Active noise reduction is a successful addition to passive eardefenders for improvement of the sound attenuation at low frequencies. Assessment methods are discussed, focused on subjective and objective attenuation measurements, stability, and on high noise level applications. Active noise reduction systems are suitable for integration with an intercom. For this purpose the intelligibility in combination with environmental noise is evaluated. Development of a system includes the acoustical design, the feedback amplifier, and the speech input facility. An example of such a development is discussed. Finally the performance of some commercial systems and a laboratory prototype are compared.
Keywords: Active noise reduction, speech communication, environmental noise, protectors
|How to cite this article:|
Steeneken H. Personal active noise reduction with integrated speech communication devices : Development and assessment. Noise Health 1998;1:64-75
| Introduction|| |
Active noise reduction is an effective tool to increase the sound attenuation of hearing protectors. Especially for the low frequency range the passive sound attenuation of an earmuff or earplug is often insufficient. Active noise reduction can provide an additional attenuation of 20-30 dB at low frequencies (below approximately 500-1000 Hz).
Two specific active noise reduction systems have been developed, one system based on an earmuff and a second system based on an ear plug. The earmuff based system offers a high additional attenuation (up to 25 dB) and can be used with very high noise levels (up to 160 dB SPL).The earplug based system is small and can be used in combination with a gasmask or with a pilot helmet.
A specially developed speech interface allows for injection of speech signals from an intercom system, thus offering a high intelligibility due to the improved sound attenuation and the low acoustic distortion.
Assessment methods of ANR systems differ from methods as used for passive hearing protectors. Due to noise introduced by the electronic system no measurements at the threshold of hearing can be performed. Objective and subjective assessment of the attenuation and the speech quality will be discussed.
Principle of Active Noise Reduction
Active noise reduction is based on the addition of a secondary sound signal to a primary sound signal which has to be suppressed (Lueg, 1936). If the waveform of the two signals are identical but in anti-phase the resulting sound will be zero. A perfect match is theoretical; in practice a feedback loop is used according to the block diagram given in [Figure - 1]
The resulting noise signal N'(t) at the microphone position is the sum of the primary noise signal N(t) (leaking from the outside of the hearing protector) and the secondary compensation signal from the loudspeaker. The latter signal is equal to the resulting noise signal at the microphone multiplied by the loop gain, hence:
Where β represents the frequency transfer and the efficiency of the electro acoustic transducers (microphone β1, and telephone β2), A1 represents the gain and frequency transfer of the correction amplifier and A2 the gain and frequency of the telephone amplifier. The amount of suppression is given by the denominator of equation 2. An increase of the loop-gain (β1× β2×A1×A2) results in more suppression.
The frequency transfer of the combination of the electro-acoustic transducers and the cavity under the earmuff is limited. An example of the transfer function of the amplitude and phase of such a system is given in [Figure - 2]. In view of this transfer function three ranges for the denominator of equation 2 are identified:
(1) the denominator is greater than one which results in a suppression of the primary noise,
(2) the denominator is smaller than one but greater than zero which results in an amplification of the primary noise, and
(3) the denominator becomes zero which results in an unstable system which will oscillate.
The last two possibilities should be avoided by either a lower total loop-gain or correction of the amplitude and phase response. Such a correction can be obtained by a compensation network to be included in amplifier A1. Reduction of the total loop-gain results in a smaller amount of noise suppression. Therefore, a careful design of the acoustical properties of the cavity within the earmuff, and careful selection of the transducers with an optimal frequency response is required. The frequency and phase response of the compensation network is defined in relation to the frequency response given by 131 and 132. A description of the design criteria is given by Olson and May (1953), Carme (1987), and Nelson and Elliott (1993).
Insertion of speech signals in the ANR loop is possible. An optimal method to do this is to compensate for the feed back on the speech signal. This can be performed with the circuit given in [Figure - 3]. The speech signal is then defined by:
As β1 (microphone) has a fairly flat frequency response, the speech signal will be reproduced with a nearly flat frequency response.
Assessment of ANR systems
The performance of an ANR system depends on a number of technical properties. Of course the addition of active sound attenuation is a major aspect. However, in order to specify the personal protection and safety and not mean values the following items are of interest:
- passive sound attenuation as a function of frequency,
- active sound attenuation as a function of frequency,
- variance among systems,
- variance among users,
- stability on the head,
- stability for open system during placing or removing from the head,
- sensitivity for vibrations,
- maximum sound pressure level (dynamic range),
- overload response,
- speech intelligibility of the integrated communication system.
In this overview we will focus on the sound attenuation and speech intelligibility. However, some examples will be given on the other aspects.
