GB2507113A

GB2507113A - Acoustic pick-up assemblies

Info

Publication number: GB2507113A
Application number: GB201218867A
Authority: GB
Inventors: Frank Keane
Original assignee: VITALOGRAPH IRELAND Ltd
Current assignee: VITALOGRAPH IRELAND Ltd
Priority date: 2012-10-19
Filing date: 2012-10-19
Publication date: 2014-04-23
Also published as: WO2014060602A1; GB201218867D0

Abstract

Acoustic pick-up assemblies are described, particularly for use in monitoring heart or lung sounds within a patient's body. The assemblies comprise a housing with an electret transducer 6 located therein. In a typical construction, the housing consists of a round disc-like base 4 and a lid 3, the base 4 including annular air pockets 16 for acoustic insulation and having formed in it a chamber on one side of which is the electret transducer 6 and the other side of which is formed by an elastic membrane or diaphragm 2 which may be held against, or adhered with a hydrocolloid adhesive to, the skin of a patient.

Description

ACOUSTIC PICK-UP ASSEMBLIES

This invention relates to acoustic pick-up assemblies, in particular for picking up noises generated internally of the human body, for example the heartbeat or noise caused by airflow when breathing.

For listening to heartbeat or breathing sounds, clinicians have long used a standard stethoscope. A diaphragm is held against the patient's chest, the diaphragm forming one side of a chamber which has a tubular connection to an earpiece, or more usually a pair of earpieces. The vibrations of the diaphragm are transmitted through the air in the chamber behind it and the tubular connection to the ears of the clinician. While standard stethoscopes are widely used in clinical diagnosis and monitoring, they are not suitable for continuous monitoring of a patient's head or breathing. Additionally, the audible volume of the sound is relatively low which sometimes makes it difficult for the clinician adequately to hear what is going on.

WOI01 34033 discloses a stethoscope transducer where the diaphragm forms one plate of a capacitor and circuitry connected to the capacitor is provided to product an audible signal for the user.

Piezo-electric acoustic sensors for use in stethoscopes are disclosed in JP-A-2000139904, EP-A-1 053716 and WO/0035348. None of these operate particularly satisfactorily as continuous monitoring devices on account of their being too heavy.

We have now found that improved performance and considerably improved ease of use, can be achieved by the use of electret transducers to capture the sound, the transducer being incorporated in a housing which is particularly designed to be light in weight, easily applied to a patient (optionally for long periods of time) and which acts to enable faithful capture of the noises from within the body while being relatively insensitive to ambient external noise.

According to a first feature of the present invention, there is provided an acoustic pick-up assembly comprising a housing and an electret transducer including a membrane of electret material mounted therein wherein the housing includes a chamber, one wall of which is covered by a diaphragm and with the electret transducer membrane constituting part of the opposite wall of the chamber, and wherein the housing includes, surrounding the side walls of the chamber, one or more air gaps to reduce acoustic transmission between the outside of the housing and the electret transducer.

The electret transducer used is preferably of omni-directional type, most conveniently (because they are available as ready-made components) an electret condenser microphone insert unit. Such a unit may conveniently be mounted on one side of a printed circuit board while preferably the other side of the printed circuit board carries areas to which wires may be attached, for example audio cable connectors or wires connecting the electret condenser microphone insert into a circuit forming part of a wireless signal transmission module.

Preferably the side of the electret transducer remote from the chamber is sealed to the housing via a permanently soft sealant material, for example silicone-based medical grade putty.

The diaphragm, which is applied to the skin of a patient during use, is preferably formed of a material having an acoustic impedance similar to that of human skin. The diaphragm does not act in any capacitative fashion, but rather merely acts faithfully to transmit the acoustic vibrations from the skin into the chamber where they are picked up by the electret transducer. A preferred material for the diaphragm is a polyurethane polymer elastic membrane. The use of such a membrane is preferred because it is tough and also biocompatible. Such materials are used in wound dressings. The elastic membrane preferably has a thickness of 0.05 to 0.25 mm and it is preferably a membrane of high tensile strength and substantial elongation before break.

