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Perception-based protection from low-
frequency sounds may not be enough
             Alec N. Salt, Ph.D.
                     &
       Jeffery T. Lichtenhan, Ph.D



            Department of Otolaryngology
            Washington University School of Medicine
            St. Louis, Missouri, USA




                                                InterNoise, New York, August 2012
Human Hearing at Low Frequencies



                 120

                 100
                                                                      Hearing (perception) is
                                                                      insensitive to very low
                 80
Level (dB SPL)




                 60
                           109 dB SPL at 5 Hz


                                                                        frequency sounds
                 40

                 20

                  0
                       1              10               100     1000
                                           Freq (Hz)




                       As a result, some have concluded that:
                       Subaudible, low frequency sound and infrasound from wind turbines do not present a
                       risk to human health. (AWEA, CanWEA Report Colby et al. 2009)


                       Our data show that the ear is sensitive to infrasound frequencies
                       that are not heard. The electrical responses of the ear can be
                       substantially larger than to any other type of acoustic
                       stimulation.




                                                                                                InterNoise, New York, August 2012
Measurements from the                               Low
  inner ear with tonal                           Frequencies
        stimuli

For most sounds, electrical
responses from the ear are never   High
larger than a few mV.              Frequencies

From an electrode in endolymph
near the apical (low frequency)
end of the ear we measure
enormous (19 mV) electrical
responses with a 5 Hz
(infrasound) stimulus.

Much larger responses than with
sounds in the normal audible
range.



                                                 InterNoise, New York, August 2012
Response Amplitude with Tone Level
    500 Hz perceptual threshold: 18 dB SPL




                                              Linear
                                             1 dB/dB
                                             (10X per
                                              20 dB)



                                             InterNoise, New York, August 2012
Response Amplitude with Tone Level
    500 Hz perceptual threshold: 18 dB SPL




                                             Responses
                                             saturate at
                                                4 mV




                                             InterNoise, New York, August 2012
Response Amplitude with Tone Level
    50 Hz perceptual threshold: 53 dB SPL


                                             Responses
                                            saturate at
                                              10 mV




                                             InterNoise, New York, August 2012
Response Amplitude with Tone Level
    5 Hz perceptual threshold: ~ 124 dB SPL

                                               Responses
                                              saturate at
                                                >17 mV
                                              Larger voltages mean
                                              larger transduction
                                              currents, more ion
                                              transport, more
                                              metabolic demand.

                                              The system is being
                                              driven harder.




                                                InterNoise, New York, August 2012
The large responses to infrasound (5 Hz) are suppressed by
             higher frequency tones (500 Hz)




                                                  InterNoise, New York, August 2012
Initial Conclusions

●
    The ear generates larger responses to infrasound than it
    does for low frequency sounds in the audible range.
●
    Audible low frequency sounds suppress the response to
    infrasound.




                                                     InterNoise, New York, August 2012
Measurements with low-
   pass filtered frozen noise
Some types of low pass filtered noise have been
shown to be very annoying (Krahé, 2008,2010)

 Changing cutoff frequency alters
 high frequency content but does
 not affect sound below 125 Hz.

 As filter cutoff is reduced below
 1 kHz, sound becomes quieter,
 especially when measured as
 dBA.

 125 Hz cutoff is 56 dBA, which
 is -33.6 dB re. 8 kHz cutoff
 noise.


                                                  InterNoise, New York, August 2012
Responses measured
simultaneously from
 the ear canal (mic)
 and from the inner
         ear.
Responses from the ear
are larger when high
frequency components
are absent.

Similar to previous
results with tones.




                         InterNoise, New York, August 2012
Responses with level



                       With higher
                       frequencies
                       present,
                       responses
                       saturate




                          InterNoise, New York, August 2012
Responses with level

                  Without higher
                  frequencies,
                  responses keep
                  growing




                       InterNoise, New York, August 2012
Same Responses plotted vs A-weighted sound level




                                        InterNoise, New York, August 2012
Responses vs A-weighted sound level

                41 dBA with 125 Hz cutoff
                stimulates the ear to the same
                degree as 85 dB wide band noise




                                       InterNoise, New York, August 2012
Responses vs A-weighted sound level


                   46 dBA (or higher) with 125 Hz
                   cutoff stimulates the ear more
                   than ANY wide band noise level.