With the introduction of hearing protectors with active noise reduction, which may introduce some system noise at the users ear, the assessment of the sound attenuation according to the standard measuring methods (ISO4869-1) is no longer valid. The ISO method is based on the threshold of perception and, thus, limited to low sound levels. The noise introduced by the ANR systems will interfere with the measurement. Also the sound attenuation of ANR systems may be level dependent. Hence, measurements should be performed at various levels. Three alternative methods for measuring the sound attenuation are in use:
(1) By comparing the sound pressure level measured under the earmuff with the ANR system switched on and off. The level difference between the two measurements gives the sound attenuation. The measurements are performed by making use of the sense-microphone included in the ANR-loop.
(2) Similar measurements as described under (1) by making use of an additional microphone, positioned close to the entrance of the ear canal.
(3) By subjective matching of the loudness of two sound levels, representative for the additional attenuation of the ANR system.
The active sound attenuation can be obtained by measuring the difference between the sound pressure levels under the earmuff shell with the ANR system switched on and off. As measuring microphone the loop microphone or an additional microphone placed near the entrance of the ear canal can be used. By means of a positioning system the miniature microphone is placed near the entrance of the ear canal [Figure - 4]. This method is called MIRE (Microphone In Real Ear) and is considered to become a new international standard (Technical Committee CEN/TC 159).
Preferably, the noise level and spectrum used for the measurements are identical to the noise level and spectrum of the real application. As ANR systems may have a level dependent attenuation it is advised to determine the attenuation as a function of the noise level.
The attenuation is measured as a function of the frequency. Usually a resolution of 1/3 octave band is used. For this purpose the output signal of the microphone used for the measurements is analysed by a spectrum analyzer. In order to obtain representative results and to get information on user dependency, various subjects are used.
The standardized method for the subjective assessment of the attenuation of hearing protectors is based on a shift of the hearing threshold level if a hearing protector is applied. However, the determination of the hearing threshold in conjunction with an ANR system is not possible as the system itself introduces some noise with a level above the hearing threshold. Therefore, the following method was developed where the subject (with an ANR system for each ear) is placed in a diffuse sound field which alternates periodically between two levels (typically every second). An example of this level alternation is given in [Figure - 5].
During the highest sound pressure level the ANR system is switched on, while during the low sound level the ANR system is switched off. The subject will hear a smaller difference between the two sound levels as the ANR system attenuates only the highest level. The subject is asked to match both levels for equal loudness by adjusting the level difference AL between the two signals. The resulting difference in sound level outside the earmuff is equal to the subjective attenuation provided by the ANR system. The adjustment can be made by changing the sound level during the "ANR-off" interval. Since the subject adjusts for a continuous signal, the on/off rhythm is indicated with a light signal. A study (Steeneken and Langhout, 1985) showed that the accuracy lies within 1-3 dB.
The measurements are performed in a specific room with a diffuse sound field. The test signals that are used consist of noise bands with a bandwidth of 1/3 octave. Measurements are performed in one octave steps. The absolute signal level can be adjusted to any level which is high enough not to interfere with the system noise. However, as the noise reduction of ANR systems may be level dependent, the measurements should be performed systematically as a function of the level.
Comparison of subjective and objective measuring results
A comparison between subjective and objective attenuation measurements was made. The subjective attenuation was measured with four subjects and various signal levels. For one of the conditions the 1/3 octave band signal level was 110 dB SPL. The mean attenuation for these conditions, as a function of frequency with one octave steps, is given in [Figure - 6].
The objective attenuation was measured with the loop microphone as well as with a special electret microphone positioned close to the entrance of the ear canal. For the objective measurement a pink noise (level 105 dB SPL) was used. The results indicate that the attenuation values obtained with the subjective method and those obtained with the ear microphone (MIRE) are in close agreement. The attenuation values obtained with the loop microphone are somewhat higher (2-5 dB). Obviously, the sound field under the earmuff is not homogeneous and is minimal at the sensing position of the loop microphone.
Speech transmission quality
The speech quality depends on the method used for the injection of the speech signal. Some systems make use of the method given in [Figure - 2] while others inject the speech signal at the sense microphone input. Some designs make use of a correction amplifier.
As the speech transmission quality is defined by the design of the ANR system, the speech injection method, and the suppression of background noise, it is important to assess the speech intelligibility in a representative condition.
This assessment can be done with subjective measures (by making use of speakers and listeners) or by objective methods (by making use of a measuring device). In this study an objective method (the Speech Transmission Index, STI) is used (Steeneken and Houtgast, 1980; IEC 268-16).
The STI is obtained by applying a specific speech-like test signal at the audio input and by analysis of this transmitted test signal through the same measuring microphone as used with the MIRE attenuation measurements.
The STI for a specific communication system with ANR as a function of the noise level is given in [Figure - 7]. The STI is given for two conditions: ANR switched on and off. Hence, the effect of the ANR on the STI-value can be obtained by comparing the two conditions. Additional to the STI-value also a qualification (based on STI) is given. The improvement of the speech transmission quality is obvious. It is shown that for a constant speech intelligibility (STI=0.7) a 10-dB higher noise level can be applied. Hence the effective gain in this situation and for this type of noise is 10 dB.
