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Acoustics Terminology for Musicians © 2023 by Robert Willey is licensed under CC BY-NC-SA 4.0
TRANSCRIPT:
Chapter 5: The Ear: Intensity and Loudness Levels
Welcome to the Acoustics Terminology for Musicians podcast, brought to you by your host, Robert Willey.
[ image: BackusCover.png ]
The order that the terms are introduced in this podcast follows the organization of John Backus’ book, The Acoustical Foundations of Music. The fifth chapter is titled “The Ear: Intensity and Loudness Levels”. This is the first chapter in Part II of the text, which is concerned with how the brain interprets the signals received from the ears.
[ image: OuterEar.png public domain https://commons.wikimedia.org/wiki/File:Gray907.png ]
The ear can be divided into three main parts. The outer ear consists of the flexible part of the ear that we see called the pinna, which is made of cartilage covered with skin. The pinna funnel the pressure wave into the auditory canal. When it reaches the end it causes the eardrum to vibrate. Children who get lots of ear infections may have small tubes put through their ear drums to help ventilate the middle ear. The dimensions of the auditory canal resonates frequencies between 2000 to 5000 Hz, making speech easier to hear.
[ image: MiddleEar.png from Bruce Blaus “Medical gallery of Blausen Medical 2014”, offered under Creative Commons Attribution 3.0 Unported license]
The middle ear has the three ossicles—the smallest bones in the body of mammals. Together they act like a crowbar to increase the amplitude of vibrations coming from the eardrum on one side, and pass them on to the oval window on the other side. Ligaments attached to the bones help clamp down the vibrations if they get too intense in order to protect the inner ear. Unfortunately they are not able to compensate for the extremely loud noises in the modern environment, such as gunshots, jet engines, and rock concerts.
[ image: Nasopharanx.png from Bruce Blaus “Medical gallery of Blausen Medical 2014”, offered under Creative Commons Attribution 3.0 Unported license]
The Eustachian, or auditory tube leads from the middle ear to the nasopharynx. When you swallow, chew, or yawn the tube opens and equalizes the pressure between the environment and the middle ear, which you may need to do when you change altitude in an airplane. If the Eustachian tube gets infected fluid may build up in the middle ear.
Imagine that you are standing next to a swimming pool and yell at someone underwater. 99% of the energy in the pressure wave you project will be reflected off the water, since water is denser than air. The inner ear is filled with fluid, which is why we need the amplification of the middle ear to make the transfer of energy through the oval window into the middle ear more efficient.
[ image: InnerEar.png from Bruce Blaus “Medical gallery of Blausen Medical 2014”, offered under Creative Commons Attribution 3.0 Unported license]
The vibrations of the oval window send a pressure wave into the middle ear, where it travels down the cochlea. The middle ear also reacts to changes in head position. You may see medical offices that specialize in treating hearing and balance issues, since both involve the function of the middle ear.
[ image: Cochlea.png inspired by https://www.researchgate.net/figure/Illustration-of-Greenwood-map_fig4_352295897 ]
The basilar membrane inside the cochlea is lined with hair cells. Those closest to the entrance vibrate in response to the highest frequencies, and the cells at the far end respond to the lowest frequencies. Each region on the basilar membrane resonates to a different frequency, from 20 Hz to 20 kHz. The vibration of the hair cells causes electrical signals to travel through the auditory nerve to the brain. These signals are interpreted by the brain as sound.
Our ears are able to respond to pressure waves with an enormous range of amplitudes. Softest sounds, described as being at the threshold of hearing, are what we can just barely hear, like a mosquito buzzing three meters away. The loudest sounds include those we might hear at a popular music concert.
[ image: LinearVsLogScale.png ]
In order to make it easier to compare those levels on a graph we use a logarithmic scale, which has units based on powers of ten. A linear scale would divide the axis on a graph into equal parts.
[ image: SoftestToLoudest.png ]
The decibel, written as a lowercase “d” followed by a capital “B” is a unit used to measure the intensity or power of a sound by comparing it to a reference level, such as the threshold of hearing. It uses a logarithmic scale, which corresponds more closely to how the ear responds to pressure waves than a graph using a linear scale would suggest.
The smallest change that listeners can generally hear is a 3dB increase, which is surprising since that is equal to a doubling of power. Listen to a bit of pink noise played twice. The second time it is 3dB louder. Can you detect the difference?
[ audio: 3dBComparison.mp3 ]
A 10dB increase in amplitude is perceived by listeners as a doubling of volume. Here is the pink noise again, and then played a second time 10dB louder. Does that sound about twice as loud to you?