                                     InterNoise, New York, August 2012
From Rand and Ambrose, 2011


           Responses vs A-weighted sound level


                              46 dBA (or higher) with 125 Hz
                              cutoff stimulates the ear more
                              than ANY wide band noise level.




                                                InterNoise, New York, August 2012
Low frequency sounds are strongly stimulating the
            ear at low dBA levels.


 Ear

                      Although an influence on the
 Hearing             body is not be mediated though
                             perception……

Brain




                                             InterNoise, New York, August 2012
Low frequency sounds are strongly stimulating the
            ear at low dBA levels.


 Ear
                         We also need to consider
                          scientifically-plausible
 Hearing              mechanisms that do not involve
                      perception of the low frequency
                                   sound.
Brain

                     There are at least 3 possibilities




                                              InterNoise, New York, August 2012
1) Infrasound-induced amplitude modulation
Low frequency and infrasound cause amplitude modulation of sounds
you can hear that is well-established in auditory neuroscience,
described as low frequency biasing.
This is BIOLOGICAL in origins and cannot be measured with a sound
level meter.

                                                     Data recorded
                                                     from single
                                                     auditory nerve
                                                     fiber.

                                                     Salt & Lichtenhan
                                                     2011




Expected symptoms: Pulsating sounds, annoyance, stress .



                                                           InterNoise, New York, August 2012
2) Endolymphatic Hydrops (fluid disturbance)
 Low frequency sound at non-damaging levels for just 3 minutes causes a
swelling of the endolymphatic space - endolymphatic hydrops.
 As the most compliant part of the endolymphatic system is the saccule, this
could lead to saccular disturbance.
 Normal         Mild            Severe
               Hydrops          Hydrops




Expected Symptoms: fullness, unsteadiness, tinnitus,
  “seasickness”, possibly vertigo.

                                                                InterNoise, New York, August 2012
3) Non-perceived neural pathways
   5% of auditory nerve fibers connect multiple outer hair
   cells (the source of our large responses) to the brain.
   Similar fibers that innervate multiple hair cells in birds
   respond strongly to infrasound.




   Auditory
   Pathway
    Pathway from
   Outer Hair Cells


   Expected Symptoms: Sleep disturbance, stress, leading to
   secondary effects such as elevated blood pressure and
   memory disturbances.


                                                      InterNoise, New York, August 2012
To Summarize

There are at least 3 processes, each with
supporting scientific data, by which low
frequency stimulation could influence
people.
The assertion that effects of low frequencies can
ONLY be mediated by hearing the sound is simply
untenable.




                                            InterNoise, New York, August 2012
Final Conclusions

The ear generates larger responses to infrasound than it does for low
frequency sounds in the audible range.

Audible low frequency sounds suppress the response to infrasound.

Optimal masking of low frequency responses to noise occurs with
frequencies of 150 Hz -1.5 kHz.

There are a number of scientifically plausible pathways, unrelated to
perception, by which low frequency sounds could influence someone.

We need to better understand how low frequencies affect the ear
before we dismiss their influence on people.

                                                           InterNoise, New York, August 2012
Additional Slides for questions



     Measurements
   from the inner ear
     of guinea pigs
Guinea pigs vs Humans

  Guinea pig hearing is about 15 dB less sensitive than humans
                                BUT
  We make our measurements with the middle ear open,
  which makes them ~10-15 dB more sensitive at low frequency.


The sensitivity we measure will be comparable to humans.
                                                    InterNoise, New York, August 2012
Demonstration that
     measured responses to
         infrasound are
     generated locally in the
           apical turns
Response changes are shown for an injection of toxic KCl
(150 mM) into the cochlear apex. The calculated elevation
of K at various locations is shown at the top. K
progressively moves from apex to base with time.

