GB2413233A

GB2413233A - Bass reflex or ABR loudspeakers with positive feedback

Info

Publication number: GB2413233A
Application number: GB0408240A
Authority: GB
Inventors: Peter Alexander Fryer
Original assignee: B&W Loudspeakers Ltd
Current assignee: B&W Loudspeakers Ltd
Priority date: 2004-04-13
Filing date: 2004-04-13
Publication date: 2005-10-19
Anticipated expiration: 2024-04-13
Also published as: GB2413233B; GB0408240D0

Abstract

A bass reflex loudspeaker system comprises a loudspeaker enclosure 2, a loudspeaker drive unit 4 mounted in the loudspeaker enclosure, means 8, 10, 12 to provide an electrical drive signal to the loudspeaker drive unit, and a tuning port 6 or passive diaphragm (7, Fig 2) in the enclosure. The loudspeaker drive unit 4 and the tuning port (6) or passive diaphragm are tuned to frequencies appropriate for a bass reflex loudspeaker enclosure of substantially greater internal volume than the internal volume of the loudspeaker enclosure actually used. A transducer 14 is provided to produce an electrical signal related to the sound waves present at the tuning port or passive diaphragm, and the electrical signal from the transducer is included as positive feedback in the electrical drive signal to the loudspeaker drive unit.

Description

- 1- 2413233 Loudspeaker Systems This invention relates to bass reflex

loudspeaker systems.

A bass reflex enclosure constitutes a known way of improving the bass response of a loudspeaker system. For that purpose, a tuning port or a tuned passive diaphragm forming an auxiliary bass radiator is provided. Whilst a bass reflex enclosure can significantly improve the bass response for a given size of enclosure, nevertheless bass reflex enclosures are often too large for modern homes.

This arises from the fact that a large physical size is needed to obtain a sufficiently low resonance frequency and suitable frequency response for the enclosure.

It is an object of the invention to provide a bass reflex loudspeaker system which allows a relatively small loudspeaker enclosure to be used.

The present invention provides a bass reflex loudspeaker system comprising: a loudspeaker enclosure; a loudspeaker drive unit mounted in the loudspeaker enclosure; means to provide an electrical drive signal to the loudspeaker drive unit; and a tuning port or passive diaphragm in the enclosure; characterized in that the loudspeaker drive - 2 unit and the tuning port or passive diaphragm are tuned to frequencies appropriate for a bass reflex loudspeaker enclosure of substantially greater internal volume than the internal volume of the loudspeaker enclosure actually used, in that a transducer Is provided to produce an electrical signal related to the sound waves present at the tuning port or passive diaphragm, and in that the electrical signal from the said transducer is included as positive feedback in the electrical drive signal to the loudspeaker drive unit.

The positive feedback signal from the sound waves present at the tuning port or passive diaphragm employed in conjunction with the low tunings of the loudspeaker drive unit and the port or diaphragm has the surprising effect of enabling the acoustic performance of a bass reflex system with an enclosure of larger size to be achieved. In this way, the acoustic performance of a relatively large bass reflex enclosure can, surprisingly, be achieved with an enclosure of relatively small physical size.

The loudspeaker drive unit and the tuning port or passive diaphragm may be tuned to frequencies appropriate for a bass reflex enclosure of internal volume more than one and a half times and less than ten times the internal volume of the loudspeaker enclosure actually used.

The loudspeaker drive unit and the tuning port or passive diaphragm may be tuned to frequencies appropriate for a bass reflex enclosure of internal volume more than twice and less than five times tile internal volume of the loudspeaker enclosure actually used.

The loudspeaker drive unit and the furling port or passive diaphragm may instead be tuned to frequencies appropriate for a bass reflex enclosure of internal volume more than four times and less than eight times the internal volume of the loudspeaker enclosure actually used.

The transducer may convert either air acceleration or dynamic pressure at the tuning port or passive diaphragm into an electrical signal. This constitutes a simple means of obtaining the feedback signal. The acceleration of air and the dynamic (that is, time- varying) pressure are proportional to one another so either may be monitored.

The transducer may comprise a microphone located at the tuning port.

The microphone may be located at the mouth of the tuning port.

When a passive diaphragm is provided, the transducer preferably converts the acceleration of the passive diaphragm into an electrical signal. - 4

The transducer may comprise an accelerometer provided on the passive diaphragm.

