GB2075849A - Improvements in or relating to hearing protectors - Google Patents

Abstract

Hearing protectors having an earshell 20 and ear cushion 21 encloses a volume of air. A membrane 22 in the earshell 20 has a predetermined stiffness and mass so as to flatten the attenuation response of the protector (within a range of about 10dB) from low frequencies (200Hz or less) up to frequencies important for understanding speech (at least 2kHz). Alternate or additional structures for flattening the response include further membranes and barriers dividing the enclosed volume of the shell with interconnecting tubes. An ear plug 30 is also disclosed, having a channel or opening 32 closed by a membrane 33. <IMAGE>

Description

SPECIFICATION Improvements in or relating to hearing protectors The present invention is concerned with hearing protectors. hearing protectors, also sometimes called ear protectors or ear defenders, normally exhibit a non-uniform characteristic of attenuation versus frequency. Using an electric circuit analogue, the frequency response characteristic of the attenuation' of typical hearing protectors approximates to that of an inductance-capacitance low-pass filter, but with substantial insertion less at low frequencies below a turnover frequency in the response. This response characteristic arises from the physical form of the normal hearing protector.Considering an external protector of the kind which fits around the ear-o.f the wearer, the protector usually comprises an earshell of substantially rigid material enclosing a predetermined volume of air exterior to the ear drum of the wearer. The shell has a predetermined mass and as usually seated about the ear of the wearer by means of an ear cushion. Using established principles of eiectro-acoustic and electro-mechanical analogies, it can be seen that the enclosed air volume within the earshell is equivalent to a shunt capacitance, whilst the mass of the earshell, is equivalent to a series inductance.The compliance of the ear cushion on the other hand provides a series capacitance, and this together with the shunt capacitance of the earshell provides a substantially level frequency response characteristic of the attentuation at very low frequencies. At a (usually fairly low) turnover frequency, the earshell series inductance becomes predominant and the attenuation begins to increase at, theoretically, 12 dB/octave. In practice, an-upper limit to attentuation is reached at higher frequencies because of various flanking transmission paths.

Normally, therefore, a plateau of attenuation is reached at frequencies above, say kHz.

Atypical frequency response characteristic of a prior art hearing protector is shown in Figure 1 of the accompanying drawings. The detailed shape and numerical values of the frequency response would vary for different forms and types of prior art hearing protector but it is normally found that the attentuation in the frequency range between, say, 1 kHz and 4kHz is greater than the attentuation at frequencies below 1 kHz.

Figure 2 illustrates in cross-section atypical form of prior art hearing protector having a rigid earshell 10 enclosing a volume V of air. An ear cushion 11 enables the ear shell 10 to be seated about the ear of the wearer. The head of-the wearer is illustrated in Figure 2 schematically at 12 with .ear 13 and ear canal 14 leading to eardrum 15. Figure 3 shows an equivalent electrical circuit of the prior art hearing protector of Figure 2. The enclosed volume V is equivalent to a shunt capacitance Cv. The mass.of the earshell 10 is equivalent to a series inductance LM and the compliance of the ear cushion 1 is.: equivalent to a series capacitance Cc.

A problem with prior art hearing protectors ia the difficulty of understanding speech when wearing the protectors. This difficulty arises.partly because wither -prior art hearing protectors typically the greatest attenuation is at those frequencies which are most important to speech intelligibility, i.e. 1 kHz to 4kHz.

On the other hand, noise in the encironment at frequencies below 1 kHz typically in the range 60 to 500 Hz is attenuated least. There is a well-known phenomenon known as "upward making" whereby relatively loud low frequency noise can mask relatively quiet higher frequency speech sounds. The result is that speech is even less intelligible than would be expected from considering the relative attentuation alone.

The resulting reduction in the intelligibility of speech heard through hearing protectors is one important reason for the reluctance of poeple to wear hearing protectors.

According to the present invention, a hearing protector is adapted to have an attenuation frequency response which is flat, within a range of about 1 OdB, from 200Hz to at last about 2kHz. With such a protector, the attention at the frequencies important for understanding speech is not substantially different from the attenuation at lower frequencies, so thatthe upward masking effect of lowfrequency noise is reduced. Although the attenuation at the higher frequencies up to at least 2 kHz may be less than with prior art hearing protectors, satisfactory attenuation at these frequencies is still achieved but with the greater advantage of improved speech intelligibility.

From a different aspect, the invention provides a hearing protector adapted to have an attenuation frequency response which is sufficiently flat to reduce the upward making effect of low frequency noise on the intelligibility of speech when the protector is worn.

