Attenuating Low Frequencies

In order to treat a room acoustically, the first thing most people do is add some foam here and there and make the midrange and high frequency  more “dead” and they then think it is more accurate. However, it is very unlikely that the low frequencies which also tend to boom in the room have been affected at all. These booming frequencies have the longest reverberation times and if these are very unbalanced from the other frequencies, the sound can appear even more muddled.

“Bass trapping” can be achieved using thick acoustic foam blocks or Rockwool in the corners of the room which tends to damp the bass and reduce the RT (reverb time). However, it is usually only effective above 80 Hz or so unless they are very large. To attenuate problem frequencies below this usually requires the use of Helmholtz or quarter wave traps (a bit like a port) or panel resonators (a little like a passive radiator) to tune to a specific frequency range. A combination of techniques is best or whilst it is possible to attenuate continuous notes with panel traps, they may take some time to respond.

Every room has frequencies which are linked to its dimensions which in certain places offer a constructive interference between direct and reflected sound or a boost in bass. At other positions, they offer destructive interference or cancellations. Whilst it is possible to EQ out a boost in bass with a “graphic equaliser”, it is not possible to prevent the cancellations.

A larger room will have multiples of these room modes at frequencies much closer together so the bass is more even. Smaller rooms tend to have a less even bass response. Having dimensions of length, width and height that are also not multiples of each other is an advantage. For serious studio or anechoic chamber design, having non-parallel walls would prevent many of these room modes building up in the first place and should form part of the early design process.

For most of us, we tend to convert an existing room for our musical needs. We would like less noise entering or escaping the room, an even bass response, a balanced and controlled reverberation and optimal siting of loudspeakers and listening positions for the best sound. Some simple rules can be followed for room treatments to make a good start but for the best results, a bespoke solution is required for your room. Every room is slightly different, has different construction techniques and materials and has different objects breaking up the sound. Start with the following tips and when you want to know more, LFS may be able to assist with a bespoke room analysis and recommendation. This is particularly important for studio design and you can click here if you want to know more.

Basic Techniques for a typical listening room

Noise insulation – This is the term used for preventing sound breaking in or out of a room. The first thing to note is unless your room is constructed very weakly, it is likely to be the gaps around doors and windows or vents which dominate the sound transmission path. Aim to seal up as many gaps as practicable to prevent this. Next is the material your room is made from. Even a thin plasterboard construction will reduce broadband noise by up to 20 dB(A) and brick constructions have the potential to improve this to 60 dB(A) but usually limited by gaps and weak spots. However, at low frequency plasterboard does very little, 3 to 10 dB(A) depending on construction and for brick, 20 to 30 dB(A) maximum. Stiff and heavy materials, completely sealed are required to enclose low frequencies. Additional layers of paster board help and a dedicated Soundbloc version of the British Gypsum product is ideal as it is more dense. Preventing a direct vibration transmission between plasterboard layers is also key. Use a Rockwool fibre insulation instead of rigid insulation to prevent this and if you are serious, dont connect both sides of the wall to the same beams, use separators or staggered beams.

Bass trapping

This may be the most essential treatment in most rooms as other controls are relatively easy to install but don’t affect the bass. Use the thickest foam bass traps you can get in the corners of the room as a start. The more the bass notes are controlled, the less the room modes are excited and it is the room modal frequencies which break out of a room the easiest. You will find a more even bass response is achieved and less noise breakout and boom.

Loudspeaker placement

Placing loudspeakers in a corner will excite all the room modes, moving out into the room will minimise this but the distance between the loudspeaker drivers/ports and the room boundaries will have an effect. Try not to place this distance at a quarter of a wavelength of a room mode if you don’t want boomy bass. Remember, the port location will be for the lowest frequencies and the speaker driver for the middle and upper bass. If you can trap the 80Hz and above frequencies then its the port output (if you have one) location that matters the most as these frequencies in the 30 to 70 Hz range are usually the most troublesome. As a guide to placement start about 1/5 of the room length away from the front wall to the front of the loudspeaker.

Seating and listening positions

If you sit at the back of a room, you get boom at the lowest room mode frequency. If you sit in the middle of the room, you get 1 Octave higher and a cancellation at the lowest note. As a simple guide start 37% in from the back wall for the most even bass response and work from there fine tuning.

First reflection points

These are the points on the side walls, floor and ceiling that the reflected sound first arrives at the listener after the direct sound. Treatment of these will reduce unwanted muddying of the timing of mid and higher frequencies for a clearer stereo image. For the first reflection points, use a mirror from your listening position on the side walls just in front of the speakers and where you can see the speakers in the mirror, place the treatment. Either use small foam panels around 50mm thick, an acoustic diffuser to prevent direct reflection or a combination. All other reflected sound will pass the listener and be reflected from the rear wall.

Rear of room treatment

This would be a good place to put more diffusers and absorption panels. If you room is fairly live with hard surfaces everywhere, some absorption could be good to the rear. Otherwise with carpets, curtains and soft sofas, diffusion to the rear may be an advantage. A diffuser is an object that mathematically spreads out the sound that arrives at its surface so it disperses in multiple directions rather than bounce back in a fixed angle. Quadratic diffusers, either 2D or 3D can be expensive but tend to make the room appear to have more acoustic space and offer a much more live sound than absorption alone. The rear wall should have diffusion at head height and if you need absorption as well, use this sparingly (unless its a studio where you want no rear reflection) on the parts of the wall above and below head height. In the real world, a heavy pleated curtain across the rear wall can prevent unwanted mid frequency reflections and you can hide some rear bass trapping behind it!

None of this treatment need be expensive. Many acoustic material and panel manufacturers charge a lot for this type of treatment. For example,  a manufacturer of acoustic panels on Ebay we spotted appeared reasonable value at £200 for 4 acoustic panels, each approximately 60cm square. We managed to build copies of these for £30 worth of material so if you are good at DIY or just fancy having a go, you could save a lot of money.

Acoustic foam is also usually at a premium price but as long as it is open cell, fire resistant and approximately 15 to 40 Kgs per cubic metre in density, you can usually find a general foam supplier which will be significantly cheaper and will cut them to size for you neatly.


LFS will update this page with some free designs for simple acoustic treatments and provide some more specific designs at low cost in time. Please come back regularly to see updates or contact us click here if you have any questions.