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How close may a loudspeaker be placed to a wall?

Peter Strassacker Dr. Peter Strassacker
Loudspeaker design since 1977
Books on material research and field theory
Design of loudspeakers like Lagrange 98, Pascal, Sub 10-60

Hints how to place speakers
Peter Strassacker interviewed by
Dennis Frank (7/2004).
 
 

Dennis:
Peter, a lot of customer would like to place their speakers as close to the wall as possible. How close may they be placed to the rear wall? How close may they be placed to the side wall?

Peter:
Loudspeakers are usually set-up neutrally. This means that they are designed in such a way that their sound reproduction is neutral when they are placed away from walls. In a living room we have a rule of thumb: 1 metre away from the wall.

If this distance is reduced considerably then we have a boost in bass due to the reflections of the wall. The low frequencies that usually disperse uniformly, will then be radiated within a part of the room or in a corner.

Since the radiated energy remains almost the same, the reflection coming from the wall is boosted by 4 to 6 dB, in a corner it will be even up to 10 dB.

Dennis:
Is that the reason why we consider this to be a boomy bass? What about the bass reflex tube? Is it important whether it is facing the rear or the front?

Peter:
Generally it's not important whether the bass reflex tube is pointing to the rear or tho the front. That was confirmed by Visaton using the VOXen placed 10 cm from the wall.

If you have a solid rear wall (bricked or concrete) it is often helpful to have the bass reflex tube just 2 or 3 cm away from the wall. The small gap between cabinet and wall acts like an extension to the bass reflex tube; this results in a lower tuning and, therefore, reduces the sound pressure level in the low bass region by 1 - 3 dB. This partly compensated by a rise of 4 - 6 dB due to the proximity of the wall.

Dennis:
The bass boost takes place at the side walls too.

In magazine like Stereoplay we often read about loudspeakers, like e.g.. JMLab, with a slight volume boost at 100 Hz. What's the purpose of this boost?

Peter:
This is an effect of the globalisation. In the USA and Japan the walls are often relatively soft and absorb, therefore, frequencies around 100 Hz. To make sure that the US Americans and Japanese also enjoy these frequencies the loudspeaker design is done in such a way that this absorption is compensated. For Europeans this is not pleasant since this frequency range is boosted twice: by the design of the speaker and by placing it close to the wall.

Dennis:
This means that a Pascal with a restrained bass is perfectly suited for European solid walls.

What happens to three-dimensionality when speakers are placed close to a wall?

Peter:
The human ear in conjunction with our brain forms a three-dimensional picture using the first sound waves that reach both out ears. If loudspeakers are placed 20 to 50 cm away from the wall then we hear first the front radiating waves and with a delay of 20 to 50 cm the reflections coming from the wall. This is bad. Our brain is not capable of differentiating waves that arrive at our ears that close to one another. The second sound wave should have a delay of 1 metre. Only then the brain forms a three-dimensional picture.

Dennis:
Does this mean that the distance from the side wall to the speaker should be - if possible - 1 metre?

Peter:
That's correct.

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