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The square grid of the graph relating phons and decibels is a plot of intensity level versus frequency—both physical quantities. These answers, like all information extracted from [link], have uncertainties of several phons or several decibels, partly due to difficulties in interpolation, but mostly related to uncertainties in the equal-loudness curves.
Further examination of the graph in [link] reveals some interesting facts about human hearing. Hearing tests are performed over a range of frequencies, usually from 250 to 8000 Hz, and can be displayed graphically in an audiogram like that in [link]. The cochlear implant was pioneered in Melbourne, Australia, by Graeme Clark in the 1970s for his deaf father.
Why can a hearing test show that your threshold of hearing is 0 dB at 250 Hz, when [link] implies that no one can hear such a frequency at less than 20 dB?
What are the closest frequencies to 500 Hz that an average person can clearly distinguish as being different in frequency from 500 Hz? Can the average person tell that a 2002-Hz sound has a different frequency than a 1999-Hz sound without playing them simultaneously?
If your radio is producing an average sound intensity level of 85 dB, what is the next lowest sound intensity level that is clearly less intense?
Can you tell that your roommate turned up the sound on the TV if its average sound intensity level goes from 70 to 73 dB? Based on the graph in [link], what is the threshold of hearing in decibels for frequencies of 60, 400, 1000, 4000, and 15,000 Hz? What sound intensity levels must sounds of frequencies 60, 3000, and 8000 Hz have in order to have the same loudness as a 40-dB sound of frequency 1000 Hz (that is, to have a loudness of 40 phons)? What is the approximate sound intensity level in decibels of a 600-Hz tone if it has a loudness of 20 phons? A person has a hearing threshold 10 dB above normal at 100 Hz and 50 dB above normal at 4000 Hz. A child has a hearing loss of 60 dB near 5000 Hz, due to noise exposure, and normal hearing elsewhere. What is the ratio of intensities of two sounds of identical frequency if the first is just barely discernible as louder to a person than the second?
Transformer a€?humminga€? has been known to soothe people (which makes it a sound) but generally it is thought to be a nuisance (which makes it a noise).
The causes and reduction of transformer noise has been the subject of many articles for at least two generations.
The remedies we use to counter possible objections to transformer noises are varied and in some cases, expensive, because we cannot produce a blanket remedy to cover all situations. Most of us have excellent relative pitch, which means that we can tell whether one sound has a different frequency from another.
The reason is that each instrument produces a distinctive set of frequencies and intensities. Phons differ from decibels because the phon is a unit of loudness perception, whereas the decibel is a unit of physical intensity. To find the loudness of a given sound, you must know its frequency and intensity level and locate that point on the square grid, then interpolate between loudness curves to get the loudness in phons. This is particularly true for frequencies above 8000 Hz, which are rare in the environment and are unnecessary for understanding conversation or appreciating music. The hearing threshold is measured in dB relative to the normal threshold, so that normal hearing registers as 0 dB at all frequencies. The air column in the ear canal resonates and is partially responsible for the sensitivity of the ear to sounds in the 2000 to 5000 Hz range. Pressure waves moving through the cochlea cause the tectorial membrane to vibrate, rubbing cilia (called hair cells), which stimulate nerves that send electrical signals to the brain. Conductive losses in the middle ear can be partially overcome by sending sound vibrations to the cochlea through the skull. If you could measure distances over the same range with a single instrument and the smallest distance you could measure was 1 mm, what would the largest be? Note that many AC electrical appliances produce 60 Hz, music is commonly 400 Hz, a reference frequency is 1000 Hz, your maximum sensitivity is near 4000 Hz, and many older TVs produce a 15,750 Hz whine.


