Sound Advice : Recording
Hugh Robjohns
There are many good reasons why soffit-mounted speakers, such as this beautiful set of Kinoshitas in South Africa’s BOP Studios, are used. But not all speakers are designed to be placed so close to the wall!There are many good reasons why soffit-mounted speakers, such as this beautiful set of Kinoshitas in South Africa’s BOP Studios, are used. But not all speakers are designed to be placed so close to the wall!
Most of the advice I’ve seen about speaker placement suggests keeping them well away from the walls, but I was recently advised to place my monitor loudspeakers directly against the back wall to give a better sound in the room. Is this valid and, if so, what is the thinking behind this approach?
Jerry Jones, via email
SOS Technical Editor Hugh Robjohns replies: The ideal speaker placement depends on the design of the speaker, the dimensions of the listening room, the efficacy of any acoustic treatment, and the location of the listening position within the room. In most cases, though, the positioning of loudspeakers inherently involves some level of compromise in the overall performance. Sometimes, placing speakers directly against the back wall does indeed provide the best balance but, often, positioning the speakers away from the wall by a small distance might give better results. The only way to know is to try different positions and listen critically!
Let’s consider the loudspeaker issues first. Most loudspeakers are designed to give a tonally balanced sound when placed away from the room boundaries (the so-called ‘free space’ condition), but different manufacturers adopt different ‘ideal’ placement distances. Many monitor speakers incorporate EQ facilities to reduce the bass output if the speaker is placed near room boundaries, because a compact loudspeaker radiates more or less omnidirectionally at low frequencies. The consequence of this is that if a speaker is placed against a back wall, the portion of sound that would have gone away from the listener is bounced instead straight towards them. This is often referred to as a ‘half space’ condition and it produces a 6dB increase in the SPL of low frequencies measured in the room. Place the speaker in the corner of two walls and the area it radiates into is a ‘quarter space’, with a 12dB rise in LF output compared to the ‘free space’ condition.
So placing a speaker near or against the room boundaries changes the spectral balance very significantly, and different manufacturers therefore optimise their designs for use in different placement conditions. Some also provide EQ facilities to adjust the balance to allow use in different conditions. It is therefore vitally important to place or configure the speaker according to the manufacturer’s specifications.
Thinking more about the room now, solid room boundaries effectively act like mirrors to low-frequency sound. If a loudspeaker is placed away from the wall, the direct and reflected LF sound waves travel to the listener over different path lengths, and thus arrive with different phases. The inevitable outcome is a degree of cancellation resulting in one or more deep notches in the LF frequency response, the precise tuning being dependent on the spacing between speaker and boundary wall(s). As the speaker is moved further from the wall, the notch moves lower in frequency, and vice versa.
As a rough rule of thumb, it’s usually best to keep the speaker within 1 metre of the back wall, to push any notches up above about 80Hz and into a region where back-wall reflections can be controlled reasonably well by conventional bass-trapping treatments. The real danger zone is when the speaker is between 1 and 2.5 metres from the back wall, as this can result in substantial notches between about 35 and 80 Hz, which require more complex bass-trapping techniques to resolve. A back-wall spacing of more than 2.5 metres puts the notches below the lowest usable audio frequencies, so it’s not really a problem any more.
Another consideration is the way in which the speaker’s position stimulates the room’s standing waves. Often, moving the speaker a few centimetres forwards or backwards, side to side, or even up and down, can have a dramatic effect on how it interacts with the room and the resulting consistency of the bass response. Ideally, the distance between the speaker and back wall should be different from that between the speaker and side wall, and also between the speaker and ceiling/floor.
This might suggest that the ideal would be to mount the speaker in the back wall. Providing it can be equalised appropriately, that approach can work well, hence the popularity of ‘soffit-mounted’ speakers in commercial studios. However, soffit-mounting is not practical for most project studios, and it involves a raft of technical challenges that we don’t need to get into here.
One down side of placing the speaker very close to the back wall is the effect it has on stereo imaging and the perceived depth of the soundstage. In general, the further the speaker is from the back wall, the greater the impression of soundstage depth becomes. So there are lots of interacting aspects of the overall in-room speaker performance that have to be balanced — compromises are inevitable.
When we try to optimise speaker positioning in our Studio SOS visits, our normal practice is to listen to familiar music with a varied bass line to assess the balance in the room. We then move the speakers closer or further from the wall (and adjust their angle and height) as necessary to optimise the performance. No two rooms are ever the same, and the only reliable technique is the time-consuming one of moving the speakers in roughly 10cm increments and reassessing the balance, the consistency of the bass, and the soundstage depth and imaging.
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