Ole Witthøft

The wall behind your speakers works against good sound

The sound from your speakers is created by the interaction between the acoustics of your living room and the speakers themselves. Here we look at the importance of the wall just behind the speaker, and the engineers document with some measurements that don't usually appear in hi-fi magazines or anywhere else.

 

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Measurement setup. We want to see how the acoustics work when the loudspeaker is placed on a large surface. The measurement setup should mimic the conditions the speaker encounters when hung on a wall.

 

Q113 Revolution is made as a speaker to be hung directly on the wall, and therefore it is quite relevant to look at the cooperation between wall and speaker. The very fact that there is a (relatively large) surface near the speaker has an impact on the sound in the living room. The wall behind the loudspeaker acts as a kind of extension of its front plate, resulting in an amplification of the sound at the low frequencies. This is called the bass range. The theoretical amplification of the low frequencies is 6 dB, and if the speaker is designed to work near a wall, you get 6 dB of sound pressure free from the laws of nature. If, on the other hand, the loudspeaker is designed to be placed more freely in the living room, placing it on a wall will result in 6 dB too much bass and a dark and mumbling music reproduction. So there is always good reason to experiment with the positioning of your speakers in the living room. However, wall bass boosting is not the subject of this little article series. It's something far more unruly.

 

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Omnidirectional radiation. The starting point for our measurements is that a loudspeaker emits sound in all directions at the lower frequencies. Illustration: Genelec.

 

Backward sound sent forward

We have previously looked at the sound dispersion in loudspeakers and here it appeared that bass tones disperse equally in all directions, i.e. in a spherical shape. Here, just as much sound is emitted in front of the speaker as behind it. When the loudspeaker is hung on a wall, the bass tones that the loudspeaker sends out behind it are reflected by the wall behind the loudspeaker and are thus thrown back into the room. This is the reason for the extra 6 dB of sound pressure in the bass range. However, the reflected sound that is thrown into the room via the rear wall is slightly delayed compared to the sound that the speaker plays directly into the room. This is because the speaker units are located at a certain distance from the back wall, and it takes time for the sound to travel those centimetres from the speaker faceplate, to the wall and back into the room. Nonsense, you may think. A few centimetres travelling at 1000 km/h can't change anything, can it? But it can! I would argue that if you get these reflections at home somewhat under control, you will experience a greater advance in sound quality than an amplifier twice your price could dish out.

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Ole Witthøft
Ole is the founder of System Audio. His 3 greatest passions are music, design and technology. Every day, Ole is working on some kind of projects, and you find him in the workshop, in the production, behind a computer or on one of his many presentations around the world.

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