Doc modding Marantz imperial 7

[karatestu]

It's odd that the distance between the cab and the panel above when up firing is exactly the same as when i have down fired these speakers as the wooden legs in the corners are the same ones.

[karatestu]

A good explanation of how a speaker radiates at different frequencies (with easy to understand graphics).

https://blog.soton.ac.uk/soundwaves/fur ... udspeaker/

[karatestu]

Howz about a woofer enclosure made out of tyres stacked up ? Could work for an upfiring 12" clamshell isobaric pair. What would a rubber enclosure sound like ? Would it flex too much. Depending on depth of tyres it would need three or four per channel. Maybe a boffle type of thing could be going on with the tyre side walls.

[karatestu]

I have messed with a leaky bass box not too long ago and liked what it did. That was just a small 6mm hole drilled through the back panel. The theory says it lowers the magnitude of the resonant frequency peak, makes the enclosure seem bigger and increases Fs (frequency of resonance) slightly.

Well i have tried it again but this time with a 5" hole on the back panel. Stuffed it with some slightly over sized foam which moulds to the edges of the circular cut out.

It has had quite an effect on the bass. I am pretty sure this is going to be a permanent feature of my speakers but obviously done in a neater fashion when final build happens. Yes i keep saying that and then i decide to mess about with something else

The beauty of this is it can be tweaked even after the speakers are finished by altering the type of stuffing and thickness of it. I could also make it so it can be blanked off fully when not required and maybe other partial blanking plates to change the size of the hole to suit. A bit like a tone control or bass boost for quiet listening . Some of the electronic music i listen to can be a bit too bass heavy at times - well this aperiodic vent has tamed it no question about it. But i also listen to electric and acoustic jazz and other well behaved music where sometimes i wish i had a bass tone control.

Yeah shoot me now . I don't think i have veered too far from the sealed box loading. It is not a tuned vent like a bass reflex (although it does leak a small amount of out of phase sound through it). I have not put any stuffing in the box so my hfs credentials are hopefully still in tact

All in the pursuit of tight, deep, tuneful and room filling bass which is full of the texture and detail that i know real bass has (guitar, acoustic or electronic). The plus of all this is the reduction of cabinet volume required. I have some scope now to decrease the volume which is a good thing when it comes to getting these past the wife when i do the final build.

There is no need to but an aperiodic vent on the 5.25" mid bass drivers as their required cabinet volume is tiny in comparison and their zmax at Fs is relatively well behaved. Only thing is that if the mid bass share an enclosure with the 12" bass drivers then they will have to be aperiodically vented. I will cross that bridge when i come to it as current thinking is along the lines of a separate enclosure with up and down firing 5" drivers which is above the up firing 12 " driver (as per my hand drawn sketch i posted recently). If that design turns out to be crap then the drivers may be all sharing an enclosure.

Time will tell. Anyway, back to the tunes

[karatestu]

My cheap crap chipboard enclosure panels are vibrating a lot less too

[karatestu]

Discs i have been using to test the bass abilities of my speakers. It really pays to put some torturous music through any speakers to see if they fall down. No point testing for how your speakers handle bass when there isn't much bass content in the music.

Herbie Hancock - Secrets
Portishead - Dummy
Morcheeba - Big calm
Gorillaz - Demon days
Jamiroquai - Funk Odyssey
Beck - Odelay
Air - Moon Safari
Chemical Brothers - We are the night

I would have mentioned Bob Marley but the bass is weak relative to the above, which surprised me.

[karatestu]

I had to copy the following about cardioid bass loading. My little experiment with taking the back panel off my bass cube's was a very crude example of what follows below. The stuffing was not tuned and probably not enough but it had a very interesting effect. Unlike open baffle where the main benefit is the way dipole works in limiting the influence of the room on bass passive cardioid operates my lowering the rear output and with a time delay equal to the distance between the two sides of the driver.



"The benefits of cardioid or quasi-cardioid response in a listening room have been addressed by Backman, (AES paper 5867, 2003). Cardioid bass and monopole bass both are capable of taking advantage of room pressurization below the room fundamental where as a dipole woofer does not. Whether you view that as a positive or negative depends on what you are trying to accomplish. However, all dipole and cardioid sources have fundamentally a 1st order gradient response with reduction in efficiency of 6dB/octave, compared to a monopole, as the frequency drops and must be equalized for flat response. Again, whether this requirement is viewed as a draw back depends on how it is weighted against the other design factors.

