# Presentation: 076276

## “Augmented” HPDs:Active Noise Reduction, Level-Dependent, Sound Transmission, Uniform Attenuation, and Adjustable Devices

**U.S. EPA Workshop on ****Hearing Protection Devices****March 27-28, 2003****Washington, DC**

## Presentation Objectives 

- Briefly cover HPD augmentation technologies, their basic design and function, and their intended benefits.
- Provide example attenuation and other relevant data for a few key HPD augmentation technologies.
- Cover issues regarding the applicability of the REAT testing standards for each augmentation technology. (We will raise more questions than provide answers.)
- Present methodologies for obtaining attenuation data for ANR-based and level-dependent HPDs, as examples.
- Handout contains additional slides and reference listing.

## Conventional Passive HPDs

## Conventional Hearing Protectors

- Reduce noise at the ear by passive means.
- Attenuation is the same regardless of ambient sound level ("level-independent or amplitude-insensitive").
- Attenuation increases as a function of frequency.

## Hearing Signals and Speech with Conventional Hearing Protection in Noise:  Research Evidence

- Normal hearers:  protector usually does not degrade and may improve detection and recognition in high level noise.
- Crossover point is between 80 and 90 dBA, where protector's presence moves from disadvantage to advantage.
- Hearing-impaired:  protection often degrades hearing signals and speech in noise; further elevates already high thresholds.
- Very situation- and user-specific effects that depend on many contributory variables.

## Hearing with Hearing Protection:  Complaints

- "I cannot hear warning signals in the vicinity."
- "The  machine noise cues that I listen to for feedback sound strange with protectors on."
- "I missed hearing my co-worker shouting LOOK OUT."
- "Every time my machine is on a quiet cycle it's inconvenient to take my protector off so I can hear."
- "I need to hear others near me on the firing range but can't remove my protector because I don't know when the next shot will be fired."

## HPD Selection Dilemma(or the plaintiff’s foundation for litigation)

**Underprotection**
- “The hearing protector provided inadequate noise attenuation for defending the worker’s ears against the damaging effects of noise, so he lost his hearing.”
**Overprotection**
- “The hearing protector provided more attenuation than needed for the noise problem ... and therefore was the proximate cause of the accident when the worker did not hear the backup alarm and was run over.”

## Intended Advantages of Augmented HPDs

- Avoidance of “under- or over-protection.”
- Protection that is tailored to ambient levels.
- Protection that is tailored to job demands.
- Protection that is tailored to user.
- Improved signal detection and interpretation.
- Improved speech communications.
- More “natural” hearing.
- Reduction of noise-induced annoyance.
- Improved user acceptance and wearability.
*These advantages are not always realized in research studies or in actual practice.*

## Augmented HPDs:  Primary Categories (per Casali & Berger, 1996, with modifications)

- Passive (Non-Electronic)
- Level-Dependent (Amplitude-Sensitive) Devices
- Uniform (flat) Attenuation Devices
- Adjustable (as to attenuation) Devices
- Active (Electronic)
- Level-Dependent (Amplitude-Sensitive) Sound-Transmission Devices
- Also called Sound Restoration Devices
- Active Noise Reduction (ANR) Devices
- Adjustable (as to hearing-assistive, filtering) Devices

## HPD Attenuation Measurement:Insertion Loss (IL) versus Noise Reduction (NR)

- Insertion Loss = A – A'
- (same location, different time, before and after barrier insertion)
- (applies to REAT and physical tests)

- Noise Reduction = C' - A'
- (different location, same time, on either side of barrier)
- (applies only to physical tests)

## Psychophysical Methods:  REAT(Real-Ear Attenuation at Threshold)

- "Subjective"  binaural absolute threshold shift
- (occluded - unoccluded:  an Insertion Loss metric)
- Basis for ANSI S3.19-1974; ANSI S12.6-1997
- 1/3-octave noise stimuli
- Advantages:
- accounts for bone conduction pathways to ear
- recognized as most accurate method
- Disadvantages:
- measurement contamination due to physiological noise (overestimate of attenuation at low frequencies)
- quiet ambient environment necessary
- inter- and intra-subject variability
- REAT is not applicable to certain augmented HPD technologies
- As a result, the data produced are not representative of the device’s performance across different application conditions.

