There are a total of 5 mute (sound reducer) parts installed on the double bass, all of which were obtained at a home improvement store except for No. 2.
1: F-shaped sponge to block the sound hole entrance
2: ULTRA USA mute clipped to the bridge
3: Sponge to clip on the bridge
4: Sponge to put on the end of the endpin
5: Sponge clipped to the strings near the tailpiece
The F-shaped sponge to block the soundhole entrance (1) is just an F-shaped sponge cut from a sponge bought at a home improvement store, but I found that it cut the bass sound significantly.
The ULTRA mute (#2) was purchased on the Internet for approximately 3,000 yen(12 bucks). Compared to the F-hole sponge, it is easier to put on and take off because of its inserting design into the bridge.
The effect of clipping the sponge to the upper and lower bridge in #3 is subtle compared to #1 and #2 which significantly reduces the sound. There was concern that the clips would vibrate with the string vibration and generate noise, but since they were plastic, there was no problem. However, if the clips were metal, it may cause noise.
I don’t know if the sponge at the end of the endpin 4 is effective, but since I live in an apartment, I put it on the floor in hopes that it will mute as much as possible. The amplitude of vibration on the floor changes a little with and without.
I put the sponge on the tailpiece(No.5). As well as No. 4, I am not sure if the sponge on the tailpiece is effective (theoretically, the sponge also has a mute effect because the tailpiece can contribute to the acoustic effect).
Although not on the list, sound-absorbing material attached to the wall was also effective there as it picked up the extra echoes from the double bass. Since there is not much furniture in the room, the sound tends to bounce back, but a house with lots of furniture may not need it.
2024/10/17 note: I have not tested the idea because I am not currently using double bass, but a sponge between the backside of the fingerboard and the top material can have a certain mute effect. I think it is one of the options.