Now on the Internet there are many opportunities to test the acuity of your hearing online. To do this, run a video with sound, the frequency of which increases. The creators of the test recommend checking with headphones so that extraneous noise does not interfere. The range of sound frequencies in the video begins with such high values that only a few can hear. Further, the sound frequency gradually decreases, and at the end of the video, a sound is heard that even a person with a hearing impairment will hear.
Throughout the video, the user is shown the value of the frequency of the sound that is being played. The test conditions suggest that the video must be stopped at the moment when a person can hear the sound. Next, you should look at what mark the frequency stopped. Its value will make it clear that the hearing is normal, better than most people, or you should consult a doctor. Some tests show how old the limit frequency a person could hear is.
What is sound and sound wave?
Sound is a subjective sensation, but we hear it because something really exists gets into our ear. This is a sound wave. Physicists are interested in how the sensations we experience are related to the characteristics of a sound wave.
Sound waves are longitudinal mechanical waves with a small wave amplitude, the frequency range of which is 20 Hz-20 kHz. A small amplitude is when the pressure change due to compression-rarefaction is much less than the pressure in this medium. In air in the areas of compression-rarefaction, the pressure change is much less than atmospheric. If the amplitude is of the same order or greater than atmospheric pressure, then these are not sound waves, but shock waves, they propagate at a supersonic speed.
Audibility of sounds
We have already figured out what is the range of sound frequencies, but what lies beyond its borders? If the frequency is less than 20 Hz, such waves are called infrasound. If more than 20 kHz, these are ultrasonic waves. Both infra- and ultrasound do not cause auditory sensations. The boundaries are quite blurred: babies hear 22-23 kHz, unhealthy people can perceive 21 kHz, someone hears 16 Hz. That is, the younger the person, the higher the frequency he can hear.
Dogs hear higher frequencies. Trainers use this ability, they give commands with an ultrasonic whistle that is not heard by people. The figure shows the frequency ranges available for perception by different animals.
Sound like a cop weapon
We give an example of a case that shows that the range of sound frequencies heard by a person is approximate and depends on individual characteristics.
In Washington, police found a way to non-violently disperse youth. Boys and girls constantly gathered near one of the metro stations, talked. The authorities considered that their aimless pastime was hindering others, because too many people were gathering at the entrance. The police installed the Mosquito device, which made a sound at a frequency of 17.5 kHz. This device is designed to repel insects, but manufacturers assured that sound waves of a given frequency are perceived only by adolescents from 13 and not older than 25 years.
Thanks to the device, they managed to get rid of the youth, but a 28-year-old man heard a sound and complained to the city administration. Local authorities had to stop using the appliance.
Wavelength range
Sound waves in different environments have different characteristics. The wavelength and speed of wave propagation are different. In air (at room temperature) the speed is 340 m / s.
Consider waves with frequencies in the audible range for us. Their minimum length is 17 mm, the maximum is 17 m. The sound with the shortest wavelength is on the verge of ultrasound, and with the longest it is approaching infrasound.
Sound wave speed
It is believed that light spreads instantly, and for the propagation of sound it takes a certain amount of time. In fact, light also has speed, it’s just the ultimate, faster than light, nothing moves. As for sound, the most interesting is its distribution in air, although the speed of a sound wave in denser media is much higher. Recall the thunderstorm: at first we see a flash of lightning, then we hear the sound of thunder. The sound is delayed because its speed is many times lower than the speed of light. For the first time, the speed of sound was measured, fixing the time interval between a shot from a musket and sound. Then they took the distance between the instrument and the researcher and divided it by the time of the "delay" of the sound.
This method has two drawbacks. Firstly, this is the error of the stopwatch, especially at a close distance to the sound source. Secondly, this is the reaction rate. With such a measurement, the results will not be accurate. To calculate the speed it is more convenient to take the known frequency of a certain sound. There is a frequency generator, a device with a range of audio frequencies from 20 Hz to 20 kHz.
It is turned on at the desired frequency, the wavelength is measured during the experiment. Multiplying both quantities, we get the speed of sound.
Hypersound
The wavelength is calculated by dividing the speed by frequency, therefore, with increasing frequency, the wavelength decreases. It is possible to create oscillations of such a high frequency that the wavelength will be of the same order as the mean free path of gas molecules, for example, air. This is hypersound. It does not spread well, because air ceases to be considered a continuous medium, because the wavelength is negligible. Under normal conditions (at atmospheric pressure), the mean free path of molecules is 10 -7 m. What is the frequency range of the waves? They are not sound, because we do not hear them. If we calculate the frequency of hypersound, it turns out that it is 3 × 10 9 Hz and higher. Hypersound is measured in gigahertz (1 GHz = 1 billion Hz).
How the frequency of sound affects its pitch
The range of sound frequencies affects the pitch range. Although the pitch is a subjective sensation, it is determined by the objective characteristic of sound, frequency. High frequencies produce high sound. Does sound pitch depend on wavelength? Of course, speed, frequency and wavelength are interconnected. However, sound of the same frequency will have a different wavelength in different environments, but it will be perceived the same.
We hear a sound because pressure changes make our eardrum oscillate. The pressure changes at the same frequency, so it does not matter that the wavelength is different in a different medium. Due to the same frequency, we perceive the sound as high or low, even in water, even in air. In water, the speed of sound is 1.5 km / s, which is almost 5 times greater than in air, therefore, the wavelength is much longer. But if the body vibrates at a constant frequency (say, 500 Hz) in both environments, the pitch will be the same.
There are sounds that have no pitch, for example, the sound "shhhh". Their frequency fluctuations are not periodic, but chaotic, so we perceive them as noise.