Ear spacing

In psychoacoustics, the ear distance indicates the distance between the microphones that is optimal for binaural sound recording . The microphone arrangement should correspond to the position of the ears of a human head in order to create a good stereo impression .

theory

The true distance between the ears, as the crow flies, from eardrum to eardrum is about 14 cm. A number of approaches have been taken in the literature to calculate the interaural (i.e. "ear to ear") signal differences between the two eardrums. To determine the "head diameter", the head is usually assumed to be spherical. Some authors assume an ear distance of 17.5 cm, which is also used in the ORTF stereo system. The spherical surface microphone KFM 6 from Schoeps has a diameter of 20 cm, the KFM 360 of 18 cm.

To model the distance between the ears, it is necessary to decide whether the entrance to the external auditory canal or the eardrum is the correct measuring point , whether the human head is assumed to be a sphere and whether the ears are aligned as precisely at right angles to this sphere. The ear canal entrances are offset backwards at approximately ± 104 °. The ear canal is an average of 2.5 cm long, with an average diameter of 7 to 8 mm. The eardrum is inclined at about 45 ° at the end of the ear canal. The sound pressure fluctuations in the ear canal are picked up by the eardrum and transmitted to the inner ear via the ossicles .

Measurements

Pulse measurements at the ear canal input have resulted in a maximum phase delay around the head of Δt = 0.63 ms = 630 μs.

This corresponds to

{\ displaystyle \ Delta t = {\ frac {a \ cdot sin \, \ theta} {c}}}

${\ displaystyle \ Leftrightarrow a = {\ frac {\ Delta t \ cdot c} {sin \, \ theta}}}$

With

a sound incidence angle θ of 90 °, i.e. H. exactly from the side
the speed of sound c = 343 m / s at 20 ° C.

a distance of a = 0.216 m = 21.6 cm. This path, which is effective for the sound, is called effective ear spacing. For the interaural phase delay difference IPD , imagine that there two pressure microphones in a straight line form an “ear” or microphone base .

The frequency f = c / λ = 343 / 0.216 = 1588 Hz belongs to the wavelength λ = 21.6 cm, i.e. around f = 1600 Hz. The lowest frequency at which a phase shift of φ = 180 ° occurs is therefore f = 800 Hz.

Localization sharpness

It is no coincidence that the frequency range with the lowest localization sharpness is between 800 and 1600 Hz. This corresponds exactly to Blauert's direction-determining rear band ( median plane ) and the less sensitive area of the curves of the same volume level around 1000 Hz.

This is about the interaural transit time difference ITD , the ear distance and frequencies that belong to λ and λ / 2, i.e. to head-related interaural signal differences in natural hearing. These differences have nothing to do with the Inter Channel - level differences to do that for the two speakers at the Stereofonieaufnahme be generated. The interaural level difference ILD is frequency-dependent in a very complex manner with the sound incidence angle θ , in contrast to the interaural transit time difference or the corresponding interaural phase difference . The values found in lateralization attempts via headphones must not be output for loudspeaker signals in stereophony.

The shadowing of the auricles by the head with the ear distance d = 21 cm as an obstacle results in an effective sound shadow only from a very high frequency f = 5343 / 0.21 = 8200 Hz (empirical formula with the speed of sound c = 343 m / s at 20 ° C). If the diameter of the obstacle is only twice as large as the wavelength, then the sound is still almost completely diffracted.

literature

Jens Blauert: Spatial hearing. S. Hirzel-Verlag, Stuttgart 1972, ISBN 3-7776-0250-7

Web links

The ear gap - which one? (PDF file; 168 kB)
Microphone distance = ear distance = best results? (PDF file; 24 kB)

Individual evidence

↑ z. B. Dickreiter "Handbook of Microphone Technology"

[1] z. B. Dickreiter "Handbook of Microphone Technology"