GB2265520A

GB2265520A - Motional feedback control of loudspeakers using simulated acoustical impedance

Info

Publication number: GB2265520A
Application number: GB9306098A
Authority: GB
Inventors: Maximilian Hans Hobelsberger
Original assignee: Individual
Current assignee: Individual
Priority date: 1992-03-24
Filing date: 1993-03-24
Publication date: 1993-09-29
Anticipated expiration: 2013-03-24
Also published as: DE4308961A1; AT406105B; CH685657A5; GB9306098D0; GB2265520B; ATA55793A; DE4308961B4

Description

2265520 1 Device for active simulation of an acoustical impedance

Description

In some applications in the f ield of acoustics devices are needed which reflect acoustical waves in a specified way. Often these devices should not reflect any acoustical waves.

At high frequencies this specified behavior, e.g. no reflection, can be achieved by simple, passive constructive means, i.e.the of use absorptive materials. However at low frequencies the dimensions of absorptive structures get large and impractical.

The devices according to the invention allow the active simulation of an acoustical impedance. By using these devices a specified behavior of reflection can be achieved. Especially at low frequencies their dimensions are low in comparison to those of passive devices.

One important use is in loudspeaker boxes to eliminate reflections and standing waves inside the housings.

The device according to claim 1 consists of an electrodynamic transducer with a membrane driven by a coil which is placed in a magnetic field. The membrane is equipped with a pressure sensor on its surface. The gas pressure on the surface is measured by the sensor. The signal produced is forwarded to a controller, which controls the movement of the membrane via a power amplifier. The position, the speed and the acceleration of the membrane are measured by sensors too. Their signals are used by the controller to control the movement of the membrane. The controller forces the membrane to move with a speed and an acceleration depending on the momentary gas pressure on the surface according to the desired acoustical impedance function. The impedance function defines the relation between gas speed and gas acceleration and the gas pressure on the surface of the membrane. The setpoint values for the movement are derived from

2 the measured pressure values according to the impedance function either by fast calculators or by analog models.

According to claim 2 the material polyvinylidenfluorid, PVW, or other piezoelectric polymers are used for pressure sensors on the surface of the membrane.

The device according to claim 3 is a series combination of a passive and an active acoustical impedance. Typically the device consists of an e.g. cylindrical housing. The inner volume of the housing is divided into two chambers by a soundproof wall. An electrodynamic transducer is built into an opening of this wall. The membrane of the transducer separates the two chambers from each other. The membrane is equipped with pressure sensors and acts together with a controller, as active acoustical impedance. The controller controls the movement of the membrane according to the impedance function and the measured pressure. Speed and acceleration sensors give the controller information about the membrane's movement. The inner chamber which adjoins the surface of the membrane's pressure sensor is connected to the outer space via openings in the casing. These openings are shaped and stuffed with sound absorbing material in a way, that sound with higher frequencies is absorbed. Sound with lower frequencies can pass this filter. It will be reflected or absorbed by the active impedance according to the desired impedance function. The advantage of this series arrangement is that the control loop is not excited by high frequencies.

Claims

Claim 4 describes devices with an open inner chamber. The chamber which

adjoins to that membrane's surface, which has no pressure sensor, is connected to the outside

Claim 5 and 6 concern the application of the invented devices in loudspeaker systems with closed housings. The devices are used to eliminate standing waves and sound reflections inside the housing.

A 1 3 Claim 7 describes a loudspeaker system whose inner volume consists of a bundle of parallel pipes. The diameter of the pipes is small in comparison to the wavelength of the reproduced sound. That ensures that no standing waves can arise in perpendicular direction to the axis of the pipes. The pipes are closed at one of their endings by devices which simulate sound impedances. The impedances match the impedances of the pipes to avoid sound reflections.

Claim 8 concerns a different placement of the pressure sensor. The sensor is not attached to the membrane, but is placed near the membrane.

Description of the drawing The figure shows a loudspeaker system according to claim 6. The inner volume of the housing G is divided by two walls into three chambers V1, V2 and V3. An electrodynamic transducer TR is built into the wall T2. Its membrane M is equipped with a pressure sensor S. The signal produced by the sensor is applied to a analog model AM of the impedance. An other sensor measures V the speed of the membrane. The output signal of the analog model is applied as setpoint-value for speed to the controller R. The controller drives the inner transducer via a power amplifier A. The other inner wall Tl has openings D to the other chamber V1 which are stuffed with flow resistive material. The chamber V1 adjoins the loudspeaker L.

L4 Claims l.) Device to simulate a selectable acoustical impedance, characterized by the claimed f acts, that an electrodynamic transducer is built into the opening of an acoustically closed housing in a way, that its membrane separates the inside from the outside, that on the outer surface of the membrane of the transducer a pressure sensor is placed which measures the gas pressure on this surface and which produces a signal indicative of this gas pressure, that the movement of the membrane is measured by other sensors, that the transducer's membrane is driven by an electronic controller via a power amplifier, that this controller controls the movement of the membrane using the signals of the speed and acceleration sensors, that the controller's setpoint-values for the membrane's movement are derived from the signal of the pressure sensor, that this is achieved by applying this signal to an electronic model or to a calculator which produce the setpoint-values according to the desired impedance function, and that the membrane is forced by the controller 2.) Device according to claim 1, characterized by the claimed fact, that the material polyvinyl idenf luorid, PVW, or other piezoelectric polymers are used for the pressure sensors on the membrane's surface.

3.) Device to simulate an acoustical impedance, characterized by the claimed facts, that the device consists of a soundproof housing, whose inner volume is divided by two soundproof walls into two chambers, that an electrodynamic transducer is placed in one opening of one inner wall in a way, that it's membrane separates the the two chambers, that one chamber has openings opposite to the membrane connecting to the outside of the housing, that these openings are shaped and stuffed with material to create a flow resistance for gas, that a gas 1 pressure sensor is placed within this chamber with the openings upon the adjoining membranes surface, that other sensors are used to measure the movement of the membrane, that a controller drives the transducer via a power amplifier and controls the membrane's movement based upon the signals of the movement sensors, that the controller uses setpoint values for the movement which are derived from the pressure signal according to a chosen impedance function, and that the controller forces the membrane to move in a way that the relations between the pressure on the surface and the membrane-s speed and acceleration almost match the desired impedance function 4.) Device according to one of the preceding claims, characterized by the fact, that the chamber, which adjoins that transducer-s surface which is not equipped with a pressure sensor, is not closed, but is connected to the outside of the housing via openings.

5.) Application of a device according to one of the preceding claims in a loudspeaker 'system with closed or partly open housings, characterized by the claimed fact, that the device for active simulation of an acoustic impedance is used within the housing.of the loudspeaker system in a way, that its membrane's movement influences the inner chamber of the system's housing.

6.) Loudspeaker system with closed housing, characterized by the claimed facts, that the housing is shaped like a pipe, that the loudspeaker is mounted at one end of the pipe, that the other end of the pipe is closed by a device which actively simulates an acoustical impedance, and that the impedance function is chosen to eliminate wave refections within the pipe or to otherwise support the loudspeaker's reproduction of sound.

7.) Loudspeaker system with closed housing, characterized by the claimed facts, that the inner chamber consists of a bundle of thin pipes, which are open at the end which adjoins to the c loudspeaker's membrane, and which are closed at the other end by devices which actively simulate an acoustical impedance.

8.) Device according to one of the preceding claims, characterized by the claimed fact, that the pressure sensor is not attached to the membrane, but is placed near the membrane.