As a professional sound quality solution company, Acoutec found that the emerging automotive active sound quality control solution is actually an interdisciplinary and cross-domain concept. Most of the current professional divisions do not fully cover the basics involved in this concept. Therefore, Acoutec will introduce some basic knowledge points in multiple sessions. With the goal of helping everyone understand the sound quality, learn and grow together.
The complete knowledge map of this series is as follows:
In this issue, we mainly explain the mechanism of hearing. That is, the green part of the map.
1. Overview of human ear structure
To have hearing, there must be a complete “hearing perception system”-the human ear, which is like a transducer on our body that converts sound waves into signals that our brain can accept. The figure above is a schematic diagram of the human ear. From the physiological point of view, including Pinna, Auditory canal, Tympanic membrane, Ossicles, Semicircular Canals, Auditory nerve, Cochlea and Eustachian tube.
2. Pinna
For point sound sources, the inverse square law is followed when sound is propagating outwards (follow us, we will introduce this law later when introducing sound propagation). This means that for a given sound intensity, the larger the pinna, the greater the sound energy received. The pinna acts as a preamplifier for the human ear and is used to enhance the sensitivity of listening. Therefore, ears not only attract wind, but also voice.
P.S.
- Someone once asked us which artificial head is strong. One of the criteria in our reference answer is to compare the pinna of a dummy.
- In addition, the pinna has a very important role to help people perceive the spatial location of the sound source.
3. Auditory canal
The picture above is the hearing curve, which is the is loudness curve we often say. It can be seen from this figure that there is an obvious depression between 2000-5000Hz, and there is a peak between 3500-4000Hz, which is caused by the ear canal resonance. There is another peak around 13500Hz, which is caused by the third-order resonance of the auditory canal. High sensitivity response between 2000-5000Hz is an important condition for human voice perception.
If the auditory canal is modeled as a closed cylinder as shown in the figure above, a peak at 3700 Hz appears on the iso-loudness curve, and it can be inferred that under normal body temperature conditions, the length of the canal should be 2.4 cm. The peak around 13kHz is the third harmonic of this resonance frequency. In the human auditory perception system, the auditory canal acts as a closed tube oscillator for amplifying sound between 2000-5000 Hz.
4. Tympanic membrane
The tympanic membrane is an important organ between the ear canal and the inner ear. It receives vibrations from the ear canal and transmits it to the oval window through the ossicles (the oval window is the portal of the inner ear). The area of the tympanic membrane is 15 times larger than the oval window.
5. Hearing sensitivity and dynamic range
In fact, the structure of the outer and middle ears determines the superior sensitivity and wide dynamic range of the human auditory system.The pre-amplifications of the external ear and middle ear to the signal are as follows (the following data are approximate, not exact values):
- The auditory canal doubles the sound in the 2000-5000Hz range;
- The tympanic membrane magnifies the sound by 15 times;
- The ossicles magnify the sound by a factor of three.
In the range of audible frequency (20-20kHz), the human ear can respond to the fluctuation of sound waves in real time. The smallest pressure change that the human ear can perceive is one billionth of an atmosphere (hearing threshold). This incredible sensitivity depends on the magnification produced by the structure of the outer and middle ears. Another thing worth mentioning is the dynamic range of the human ear, which is the ratio between the hearing threshold and the pain domain, up to 130dB. The pain field is determined by the self-protection structure of the human ear.
The above picture is a schematic diagram of the human ear protection structure. When a large sound is introduced into the auditory canal, the muscle tension tightens the tympanic membrane, and through the tendon that connects the ossicle hammer and anvil, the ossicle is reset and the patella is retracted. To reduce the force acting on the oval window in the inner ear portal. The stapedius and tympanic muscles protect the human ear from loud sounds. As you get older, these muscles become harder and the extent to which the cheekbones come back becomes smaller, so in general, older people are less tolerant of loud sounds than young people. Therefore, the elderly do not like to listen to loud music.
The ossicle is composed of Hammer, Anvil, and Stirrup. It can be reduced to a lever model. Under the best conditions, the leverage of this lever is about 3 times, but it will also be affected by the action of muscles (protecting human ears from injury).
6.Inner ear (semicircular canal and cochlea)
The picture above shows the inner ear. The inner ear includes two important organs, the semicircular canal and the cochlea. The semicircular canal detects three-dimensional vibration signals at all times, and controls body balance based on these signals. The cochlea is the human body’s microphone. It converts the sound pressure from the outer ear into electrical signals and transmits them to the brain through the auditory nerve. The shape of the cochlea is like a snail shell. This shape determines the resolution of the pitch that people can hear.
2.75 turns of the cochlea, with a total length of about 3.2cm, can analyze about 1500 independent pitches using 16k-22k hair cells. That is to say, every 0.002cm cochlea can analyze a pitch, which is a very amazing ability.
In the next issue, we will introduce the concepts of pitch, loudness and timbre.
We are not the creators of these theories, but the communicators. Based on the learning of this knowledge, we have concluded some potential connections and laws between physiology and psychology, helping everyone to improve the sound quality of products and constantly optimize the user experience.
For more information about vehicle sound quality control and management solutions, please contact ACOUTEC, sales@acoutec.com.