By how many factors of 10 will low-intensity sounds need to be amplified to seem normal to this person?
Note that smaller amplification is appropriate for more intense sounds to avoid further damage to her hearing from levels above 90 dB. How much more intense must a 100-Hz tone be than a 4000-Hz tone if they are both barely audible to this person?
How much more intense is a 5000-Hz tone than a 400-Hz tone if they are both barely audible to the child? It has come to prominence again, mainly because transformers are placed closer to the populacea€”in high rise office buildings, apartments, shopping malls and in their gardens. It is absolutely necessary to consider each case on its merits, to apply the general rules of acoustic technology and to be familiar with the causes of transformer noise.
Finding out you have a noise problem (or vibration problem) after the placement of the unit is costly, time consuming and frustrating. Typically, we can discriminate between two sounds if their frequencies differ by 0.3% or more. We call our perception of these combinations of frequencies and intensities tone quality, or more commonly the timbre of the sound. In fact, people who have lost the ability to hear such high frequencies are usually unaware of their loss until tested. Hearing loss caused by noise typically shows a dip near the 4000 Hz frequency, irrespective of the frequency that caused the loss and often affects both ears. The ear is a transducer that converts sound waves into electrical nerve impulses in a manner much more sophisticated than, but analogous to, a microphone. The middle ear converts sound into mechanical vibrations and applies these vibrations to the cochlea.
The membrane resonates at different positions for different frequencies, with high frequencies stimulating nerves at the near end and low frequencies at the far end. Hearing aids for this purpose usually press against the bone behind the ear, rather than simply amplifying the sound sent into the ear canal as many hearing aids do. The external components are a microphone for picking up sound and converting it into an electrical signal, a speech processor to select certain frequencies and a transmitter to transfer the signal to the internal components through electromagnetic induction. Note that smaller amplification is appropriate for more intense sounds to avoid further hearing damage. It is becoming even more necessary to locate these units carefully and some planning, preferably ahead of time, is needed. The techniques can be explained simply enough for anyone to understand and the rules are, in the main easy to apply. How is our hearing related to the physical qualities of sound, and how does the hearing mechanism work? At a given frequency, it is possible to discern differences of about 1 dB, and a change of 3 dB is easily noticed.
It is more difficult to correlate timbre perception to physical quantities than it is for loudness or pitch perception. The shaded region in [link] is the frequency and intensity region where most conversational sounds fall. The most common form of hearing loss comes with age and is called presbycusis—literally elder ear.
The lever system of the middle ear takes the force exerted on the eardrum by sound pressure variations, amplifies it and transmits it to the inner ear via the oval window, creating pressure waves in the cochlea approximately 40 times greater than those impinging on the eardrum. The complete operation of the cochlea is still not understood, but several mechanisms for sending information to the brain are known to be involved.
Damage to the nerves in the cochlea is not repairable, but amplification can partially compensate. Attending rock concerts with noise levels at eardrum rattling levels is totally stimulating to many people.
When we do hear low-frequency vibrations, such as the sounds of a diving board, we hear the individual vibrations only because there are higher-frequency sounds in each. Pitch perception is directly related to frequency and is not greatly affected by other physical quantities such as intensity.


Such loss is increasingly severe at higher frequencies, and interferes with music appreciation and speech recognition. For sounds below about 1000 Hz, the nerves send signals at the same frequency as the sound. Put those same people in a different environment, possibly next to a transformer, and there will be wild protests. Musical notes are particular sounds that can be produced by most instruments and in Western music have particular names.
Terms such as dull, brilliant, warm, cold, pure, and rich are employed to describe the timbre of a sound. A 40-phon hearing loss at all frequencies still allows a person to understand conversation, although it will seem very quiet. They react to intense sound in a few milliseconds and reduce the force transmitted to the cochlea. Another common failure in the cochlea is damage or loss of the cilia but with nerves remaining functional. Dogs can hear sounds as high as 30,000 Hz, whereas bats and dolphins can hear up to 100,000-Hz sounds. The ear has its maximum sensitivity to frequencies in the range of 2000 to 5000 Hz, so that sounds in this range are perceived as being louder than, say, those at 500 or 10,000 Hz, even when they all have the same intensity. So the consideration of timbre takes us into the realm of perceptual psychology, where higher-level processes in the brain are dominant. The curves were determined by having large numbers of people compare the loudness of sounds at different frequencies and sound intensity levels.
A person with a 60-phon loss at all frequencies will hear only the lowest frequencies and will not be able to understand speech unless it is much louder than normal. This protective reaction can also be triggered by your own voice, so that humming while shooting a gun, for example, can reduce noise damage. There is a structure to the cilia, and there are connections between nerve cells that perform signal processing before information is sent to the brain. Cochlear implants that stimulate the nerves directly are now available and widely accepted. Since it is necessary to place electrical apparatus alongside a wide spectrum of people we have to accept the inevitable, that even under normal conditions, somebody will always complain. You may have noticed that dogs respond to the sound of a dog whistle which produces sound out of the range of human hearing. Some people can identify musical notes, such as A-sharp, C, or E-flat, just by listening to them.
Sounds near the high- and low-frequency extremes of the hearing range seem even less loud, because the ear is even less sensitive at those frequencies. Intensity information is partly indicated by the number of nerve signals and by volleys of signals. We shall not delve further into them; rather, we will concentrate on the question of loudness perception. For example, a 15-dB sound at 4000 Hz has a loudness of 20 phons, the same as a 20-dB sound at 1000 Hz. The conversational speech region also has a gender component, in that female voices are usually characterized by higher frequencies. The brain processes the cochlear nerve signals to provide additional information such as source direction (based on time and intensity comparisons of sounds from both ears). The curves rise at both extremes of the frequency range, indicating that a greater-intensity level sound is needed at those frequencies to be perceived to be as loud as at middle frequencies. So the person with a 60-phon hearing impediment might have difficulty understanding the normal conversation of a woman. For example, a sound at 10,000 Hz must have an intensity level of 30 dB to seem as loud as a 20 dB sound at 1000 Hz.



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