Patent Application
20060013409
Hearing protectors are designed to shield the user from loud external sounds. They accomplish this goal by blocking the ear and reducing the intensity of external sounds reaching the ear canal. Such protection helps to safeguard the hearing of people who are routinely exposed to high-level noise.
One problem with typical hearing protectors is that they reduce all sounds. While protecting the user from loud, damaging sounds, they also reduce the user's ability to hear important sounds at normal intensity. The loss of normal-level auditory stimuli isolates the wearer from the surrounding environment, thereby delaying or even preventing the wearer's reaction to low-level sounds (e.g., the wearer cannot hear spoken communication from people nearby). Such isolation can cause the wearer to remove the hearing protection, which then leaves the wearer vulnerable to unexpected loud sounds.
In order to address this problem, many hearing protection systems offer “hear-through” capabilities. In such systems, microphones are located near the two ears. These microphone signals are automatically controlled for sound level and fed electronically through the hearing protection and presented at the ear canal. By automatically controlling the signal level, these hear-through systems allow normal-volume sounds to reach the ear unchanged while attenuating loud-volume sounds to prevent hearing damage.
Current hear-through systems degrade the user's ability to localize sounds. Humans estimate where sounds come from by sensing acoustic characteristics of the signals received at the ears. Some of these characteristics are related to differences between the signals at the two ears (interaural differences), and others are related to the spectral shaping imposed by the head and pinna (outer ear) through head-related transfer functions (HRTFs). Signals from the pick-up microphones in current hear-through systems are filtered by HRTFs that differ substantially from the natural ones that characterize a person's open ears. Without appropriate HRTFs, the user's ability to localize sound sources in space degrades.
Current hear-through systems preserve some but not all of the important HRTF cues. By simply locating the microphones near the two ears, the natural interaural (level and time difference) cues are approximately retained. The pinna-related HRTF cues, however, are generally lost. Specifically, when the hear-through protector consists of muffs that completely cover the external ear, microphone signals taken from outside the muff can lose all cues provided by the pinnae. When the hearing protection consists of earplugs, the plugs often fill the conchae and microphone signals taken from outside the plug lose important concha-reflection cues. The concha is the largest and deepest concavity of the external ear. This loss of pinna HRTF cues reduces the listener's ability to determine the elevation and the front-back orientation of a sound source.
Systems and methods are described for re-introducing some pinna cues into hear-through hearing-protection systems, such as muffs or ear plugs. Two or more microphones are provided at each ear to create spectral features (e.g., notches) that depend on the location of the source to mimic those generated naturally by the user's pinnae. As with the currently available hear-through systems, the interaural HRTF cues are approximately preserved by placing the microphone clusters near the two ears.
In certain embodiments, left and right omnidirectional pick-up microphones are replaced with left and right clusters of microphones with the goal of generating a hear-through hearing protection system that preserves pinna-dependent localization cues. The system can specifically apply the location-dependent frequency-response capabilities of multi-microphone systems to the task of reproducing human spectral localization cues. The methods described herein are generally referred to as simulated-pinna processing.
According to certain embodiments, a device for mimicking directional cues of an acoustic signal includes a circumaural muff with first and second spatially-separated microphones outside a muff (or other hearing protection device) for receiving the acoustic signal and communicates respective first and second signals. The first signal is substantially similar to the second signal but shifted in time relative to the second signal due to the displacement between the microphones. A circuit can process and combine the first and second signals in accordance with the time shift, the lateral displacement of the microphones, and the predetermined direction. An amplifying circuit can be provided to amplify the resultant processed signal. A driver can be provided inside the hearing protection device for receiving the amplified signal electromechanically and transmitting a second acoustic signal into the interior of the hearing protection device.
A processor can receive the electrical signals and process the signals in accordance with a source-location dependent frequency notch in the frequency spectra. The processor combines the electrical signals and introduces directional cues into the combination of electrical signals. The device can further include an amplifier for amplifying the processed electrical signal. A driver is provided on the inside of the hearing protection device for receiving the combination of electrical signals. The driver transmits a second acoustic signal into the interior of the hearing protection device. The frequency notch is a result of a destructive interference caused by the propagation delay between the first and second acoustic microphone signals and the signal processing used to combine these signals.
Such hear-through hearing protectors can be used for industrial and military purposes, target shooting, hunting, or for other applications. Other features and advantages will be appreciated to one skilled in the art.
Referring to
Ear muff 120 includes a first microphone 130, a second microphone 140 spaced from first microphone 130, and a processing circuit 170. These microphones can be located physically on the outside of the muff, or they can be outside the muff but not necessarily physically on the muff. In one embodiment, processing circuit 170 includes an adder circuit that adds the signals produced from microphones 130 and 140.
