The human ear is an impressive instrument, and our brain, the “processing unit“ behind it, even more so. To be able to interpret the results of acoustic measurements, it is important to understand how we register and process sound.
Sounds from outside reach our brain in the following ways:
Pinna: determines the frequency and direction of sound
Ear canal: increases the frequency of sound to up to 4000Hz
Eardrum: protects the middle ear from moisture, regulates the ear pressure through the Eustachian tube, through which air from the atmosphere can flow via the nasal cavity.
Hammer, anvil and stirrup: Ossicles that transmit sound to the inner ear and cochlea via the stapedius muscle. This muscle contracts in certain situations to protect our inner ear. After a rock concert or a party, we sometimes hear sounds for a while damped or a ringing in our ears, but these effects usually subside until the next day.
Inner ear and cochlea: A liquid inside the cochlea, our organ of balance, transmits sound waves via the basilar membrane to tiny hair cells (cilia), which are connected to the auditory nerve. For every sound frequency (so-called Bark scale), there is a special group of cilia. If these hair cells bend or break due to the exposure to very loud noise, we can suffer from a permanent hearing damage.
The human ear is not constructed in a linear way. Depending on the noise volume, the pitch of the sound is transmitted on various levels:
What is the meaning of A-B and C weight during a sound measurement?
Depending on the noise volume, the human ear is more or less sensitive to low or high frequencies. The sensitivity is displayed in so-called isophone curves, which show noise levels that we perceive as equally high. To be able to perform a valid sound measurement, the measured values must be weighted, for which filters are being used.
The A-filter weakens all frequencies under 1000Hz and is used at a sound pressure level of up to 55dB.
The C-Filter is used for sound pressure of 100dB and more. For the range between 55 and 100dB, there is in an additional B-filter, which is hardly used in practice, as the A-weighting delivers sufficiently good results in this area.
The isophone curves shown above indicate at which sound pressure level we perceive sound as equally loud. The dotted line represents the limit of our hearing capabilities. Below this line, we don’t hear anything. The difference between a sound of 4000Hz and 20Hz (medium/high sound and bass sound) is only 75dB!
Therefore, a weighting using a filter (A-B-C) is important when we measure sound levels.
Sounds within a range of the Bark-frequency scale can mask noises within the adjacent range. This means, that we can’t hear certain noises when we are under the influence of other sounds. If, for example, we have a conversation at a station and a train passes, we are not able to understand any longer what the other person says. But masking can also be useful, for example when a building is too quiet. In such a case, a buzzing on a low frequency level can be very helpful.
Masking in our ear happens in an asymmetrical way: low frequencies mask other sounds more than high frequencies, which can be seen in the diagram below.
We define the direction of sound in three different ways:
1-By defining the difference in volume between both ears
2-By defining the sound colour (this happens in our pinna and our brain)
3-By defining the time difference between the sound perceived by our two ears.
This ability helps us especially in survival situations: as this part of our sensory system is particularly well developed, we are able to know from which direction a possible danger is threatening us.
Identifying a pattern: Just like we are able to deduct the nature of an object from a primitive shape (we compare the imagine we see to images from our memory), we are able to understand what another person says even under bad acoustical conditions, such as reverberation or noise. We do so by recognizing the voice of a person and interpreting words that we don’t hear properly. It is no surprise, though, that this uses up a lot of our energy.
In this video Julian Treasure explains why we should design more for our ears and not only for our eyes. Architects and designers mainly emphasize what we can see, while sound strongly influences us. By designing more for sound, we can improve our health, social behavior and productivity.