If it is desired to use the acoustic pick-up assembly for continuous monitoring, the side of the diaphragm away from the chamber may carry an adhesive coating serving to stick the pick-up assembly to the skin. This is preferably a hydrocolloid gel type of adhesive as used in wound dressings.

Such materials are well-known to give rise to few problems, if any, when they are adhered to the skin for long periods of time, even in the case of sensitive skins. They absorb any exudates from the skin and retain their adhesion to the skin even if the skin is slightly moist.

The design of the housing may vary widely, but, as noted above, it defines a chamber which is sealed on one face by the diaphragm and where the opposite wall of the chamber to the diaphragm has the electret transducer set in it. A highly preferred structure for the housing is that of a generally cylindrical chamber having means for mounting the electret transducer at one end of it, for example by way of a printed circuit board as described above, and where the chamber is surrounded on all of its sides save that destined to face the skin of the patient by outer housing parts which are suitably acoustically isolated from the chamber itself. The acoustic isolation is easiest achieved by providing sealed chambers or pockets surrounding the chamber which has the electret transducer on one side. The housing is preferably a simple lightweight plastics moulding, preferably of ABS copolymer.

Care should be taken in the design of acoustic pick-up assemblies in accordance with the present invention to reduce the amount of ambient noise which can reach the chamber and which accordingly can be picked up by the electret transducer. This may be done by ensuring that there is very little acoustic transmission from the outside of the housing to the chamber within it. Acoustic transmission is minimal across boundaries between materials having substantially different acoustic impedances. Thus, in the case of a housing made of ABS plastics, acoustic transmission is very small into the inner chamber because of the major difference of acoustic impedance between air and the ABS. For example, air typically has an acoustic impedance of 0.000429 MRyals while the acoustic impedance of ABS copolymer is usually around 2.3 MRyals. Likewise, if a silicone putty is used to mount the PCB, very little sound transmission takes place through it because its acoustic impedance is usually around 1.45 MRyals.

In contrast, the area in which acoustic transmission is highly desired is across the diaphragm which is held in contact with the skin of the patient.

The acoustic impedance of skin is usually in the range of 1.53 to 1.68 MRyals, and a typical acoustic impedance of the preferred polyurethane diaphragm materials is around 1.55 MRyals. Accordingly, there is a good pick up of sound from the body cavity when, for example, an acoustic pick-up assembly according to the present invention is adhered to the patient's chest.

The invention is illustrated by way of example with reference to the accompanying drawings which show an acoustic pick-up assembly according to the present invention diagrammatically, or parts thereof.

In the drawings: Figure 1 is a perspective view of an acoustic pick-up assembly in accordance with the present invention; Figure 2 is a sectional view of the assembly shown in Figure 1; Figure 3 is an exploded view of the assembly; and Figure 4 is a plan and side view of the electrically active components of the pick-up.

Referring to Figures 1 to 3, the assembly consists of a housing 1 of generally cylindrical shape having adhered to one side of it a polyurethane membrane sheet 2. Housing 1 consists of a generally cylindrical multi-compartmented component 4 with a central chamber 12 and a lid 3. Component 4 has a series of annular walls surrounding chamber 12 as shown, some of which are slotted to receive an electrical signal cable 5. The signal cables terminates at one end in two wires which are soldered on to connection areas 9 on the back of a round printed circuit board 7 shown in more detail in Figures 3 and 4. The front of the printed circuit board 7 carries an electret condenser microphone insert 6 of known type. At its other end, the lead 5 terminates in a standard jack plug 8.