It will be appreciated that speech may be rendered more intelligible with the hearing protector of the present invention if the frequency response of attenuation is flat within a 10 dB range up to-the stated minimum frequency 2kHz. Indeed, if the environmental noise to be attenuated is primarily high frequency noise, typically 3 kHz and above, then it can be advantageous for the hearing protector of the present invention to be formed with an increasing attenuation above 2 kHz. On the other hand, if the environmental noise is mostly at low frequencies, i.e. 1 kHz and below, then a useful further increase in the intelligibility of perceived speech can be achieved if the substantially flat, as defined, response is extended to 3 kHz or more.

The attenuation frequency response of the protector must be flat, as defined, down at least to a frequency of 200 Hz in order to provide at least to some degree the stated advantages of this invention However, desirably the flat response may be extended to lower frequencies, for example, 100 Hz, 60 Hz or less.

In certain circumstances, the flat response of the hearing protector may be extended above 2 kHz to as much as 8 kHz.

Conveniently, the hearing protector comprises a body arranged, when the protector is worn, to enclose a volume of air exterior to the ear drum of the wearer, said body having sufficient mass relative to the enclosed volume, and/or the interconnection between the body and the wearer being sufficiently stiff, such that the protector has a predetermined attenuation at frequencies about 200 Hz, and high frequency transmission means fitted in the body arranged to permit higher frequencies above 200 Hz to at least about 2 kHz to be transmitted to the enclosed volume to provide said flat response.

Preferably, said high frequency transmission means comprises a diaphragm in an opening through the body between the outside environment and the enclosed volume, the diaphragm having predetermined stiffness and mass such as not substantially to reduce the attenuation of the protector at frequencies below about 200 Hz.

The protector may include two or more said diaphragms having different predetermined stiffnesses and masses to provide reduced attenuation over an increased spread of frequencies above 200 Hz.

Alternatively, or as well, the hearing protector may include barrier means dividing said enclosed volume into a first part in direct communication with the eardrum of the wearer and a second part, tube means communicating between the said first and second parts and defining a column of air interconnecting said parts and having a predetermined length and cross-section, the dimensions of said column of air and the volume of said second part relative to said first part being selected to reduce the effective size of the enclosed volume of air at frequencies above 200 Hz so as to reduce attenuation at such frequencies.

The present invention can be applied to the various existing kinds of protectors, for example, earpiugs, supra-aural-type hearing protectors and circum-aural protectors, and including hearing protectors attached to or built into helmets or the like and protectors fitted with earphones for communication purposes.

Thus, referring to the above examples of the invention, the body may define an earshell for an external hearing protector fitting around or against the ear of the wearer. Alternatively, the body may define an earplug fitting in the ear canal of the wearer.

Examples of the present invention will now be described with reference to the accompanying drawings in which: Figure 4 is a graphical representation of the typical frequency response of attenuation of a hearing protector embodying the present invention; Figure 5is a cross-sectional schematic diagram illustrating one form of hearing protector embodying the present invention; Figure 5a is an analogous electrical circuit equiva lent to the protector of Figure 5; Figures 6, 7 and 8 are cross-sectional schematic diagrams of three further embodiments of the - present invention; and Figures 6A, 7A and 8A are corresponding analog~' gous electrical circuit diagrams equivalent to the embodiments of Figures 6, 7 and 8.

Referring to Figure 4, the attenuation of a hearing protector embodying the present invention is shown to be substantially flat, i.e. within a range 10dB about a low frequency attenuation x at 100 Hz. The attenuation is maintained within + 5dB of the attentuation x up to a frequency of at least about 2 kHz. At higher frequencies, the attentuation may increase as shown in Figure 4 and as normal for hearing protectors due to the combined effect of the mass of the earshell or body of the protector and the enclosed volume of air.

In some arrangements it may be desirable to extend the flat response region to higher frequencies than 2 kHz, as indicated by the dotted line in figure 4.

Minor peaks or troughs in the response over the substantially flat region are of little consequence provided there is not a total variation of more than about 10do. The frequency response should be flat down to a frequency of no more than 200 Hz to obtain the advantages of the present invention, but the flat response is normally extended to lower frequencies, e.g. 100Hz or 60Hz.

Referring now to Figures 5 and 5A, an external or circum-aural ear protector is illustrated in crosssection. The ear protector has a body 21 defining an earshall which has a predetermined mass. The protector is provided with an ear cushion 21 which allows the protector shell 20 to fit against the head of the wearer around the ear providing a satisfactory sound proof seal. A membrane or diaphragm 22 is fitted in an opening 23 of the earshell 20.

