APP Acoustic Performance Plug

Report January 1999, ©Mats Andersen, Qln Acoustics AB

General
This summary is not intended to convince you about the excellence of Qln G3. You are
already aware of its virtues and know of the sound quality it is capable of. Instead we
would like to give a technical background to how it actually works.

The design philosophy
Our goal was to create a loudspeaker which combines outstanding sound quality with
Scandinavian design and excellent finish, but with an appearance possible to integrate
into a modern and aesthetically decorated home. Qln G3 is only 11 centimetres wide
and the transducer units are strikingly small. Normally, this means limited bass
reproduction and a weakened sound. This is not the case with Qln G3. It retains
transparency and dynamics also at high sound pressure levels.
The inearity in the low frequency range comes from three co-operating solutions.
First of all the speaker unit itself provides excellent performance in terms of
controlled behaviour throughout its frequency range. It has a rigid chassis and a
high-precision suspension.

The cabinet
Secondly the cabinet is a quarter-wave pipe. This means that it can be easily tuned to
match the resonance frequency of the speaker unit and even extend the range a bit.
The tall and lean proportions combined with the placement of the units also provide
for full and controlled damping of harmonics to the pipe resonance.

The secret
This is the little secret of this loudspeaker; our invention APP, the Acoustic
Performance Plug. By attaching a defined amount of air to the cone diaphragm we can
increase the moving mass at low frequencies. The diaphragm is better coupled to the
surrounding air, which increases movement control and the efficiency at low
frequencies. The audible result is an agile loudspeaker with a well-defined transient
response, hence providing a clean mid-range and a more natural sound reproduction.

The solution
In practice the APP is an open cavity placed behind the low-frequency units. It
contains mechanical impedance that can be compared to an RC link. Below the
resonance frequency, where the diaphragm would normally operate under poor
loading conditions, it adds an acoustic resistance that limits the travel of the cone.
By tuning the dimensions and the mechanical impedance it is matched to the
characteristics of the speaker unit. The result is a reduction of intermodulation and
doppler distortion products, and the bass reproduction remains clean, tight and
controlled.

APP is only active at low frequencies, around the resonance point. The added
acoustical mass does not load the diaphragm at higher frequencies, nor does it limit
rapid movements of the cone. With this acoustical solution we can combine a deep,
and well-controlled bass with high definition and transient response.

Mölndal, January 1999
Mats Andersen