## Physical Methods:  MIRE (Microphone in Real-Ear) 

- "Objective”
- Basis in part for ANSI S12.42-1995; Basis in full for MIL-STD-912.
- Miniature microphone in human ear canal; spectrum analyzer.
- Easily implemented with earmuffs;  more difficult with earplugs.
- High-level broadband noise stimuli, usually pink or white noise.
- Advantages:
- quick measurement
- no rigorous requirement as to quiet ambient conditions
- no contamination due to physiological noise
- Disadvantages:
- special equipment required
- does not account for bone conduction pathway

## Physical Methods:  HATS (Head and Torso Simulator - Manikin) and ATFs (Acoustical Test Fixtures)

- Basis in part for ANSI S12.42-1995.
- Microphone in simulated ear canal or special coupler; spectrum analyzer.
- Easily implemented with earmuffs;  more difficult with earplugs.
- High-level broadband noise stimuli, usually pink or white noise.
- Advantages:
- quick measurement
- no contamination due to physiological noise
- Disadvantages:
- validity issues -- transfer functions must be quantified
- acoustic isolation of HATS “skull and torso” is ill-defined
- does not account for bone conduction pathway

## UNIFORM (FLAT) ATTENUATION HPDs

**UNIFORM (FLAT) ATTENUATION HPDs**

## Uniform Attenuation vs. Conventional HPDsREAT Data

- CONCLUSION: Uniform attenuation HPDs of the passive, level-independent type are accommodated by REAT tests.

## Noise Level-Dependent

- Hearing Protection Devices

## Level-Dependent Hearing Protectors

- Also called ”amplitude-sensitive.”
- Most are earmuffs; a few earplugs exist.
- In low-to-moderate noise levels, reduced attenuation (or electronic amplification) is provided to allow better hearing.
- In high noise levels, attenuation increases (or amplification decreases).
- May be active (electronic) or passive (non-electronic).

## Level-Dependent Passive HPDs

## Level-Dependent Passive HPDs

- Rely on passive attenuation from the structure of the protector.
- A controlled leakage path is intended to flank the protector's structure at lower sound levels; leakage decreases at high sound levels, providing more attenuation.  This is achieved by nonlinear elements:
- Dynamically-functional valve (inertia and friction problems).
- Orifice in a disc which occludes duct passing through muff (no moving parts).
- At sound levels below about 110-120 dB, most level-dependent HPDs behave as a "leaky" protector, offering little attenuation below about 1000 Hz.
- Some orifice devices offer roughly flat attenuation.

## Level-Dependent Passive HPDs

- Orifice-Type:  E•A•R Ultra 9000

## Orifice-Type Level-Dependent Passive HPDs

- Rely on nonlinear acoustical behavior in an orifice:
- At low intensity, sound waves exhibit primarily laminar flow and pass unimpeded.
- At high intensity, above about 120 dB, turbulent flow occurs and attenuation increases due to increase in acoustic resistance at the orifice.
- Because these devices do not become level-dependent until 110-120 dB and provide only moderate attenuation below that, they are best suited for isolated impulse noise, like gunfire.
- Examples:
- "Gunfender" earplug (British Military, 1969)
- "E-A-R Ultra 9000" earmuff (Cabot Safety, 1990)

## Level-Dependent Passive HPD:Orifice Type

- (Allen and Berger, 1990)

## Level-Dependent Sound-Transmission Active HPDs

**Level-Dependent Sound-Transmission Active HPDs**

## Level-Dependent Sound-Transmission Active HPDs(also called “Sound Restoration” devices)

- External microphone transduces sounds external to the protector.
- Output-limited amplifier provides gain and powers earphone; gain is typically user-adjustable.
- Earphone output level is typically limited to 82-85 dBA.
- At ambient levels above a preset value (e.g., 115 dBA), electronics cease to function and device essentially reverts to a passive protector.
- A filtered passband may be incorporated to pass only critical speech and warning signal frequencies.

## Level-Dependent Sound-Transmission Active HPDs (also called “Sound Restoration” devices) 

## Level-Dependent Sound-Transmission Active HPDs:  Design Preferences

- Flat frequency response and distortion-free amplification.
- Safe cutoff sound level and sharp attenuation transition at this level.
- High signal/noise ratio at sound levels below cutoff.
- Fast response time of electronics to transients or impulses.
- Bandwidth of passband to accommodate desired signals.
- Low residual electronic noise.
- Minimal wind noise effects.
- Dichotic (two microphones, one for each ear) preferable to diotic (same microphone signal to both ears).

## Level-Dependent Sound-Transmission Active HPDs:  Applications

- Can improve hearing of impaired listeners in quiet or moderate noise, acting like a hearing aid.
- Hearing improvements may not be realized for normal hearers:   Even though amplified sounds are louder, the residual electronic noise will also be amplified and audible, resulting in signal masking.
- Some are well-suited to impulse noise (e.g., gunfire) applications and noises with short on-durations.
- Not well-suited to noises with long on-durations because continual distortion products through earphone are objectionable.