Lower Panel: Compound APs are suppressed progressively
with frequency, showing the basal movement.

Middle Panel: Response from turn 3 with 4.8 Hz
stimulation is ablated before CAP thresholds start rising.
This confirms the response is locally generated near the
recording site.




                                                             InterNoise, New York, August 2012
Infrasound
  Production by Dun
 Law Wind Farm, UK
     From Styles et al. 2005,
     Keele University


Peak spectral output at ~ 0.5 Hz
with harmonics up to ~ 7 Hz.

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Salt Internoise2012

  • 1. Perception-based protection from low- frequency sounds may not be enough Alec N. Salt, Ph.D. & Jeffery T. Lichtenhan, Ph.D Department of Otolaryngology Washington University School of Medicine St. Louis, Missouri, USA InterNoise, New York, August 2012
  • 2. Human Hearing at Low Frequencies 120 100 Hearing (perception) is insensitive to very low 80 Level (dB SPL) 60 109 dB SPL at 5 Hz frequency sounds 40 20 0 1 10 100 1000 Freq (Hz) As a result, some have concluded that: Subaudible, low frequency sound and infrasound from wind turbines do not present a risk to human health. (AWEA, CanWEA Report Colby et al. 2009) Our data show that the ear is sensitive to infrasound frequencies that are not heard. The electrical responses of the ear can be substantially larger than to any other type of acoustic stimulation. InterNoise, New York, August 2012
  • 3. Measurements from the Low inner ear with tonal Frequencies stimuli For most sounds, electrical responses from the ear are never High larger than a few mV. Frequencies From an electrode in endolymph near the apical (low frequency) end of the ear we measure enormous (19 mV) electrical responses with a 5 Hz (infrasound) stimulus. Much larger responses than with sounds in the normal audible range. InterNoise, New York, August 2012
  • 4. Response Amplitude with Tone Level 500 Hz perceptual threshold: 18 dB SPL Linear 1 dB/dB (10X per 20 dB) InterNoise, New York, August 2012
  • 5. Response Amplitude with Tone Level 500 Hz perceptual threshold: 18 dB SPL Responses saturate at 4 mV InterNoise, New York, August 2012
  • 6. Response Amplitude with Tone Level 50 Hz perceptual threshold: 53 dB SPL Responses saturate at 10 mV InterNoise, New York, August 2012
  • 7. Response Amplitude with Tone Level 5 Hz perceptual threshold: ~ 124 dB SPL Responses saturate at >17 mV Larger voltages mean larger transduction currents, more ion transport, more metabolic demand. The system is being driven harder. InterNoise, New York, August 2012
  • 8. The large responses to infrasound (5 Hz) are suppressed by higher frequency tones (500 Hz) InterNoise, New York, August 2012
  • 9. Initial Conclusions ● The ear generates larger responses to infrasound than it does for low frequency sounds in the audible range. ● Audible low frequency sounds suppress the response to infrasound. InterNoise, New York, August 2012
  • 10. Measurements with low- pass filtered frozen noise Some types of low pass filtered noise have been shown to be very annoying (Krahé, 2008,2010) Changing cutoff frequency alters high frequency content but does not affect sound below 125 Hz. As filter cutoff is reduced below 1 kHz, sound becomes quieter, especially when measured as dBA. 125 Hz cutoff is 56 dBA, which is -33.6 dB re. 8 kHz cutoff noise. InterNoise, New York, August 2012
  • 11. Responses measured simultaneously from the ear canal (mic) and from the inner ear. Responses from the ear are larger when high frequency components are absent. Similar to previous results with tones. InterNoise, New York, August 2012
  • 12. Responses with level With higher frequencies present, responses saturate InterNoise, New York, August 2012
  • 13. Responses with level Without higher frequencies, responses keep growing InterNoise, New York, August 2012
  • 14. Same Responses plotted vs A-weighted sound level InterNoise, New York, August 2012
  • 15. Responses vs A-weighted sound level 41 dBA with 125 Hz cutoff stimulates the ear to the same degree as 85 dB wide band noise InterNoise, New York, August 2012
