GB2516469A

GB2516469A - Loudspeaker improvements

Info

Publication number: GB2516469A
Application number: GB201313141A
Authority: GB
Inventors: Richard Warrington George
Original assignee: Individual
Current assignee: Individual
Priority date: 2013-07-23
Filing date: 2013-07-23
Publication date: 2015-01-28
Also published as: GB201313141D0

Abstract

A deflectable membrane 34 is mounted in spaced relationship to, and covering the output side of the reflex port 19. The deflectable membrane may be a polyurethane elastic film, a semi rigid membrane or rigid membrane with a flexible surround (34, 36 fig 2). A means of adjusting the degree of coupling between the reflex port and the deflectable membrane is also included 40, 42. This is said to more efficiently couple sound waves emitted from the reflex port with the surrounding air.

Description

Loudspeaker Improvements

Background

The present invention relates to a means of improving the efficiency of bass reflex speaker systems, such speakers are well known and have been used for many years.

Normal sealed or infinite baffle type speakers do not utilise the energy from the rear of the drive unit. A bass reflex speaker system improves the efficiency of a speaker by utilising the low frequency sound waves from the rear of the drive unit. It achieves this by utilising a duct, the duct acts as a Helmholtz resonator which inverts the phase of sound waves over a limited frequency range emitted from the rear of the drive unit. These sound waves are then emitted outside the cabinet via a reflex port in the cabinet wall; these sound waves are in phase with the driver output and serve to augment the low frequency output of the driver thus improving system efficiency and low frequency sound output of the speaker system.

This system has a number of disadvantages including limited or no resistance to speaker cone movement at low frequencies below those of the tuned duct/reflex port frequency, the duct can also pass sound waves outside its tuned frequency range.

In addition to this the air velocity in the port can be very high, this combined with the compressible nature of air and turbulent flow can result in poor coupling between the sound output of the port and the surrounding air resulting in less than ideal transfer of sound energy from the outlet of the reflex port to the surrounding air.

Summary of the invention

An objective of the present invention is to provide a bass reflex type speaker system with ducted port that has improved coupling between sound waves emitted from the reflex port and the surrounding air thus giving improved efficiency and low frequency sound output.

Additional objects and advantages will be set forth in the following description The objects of this invention are achieved by mounting a deflectable membrane to a bass reflex loudspeaker such that it is in spaced relationship to and covering the outlet of the bass reflex port such that a cavity is formed between the bass reflex port outlet and the deflectable membrane, said bass reflex type speaker system comprising a cabinet, a driver mounted in the cabinet for transmitting sound waves inside the cabinet, a bass reflex port in a wall of the cabinet, a duct open at both ends mounted inside the cabinet with one end connected to the bass reflex port.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.

Figure 1 is a vertical section through a first embodiment of a bass reflex speaker system according to the invention with an elastic membrane sound radiator.

Figure 2 is a vertical section through a first embodiment of a bass reflex speaker system according to the invention with a suspended rigid sound radiator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings.

Figure 1 shows a vertical section through a 2 way bass reflex speaker systemlO, comprising a speaker cabinet 11, a bass/mid driver 12, a high frequency driver 16, and a bass reflex output port 19. For the sake of clarity components that do not relate to the invention such as the crossover filters, cabling and damping material are not shown in Figure 1 or Figure 2 The bas/mid-range driver 12 has a speaker cone 13 driven by a voice coil (not shown) in a magnet structure 14, which is supported by an acoustically open metal basket 15. The open end of the speaker cone 13 is connected to the basket 15 via a soft ring called a surround, which forms a seal between the speaker cone 13 and the outside of cabinet 11, but allows in/out movement of the speaker cone. The surround usually has the shape of a half-toroid, so the material can roll instead of stretching when the speaker cone 13 moves. The narrow end of the speaker cone 13 is connected to the magnet structure via a membrane with concentric corrugations called a spider. The surround and the spider allow a pure reciprocating motion of the speaker cone, so that when the voice coil of driver 12 is connected to output terminals of an audio amplifier, the front of the speaker cone 13 radiates directly to the outside of the cabinet, while the rear of the speaker cone 13 radiates via the open basket into the enclosed cabinet 11. The tweeter 16 has a dome 17 driven by a voice coil (not shown) inside a magnet structure 18. The dome 17 is supported by a surround, but usually there is no spider. When the voice coil of the tweeter is connected to output terminals of an audio amplifier, the front of the dome 17 radiates directly to the outside of the cabinet. The rear of the dome 17 radiates into a closed chamber housing the magnet structure, so the tweeter does not affect the sound pressure inside the cabinet 11.