The mode of assembly of the acoustic pick-up assembly shown in the drawings is straightforward: First of all, the electrical assembly shown in Figure 4 is constructed by soldering the electret transducer 6 to one side of the PCB 7 and soldering the ends of the wires in lead 5 to appropriate terminal areas 9 on the other side of the PCB. The PCB7 is then located centrally in the circular recess in the middle of the top of component 4, and the leads positioned in slots 10 formed in the walls shown in component 4. This locates electret condenser microphone insert 6 as one end wall of cylindrical chamber 12. A quantity of medical putty 11 is then applied to the back of the PCB 7 usually about 0.5 gm, and lid 3 then pressed on to the component 4 to close off the back of the PCB 7. As can clearly be seen in Figure 2, an annular wall 13 forming part of the lid 3 fits into the outermost annular air gap formed in component 4, and denoted 16 in Figures 2 and 3, while an inner annular wall 14 is a fit in on intermediate annular groove 15 in component 4.

Lid 3 may be secured into component 4 by means of an appropriate adhesive. As the lid is engaged on component 4, the quantity of medical putty 11 is pressed into place to prevent any direct air passage between the interior of the lid 3 and the location of lead 5. This leaves, when the lid 3 is fully in place, sealed air-containing chambers 16, 17, 18 extending all around the central chamber and below the periphery of the PCB 7, all of these serving to reduce the quantity of ambient noise penetrating into the main chamber 12 of the pick-up assembly.

The lid 3 may be secured in place by an appropriate adhesive.

Finally, the diaphragm 2 is adhered to the base of component 4 by an appropriate permanent rubbery adhesive composition.

The diaphragm 2 may be, if desired, a laminated structure consisting of a layer of elastic polyurethane film, a layer of a hydrocolloid adhesive, for example about 0.5 mm thick and a release paper sheet covering the hydrocolloid adhesive layer and shielding it from picking up dust or sticking to anything until the pick-up assembly is put to use by sticking it on to the skin of a patient.

The electric transducer 6 may be selected from a wide variety of such transducers commercially available. A preferred transducer is an omni-directional type having a sensitivity of around 38 db and a flat low frequency response down to around 20 Hz.

Claims

CLAIMS1. An acoustic pickup assembly comprising a housing and an electret transducer including a membrane of electret material mounted therein wherein the housing includes a chamber, one wall of which is covered by a diaphragm and with the electret transducer membrane constituting part of the opposite wall of the chamber, and wherein the housing includes, surrounding the side walls of the chamber, one or more air gaps to reduce acoustic transmission between the outside of the housing and the electret transducer.
2. An acoustic pickup assembly according to Claim 1 wherein the transducer is omni-directional.
3. An acoustic pickup assembly according to Claim 1 or 2 wherein the electret transducer is constituted by an electret condenser microphone insert.
4. An acoustic pickup assembly according to Claim 3 wherein the electret condenser microphone insert is assembled to one side of a printed circuit board and the other side of the printed circuit board includes connectors for audio cable or to a wireless signal transmission module.
5. An acoustic pickup assembly according to any one of Claims 1 to 4 wherein the side of the electret transducer remote from the chamber is sealed to the housing via a permanently soft sealant material.
6. An acoustic pickup assembly according to Claim 5 wherein the sealant is silicone-based medical grade putty.
7. An acoustic pickup assembly according to any one of Claims 1 6 wherein the diaphragm is formed of material having an acoustic impedance similar to that of human skin.
8. An acoustic pickup assembly according to any one of Claims 1 to 6 wherein the diaphragm is formed of a polyurethane polymer elastic membrane.
9. An acoustic pickup assembly according to any one of Claims 1 to 8 wherein the side of the diaphragm away from the chamber carries an adhesive coating allowing the pickup assembly to be adhered to the skin.
10. An acoustic pickup assembly according to Claim 9 wherein the adhesive coating is a hydrocolloid adhesive coating.
ii. An acoustic pickup assembly according to any one of Claims ito 10 wherein the housing includes, to the sides of the chamber, one or more air gaps or sealed air pockets to reduce the transmission of ambient noise.
12. An acoustic pickup assembly according to any one of Claims ito ii wherein the housing is made of a plastics material.

i3. An acoustic pickup assembly according to Claim i2 wherein the plastics material is an acrylonitrile butadiene styrene copolymer.