Exam Accommodations folks, talk to me now!

• Midterm is next week!

• Study guide is online, as is a sample of what the exam’s format will look like. See https://savethevowels.org/docs/l113_studyguide.html

• 41 questions, fill-in-the-blank and short answer, given in 50 minutes.

• I reserve the right to ‘curve’ the exam or not, as seems appropriate based on the grade distribution

Amplitude and Decibels

Will Styler - LIGN 113

Today’s Plan

• Filtering

• Measuring Sound Pressure

• Distance and Pressure

• Units for measuring pressure

• Decibels

• Amplitude related hearing damage!

Filtering

We’re just going to give you four terms

• High Pass

  • “Let frequencies higher than this pass”

• Low Pass

  • “Let frequencies lower than this pass”

• Band Pass

  • “Let frequencies in this band pass”

• Band Stop (also ‘Band Reject’ and ‘Notch’)

  • “Let only frequencies outside this band pass”

“>

Filters aren’t perfect

• They always have a ‘cutoff’ and a ‘slope’

  • Your book calls slope ‘rejection rate’, but you’ll also hear ‘roll-off’

• The slope dictates how quickly sounds outside the cutoff are attenuated

• There’s no such thing as a perfect filter

  • Where the slope is vertical

Filtering is important!

• It’s a part of lots of hearing aids’ and CIs’ processing

• We can model hearing loss by filtering

• It’s a basic component of signal processing

… and now, for something completely different

Now we’ve talked about all the basic properties of sounds

• As well as the process of combining them and modifying them

• Now, how should we measure that Y-axis anyways?

Amplitude

Sound Amplitude is pressure

• Greater compression, greater amplitude

Measuring Sound Pressure

Where do we measure on the wave?

We use Root-Mean-Square Amplitude

• Eliminates asymmetry between up and down

• Takes the mean amplitude for the sound overall!

• This captures the fact that sounds often vary in amplitude!

How far away do I measure from?

Distance matters!

• If the distance from the source is doubled, pressure is cut in half!

There’s a bit more to it than this!

• Moving the medium damps sound independently of this

  • Lower frequencies are damped less in air, hence seeming to ‘travel’ farther

• Vertical differences in air temperature can ‘bend’ sound up and down

  • When the air is warmer near the ground, sound bends upwards

We know this to be true!

• Distant sounds are quieter

• You don’t feel the pressure bursts from distant fireworks

• There is a ‘safe distance’ from an explosion

Amplitude measures without distances are meaningless

Amplitude measures without distances are meaningless

• Anything’s safe for hearing if you’re far enough away

  • “Just grab their measurement device and walk ’til it’s legal”

• So, it’s critical to give amplitudes with a distance

  • “20 Pascal at 1 meter”
  • “… measured at 90dB at 2 meters from the source”
  • “Less than 50dB at the property boundary”

OK, so what units should we use?

Let’s start with a simple unit

• Pascal - Newtons per square meter

• Where a newton is the force required to move a kilogram 1 meter per second per second

What’s zero sound pressure?

• Nothing really is

• Quietest places on Earth (anechoic chambers) are ~1.9x10^-6 Pascal

• The quietest 1kHz sound we can hear is 2.00×10⁻⁵ Pascal

• We’re humans, so we’re choosing the human audibility threshold as ‘zero’

What’s the maximum pressure?

• The Planck Pressure?

  • The highest possible force in the smallest possible area

  • 4.63309*10¹¹³ Pascal

• In Earth’s atmosphere, the loudest possible waves are around 1.01x10⁵ (101000) Pascal

• Humans experience pain in the 200 Pascal Range

• Humans experience long term hearing damage > 0.36 Pascal

So our range as humans is…

• 200 Pascal highest tolerable sound with pain

• 63 Pascal for a trumpet half a meter away

• 20 Pascal can cause instant hearing damage

• 2 Pascal for a jackhammer a meter away

• 0.36 Pascal causes hearing damage over long term exposure

• 0.02 Pascal for a home TV a meter away

• 0.002 for conversation

• 0.00002 Pascal lowest detectable sound by humans

That’s kind of awful

• Most of the scale is used for ‘too damned loud’

• Any time you’re using 0.00002, you’re going to make mistakes

Enter the Decibel!

• Decibels are logarithmic

• We’ll use dB SPL (Sound Pressure Level)

• We use a reference value of 0.00002 Pascal

  • The lowest audible amplitude

dB SPL can be calculated from Pascal

• dB = 20*log10(Pascal/0.00002)

  • log10 is NOT the same thing as log or ln!
  • This will mess up your homeworks!