The positive feedback signal may be processed to compensate for nonlinearities resulting from the small size of the enclosure. By this means, as explained more detail later, distortion which would otherwise require the sensitivity of the loudspeaker drive unit to be reduced can be avoided.

Bass reflex loudspeaker system constructed in accordance with the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic illustration of a first bass reflex loudspeaker system embodying the invention; and Figure 2 is a diagrammatic illustration of a second bass reflex loudspeaker system embodying the invention.

Referring to the accompanying drawings, Figure 1 shows a bass reflex loudspeaker system 1 comprising a loudspeaker enclosure 2, a loudspeaker drive unit 4 mounted in the loudspeaker enclosure 1, and a tuning port 6.

An audio input 8, a summation amplifier 10 and a power am,olifier 12 constitute means to provide an electrical drive signal to the loudspeaker drive unit 4.

The audio input 8 Is connected to one input of the summation amplifier 10, and the output of the summation amplifier is connected to the input of the power amplifier 12. A transducer in the form of a microphone 14 is provided at the tuning port 6 and connected to the input of a signal amplifier 16, the output of which is connected to a second input of the summation amplifier 10. By this arrangement, an electrical signal related to the sound waves present at the tuning port is produced by the microphone 14 and is included as positive feedback in the electrical drive signal to the loudspeaker drive unit 14. The dynamic pressure at the tuning port (proportional to the acceleration of air in the port) is converted into an electrical signal by the microphone 16.

As illustrated, the microphone 14 is located outside the mouth of the tuning port 6 but other positions are possible, such as inside the mouth of the tuning port.

The loudspeaker drive unit 4 and the tuning port 6 are tuned to frequencies appropriate for a bass reflex loudspeaker enclosure of substantially greater internal volume than the actual internal volume of the loudspeaker enclosure 2 actually used, for example, about three times or about five times greater. The matching of the - 6 - resonant frequency of the loudspeaker drive unit and that of a port or passive diaphragm to the size of the loudspeaker enclosure is well known to those skilled in the art and does not therefore need to be explained further here. Theoretically, the loudspeaker drive unit and the tuning port could be tuned to frequencies appropriate to a hypothetical enclosure any number of times larger but, in practice, the loudspeaker drive unit and the tuning port cannot be tuned so low that the enclosure is no longer able to accommodate them physically.

Figure 2 shows a second bass reflex system 1' in which identical elements to those used in Figure 1 are given identical reference numerals. Only the differences from Figure 1 will therefore be described. In Figure 2, a passive diaphragm 7 is provided and an accelerometer 15 located on the passive diaphragm is provided to convert the acceleration of the passive diaphragm into an electrical signal.

In these two loudspeaker systems, the low tuning of the loudspeaker drive unit and the tuning port or passive resonator create a low resonant frequency and the positive feedback derived from the tuning port or the passive radiator modifies the frequency response about that resonant frequency to correspond to that of the hypothetical larger enclosure. The voltage boost created - 7 by the feedback signal In the drive signal to the loudspeaker drive unit and which adapts the frequency response of the loudspeaker driver unit in the smaller enclosure to that in the hypothetical larger enclosure consists of low frequencies only.

When tuning port output acceleration or pressure, or the passive diaphragm acceleration or pressure are used to provide positive feedback, the low frequency performance which would otherwise be unavailable in too small a enclosure is instead achieved. As explained, to create the performance obtained by the same loudspeaker drive unit in a hypothetical larger enclosure it is necessary to tune the loudspeaker drive unit and the port or passive diaphragm so that resonance frequencies appropriate for the larger enclosure are obtained and this can be achieved appropriate tuning of the loudspeaker drive unit, and dimensioning the tuning port to have a larger air mass associated with it or by giving the diaphragm greater mass. The appropriate amount of port or passive diaphragm pressure or acceleration positive feedback is used to achieve the desired frequency response corresponding to that of the hypothetical large enclosure with the same loudspeaker drive unit. The same frequency response and sensitivity that would have occurred with the same loudspeaker drive unit in the hypothetical large enclosure are then obtained, but now with the larger real tuning port air mass or passive diaphragm mass in the smaller enclosure.

The voltage feedback curve follows the port or passive diaphragm output acceleration curve and therefore the extra voltage provided by the feedback reaches a peak exactly at the port or passive diaphragm enclosure tuning frequency and falls off at 12 dB per octave on each side of this frequency.