A broadband (e.g., 20-20 kHz) acoustic source 110 transmits an acoustic signal with wavelength λi. The acoustic signal is received by microphones 130 and 140, which produce electrical signals representative of the acoustic signal. These electrical signals are substantially similar to one another but shifted in time (or phase) because of the microphone spacing. For instance, the signal produced by 140 is a time-shifted example of the signal received at microphone 130 due to the additional time for the acoustic signal to arrive at microphone 140. The phase difference ΔΦ between the signals produced by microphones 130 and 140 is a function of distances r1 and r2 from source 110 to the microphones. The difference between r1 and r2 is a function of d, θ (or ym), and the spacing between microphones 130 and 140.
An adder circuit can be used to combine the signals produced by microphones 130 and 140. For far-field sources ,where r1 and r2 are significantly greater than (e.g., 25 times greater than) the microphone separation, the resultant power of the combined signal is approximately dictated by the following equation:
Since microphone simulated-pinnae microphone spacings are very small (˜1-2 cm), most sources exhibit ‘significantly large’ enough r1 and r2 such that the above relation approximately holds.
For a specific source location (with corresponding r1 and r2 ), the combined power exhibits peaks and valleys as a function of λi. The valleys, referred to here as spectral notches, change with source location.
In another embodiment, the processing circuit may include a delay circuit applied to the output of one microphone prior to microphone signal adding. The phase difference ΔΦ between the signals produced by microphones 130 and 140 is a function of the distance between microphones 130 and 140. The delay circuit changes the phase difference ΔΦ between the signals produced by the microphones. As a result, a shift in the location-dependent spectral notches occurs. The delay can be adjusted to shift the spectral notches so that they resemble naturally occurring pinnae-cue spectral notches. Naturally occurring spectral notches can be empirically measured using a KEMAR® manikin (KEMAR is a registered trademark of Knowles Electronics, Inc.).
Referring to
The resultant signals processed by filters 360, 370, 380, and 390 are summed by an adder circuit 350 to generate a simulated-pinna output signal 395. The placement of microphones 210, 220, 230, and 240 and the selection of system filters 360, 370, 380, and 390 generate an overall system response that changes with the arrival direction of any given source. By selecting these microphone placement and the filter parameters accordingly, the resulting system can mimic spectral HRTF sound localization cues.
Other microphone configurations can be chosen. Given a particular microphone configuration and a set of microphone filters, the simulated-pinna output signal 395, YSP, for a specific source location is dependent upon the source location, the microphone placement, and the simulated-pinna filters. The desired, naturally-occurring HRTF, represented by variable YHRTF, is only dependent upon the source location. In the present embodiment, location depends upon azimuth, which refers to the horizontal-plane angle between the source location and straight ahead of the listener, and elevation, which refers to the angle between the source location and a horizontal plane. So, for example, the YHRTF of (0°,45°) will be different from the YHRTF of (0°, 0°).
These two signals may be represented as:
Ysp=Ysp(loc,mic,filter) and YHRTF=YHRTF(loc),
where loc=(azimuth, elevation) source location,
The simulated-pinna system can be designed by selecting the microphone placement and the simulated-pinna filters to minimize the differences between the spectral features of Ysp and YHRTF that are most useful for source localization.
One general approach to this problem is to use a general error criterion such as:
E(loc,mic,filter)=feature_error[Ysp(loc,mic,filter), YHRTF(loc)],
where ‘feature_error[A,B]’ measures the error in important spectral source-localization features between the two signals A and B. A location-averaged error may then be formed by averaging over source location:
EAVG(mic,filter)=AVERAGEloc[E(loc,mic,filter)].
Given this average error, it is then possible to select the ‘mic’ and ‘filter’ parameters of the simulated-pinna system to minimize EAVG(mic,filter). This general solution is very flexible in that the function ‘feature_error[A,B]’ can be designed to optimize different combinations of important spectral source localization features such as spectral notches, spectral resonances, etc. This flexibility may require complex simulated-pinna filters that may utilize digital signal processing (DSP) in their implementation. Characteristics of the filters can be determined by simulation, or empirically by using a grid of sources and taking actual measurements.
The system thus senses the angle of elevation and provides an elevational spectral notch as a cue to the wearer of the hearing protector.
While different types of processing can be used, in the particular example here, a first stage of processing includes a delay circuit 540 for delaying the top microphone signal 525 and a summer 570 for summing delayed signal 550 with the bottom microphone signal 560. The first stage of processing produces an elevation dependent notch that is controlled through a combination of microphone separation and choice of delay. For example, a microphone separation of 1 cm and a top-microphone delay of 70 μsec leads to the behavior shown in
This simulated-pinna system is somewhat simplified compared to the system in
The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of the equivalency of the claims are therefore intended to be embraced therein.