## Level-Dependent Sound-Transmission Active HPDs:Theoretical vs. Actual Performance (Maxwell, et al., 1987)

**Level-Dependent Sound-Transmission Active HPDs:****Theoretical vs. Actual Performance**** **(Maxwell, et al., 1987)

## Issues in Applying Current REAT Standards to Level-Dependent Devices

- Passive L-D HPDs:  because test is run at threshold, attenuation is valid only for the HPD’s performance in quiet.
- Attenuation at higher sound levels should be higher, but is not represented in the data.
- Active L-D HPDs:  because test is run at threshold, attenuation is valid only for the HPD’s performance with electronics turned off (otherwise, electronic hum and amplification would affect thresholds).
- Attenuation data are valid only when HPD is functioning as a passive attenuator, and this yields “best-case” attenuation.
- With sound transmission circuit on, attenuation will likely be lower because of electronic pass-through of signal.

## Testing Needs for Level-Dependent HPDs

- Passive L-D HPDs:
- Need to quantify attenuation as a function of transient pressure pulse onset-to-peak (“attack”) time and peak-to-ambient (“decay”) time for a “standard” rapid burst, such as that produced by gunfire.
- Probably should utilize both Type A (damped) and Type B (undamped) impulses.
- Quick response time performance is of critical importance in these nonlinear devices.
- The need for high level (_>_120 dB) impulse attenuation data precludes use of human subjects (that is, no REAT or MIRE tests).

## Impulses:  Damped and Undamped

## Testing Needs for Level-Dependent HPDs

- Electronic L-D HPDs:
- Same needs as for Passive L-D HPDs, plus:
- Need to quantify frequency response of mic-amp-earphone circuit.
- Need to quantify under-HPD sound level for impulsive and steady-state sounds which exceed 85 dBA.
- Need to quantify distortion characteristics of circuit.
- Need to quantify performance degradation as a function of battery charge.

## Performance of Level-Dependent Sound-Transmission Muffs against Gunfire (Murphy & Franks, 2002) 

- Subjected 6 electroacoustic earmuffs and one assistive listening device (hearing aid) to gunfire noise, using an acoustic manikin.
- Results:
- Peak SPL attenuation ranged from about 22 dB to 34 dB.
- Peak SPL attenuation was slightly dependent upon gain setting.
- Peak SPL attenuation differences between Maximum Gain vs. Gain Off settings were within about 3 dB for most devices.

## Peltor T7-SR Electronic Muff Gain Control Evaluation (Casali & Wright, 1995)

- Subjects performed a signal detection experiment, and set gain control to maximize their perceived performance in 75, 85, and 95 dBA continuous pink noise.
- Gain control effects were measured on KEMAR manikin at subject's chosen gain settings.
- MANOVA applied to dBC, dBA, and 1000 Hz measurements under earmuff, with gain status (on/off) as a variable.
- Gain on vs. gain off differences were largest (5 dB) at 1000 Hz.
- Overall, the contribution of the gain to the noise exposure, as measured by noise dose, was negligible.

## OSHA Noise Dose Under Peltor  T7-SR Electronic Earmuff

- (Casali & Wright, 1995)

## Foil Blaster for Producing 170 dB Peak SPL

- (Zera, 2002)

## Foil Blaster for Producing 170 dB Peak SPL: Video Clip (Zera, 2002)

- Foil Blaster for Producing 170 dB Peak SPL: Video Clip (Zera, 2002)

- Foil

- Locking ring

- Puncturing needle

## Shock Tube – Impulse Generator 

- (Vergara, Gerges, & Birch, 2002)

- Diaphragm

- Volume of pressurised air (adjustable)

- Piston

- Piston is pushed forward to discharge the pressurized air

**Generator**

**Shock tube of 12m length**

## Slide 39

## Active Noise Reduction HPDs

- Muff Type

- Open Back type

## Active Noise Reduction HPDs

## Analog ANR Earmuff/Headset Block Diagram

**Analog ANR Earmuff/Headset Block Diagram**

## ANR Digital Asynchronous HPD

- (Denenberg & Claybaugh, 1993, w/ modifications)

## Active Noise Reduction (ANR) in HPDs

- Patent dates back to 1936 (Paul Lueg)
- First prototyped in earmuffs in 1957 (Meeker-WPAFB, 1957)
- Relies on principle of “destructive interference.”
- “Anti-noise” is introduced through earphone, out-of-phase with but equal amplitude to offending noise.
- Electronic (active) cancellation is most effective below about 1000 Hz.
- ANR compensates the typically high-frequency attenuation bias of earmuffs.
- May be open-back (no passive benefit) or closed-back.
- ANR has been incorporated into earplugs.