The inside of the cabinet 11 communicates acoustically with the outside only through the bass reflex output port 110, via an opening 22 in a duct 20 with wall 21 made of cardboard or plastic. The duct 20 and the interior of the cabinet form a Helmholtz resonator with a resonance frequency determined by the compliance of the air volume inside the cabinet 11 and the air mass inside the duct 20. Sound with frequency equal to the resonance frequency of the Helmholtz resonator, passes through the port 19 with a phase shift of 180 degrees, sound pressure at frequency from the port adds directly to the sound pressure from the front of the speaker cone 13. Sound of all other frequencies is attenuated. By selecting a resonance frequency about 1/2 octave lower than the roll-off frequency of driver 12, it is possible to get flat response to a bass frequency 1/2 octave lower than for a speaker with a closed cabinet without need for increased amplifier power, which is the object of a bass reflex speaker system.

According to the present invention the coupling between the sound waves emitted from the bass reflex port 19, and the surrounding air 30, is improved by interposing a deflectable membrane 34 mounted in spaced relationship to the port 19, on a frame 32, thus forming a cavity 38 between the deflectable membrane 34 and the output of the reflex port 19. The membrane34, preferably having a larger surface area than the surface area of the bass reflex port 19. In this way sound waves emitted from the bass reflex portlY act across the larger area of the deflectable membrane 34, thus improving coupling between the output of the bass reflex port 19 and the surrounding air 30. The overall effect being to increase efficiency and low frequency sound output of the speaker system.

The deflectable membrane 34 is preferably made from elastic, acoustically self-damping film with low mass, in practice films made from polyurethane or silicone elastomer exhibit excellent characteristics for this application.

The deflectable membranes 34 can also be made from a substantially inelastic material, permeable materials or solid material such as shown in Figure 2. The inelastic membrane 34 can be flat as shown or shaped, the inelastic membrane 34 having been made deflectable by means of a surround 36, which is formed around the periphery of the membrane 34. The surround 36 allows reciprocating deflection of the membrane 34 in the same way as the surround for an ordinary speaker cone. The surround 36 can be formed in the same material as the membrane, or it can be made of a different material by gluing to the membrane 34.

Outside the surround 36 are sections 32 for mounting and sealing the membrane 34 with surround 36 in spaced relationship to the bass reflex port 19.

The deflectable membrane 34 may or may not be sealed to the reflex port outlet 19 depending on the degree of coupling required between the reflex port sound output and the deflectable membrane 34 Additionally a port or ports 40, can be included between the sealed cavity 38 and the surrounding air 30, an adjustable slide 42 or other means may be provided to allow the user to adjust cross sectional area of the port or ports 40 and thus the leakage of sound waves from the cavity 38 to the surrounding air 30 via the port or ports 40 thus adjusting the degree of sound wave coupling between the reflex port output 19 and the deflectable membrane 34, allowing the user to adjust the sound output of the deflectable membrane 34.

The present invention can also be optimised to offer additional benefits, in the first instance the compliance of the deflectable membrane 34 can be such that it provides resistance to excessive travel of the driver cone 12 at frequencies below the lower cut off frequency of the duct 20 and bass reflex port 19. In this way damage to the driver can be prevented in the event of overdriving of the speaker at low frequencies. In the second instance the deflectable membrane can by virtue of optimising its mass and compliance be tuned to resonate at some frequency such that additional low frequency sound reinforcement can be achieved.

To allow tuning of the speaker system to suit particular room conditions the membrane 34 and mounting frame 32 may be supplied separate from the speaker, such that the user can attach or detach the frame and membrane to the speaker to tune the bass output to suit taste, room conditions or speaker location.