851 SP3
Active Member
I added them to the PB album about my passive.
Adding weight to passive radiators...feedback
- Thread starter Hifigogo
- Start date
....then leave them alone and enjoy them.
By Elder Statesman, do you mean the guy with 7 posts?
The stuff I've used is called mortite. Mass loading passives is not a new concept. Strange responses in here.
BECtoo
Well-Known Member
Some of you may be interested in a bit of technical info on passive radiators. This is from Jerry McNutt an Eminence Engineer who designs things like this for a living. I don't think he would mind me quoting him here.
"A passive radiator works just like a port. The ID of the “port” is the same as the moving diameter of the cone. For example say you have a 15” passive radiator and the moving diameter of the cone(about ½ of the surround to ½ of the surround) is 13.75”, you would tell your favorite box simulation program that you have a port with an ID of 13.75”. The box program tells you that to tune your 3 cft box to 30 Hz with your 13.75” diameter port, that the port needs to be 290” long. To calculate the required mass that the passive radiator needs you calculate the volume of the portformed by the 13.75” diameter that is 290” long. Once you know the volume of the port, you know the port is full of air and air has mass, so you calculate how much the mass of the air in theport is by using the volume of the port and the density of air. Say the mass of the air in the port works out to 345 grams; that is how much mass you will need to have on your passive to make ittune the box to 30 Hz. Sounds complex, but it is not really once you do the calculations a few times or make a spread sheet to calculate the mass for you.
The free air Fs of the passive (Fp), with the correct mass added, must be substantially lower than the box tuning. Singe digit Fp’s are common and work well. If you try to use a very stiff suspension on the passive and wind up with it having a free air Fp of like 25 Hz and you are trying to tune the box to 30 Hz, the passive will introduce a big notch in the response. That is why most passives are so floppy, they are trying to keep their Fp well below box Fb.
The need to use a floppy suspension and then pile on hundreds of grams of weight can cause things to rip pretty easy. Great care must be taken to use parts that are strong enough to handle the high moving mass and yet be very soft to keep the Fp down. We use special spiders with reinforced necks to help with the high moving masses. Rubber surrounds work well as do foam surrounds, we even make some passives with very soft cloth surrounds for MI applications, but for HiFi, rubber and foam rule." Jerry McNutt
Bob Crites
"A passive radiator works just like a port. The ID of the “port” is the same as the moving diameter of the cone. For example say you have a 15” passive radiator and the moving diameter of the cone(about ½ of the surround to ½ of the surround) is 13.75”, you would tell your favorite box simulation program that you have a port with an ID of 13.75”. The box program tells you that to tune your 3 cft box to 30 Hz with your 13.75” diameter port, that the port needs to be 290” long. To calculate the required mass that the passive radiator needs you calculate the volume of the portformed by the 13.75” diameter that is 290” long. Once you know the volume of the port, you know the port is full of air and air has mass, so you calculate how much the mass of the air in theport is by using the volume of the port and the density of air. Say the mass of the air in the port works out to 345 grams; that is how much mass you will need to have on your passive to make ittune the box to 30 Hz. Sounds complex, but it is not really once you do the calculations a few times or make a spread sheet to calculate the mass for you.
The free air Fs of the passive (Fp), with the correct mass added, must be substantially lower than the box tuning. Singe digit Fp’s are common and work well. If you try to use a very stiff suspension on the passive and wind up with it having a free air Fp of like 25 Hz and you are trying to tune the box to 30 Hz, the passive will introduce a big notch in the response. That is why most passives are so floppy, they are trying to keep their Fp well below box Fb.
The need to use a floppy suspension and then pile on hundreds of grams of weight can cause things to rip pretty easy. Great care must be taken to use parts that are strong enough to handle the high moving mass and yet be very soft to keep the Fp down. We use special spiders with reinforced necks to help with the high moving masses. Rubber surrounds work well as do foam surrounds, we even make some passives with very soft cloth surrounds for MI applications, but for HiFi, rubber and foam rule." Jerry McNutt
Bob Crites
Well... I finally got around to trying the weights. I put a stack of washers together using epoxy and a bench vice... once it dried, i epoxied it to the rear of the passive. The total weight was 2 oz using a postal scale to check... and I could not tell the difference. I did them one at a time so I could listen and compare.