The conventional way to produce a cardioid response is through two monopole sources separated by some distance, d, operating out of phase with the input of one source delayed by Td = d/c where c is the speed of sound. This delay can be introduced through any means available over the frequency range required. A digital delay can be used for a pure time delay. A suitable analog all pass delay can also be used but yields a constant delay of a limited frequency range.

Rather than using two separate monopole sources, which add to expense, a single source mounted in an open backed enclosure can be used. In such a case, without going in to detail, the cavity of the rear enclosure must be damped. This helps damp the cavity resonance and also forms an acoustic low pass filter. The damping must be "tuned" so that the combined effects of resonance damping and low pass filtering yield the correct delay relative to the front to back length of the cavity. If the length is again d, then the damping must be adjusted so that the cavity delay at the rear exit plane is Td = d/c. When correctly tuned the opened backed system, AKA an acoustic resistance box (Holmes, AES, 1986), will tend to a cardioid at low frequency. Only when undamped it will tend to a dipole. The damped cavity is the NaO woofer as shown in A. Figures B and C are similar to each other in operation. In both cases they result in a dipole summed to a monopole. In both cases the dipole (consisting of one front driver and the rear driver in figure C) must be equalized to have the same on axis response as the monopole driver. Additionally, to form a cardioid the acoustic centers of the monopole and dipole components must be correctly aligned. Further more, since the sensitivity of the dipole and monopole as well as the on axis response must be matched, it is likely that the dipole and monopole components of configurations B and C will require separate amplifiers.

While it is possible to obtain a cardioid response over a wider frequency range with configurations B and C (since there is no cavity resonance) the additional cost of the added drivers makes them less attractive than configuration A. If additional drivers are to be used it would be better to use them as in A to increase the SPL capability of the system."

[karatestu]

Another quote

Re: Aperiodic Vents

I have heard aperiodic systems exhibit some very desirable transient response characteristics -- immediacy in drum reproduction the likes of which I have never heard in any other type of enclosure, for example.

An aperiodic vent attempts to approximate a purely resistive element, so it essentially works like an acoustic 'brake'. Its negative effect on the bottom end response of a sealed speaker comes from increasing the acoustic damping on the back surface of the driver cone, which has the same effect as additional mechanical or electrical damping (hence an aperiodic vent is usually applied when one is trying to use drivers with very low damping of its own, which manifests as a high Qts, somewhere above 0.8), but these vents can also be applied with lower-Qts drivers for an even more damped sound.

The aperiodic vent also radiates some amount of out-of-phase energy into the listening space, which is attenuated with respect to the driver's front wave but can still cause a significant amount of cancellation if there is too much radiation from the vent. These vents are usually placed on the rear of the box, which helps the box retain some sort of 'dipole moment'. What I'd really like to see done one of these days (or experiment with, myself) is use an aperiodic vent directly behind the active cone driver, with the damping varied such that there would be phase cancellation behind the enclosure but summation in front of the enclosure, to achieve a "cardioid" radiation pattern in the bass range. This technique would reduce the effect of the room on the quality of the bass reproduction from the speaker because radiation toward the front wall (and nearby corners) would be reduced.

Happy experimenting. I think aperiodic vents are much less well understood than they deserve to be, and they could be a very useful tool in the designer's toolbox."
"

[karatestu]

And this

"Aperiodic Enclosure - an otherwise sealed enclosure design, but with a vent that is stuffed with damping material, which flattens out the impedance curve of the design. The area of this resistive vent should be about 10 sq. in. per cubic ft. of enclosure volume. This design might take some experimentation with the vent stuffing, testing the impedance curve several times with different amounts of damping material until the flattest impedance curve is found. The aperiodic resistive vent damps the driver in much the same way as fully stuffing a sealed enclosure with damping material (100% fill). In this way, an optimum design may be made up to 20% (or more) smaller due to the extra damping of the resistive vent. Enthusiasts of this design often compare the performance with transmission line enclosures, advocating that the design avoids the "ringing" effects of ported enclosures while alleviating the "pressure effect" of the sealed box. Dynaudio sells a DIY insertable resistive vent called the Variovent."

[karatestu]

So for a 86 litre cabinet ( approx 3 cubic ft) at 10 sq. in (65 sq. cm) per cubic foot i need a hole (don't like the word "vent" ) of approx 190 sq.cm. It is currently 135cm and so not big enough and probably over damped.

Some tweaking required