Referring to the analogous electric circuit of Figure 5A, the mass of the earshell 20, the volume V of air enclosed by the earshell and the compliance of the ear cushion 21 are represented by a series induct ance Ls, a shunt capacitance Cv and a further series capacitance Cc respectively. This circuit assumes that the earshell 20 itself is substantially rigid so that, in the absence of the diaphragm 22, high frequency noise is substantially excluded by the earshell from the enclosed volume and lower frequencies are transmitted by body vibration of the earshell 20 causing resilient deformation of the ear cushion 21.

The effect of the diaphragm 22 is to provide a second parallel path around the inductor Ls and capacitor Cc. The second path comprises a capacitor C1 in series with an inductance L1. The capacitor C1 corresponds to the stiffness of the diaphragm 22.

This stiffness, which is substantially less than that of the earshell 20 is however considerably greater (typically three or four times greater) than the equivalent stiffness (Cc) provided by the ear cushion 21. The stiffness of the diaphragm 22 and thus the value of the equivalent capacitor C1 is chosen so as to make the impedance of the second parallel path in the diagram of Figure 5A substantially greaterthan that of the first path (Ls and Cc) at frequencies below a selected minimum frequency. This selected mini mum frequency normally corresponds to the low 'frequency from which the hearing protector is desired to have a flat response. For example, the selected frequency may be 200 Hz. Then at frequen cies below 200 Hz, the second parallel path, and hence the diaphragm 22, have substantially no effect on the attenuation provided by the hearing protector.

However, at frequencies above this minimum frequency, the diaphragm 22 permits increased transmission of sound into the enclosed air volume and thence a reduction in the attenuation of the protector. The reduction in attenuation with frequency continues to frequencies at which the mass of the diaphragm 22 becomes significant. This mass is represented by the inductor L1 and has the effect of eventually reducing the transmission of higher frequency noise.

It can be seen therefore that with proper selection of the mass and stiffness of the diaphragm 22, the attenuation provided by the protector of Figure 5 can be reduced over a selected frequency range, which for the present embodiment may be from, say, 150Hz up to, say, 3kHz.

Referring now to Figures 6 and 6A, the hearing protector illustrated in Figure 6, is similar to that of Figure 5 and has the same reference numerals for corresponding parts but is provided with a second membrane or diaphragm 24 fitted in a second opening 25 of the earshell 20. The corresponding analogous electrical circuit of Figure 6A illustrates the effect of the second membrane as a further parallel path around the inductor Ls and capacitor Cc and comprising additional capacitor C2 and inductor L2. The two membranes 22 and 24 can have different selected stiffnesses and masses thereby correspond- ing to different values of capacitance and inductance in the electrical analogy.It can be seen therefore that the arrangement of Figure 6 can be used to extend the range of frequencies over which the attenuation of the protector is maintained substantially flat. For example, the diaphragm 24 may provide the desired substantially flat response from 150Hz to 3kHz, and the diaphragm 22 the flat response from 2 kHz up to, say, 8kHz.

Figure 7 illustrates a further form of hearing protector similar to those of Figures 5 and 6. Again, the same reference numerals are used in Figure 7 for parts and features corresponding to those of Figures 5 and 6. However, in Figure 7, the enclosed volume of air within the earshell 20 is divided into two parts V1 and V2 by a barrier 26. The two part volumes V1 and V2 are interconnected by means of a tube 27 through the barrier 26. The tube 27 is arranged to define a column of air 28 which has a predetermined length and cross-section.Such a volumn of air has a predetermined mass, and provided the dimensions of the tube 28 are less than the wavelengths of the spectrum of sound energy under consideration, for the pressure of the volume V2 of air enclosed by the barrier 26 to be varied at the frequency of the sound, the entire volumn of air 28 with its known mass must be vibrated to and fro' along the tube 27. Thus, as is known from acoustic theory, the effective impedance of the column of air 28 interconnecting the volume V1 and V2 rises with increasing frequency (as the column of air becomes more difficult to move to and fro').Thus, referring to the analgous electrical circuit in Figure 7A it can be seen that the barrier 26 and tube 27 have the effect of separating the shunt capacitance Cv representing the entire volume of air enclosed by the earshell 20, into two capacitances cv1 and Cv2. Cvl is representative of the volume V1 still in direct contact with the eardrum of the wearer of the, protector. Cv2 is representative of the volume of air enclosed by the barrier 26. Cv2 is in series with an-inductor LV2 which is representative of the column of air 28. Thus, at low frequencies the column of air 28 has substantially no effect so that the ear protector of Figure 7 operates in the same fashion as that of Figure 5.However, at increasing ~ frequencies, the effective enclosed volume of the earshell of Figure 7 is reduced by the increasing irnpedance of the column of air 28. This has the effect of reducing the attenuation of the ear protector 'at higher frequencies. By proper selection of the volume V2 to be enclosed by the barrier 26 and the dimensions of the column of air 28 as defined by the tube 27, a desired flattening of the attenuation frequency response of the protector can be, achieved in accordance with the present invention. As illus trated in Figures 7 and 7A, both a diaphragm 22 and barrier 26 with tube 27 may be provided, so that a desired flattening of the response is produced by their combined effects.