## ANR HPD Testing and Labeling Issues

- Attenuation and NRR ratings _are not_ available:
- MIRE testing can be used to measure passive component and total attenuation using, e.g,  ANSI S12.42-1995, then:
- Active Component = MIRE total - MIRE passive
- REAT or MIRE testing for passive component, BUT MIRE attenuation at low frequencies is lower due to physiological noise masking effects on occluded thresholds that occurs in REAT.
- MIRE, unlike REAT, does not account for the bone conduction path.
- Bone conduction, as a flanking path, limits the performance of _all_ HPDs and thus is an important factor.
- Passive attenuation is often decreased in midrange (from about 1000 - 3000 Hz) when active circuit is on.
- Electronics produces and/or amplifies noise which increases the noise level under the protector.

## Issues with ANR HPDs relating to the NRR

- Typical ANR functional design is not compatible with fluctuating nature of REAT test signals.
- NRR ratings, in current form, cannot be computed from MIRE data. (No measure of variability, etc.)
- ANR manufacturers are “penalized” by lack of an NRR (or other) rating system, because it precludes them from selling the product as an industrial or consumer HPD.
- Yet certain manufacturers, based on the fact that they target sales applications *other than industrial (e.g., military aviation and vehicles, general aviation, consumer noise annoyance situations), *do not want an NRR.
- However, in the absence of attenuation data, will the typical consumer really know _not_ to apply the ANR device for hearing protection?

## Do ANR HPDs Need an NRR?

**Do ANR HPDs Need an NRR?**

- The high C-A (dB) values for which ANR HPDs are most effective are not so common in industrial noise.

- (Gauger, 2002)

## NCT PA-3000 Muff-Back ANR Headset Attenuation(Robinson & Casali, 1995)

## NCT PA-1000 Open-Back ANR Headset Attenuation (Robinson & Casali, 1994)

## Attenuation of Two ANR Muffs vs. Passive Muff over Foam Plug

## (Robinson & Casali, 1995)

## Open-Back ANR HPD Mean Attenuation via MIRE (L ear) (Urquhart, Robinson, & Casali, 2001)

## HUMVEE-Mounted SICPS Shelter Noise, 85 dBA

- Normal SICPS noise

- SICPS noise attenuated by open-back ANR HPD

- (Urquhart, Robinson, & Casali, 2001)

## Current ANR HPD/Headset Applications 

- Noise annoyance reduction
- Airline passenger headsets
- Personal stereo headsets
- Airplane cockpit noise
- Communications headsets
- Military noise
- Headsets for use in tanks and other vehicles and aircraft.
- Unique applications
- Telephone operator headsets
- MRI noise earphones
- Siren-canceling HPDs for emergency vehicle crews
- There is some disagreement among ANR HPD manufacturers as to the potential for industrial noise markets, and therefore, whether an NRR-like rating is necessary.

## C - A Values for Various Industries: Work Area Data

## with option as to 

**attenuation, **
**frequency response, **
**or other factors**

- "Adjustable" HPDs

## Adjustable (as to attenuation) HPDs

- VarifoneTM

- SonoCustomTM

## Issues in Applying Current REAT Standards to “Adjustable” Attenuation HPDs

- Devices with discrete settings (e.g., SonoCustomTM) can be REAT-tested and an NRR achieved for each setting.
- But this is time- and labor-intensive. (Expensive)
- Continuously-variable devices (e.g., VarifoneTM) are more problematic because they can only be tested *reliably *at the extremes of their adjustment range.
- Some of these HPDs are sold as a system inclusive of training for end-user, individual “tuning,” etc. and these factors impact attenuation performance.
- But the standards and labeling therefrom do not reflect the performance influence of these features.

## Issues in Applying Current REAT Standards to “Adjustable” HPDs

- This class of device affords flexibility in product development in that it can be fitted with “modular” augmentations.
- Tunable attenuation to match user, job, hearing-critical, and noise requirements
- Adaptive attenuation
- Frequency filtering (active or passive)
- Noise suppression circuitry
- Hearing assistive circuitry
- Each augmentation may require different testing/labeling procedures.