When the performance of the loudspeaker drive unit in a smaller enclosure is changed, according to the invention, to that in the hypothetical larger enclosure, the same total amount of air needs to be moved in the small enclosure with feedback as in the large enclosure without feedback. Therefore the cone excursion of the loudspeaker drive unit in the small enclosure is the same as that of the same driver in the larger enclosure.

Extra power is not fed into the loudspeaker drive unit at all frequencies; the extra power is centred around the port or passive diaphragm tuning frequency.

Tuning port or passive diaphragm positive acceleration or pressure feedback combined with the low tuning of the loudspeaker drive unit and the port or diaphragm gives the unexpected result of creating, in the smaller bass reflex enclosure, the performance obtained by the same driver in the hypothetical larger bass reflex enclosure. - 9 -

Distance of the transducer from the port or passive diaphragm will reduce the measured pressure and produce phase changes which vary with frequency. If, however, the distance is much smaller than the smallest wavelength in the band of application, these effects are minimal and can be disregarded. When, however, a microphone is placed at any substantial distance from the mouth of the port, there will be a changing phase relationship between the measured pressure and the port acceleration depending on the distance. The signal from the microphone will, in this case, have to be processed to remove the phase effects before the resulting signal can be used for feedback purposes. An accelerometer on a passive radiator suffers no such problems, provided the passive radiator does not exhibit so-called "break-up" in the pass band.

The positive feedback signal can, if desired, be filtered by means of a frequency selective network or it can be digitally processed electronically. For example, the positive feedback signal can be processed so as to compensate for non-linearities in the loudspeaker system.

Such compensation is particularly useful in an enclosure of such small size that the change in internal volume as the diaphragm of the loudspeaker drive unit moves would otherwise result in sound distortion unless the maximum - 10 possible output level and/or sensitivity of the loudspeaker drive unit were reduced.

The invention also embraces the monitoring of port pressure by deriving it from the total pressure lo the loudspeaker enclosure with appropriate signal processing to extract the desired signal.

The feedback signal from the diaphragm can be obtained using a closely positioned microphone instead of an accelerometer.

Claims

C L A I M S: 1. A bass reflex loudspeaker system comprising: a loudspeaker

enclosure; a loudspeaker drive unit mounted in the loudspeaker enclosure; means to provide an electrical drive signal to the loudspeaker drive unit; and a tuning port or passive diaphragm in the enclosure; characterized in that the loudspeaker drive unit and the tuning port or passive diaphragm are tuned to frequencies appropriate for a bass reflex loudspeaker enclosure of substantially greater internal volume than the internal volume of the loudspeaker enclosure actually used, in that a transducer is provided to produce an electrical signal related to the sound waves present at the tuning port or passive diaphragm, and in that the electrical signal from the said transducer is included as positive feedback in the electrical drive signal to the loudspeaker drive unit.
2. A loudspeaker system as claimed in claim 1, wherein the loudspeaker drive unit and the tuning port or passive diaphragm are tuned to frequencies appropriate for a bass reflex enclosure of internal volume more than one and a half times and less than ten times the internal volume of the loudspeaker enclosure actually used. - 12
3. A loudspeaker system as claimed in claim 2, wherein the loudspeaker drive unit and the tuning port or passive diaphragm are tuned to frequencies appropriate for a bass reflex enclosure of internal volume more than twice and less than five times the internal volume of the loudspeaker enclosure actually used.
4. A loudspeaker system as claimed in claim 2, wherein the loudspeaker drive unit and the tuning port or passive diaphragm are tuned to frequencies appropriate for a bass reflex enclosure of internal volume more than four times and less than eight times the internal volume of the loudspeaker enclosure actually used.
5. A loudspeaker system as claimed in any preceding claim, wherein a tuning port is provided and the transducer converts the air acceleration or dynamic pressure at the tuning port into an electrical signal.
6. A loudspeaker system as claimed in claim 5, wherein the transducer comprises a microphone located at the tuning port.
7. A loudspeaker system as claimed in claim 6, wherein the microphone is located at the mouth of the tuning port.
8. A loudspeaker system as claimed in any of claims 1 to 4, wherein a passive diaphragm is provided and the transducer converts the acceleration of the passive diaphragm into an electrical signal.
9. A loudspeaker system as claimed in claim 8, wherein the transducer comprises an accelerometer provided on the passive diaphragm.
10. A loudspeaker system as claimed in any preceding claim, wherein the positive feedback signal is processed to compensate for non-linearities resulting from the small size of the enclosure.