hello, last week I compared two homemade subwoofers. The first one is a 8th-order 3 chamber that I've been using for about 10 years. I has a -3dB bandwith (measured with SpectraLab 4.32) of 35 Hz to 100 Hz including the active lowpass filter. The second one is a closed box with motional feedback (piezo accelerator glued on a hardened dust cap) having a -3dB bandwith of 30 Hz to 120 Hz including the active lowpass filter, one 8 inch Kevlar driver firing on the front plus one 8 inch Kevlar driver firing on the rear. I chosed this configuration for avoiding the subwoofer cabinet moving back and forth.
To my surprise, the subjective results are completely different.
To my surprise, the closed box has a very deceptive subjective result. The room sounds "extinct" in the deep bass.
On the contrary, the 8th-order 3 chamber seems to "play" with the room, seems to "excite" the room.
I have the impression that the ears favours "packets of pressure" bouncing back and forth inside the room. Because, essentialy, this is what's happening with what's coming out of the vent of the 8th-order 3 chamber design.
I have the impression that the ear doesn't appreciate a pulsating sphere arrangement, like my dual-driver closed box tends to be.
Can you tell me more about this ?
To my surprise, the subjective results are completely different.
To my surprise, the closed box has a very deceptive subjective result. The room sounds "extinct" in the deep bass.
On the contrary, the 8th-order 3 chamber seems to "play" with the room, seems to "excite" the room.
I have the impression that the ears favours "packets of pressure" bouncing back and forth inside the room. Because, essentialy, this is what's happening with what's coming out of the vent of the 8th-order 3 chamber design.
I have the impression that the ear doesn't appreciate a pulsating sphere arrangement, like my dual-driver closed box tends to be.
Can you tell me more about this ?
Two ideas...
The obvious: Did you place them at the same room location while testing? Different locations excite different modes.
The crazy: If you model different enclosure types in hornresp and look at the impulse response spectrum, you can see that a closed box has the shortest response time, about 10ms until the sound decays, while an higher order system should have a pretty long resonating time, up to 100ms (pulled that numbers out of the air, but check yourself, they should be at least indicative of a trend). I allways assumed one cannot hear these low frequency timing things. But who knows?!
The obvious: Did you place them at the same room location while testing? Different locations excite different modes.
The crazy: If you model different enclosure types in hornresp and look at the impulse response spectrum, you can see that a closed box has the shortest response time, about 10ms until the sound decays, while an higher order system should have a pretty long resonating time, up to 100ms (pulled that numbers out of the air, but check yourself, they should be at least indicative of a trend). I allways assumed one cannot hear these low frequency timing things. But who knows?!
Forgot to mention in my first post:
About "pulsating sphere": The shape of the two boxes shouldnt be the factor that determines the sound. Wavelength is so big in the bass, that all boxes have to look like a point source for the frequencies we are discussing. A 120hz wave is about 2,8 meters long, a 30hz wave nearly 12 meters. If the woofers are less than a quarter of this length apart, they act like a point source.
About the "packets of pressure": Thats not how sound in a room works. Imagine your room as a three dimensional string and the woofer as the bow, exciting it. Once the bow moves, the whole string vibrates and even after the bow stops, the strings motion has a little decay. There are no individual packets, the whole vibration is a complex system.
About "pulsating sphere": The shape of the two boxes shouldnt be the factor that determines the sound. Wavelength is so big in the bass, that all boxes have to look like a point source for the frequencies we are discussing. A 120hz wave is about 2,8 meters long, a 30hz wave nearly 12 meters. If the woofers are less than a quarter of this length apart, they act like a point source.
About the "packets of pressure": Thats not how sound in a room works. Imagine your room as a three dimensional string and the woofer as the bow, exciting it. Once the bow moves, the whole string vibrates and even after the bow stops, the strings motion has a little decay. There are no individual packets, the whole vibration is a complex system.
I would certainly think that one can hear a slow responding subwoofer system. You have, for example, a well recorded "dry" kickdrum playing, together with a bass guitar. The resonance of the kick drum will be around 32Hz, the bass guitars' fundamental frequency will be 40Hz (low E). These two frequencies will sound horrible when playing together. That's why you want the bassdrum to sound short and defined, so as to not conflict with the bass guitar.
If you have a subwoofer with a decay time of 100ms, you will still hear a 30Hz tone sounding 100ms AFTER the bassdrum stopped, same for the bass guitar! Imaginge a fast funk piece, where many notes follow each other in rapid succession. After each note has stopped, the next one comes. But when that next note appears, you'll still be listening to the previous note decaying (a lot of similarities here with the Haas-effect). It'll sound horribly fatigeuing, because you'll be listening to enormous amounts of intermodulation distortion.
What you want, is a closed subwoofer with a large driver and a beefy amp with high damping factor, which has a firm grip on the driver and thus a firm grip on the air it's moving. You may find it sounds a bit restrainded when compared to mainstream ported or bandpass systems, but what you are actually hearing is control and lack of distortion.
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