Development of an active noise reduction system integrated in an earmuff
The development and assessment of an ANR system can be separated into the following steps:
- acoustical design,
- optimization of the required feedback amplifier,
- development of the speech input facility,
- assessment of sound attenuation and speech intelligibility by objective and subjective methods,
- field trials for conditions with high noise levels e.g. run-up sites of jet aircraft, helicopter and shooting ranges.
The acoustical design is of major importance for the final performance of the ANR system. In order to obtain a high loop-gain within the required stability criteria it is important that a fairly flat frequency response is obtained between loudspeaker and sense-microphone in combination with a smooth phase response with a minimal phase-delay in the required frequency range. In the development stage the acoustical design is considered separately. For a system built into an existing earmuff with commercial transducers (telephone cartridge and electret microphone) the lay-out is given in [Figure - 8]. The corresponding frequency transfer for the condition that the system is placed on the head of a subject is given in [Figure - 9]. The figure gives also some indication on individual differences between users as the responses for 14 subjects are given. Analysis of the phase response indicates that a frequency range between 25 Hz and approximately 800 Hz is achievable (phase between ± 60°) with a proper design of the feedback amplifier. The frequency response for the open system (not shown) indicates that the amplitude response drops with 20 dB for the lower frequencies. This guarantees a stable system during the placing on or removing from the head of a user. A systematic study on the acoustical design criteria was performed and has lead to a ANR system which can offer an additional attenuation of 25 dB in a specific frequency range.
The feedback amplifier also provides some filtering in order to define the frequency range in which the total system stability allows a high loop-gain. This can be determined by a Nyquist diagram which gives a vectorial representation of loop-gain and phase. As was discussed in section 3 the nominator of the frequency response should be above zero. This means for the diagram given in [Figure - 10] that the area around gain "-1" (0 dB) should be avoided. In the practical situation a stable system is obtained if the indicated area between -60° and 60° is avoided.
For a typical ANR system, the active sound attenuation is given in [Figure - 11]. The maximum attenuation is obtained between 50 and 250 Hz, a negative attenuation of 6 dB is obtained at 1000 Hz. This shape is typical for this type of feedback systems.
In order to investigate the individual performance we measured the active sound attenuation for four subjects and for the left and right ear separately. Based on these results the mean attenuation and the corresponding standard deviation were calculated which is also given in [Figure - 11]. In general the mean attenuation minus on time the standard deviation is used for prediction of the noise dose in combination with a specific noise spectrum. In this example the passive attenuation was not discussed, however, in the same figure the total attenuation (passive and active) is also given.
| Discussion and Conclusion|| |
A comparison of some commercial systems was made. We investigated both the passive and the active attenuation. It was found that the stability of some systems was such that the system started to oscillate when placed on the head of a subject.
Two types of oscillation were found (1) a very low frequent oscillation below 5 Hz or (2) above 1000 Hz. The sound pressure levels during these instabilities were very high. For this reason we adjust our system 6 dB below the point of instability. If the performance of systems is compared, this security range is often not included. One might get an impression of the stability by observing the amount of negative attenuation. A typical value is 6 dB around 8001200 Hz. Some systems show a value of over 12 dB. These system are generally not stable. In [Figure - 12] a comparison is given for 5 commercial ANR systems (labelled C-G) and the system discussed above (labelled A and B).
The curves clearly indicate that most systems provide an additional attenuation of 10-15 dB in a frequency range between 80 and 800 Hz. Only systems A, B and E offer a much higher attenuation. For systems A-B an additional 6 dB stability range is included. This is unknown for the other systems. But the negative attenuation values indicate the same stability.
With respect to the speech intelligibility the performance of the system described above was already given as an example in [Figure - 7]. Hence an effective gain in signal-to-noise ratio with respect to intelligibility amounts 10 dB. This is determined for a representative noise of a tank.
Recently an active earplug was developed. Such a system is much easier to integrate with an existing helmet (either for a tank or aircraft). The active attenuation amounts 15-18 dB. A study is in progress to improve the performance of this system.
This paper was an invited contribution to a special session on Non-linear and active hearing protectors at the 3rd European Conference on Protection Against Noise (PAN), 12-15 March 1998, Stockholm Sweden organised and supported by the European Commission BIOMED 2 concerted action PAN (Contract BMH 4-CT96-0110).
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TNO Human Factors Research Institute, Soesterberg, The Netherlands
Source of Support: None, Conflict of Interest: None
[Figure - 1], [Figure - 2], [Figure - 3], [Figure - 4], [Figure - 5], [Figure - 6], [Figure - 7], [Figure - 8], [Figure - 9], [Figure - 10], [Figure - 11], [Figure - 12]