Interesting XRK! Do you have a photo how you use this? Do you glue them together? Do you apply it 2cm or 6cm thick? Any information on the damping properties?
In answer to post #20, allow me to explain, GM.
The physics is λ = v/f (wavelength equals the velocity of sound divided by its frequency.)
So, for a tuning frequency of 45.5 Hz: λ = 34400/45.5 = 756 cm
Now, since the pipe is tuned to one quarter of the wavelength, the length of the pipe needs to be 756/4 = 189 cm.
EDIT: I see I haven't explained "1/4 WL of Fs = 91Hz/2", so it's back to you, GM!
The physics is λ = v/f (wavelength equals the velocity of sound divided by its frequency.)
So, for a tuning frequency of 45.5 Hz: λ = 34400/45.5 = 756 cm
Now, since the pipe is tuned to one quarter of the wavelength, the length of the pipe needs to be 756/4 = 189 cm.
EDIT: I see I haven't explained "1/4 WL of Fs = 91Hz/2", so it's back to you, GM!
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Here is an example of it used on Karlsonator. I use hot melt glue to secure it. It slices easily with a razor.
Mini Karlsonator (0.53X) with Dual TC9FDs
Use it anywhere you would apply foam or felt. Slice it thinner if needed (0.5cm thick works even).
An older (now in his mid 70's) spearerbuilder i know (not on internet) has a sub that is an JBL D130 woofer in an open back cabinet filled with rockwool, and closed from the back with a few layers of speakercloth on a metal frame. And that works very good to Fs he says (and it goes low i heared, but i did not measure it myself). The cabinet is about 40cm deep, 1m high and 50cm wide. He use the JBL to about 250Hz.
The rest of the setup is OB (an Philips 9710M8 FR and a SEAS H087 tweeter used as supertweeter (cr at 10kHz). It's his main set since 4 decades he says and altough he build a lot of speakers since then, it's still his favorite setup. And i did like it also. Crossover is diy analog active and amps are also diy class A (for tweeter and FR) and AB (for woofer) transistor amps.
How this back-damped Open back subs work technically i don't know, but it works to get a real low bass from a driver that is not fit for OB as the backwave is damped (so not cancelling) and the cabinet sounds more like an aperiodic ported than an real OB. But it sounds good.
The rest of the setup is OB (an Philips 9710M8 FR and a SEAS H087 tweeter used as supertweeter (cr at 10kHz). It's his main set since 4 decades he says and altough he build a lot of speakers since then, it's still his favorite setup. And i did like it also. Crossover is diy analog active and amps are also diy class A (for tweeter and FR) and AB (for woofer) transistor amps.
How this back-damped Open back subs work technically i don't know, but it works to get a real low bass from a driver that is not fit for OB as the backwave is damped (so not cancelling) and the cabinet sounds more like an aperiodic ported than an real OB. But it sounds good.
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Thank you Galu. I do understand the 1/4 wavelength principle.
The Fs of the driver is 100Hz according to the manufacturer. (I have seen a measurement of 94Hz so the 91 in GM's calculations might well be possible) Where I am confused is why 45.5 Hz is the tuning frequency.
Not off topic at all…
Helps reduce the hard reflection on wooden surface. Too much reflectivity and you may hear an echo or tunnel like sound.