Room Acoustics - Creating a pleasing and accurate listening environment.

By James Bellew, SoundAcoustics

Room acoustics treatment for the purpose of listening to music is partly science and partly art. The room dimensions, construction material, seating position, speaker placement, existing furniture etc. are just some of the factors that need to be considered when designing an acoustic treatment for a room. Also important is the personal taste of the listener. Some audiophiles may prefer the added sense of realism that can come from placing speakers in a relatively live space. For example, a string quartet recital may benefit from a room with some reflective surfaces. Conversely, a dance music producer would generally prefer a tightly controlled accurate space, particularly in respect to low frequencies, in order to "pull" mixes that will translate to the dance floor. What we do try to achieve in the acoustic treatment of both these environments is relatively consistent reverberation times across the audio spectrum.

Room acoustic treatment is often poorly understood. The most common error is to cover a large area of the walls with some type of thin foam (50mm or less) or carpet/drapes. The problem is that these acoustic treatments are only suitable for absorbing higher frequencies, 500 hz and above. Widespread high frequency absorption creates a dramatic change to our perception of a room or space, as a consequence it sounds "acoustically treated". As thin acoustic treatments will have no impact on lower frequencies, the result is a room that sounds extremely boomy and muddy, resonating freely at lower frequencies. The room will be acoustically unbalanced with short high frequency reverb times and long low frequency reverb times. Low frequencies interact with each other as they reflect around a room creating "room modes".

A room is a bit like a large musical instrument. When excited it will produce it's own characteristic sound. The reality is that most of us will be treating a rectangular shaped room. A room will resonate at certain frequencies and create standing waves. In respect to lower frequencies, some locations in the room the bass will sound artificially loud and in other areas it will sound like the bass has been turned down. This can happen in the space of 10cm. The result is a variation in the perceived volume of certain low frequencies. Some rooms also exhibit the "one note bass" effect where low frequencies all sound like one note. A few years ago I remixed a track many times over a period of six months trying to get the bottom end to sit nicely. It always sounded either muddy or thin on other systems. The problem was that the key this song was in resonated badly in my room - I could never get the bass right.

Personally, my aim is to create a natural listening space, still with a little life, but without any significant room modes (certain frequencies becoming louder or softer) at the listening position. The room should allow you to hear all frequencies accurately and allow your monitors or speakers to project an precise stereo image. A totally dead space is unpleasant and a bit like sensory deprivation. This characteristic in a room dedicated to music production can result in the tendency to add much EQ or FX as a way of compensating. That said, in a home theatre room some people may enjoy a gut rattling low frequency peak watching Independence Day!

An acoustic treatment package should be a balance between low, mid-range and high frequency absorption. Bass traps are designed to reduce room modes, smoothing out acoustic peaks and troughs. Broadband absorbers deal from 100 - 200Hz upwards and high frequency absorbers deal with 500Hz upwards.

Assessing the acoustics of a room. You can use a room calibration microphone and software to measure the room or alternatively you can use that most sensitive of apparatus, your ears! There are a number of room measurement programs available which are relatively inexpensive. Being a Mac user I use Fuzzmeasure running on a MacBook with 2 gig of RAM.

Firstly place your speakers away from the corners of the room and at least 50cm from the wall. Stand in the middle of the room and clap your hands. Listen for high pitched reverb or echo, the continuation of sound after the clap. Typically, rooms with a lot of reflective surfaces such as polished floor boards and glass will be particularly reverberant. Carpeted rooms with soft furniture and heavy drapes can often sound quite good in respect to mid and high frequencies.

Assessing bass resonance can be achieved by playing a track with a lot of low end that you are familiar with. My personal favorite test CD is Grace Jone's version of "Private Life" from the "Warm Leatherette" album. It features Sly Dunbar on drums and Robbie Shakespeare on bass, recorded at Compass Point Studios in the Bahamas before the days of Pro Tools! (AC/DC also recorded "Back in Black" at Compass around the same time, and also features a great drum sound). Place a midline between your speakers to the back of the room. If you're really keen you can run some masking tape on the floor, numbered at 20cm intervals for reference. Put on your test CD and start listening with your chair placed about a metre from the speakers. Gradually move your chair back along the midline and notice how the acoustics of the room increasingly influence the sound from your speakers. Are there areas where the bass becomes quieter? Are there areas where the bass becomes louder? Is the bass better defined in some spots? Does it just sound bad anywhere you sit? This is also a good method to find the best listening position or "sweet spot". In the home theatre environment it is probably important that the sound is acceptable in all the potential audience seating positions.

How much acoustic treatment a room requires ultimately comes down to personal taste and requirements. Broadband panels at the first reflection points are generally essential and dramatically improve the focus, clarity and imaging of the speakers. First reflection points can be located by placing a mirror on the wall. The position at which you can see the speakers is also the acoustic reflection point. Bass traps are best placed behind the speakers but can also be installed in the ceiling or rear corners.

Diffusion is another aspect of room acoustics which I will just briefly touch on. An acoustic diffuser will spread reflected sound in various directions and is used to help reduce standing waves and flutter. They help break up the rigid dimensions of a room by reflecting sound in a more random fashion. In a recording studio control room a diffusion panel is often installed on the rear wall. A classic example is a panel of various height wooden blocks, which looks a bit like a model cityscape. In the home a bookcase full of different sized books on the rear wall makes a great improvised diffuser.