  • 16. Responses vs A-weighted sound level 46 dBA (or higher) with 125 Hz cutoff stimulates the ear more than ANY wide band noise level. InterNoise, New York, August 2012
  • 17. From Rand and Ambrose, 2011 Responses vs A-weighted sound level 46 dBA (or higher) with 125 Hz cutoff stimulates the ear more than ANY wide band noise level. InterNoise, New York, August 2012
  • 18. Low frequency sounds are strongly stimulating the ear at low dBA levels. Ear Although an influence on the Hearing body is not be mediated though perception…… Brain InterNoise, New York, August 2012
  • 19. Low frequency sounds are strongly stimulating the ear at low dBA levels. Ear We also need to consider scientifically-plausible Hearing mechanisms that do not involve perception of the low frequency sound. Brain There are at least 3 possibilities InterNoise, New York, August 2012
  • 20. 1) Infrasound-induced amplitude modulation Low frequency and infrasound cause amplitude modulation of sounds you can hear that is well-established in auditory neuroscience, described as low frequency biasing. This is BIOLOGICAL in origins and cannot be measured with a sound level meter. Data recorded from single auditory nerve fiber. Salt & Lichtenhan 2011 Expected symptoms: Pulsating sounds, annoyance, stress . InterNoise, New York, August 2012
  • 21. 2) Endolymphatic Hydrops (fluid disturbance) Low frequency sound at non-damaging levels for just 3 minutes causes a swelling of the endolymphatic space - endolymphatic hydrops. As the most compliant part of the endolymphatic system is the saccule, this could lead to saccular disturbance. Normal Mild Severe Hydrops Hydrops Expected Symptoms: fullness, unsteadiness, tinnitus, “seasickness”, possibly vertigo. InterNoise, New York, August 2012
  • 22. 3) Non-perceived neural pathways 5% of auditory nerve fibers connect multiple outer hair cells (the source of our large responses) to the brain. Similar fibers that innervate multiple hair cells in birds respond strongly to infrasound. Auditory Pathway Pathway from Outer Hair Cells Expected Symptoms: Sleep disturbance, stress, leading to secondary effects such as elevated blood pressure and memory disturbances. InterNoise, New York, August 2012
  • 23. To Summarize There are at least 3 processes, each with supporting scientific data, by which low frequency stimulation could influence people. The assertion that effects of low frequencies can ONLY be mediated by hearing the sound is simply untenable. InterNoise, New York, August 2012
  • 24. Final Conclusions The ear generates larger responses to infrasound than it does for low frequency sounds in the audible range. Audible low frequency sounds suppress the response to infrasound. Optimal masking of low frequency responses to noise occurs with frequencies of 150 Hz -1.5 kHz. There are a number of scientifically plausible pathways, unrelated to perception, by which low frequency sounds could influence someone. We need to better understand how low frequencies affect the ear before we dismiss their influence on people. InterNoise, New York, August 2012
  • 25. Additional Slides for questions Measurements from the inner ear of guinea pigs Guinea pigs vs Humans Guinea pig hearing is about 15 dB less sensitive than humans BUT We make our measurements with the middle ear open, which makes them ~10-15 dB more sensitive at low frequency. The sensitivity we measure will be comparable to humans. InterNoise, New York, August 2012
  • 26. Demonstration that measured responses to infrasound are generated locally in the apical turns Response changes are shown for an injection of toxic KCl (150 mM) into the cochlear apex. The calculated elevation of K at various locations is shown at the top. K progressively moves from apex to base with time. Lower Panel: Compound APs are suppressed progressively with frequency, showing the basal movement. Middle Panel: Response from turn 3 with 4.8 Hz stimulation is ablated before CAP thresholds start rising. This confirms the response is locally generated near the recording site. InterNoise, New York, August 2012
  • 27. Infrasound Production by Dun Law Wind Farm, UK From Styles et al. 2005, Keele University Peak spectral output at ~ 0.5 Hz with harmonics up to ~ 7 Hz.