Acoustics 101

Applying acoustic treatment to a room, for the purpose of listening to music, is partly science and partly art.

The aim of acoustic treatment, in a room designed for listening, is to allow you to hear your speakers as accurately as possible. In an untreated room sound bounces around, recombining with the primary source (the speakers) to create new sound which often lacks clarity and balance. 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. It is desirable that the room retains a natural sense of space, which is why in professional studios you see plenty of hard surfaces combined with absorption, in the form of resonator panels. Our own HA600-75 hybrid panel exhibits the same characteristics of a resonator panel by absorbing lower frequencies and reflecting high frequencies.

Room acoustic treatment is often poorly understood.

LF60 CORNER BASS TRAP
LF60 CORNER BASS TRAP

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, generally 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.

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 particular bass frequencies will sound artificially loud and in other areas it will sound like some bass frequencies have been turned down. This can happen in the space of 30cm. Some rooms also exhibit the “one note bass” effect where low frequencies all sound like one note.

Acoustic treatment should still leave the room with a natural sense of space

A totally dead space quickly becomes an unpleasant space to be in. It is very easy to apply too much high frequency absorption to a room, and not enough bass trapping. It is important that an acoustic treatment absorbs a broad frequency range.

An acoustic treatment package should be a balance between low, mid-range and high frequency absorption.

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 30cm 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 important that the sound is acceptable in all the potential audience seating positions.

How much acoustic treatment does a room require?

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

Another aspect of room acoustics. An acoustic diffuser will spread reflected sound in various directions and is used to help reduce standing waves and flutter. They help to modify the acoustics of a room while retaining a natural sense of space rather than a room that is heavily damped. 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. The HA600/75 hybrid diffuser has been designed to provide a desirable balance of diffusion and absorption.