9d surround sound refers to 3d stereo surround sound, which means that multiple devices emit different channels at different locations, thus giving the feeling of sound coming from all directions.

A 4-unit headphone is a pair of headphones with four speaker units, each headphone has two speaker units, and 9d sound effect is just a term chosen by businesses for advertising.

There is no music effect of 9d sound effect. There are no headphones that play 9d sound effects. Usually qualified headphones, no matter how many speaker units are present (multi-core headphones), are mainly stereo sound, and the music effect depends on the combination of the headphone front end and the headphone.

Sound effect introduction:

The sound effect of 9d is better than that of 6d. 9d has stereo surround sound effect and the sound quality is particularly good, while 6d has average effect. In terms of lossless sound quality, the 6d is already very good, and the 9d has more comprehensive sound quality than the 6d, with a higher bit rate and better effects. Essentially, the two are different models of Bluetooth speakers with different powers.

But the basic functions are the same, there is not much difference. Usually it is connected to an external voice recorder or other recording equipment. The effects reflected by external playback devices are different. Common rated impedances of speakers are 4 ohms, 6 ohms, 8 ohms, 16 ohms, etc. Because speakers currently driven by transistor or integrated circuit power amplifiers are dominant.

This type of amplifier generally does not require an output transformer, so the impedance of the connected speaker is mostly in the range of 4-16 ohms. During use, the impedance of the speaker should be selected according to the requirements of the power amplifier.