Referring finally to Figure 8 and 8A, Figure 8 shows an earplug according to the present inven tion. The earplug has a body 30 which is arranged to I~fit4nthe ear canal of the wearer so as to occlude the ear canal and define an enclosed volume of air between the plug and the eardrum of the wearer.

The body 30 of the earplug has an annular flange 31 which presses resiliently against the walls of the ear canal of the wearer providing a substantially eartight connection.

Referring to Figure 8A, the earplug of Figure 8 can be represented by the same electrical configuration of shunt capacitance Cv and series inductance Ls and Cc. In this case, the inductance Ls is representative of the mass of the body 30 of the earplug, the capacitance Cc is representative of the compliance of the resilient flange 31 and the capacitance Cv is representative of the volume of air enclosed by the earplug.

In Figure 8, a channel or opening 32 is provided extending right through the earplug but is closed by a diaphragm 33. The diaphragm 33 has an effect corresponding to the diaphragm 22 of the ear protector of Figure 5. By suitable selection of the resilience and the mass of the diaphragm 33, the attenuation provided by the earplug can be substan tally flattened in accordance with the present inven tion. The stiffness pf the diaphragm 33 is repre sented by a parallel connected capacitor C1 in the diagram of Figure 8A.

Other forms and methods of flattening the re sponse of hearing protectors in accordance with the invention may be achieved by different combina tions of acoustical and or mechanical members.

Claims (11)

'ClAIMS -,

1. A hearing protector adapted to have an atte nuation frequency response which is flat, within a range of about 1 OdB, from no higher than 200Hz to at last 8bout 2kHz.

2. A hearing protector as claimed in claim 1 wherein said flat response extends down to 100 Hz.

3. A hearing protector as claimed in claim 2 wherein said flat response extends down to 60 Hz.

4. A hearing protector adapted to have an attenuation frequency response which is sufficiently flat to reduce the upward masking effect of low--frequen- cy noise on the intelligibility of speech when the protector is worn.

5. A hearing protector as claimed in any preceding claim and comprising a body arranged, when the protector is worn to enclosed a volume of air exterior to the ear drum of the wearer, said body having sufficient mass relative to the enclosed volume, and/orthe interconnection between the body and the wearer being sufficiently stiff such that theprotector has a predetermined attenuation atfrequencies about 200 Hz, high frequency transmission means fitted in the body arranged to permit higher frequencies above 200Hz to at least about 2kHz to be transmitted to the enclosed volume to provide said flat response.

6. A hearing protector as claimed in claim 5 wherein said high frequency transmission means comprises a diaphragm in an opening through the body between the outside environment and the enclosed volume, the diaphragm having a predetermined stiffness and mass such as not substantially to reduce the attenuation of the protector at frequencies below about 200 Hz.

7. A hearing protector as claimed in claim 6 including two or more said diaphragms having different predetermined stiffnesses and masses to provide reduced attenuation over an increased spread of frequencies above 200 Hz.

8. A hearing protector as claimed in any of claims 5 to 7, and including barrier means dividing said enclosed volume into a first part in direct #i~ communication with the eardrum of the wearer and a second part, tube means communicating between said first and second parts and defining a column of air interconnecting said parts and having a predetermined length and cross-section, the dimensions of said column of air and the volume of said second part relative to sald first part being selected to reduce the effective size of the enclosed volume of air at frequencies above 200 Hz so as to reduce attenuation at such frequencies.

9. A hearing protector as claimed in any of claims 5 to 8 wherein the body defines an earshell for an external hearing protectorfitting around or against the ear of the wearer.

10. A hearing protector as claimed in any of claims 5 to 8, wherein the body defines an earplug fitting in the ear canal of the wearer.

11. A hearing protector substantially as hereinbefore described.