So our range as humans is…

• 140dB highest tolerable sound with pain

• 130dB for a trumpet half a meter away

• 120dB can cause hearing damage instantly

• 100dB for a jackhammer a meter away

• 85dB causes hearing damage over long term exposure

• 60dB for a home TV a meter away

• 20dB for conversation

• 0dB lowest detectable sound by humans

• -20dB Anechoic Chamber Quiet

Yes, dB can be negative!

• Remember, 0 is our threshold for hearing!

  • … and we’re not all that good at hearing quiet things

Decibels and Damage

High Amplitude sounds cause hearing damage!

• 180+dB can break the TM and/or dislocate the ossicles

• Sounds over 132dB can rip the organ of corti off the basilar membrane

• Long term high amplitude sounds damage the stereocilia and can cause cell death

What are the ‘safe’ levels?

• Let’s Ask ASHA

  • The American Speech Language Hearing Association

What are ‘safe’ levels?

• < 70 dB is absolutely fine!

• 70-85 dB is fine in short doses < 8 hours

• 85+ dB has the safe exposure time cut in half per 3 dB

  • So 91 dB is OK for only 2 hours

• OSHA requires hearing protection for anything over 90 dB

• Over 120dB is not safe for any amount of time

  • This is instant damage!

Instant Damage Sounds

• 140-190dB for a gunshot, firecracker, artillery, nearby fireworks

• 150dB for Jet Engines and firecrackers and dynamite blasts

• Over 130dB causes immediate pain!

• 130dB for auto racing, jackhammers

• Over 120dB can cause instant hearing damage

• 110-120dB is thunder, stadia, chainsaws

• Most rock concerts are 110-120dB!

Gradually damaging sounds

• 105+ dB for unmuffled motorcycles

• 90-100dB is very possible with headphones and background noise

• 80-90dB is common on busy streets and in subways

• Restaurants can easily reach 90dB

• 85dB can cause damage with long-term exposure

  • Blenders, Smoke Alarms, Hand Saws, and Machining tools can easily be in this range!

Acoustical laws!

• In San Diego…

  • Residential Zoned Areas have a limit of 60 dB at the property line
  • Commercial Zoned Areas have a limit of 65 dB at the property line
  • Industrial Zoned Areas have a limit of 75 dB at the property line

• California limits motorcycles to 80dB (modern) and 92dB (pre-1970)

• California law limits vehicle noise to 95dB

• Both motorcycles and cars are measured at 50 feet from centerline of travel!

• This presentation does not constitute legal advice

OSHA Requirements

• OSHA has standards for noise exposure

When information indicates that any employee’s exposure may equal or exceed an 8-hour time-weighted average of 85 decibels, the employer shall develop and implement a monitoring program.

The employer shall establish and maintain an audiometric testing program as provided in this paragraph by making audiometric testing available to all employees whose exposures equal or exceed an 8-hour time-weighted average of 85 decibels.

Employers shall make hearing protectors available to all employees exposed to an 8-hour time-weighted average of 85 decibels or greater at no cost to the employees. Hearing protectors shall be replaced as necessary.

Be careful with your hearing!

• If you work in a loud setting, ask about the hearing plan

  • Many restaurants exceed the OSHA levels!

• Use noise reducing earplugs at concerts

• When you’re up against serious noise, double up

  • Earmuff style over foam plugs is more effective still!

  • Remember that electronic earmuffs are a thing

Every action hero would be completely deaf

• There is so much hearing damage in movies

• Seriously, do not fire guns in enclosed spaces without hearing protection

Be careful with your hearing!

• It doesn’t “come back” if you damage it!

One other kind of dB

About that threshold!

• We’d said that 0dB/0.00002 Pascal is the lowest detectable sound

• This isn’t quite true!

Sound Detection is frequency dependent!

This leads to the use of ‘dB HL’

• ‘Decibels of Hearing Loss’

• 0 dB HL is the normal hearing threshold at a given frequency

This means that hearing tests aren’t done in dB SPL

• We wouldn’t want ‘normal’ to look like a weird curvy line

• This way, hearing loss is expressed in terms of ‘relative to normal hearing’

So, we’ll talk about pressures in dB SPL

• … and we’ll talk about hearing loss in dB HL

Do the reading!

• The book goes deeper into the math of dB than I’m going to here

  • It also covers resonance, which I’m not getting into here

Next time

• How do we capture, measure, and play sound back?

Thank you!