vari Bass works most efficiently when positioned where the air pressure is maximum – usually in the front or back corners of the room.
There are numerous web pages available to identify the standing waves of a rectangular room. Search for “room mode calculator”, choose a calculator, then enter your room dimensions. This will present you with a frequency list of possible standing waves – a good start for experimenting with Vari Bass. Measuring the low-end response with a measurement microphone is even better.
Tune all Vari Bass to the same frequency and listen to some well-recorded drums at close to loud volume. Then quickly remove all Vari Bass from the room and listen to the same drums. Does the kick drum sound more “boomy” without the bass traps, then you’re probably identified a problematic bass frequency. If you can’t hear much difference or if the low-end sound less punchy, try another frequency and repeat the test.
Properly tuned, Vari Bass shortens reverberation time of a standing wave, both the fundamental frequency but also its overtones. This helps to clear up the low-end response of your speakers, producing a punchier and tighter low-end and upper bass.
In a normal sized room, you need four to six Vari Bass. Several Vari Bass can be stacker onto each other using the optional Vari Bass Stacker.
One box contains two Vari Bass. The dimensions of a retracted Vari Bass are 600 x 370 mm.
Vicoustic offers a whole range of different absorbers, all designed for a purpose, but the best-selling and most efficient absorber is the Cinema Round Premium.
It’s available in eight different colors and three sizes. It offers full absorption from 300 Hz and up, but still absorb pretty well at 200 Hz. One box of Cinema Round Premium contains eight panels. The dimensions of a panel are 595 x 595 x 75 mm.
Our flagship product, the Wavewood panel, has been specifically designed to treat acoustic problems without absorbing the room’s natural ambience. The non-linear cavities of the wooden front combined with thick foam, makes it both an absorber and diffuser.
It efficiently absorbs from 300Hz and up. The wooden front starts scattering sound waves in different directions from around 1000 Hz and up. About 40% of the upper mid-range and treble is diffused and reflected back into the room. This helps maintain the natural room ambience, yet defeat flutter echoes and hard reflections.
One box of Wavewood contains ten panels. The dimensions of a panel are 595 x 595 x 60 mm.
Wavewood has the same dimensions as normally sized acoustic ceiling tiles – changing old ceiling tiles to Wavewood help control the room acoustics without absorbing the natural ambience. Properly positioned Wavewood panels can really open up the room acoustics in a wonderful way.
The Multifuser DC2 is a bi-dimensional diffusion panel made of high quality expanded polystyrene – making it both efficient and light-weight. Perfect for both wall and ceiling placement.
The two-dimensional primitive root surface scatters the sound waves over a 180-degree angle.
One box contains six panels and one panel is 590 x 590 x 147 mm.
There are also two wooden versions – Multifuser Wood 36 and Multifuser Wood 64.
Both panels efficiently diffuse sound waves over a 180-degree angle.
Multifuser Wood 36 diffuses between 470–10.000 Hz – one panel is 595 x 595 x 75 mm.
Multifuser Wood 64 diffuses between 310–8.000 Hz – one panel is 595 x 595 x 135 mm.
One box contains a single panel divided into sections that can be hung on a wall, using screws and screw anchors.
Wavewood Diffuser Premium
Wavewood Diffuser Premium is a one-dimensional diffuser made of high quality expanded polystyrene and a Wavewood front. It’s light-weight and compatible with normal acoustic ceiling tiles, thus perfect for positioning in the ceiling – in the middle of the room or above the listening position. Swapping eight ceiling tiles to Wavewood Diffuser Premium makes the room sound more spacious – adding "air", if you like.
One box contains eight panels and one panel is 595 x 595 x 60 mm.
Wavewood Diffuser 60
Wavewood Diffuser 60 is the wooden version and suitable for wall positioning.
One box contains a single panel that’s 595 x 595 x 60 mm.
It can be hung on a wall using screws and screw anchors.
So you’re using a digital “room correction” system and it doesn’t really work?
That’s because it’s impossible for digital processing to combat hard reflections, long reverberation time and excessive bass energy build-up in the room. All it does, is to change the tonal balance of the speakers, often to the worse, leaving a slight artificial taste in your mouth. It’s much better to properly treat the room acoustics.
Example of an acoustically treated listening room
What’s the goal when treating the room acoustics?
The goal is to make the speakers disappear and just present a wavefront of sound, a "wall of sound", if you like. High quality full-range speakers in a well-treated listening room will give you that sensation. Trust me, it’s a great experience!
How much money should you invest on acoustic treatment?
It depends on how great you want your speakers and HiFi system to sound.
Remember… HiFi is never better than the room
For more information about our products, please visit www.vicoustic.com.
For questions regarding room acoustics and how to acoustically treat your music room, control room or home cinema room, please contact one of our product specialist or acoustical consultants.
Frederick Norén, product specialist, firstname.lastname@example.org.
Pedro Guerra, acoustical consultant, email@example.com.