[ audio: 10dBComparison.mp3 ]
[ image: FletcherMunsonCurves.png public domain from Lindosland https://commons.wikimedia.org/wiki/File:Lindos1.svg ]
Loudness is a perceptual quality measured in phons. Fletcher Munson curves were arrived at experimentally, by asking subjects to report tones as being equally loud across the frequency range. You can see in this graph that a 1000 Hz sine wave played at a sound pressure level of 20 dB has a perceived loudness level of 20 phons. The ear doesn’t respond as well to bass frequencies. For example, a 100 Hz sine wave would have to be played with a sound pressure level of 50 dB to seem to be equally loud. The Fletcher Munson curves show experimental results indicating that we are more sensitive to frequencies in the 2000-5000 Hz range.
Pressing the loudness button on an amplifier boosts the low frequencies, and is intended to be used when listening to music at a low volume level, since that is where our bass response is the weakest. At high volume levels there is not as much difference between the perception of low and high frequencies. As an engineer you need to remember that the balance of frequencies will change depending on how loud the overall volume is, so you should listen to your mixes at different volumes to see how they will sound for listeners regardless of how loud they playing the music.
[ image: NormalAudiogram.png ]
An audiogram is a graph that audiologists make to show how well a patient can hear different frequencies. The y-axis is a measure of the dB levels softest sine wave frequencies that the patient can hear. Measurements are made for each ear, resulting in two sets of connected points. Normal hearing looks close to a straight line, indicating that all frequencies are heard equally well.
[ image: NoiseInducedHearingLoss.png ]
This audiogram is from someone who has noise-induced hearing loss. Notice how much more poorly they hear midrange frequencies. Here is some speech without a cut at 4KHz simulating normal hearing.
[ audio: FilterSimulation.mp3 ]
Now here is some more speech but with a cut at 4 KHz. Does that make it sound less understandable?
Listen to this recording of speech, followed by the same speech simulating how it might sound to someone with a hearing loss of frequencies around 4Khz.
[ image: SeniorAudiogram.png ]
As we age our hearing becomes less sensitive. Here is an audiogram from a senior, which shows that they do not hear as well across the entire frequency range. Here is what that might sound like for a patient with this type of loss.
[ audio: SeniorSimulation.mp3 ]
Here is some more normal speech, and then this is a simulation for what it might sound like to reduce all the sensitivity across the range, particularly the higher frequencies.
Hearing loss is affected by your genes, so if your grandparents were hard-of-hearing then you may need hearing aids someday, too. Audiologists adjust hearing aids to boost the frequencies that have become harder to hear. They do not perfectly compensate for hearing loss, so the best thing you can do is to limit your exposure to loud sounds, and use hearing protection like foam ear plugs when you are in a noisy environment.
[ image: MusicianEarplugs.png ]
You may wish to invest in Musician Ear Plugs which cut the volume of sound without changing the tone as much as foam earplugs do.
[ image: BinauralHearing.png ]
Our brains compare the signals received from two ears, and the differences help localize the sound source. A pressure wave coming from the right side will arrive at the right ear sooner than the left ear. The difference in timing is called the Interaural Time Difference, or ITD. We can synthesize this effect by delaying a signal sent to the opposite ear. If the delay in the right ear is about 1 to 35 ms we will imagine the sound source coming from left side.
ITDs are most important for frequencies below 1,000 Hz, since those frequencies bend around the head better than higher frequencies that have shorter wavelengths.
Interaural Level Difference, or ILD, is most noticeable for frequencies above 1,500 Hz.
Listen to this demonstration of how ITD and ILD can make sounds seem to move from one side to the other. These effects will be most vivid if you listen wearing headphones, since listening with loudspeakers introduces additional time and level differences.
[ audio: HaasEffect.mp3 ]
If you’re interested you can get the link in the transcript to watch the video of the Max patch used to generate these effects.
[ Video on YouTube: “ITD vs. ILD” https://youtu.be/BuBzYt3SbbI?si=qobJ3hp_1E6faA6Z ]
Binaural hearing refers to listening to recordings made with the intention of playing them back while listening with headphones.
This episode ends with a recording of traffic sounds made with a binaural recording setup. If you listen on headphones you will get a more realistic impression than with loudspeakers.
[ audio: ayamahambho__binaural-ears-very-wide-frontal-no-head.mp3 courtesy of J.K. Chris (ayamahambho) under a Creative Commons Attribution NonCommercial 4.0 License ]
