GB2260875A - Feedback controlled noise source for acoustic testing - Google Patents

Abstract

The noise source has a rigid cone 1 supported via a flexible diaphragm 2 at an aperture 3 in a generally spherical steel enclosure 4. The enclosure 4 is hollow and partially lined with sound-absorbent material 5 to absorb noise within the enclosure. The enclosure ensures that noise from the source is propagated only from the exposed face of the cone. Within the enclosure at the aperture, a loud speaker coil drive assembly 6 is mounted for driving the cone. Rigidly fixed on the outside of the rigid cone is a conical extension 7. At its forward end, this carries an accelerometer 8 oriented to measure acceleration of the cone towards and away from the drive assembly and thereby to enable feedback control of the cone's movements. This allows for an accurately reproduced sound to be introduced into a room for room calibration purposes. <IMAGE>

Description

NOISE SOURCE The present invention relates to a noise source for use in calibrating an environment's response to a standard noise generated from the noise source.

In acoustic testing, it is frequent practice to work in anechoic chambers, which absorb practically all sound incident on their walls, and reverberant chambers, which in contrast absorb practically none of the sound incident on their walls. In reverberant chambers, standing waves are established by certain noise in the chambers. The wave patterns will differ with frequency and chamber shape. In practice, many rooms in which tests need to be carried out, such as at the end of an automotive production line, fall between these extremes, that is they reverberate for certain frequencies of noise and absorb other frequencies. At reverberant frequencies, a small excitation can result in a high noise. Thus in order to measure noise from a piece of equipment being tested, the room's response to noise over a frequency range of interest needs to be calibrated.Once response to standard noise is known, the noise generated by the equipment being tested can be measured by comparison with the calibration.

The object of the present invention is to provide an improved noise source with which a standard noise can be generated.

With a noise source, in effect including a loudspeaker, noise is generated by means of an electromagnetic drive coil. This drives in oscillation a so-called cone (which need not be truly conical in shape), from which the noise propagates into the ambient atmosphere. If a fully defined noise is to be propagated, it is important to ensure that the cone's movement faithfully follows a desired excitation of it. There are two main factors liable to disturb the cone's movement from following the desired excitation.

Firstly, due to structural resonance of the cone, its noise propagating surface may move with a different amplitude from that of a driven portion of the cone, i.e. its coil.

Secondly, due to standing waves for instance, the entire cone may move with the waves at an amplitude greater than desired.

Accordingly, the noise source of the invention comprises: a loudspeaker having a substantially rigid cone, and a cone movement transducer.

Whilst it is envisaged that the cone movement transducer may be remote from the cone per se, that is of a type adapted to measure cone movement without direct physical contact with the cone, as by electromagnetic or optical coupling; the cone movement transducer is preferably directly connected to the cone, that is rigidly fixed thereto.

The rigidity of the cone ensures that there is no flexure thereof which could allow coil movement without a corresponding movement of air adjacent the cone. In practice the cone is likely to have at least one resonant frequency, in which case, the requirement for rigidity is met by providing that the lowest resonant frequency of the structure of the cone, and the accelerometer when rigidly fixed thereto, is above the highest design frequency of the noise to be generated.

The transducer enables the movement of the cone to be monitored in order to ensure that the movement of the cone is of an intended amplitude.

The cone movement transducer enables feedback control of the cone's movement to compensate for the usual frequency dependent response of the air contained in the test room.

This causes an acoustic load on the loudspeaker which itself varies with frequency. Thus, in the absence of a feedback control system, even if the loudspeaker were supplied with an excitation voltage that was constant regardless of frequency, the resulting motion of the cone would not be constant regardless of frequency.

The noise source will normally be supplied with suitable drive and feedback control means, to operate in conjunction with a noise measurement system. However, it may be supplied separately.

The noise source will normally be used in conjunction with a drive circuit, the amplitude of which is controlled by feedback from the transducer to ensure that the cone's movement is of the intended amplitude at all frequencies of interest.

Preferably the transducer will be an accelerometer, from which acceleration, velocity or displacement can be measured. Thus noise whose strength can be measured in terms of constant acceleration, constant velocity or constant displacement of the cone can be controllably produced from the source and used for calibration of the room.

In the preferred embodiment, which is intended for calibration of a room in which tonal noise is to be measured, the cone is driven through a frequency range with a constant velocity. This constancy can be measured in terms of a constant peak to peak value or a constant RMS value.

Preferably, the cone of the noise source, the drive coil and its magnet are mounted in an aperture in a massive, hollow enclosure in order to prevent noise propagation from the rear face of the cone. Typically the enclosure has a mass of at least one thousand times that of the cone. The enclosure is preferably lined or filled with sound absorbent material to avoid the build up of reverberant noise within the enclosure. Further to avoid noise propagation from the external surface of the enclosure, its lowest resonant frequency is above the highest design frequency of the noise to be generated. In addition, the enclosure is preferably sufficiently stiff that any sound propagated from its external surface is negligible in amplitude in comparison with that propagated from the cone.

To help understanding of the invention, a specific embodiment thereof will now be described by way of example and with reference to the accompanying drawings, in which: Figure 1 is a cross-sectional side view of a noise source of the invention, Figure 2 is a block diagram of a control system for the noise source of Figure 1 and Figure 3 is a block diagram the noise source in use for calibrating a test room.