## Physical vs. REAT Testing

- MIRE
- ATF
- HATS

## Physical (microphone-based) Measurements:What HPDs (may) require them?   

- Microphone-in-Real-Ear (MIRE)
- Level-dependent earmuffs
- ANR earmuffs or supra-aural devices
- Adjustable attenuation earmuffs (also REAT)
- Microphone-in-Acoustical-Test-Fixture (ATF)
- Level-dependent earmuffs (for high level impulses)
- ANR earmuffs or supra-aural devices
- ANR or level-dependent earplugs
- Adjustable attenuation earmuffs or earplugs (also REAT)
- Microphone-in-Head-and-Torso-Simulator (HATS)
- Same as for ATF

## Issues with REAT (S12.6-1997) which limit its use with ANR or Level-Dependent Devices

- S12.6 states expressly in Abstract that it applies only to “conventional passive hearing protection devices.”
- S12.6 states expressly in Forward that it “does not pertain to physical attenuation measurements using acoustical test fixtures or microphones mounted in human earcanals.”
- See S12.42-1995 or MIL-STD-912 for MIRE.
- The levels at which passive and electronic devices must be tested (≥ 120 dB) are inconsistent with use of humans as test subjects.

## Issues with REAT (S12.6-1997) which limit its use with ANR or Level-Dependent Devices

- Test Stimulus:  S12.6 test stimulus is pulsed 1/3 octave bands of noise presented at the listener’s threshold of hearing.
- This is inconsistent with the types of noises for which ANR and passive or electronic level-dependent devices are designed.
- Generally, ANR works best to cancel low-frequency steady-state noise.
- Some specifically-tuned ANR devices exist which can cancel tonal (even high-frequency) noise which is steady-state or has a known, measurable period. (ATI and NCT examples)
- Passive level-dependent devices are intended to block relatively high levels of impulsive noise (≥ 120 dB).
- Electroacoustic level-dependent sound-transmission devices limit the levels at the ear depending upon the ambient noise level and require testing at levels ≥ 115 dBA.

## Special Issues with REAT (S3.19-1974; S12.6-1997) and MIRE (S12.42-1995) which limit their application to ANR or Level-Dependent Devices

- Certain ANR and passive/active level-dependent HPDs must be tested using test stimuli that represent the noises for which the devices were designed.
- This complicates instrumentation, test environment, and procedures.
- Clearly, this type of testing is well outside the scope of either REAT or MIRE testing standards.
- Engine/drivetrain/airfoil noise (e.g., helicopter rotor, military vehicles)
- Gunfire and explosive impulses
- Narrow-band, modulated siren noise

## Siren-Canceling ANR Headset Testing(Casali & Robinson, 1994)

**Siren-Canceling ANR Headset Testing**(Casali & Robinson, 1994)

## Siren-Canceling ANR Headset Testing(Casali & Robinson, 1994)

**Wail at 90 dB(Linear)**

**Wail at 100 dB(Linear)**

## How do MIRE and REAT Results Compare when Testing Conventional HPDs? (Casali, Mauney, & Burks, 1995)

- For one fitting of each of 6 earmuffs, subjects underwent both MIRE and REAT attenuation tests.
- MIRE was measured via both Insertion Loss and Noise Reduction techniques.
- Noise Reduction data were corrected for the Transfer Function of the Open Ear.
- 10 subjects, 3 trials each -- this allowed NRRs to be calculated.
- This paradigm afforded a comparison of MIRE data with REAT data for a common fitting of HPD.

## (Casali, Mauney, & Burks, 1995)

## (Casali, Mauney, & Burks, 1995)

## (Casali, Mauney, & Burks, 1995)

## (Casali, Mauney, & Burks, 1995)

## (Casali, Mauney, & Burks, 1995)

## A Misinterpretation with the EPA Labeling Rule

- EPA (40 CFR Part 211) label states that “Although hearing protectors can be recommended for protection against the harmful effects of impulsive noise, the NRR is based on the attenuation of *continuous* noise and may not be an accurate indicator of the protection attainable against *impulsive* noise such as gunfire.”
- This statement was included because level-dependent devices were felt by manufacturers to be under-rated by the NRR.  (Their NRR rating was based on attenuation at threshold, not at high ambient noise levels where their attenuation increased.)

## A Misinterpretation with the EPA Labeling Rule, continued...

- “... and may not be an accurate indicator of the protection attainable against *impulsive* noise such as gunfire.”
- Misinterpretation of this statement ensues:
- A common interpretation is that HPDs do not work well in impulsive noise.
- This is not true, except in extremely high levels (170+ dB) where muffs may separate from the head.
- In fact, NRR is a reasonable indicator of attenuation of impulses, and the REAT method actually underestimates the protection afforded at sound levels considerably higher than the levels at threshold.

## Acknowledgements

- The various Virginia Tech research studies discussed herein were sponsored by NIOSH, U.S. Bureau of Mines, U.S. Army Research Lab, and several hearing protection manufacturers.