The noise source shown in Figure 1 has a rigid cone 1 supported via a flexible diaphragm 2 at an aperture 3 in a generally spherical steel enclosure 4. The enclosure 4 is hollow and partially lined with sound-absorbent material 5 to absorb noise within the enclosure. The enclosure ensures that noise from the source is propagated only from the exposed face of the cone. Within the enclosure at the aperture, a loud speaker coil drive assembly 6 is mounted for driving the cone. Rigidly fixed on the outside of the rigid cone is a conical extension 7. At its forward end, this carries an accelerometer 8 oriented to measure acceleration of the cone towards and away from the drive assembly. Both the cone and the extension are of aluminium.

Their structure - including the accelerometer - is such that the lowest resonant frequency of the structure is above the highest design frequency of the noise to be generated by the noise source. Similarly, the lowest resonant frequency of the enclosure is above this highest design frequency.

Additionally, the enclosure is sufficiently massive and stiff not to radiate noise when the cone is driven.

Typically the enclosure weighs of the order of 10 kg, whilst the cone structure weighs 5 grams. The enclosure does have a small aperture 9 remote from the cone to allow equalisation of the air pressure inside and outside the enclosure. Leads 10 to the drive assembly pass through this aperture. Leads 11 to the accelerometer are of finely braided copper wire, so as not to mechanically load the cone. They are anchored on the top surface of the enclosure whence they are continued as conventional leads.

The enclosure is mounted on a base plate 12 via three resilient isolation mounts 13. A protective framework 14 with a carrying handle 15 extends up from the base past the enclosure. An acoustically transparent cover 16 is mounted within the framework.

Turning now to Figure 2, the noise source and in particular the drive assembly 6 is there shown connected to a signal generator 21 and a controller 20 via a drive circuit 22, comprising a servo unit 23 and a power amplifier 24. The accelerometer 8 is connected in a feedback loop to the servo unit 23. This arrangement - when set up with appropriate gain - provides that the cone is driven with an amplitude whose function with frequency is determined by the controller 20. In practice, the amplitude controller 20, the servo unit 23 and the power amplifier 24 can be incorporated as a single unit.

Figure 3 shows the noise source set up in a room 30 to be calibrated. The controller 20 causes the signal generator to sweep through a frequency range of 200 to 5,000 Hz in one minute for instance. Further the servo unit 23 is controlled to drive the noise source with a cone displacement such that its movement has constant RMS velocity throughout the frequency range. At least one microphone 32 in the room measures the noise as apparent in the room, and its amplitude/frequency map is memorised in a processing unit 33. The room is calibrated by running the noise source through the frequency range and recording the response map.

Then an equipment to be tested, for instance a gearbox 34, is tested through its speed range - typically up to 3,000 rpm. Its noise is recorded by the microphone(s). A spectral analysis is made of the noise at many selected speeds within a range of interest. The results of the analysis are modified in accordance with the calibration map, which is made under conditions of controlled acoustic input to the room, to take account in the analysis of the variations in responsiveness of the room from hour to hour and day to day. The final analysis is a true analysis of the noise produced by the gearbox, with reference to a standard reference test room condition.

It should be noted that whilst the signal generator 21, the drive circuit 22 and the controller are for convenience shown in Figure 3 in the test room, normally they would be located outside the room.

The noise source of the invention provides the facility to re-calibrate the room whenever acoustic conditions in the room change, such as when there is a change of temperature.

The re-calibration can be carried out on a routine basis and under computer control.

Claims (14)

CLAIMS:

1. A noise source comprising: a loudspeaker having a substantially rigid cone, and a cone movement transducer.

2. A noise source as claimed in claim 1, wherein the transducer is of a type adapted to measure cone movement without direct physical contact with the cone.

3. A noise source as claimed in claim 1, wherein the transducer is rigidly fixed to the cone.

4. A noise source as claimed in any claim 3,wherein the transducer is an accelerometer.

5. A noise source as claimed in any preceding claim, wherein the lowest resonant frequency of the structure of the cone, and the accelerometer when rigidly fixed thereto, is above the highest design frequency of the noise to be generated by the noise source.

6. A noise source as claimed in any preceding claim, wherein the cone of the noise source, a drive coil for the cone and its magnet are mounted in an aperture in a massive, hollow enclosure in order to prevent noise propagation from the rear face of the cone.

7. A noise source as claimed in claim 6, wherein the enclosure has a mass of at least one thousand times that of the cone.

8. A noise source as claimed in claim 6 or claim 7, wherein the enclosure is lined or filled with sound absorbent material to avoid the build up of reverberant noise within the enclosure.

9. A noise source as claimed in claim 6, claim 7 or claim 8, wherein the lowest resonant frequency of the enclosure is above the highest design frequency of the noise to be generated.

10. A noise source as claimed in any one of claims 6 to 9, wherein the enclosure is sufficiently stiff that any sound propagated from its external surface is negligible in amplitude in comparison with that propagated from the cone.

11. A noise source as claimed in any preceding claim in combination with drive and feedback control means for the noise source and preferably a noise measurement system.

12. A noise source as claimed in claim 12, wherein the drive means includes a drive circuit which is adapted to drive the cone through a frequency range with a constant velocity.

13. A noise source substantially as hereinbefore described with reference to the accompanying drawings.

14. A method of calibrating a room with a noise source comprising a loudspeaker having a substantially rigid cone, and a cone movement transducer, the method consisting in the steps of: actuating the noise source through a predetermined frequency range with a predetermined cone amplitude function; controlling via a feedback loop utilising the transducer the amplitude of movement of the cone such that the cone describes the said amplitude function; and recording the response of the room to the noise as a map of response against frequency.