AMT sub - using dynamic woofer

[mige0]

As some already may know, I love presentation of upper frequency department by AMT in dipole configuration (in particular attached to some diffraction alignment device ).

Looking for a solution to do a sub in dipole configuration, it seemed logically to give AMT principle a try here too.
The main disadvantage of dipole operation at the bottom 2-3 octaves is its very low SPL output – which is even a bigger problem if we seek after small footprint too.
One way to counter „natural“ SPL loss towards lower frequencies is to increase power invested and also to increase on moving diaphragm area.

Now – „AMT sub“ isn't any new – though probably not widely recognized as such.
Basically any Ripol, N-sub or W-sub utilizes AMT principle.

Despite being a total no-no from a lot of technical considerations as well as questionable in-room usability below modal region, those dipole sub's have gained some interest among hard core dipole lovers due to their mere sonic performance.

Michael

[mige0]

Standard Ripol / N-dipol / BMC usually looks like this:







Now an easy way to incerase Sd to push max-SPL is to simply stack them:




Above should have roughly +6dB of max-SPL compared to a single chassis Ripol and it already looks a little bit closer to AMT pleating



An alternative way to stack two woofers is to put them one behind the other :





Above should also increase max-SPL by +6dB IMO but also shift low frequency corner of max-SPL down – which in fact would be very appreciated
Top frequency peaking would shift down too - giving lower usable bandwidth unfortunately.


My last config : “X-AMT” - to give it a spacy name






IMO this kind of “X-AMT” configuration would again increase max SPL and also preserve down shift of max-SPL low frequency corner.
The middle two "pleats" are loaded by three times Sd this way.



So, what do you think, elsewhere seen or experimented with ?

Michael

[mige0]

Analysing and measuring AMT subwoofers turns out to be a tricky game.



1.at low frequencies a room behaves “different”, as room modes rather dominate what we can get. On the other hand, a free field FR isn't all that meaningful either – not even to speak from measurements demands to get it.
2.with AMT subwoofer – as is with any framed or unframed open baffle construction – the FR depends on the proximity to the source. Very much like the proximity effect of directional microphones
3.in general *all* speaker constructions not acting as a single point source will show more or less pronounced CMP behaviour. AMT subwoofer – like any framed or unframed OB – being rather a two point source, clearly falls into this category, as do Horns, TL, TH...


Actually, even points 1.) and 2.) are a consequence of CMP behaviour in one way or another:

http://www.kinotechnik.edis.at/pages...ts_of_View.pdf

So its not exactly tricky because AMT-subwoofer principle is any complicated to understand but rather because with constructions incorporating delay, the kinda universal tool of „frequency response concept“ gets flawed.

In short: our believe that “flat is flat” once we managed to tune a speaker to flat FR gets absolutely meaningless once CMP enters the picture:

Spectral distribution becomes a way more complex function of the *place* we stick the mic out and also being a function of the *time* we look at - than is properly reflected by simple “FR concept”.


All that said, we nevertheless can draw some *limited* conclusions looking at FR plots of H-baffles and AMT-subwoofer arrangements below.

I'll show some simulations plus “close to mouth” measurements.
Also shown are measurements that are taken close to mouth but are “dipole inverse-EQed” - meaning – the speakers are fed by a signal that accounts for the dipole-typical constructive and destructive comb filter FR.
This allows to take in-door measurements that reflect free space / far-field / non-proximity frequency response of dipoles (a very handy trick at those freezing temperatures out there right now ).
Dual-sub's are wired in parallel.


After this rather long wind up – lets jump right into a bunch of „really“ pretty pictures = a lot of heavily flawed FR plots.


Michael

[mige0]

Below for reference a simple H-baffle subwoofer

Chassis is a 15” with Fs = 40Hz and Qts = 0.4
(Selenium 15PW5)


The H-frame subwoofer is roughly 40x40x80cm (WxHxD)


This is how it roughly looks like (top opened):







This is how it measures:
RED Trace: close mouth
BLUE Trace: close mouth measurement “dipole inverse-EQed”
GEEN Trace: dipole comb filter






and this is how AJ-Horn and Hornresp simulate:








Michael

[mige0]

Next is a Dual-Ripol subwoofer

Chassis is 2x15” with Fs = 40Hz and Qts = 0.4
(Selenium 15PW5)


The Ripole-subwoofer is roughly 40x40x40cm (WxHxD) which is half the volume of the H-frame subwooofer housing two chassis.


This is how it roughly looks like (top opened):







This is how it measures:
RED Trace: close mouth
BLUE Trace: close mouth measurement “dipole inverse-EQed”
GEEN Trace: dipole comb filter




Michael

[mige0]

Next is a Dual-Ripol subwoofer with the chassis arranged one behind the other

Chassis is 2x15” with Fs = 40Hz and Qts = 0.4
(Selenium 15PW5)


This Ripole-subwoofer is roughly 30x40x80cm (WxHxD) which is about the volume of the H-frame subwooofer but housing two chassis instead.


This is how it roughly looks like (top opened):







This is how it measures:
RED Trace: close mouth
BLUE Trace: close mouth measurement “dipole inverse-EQed”
GEEN Trace: dipole comb filter




Michael

[mige0]

Last is a N-Frame subwoofer with the chassis arranged one behind the other

Chassis is 2x15” with Fs = 40Hz and Qts = 0.4
(Selenium 15PW5)


This N-frame subwoofer is roughly 40x40x80cm (WxHxD) which is about the volume of the H-frame subwooofer but housing two chassis instead.


This is how it roughly looks like (top opened):







This is how it measures:
RED Trace: close mouth
BLUE Trace: close mouth measurement “dipole inverse-EQed”
GEEN Trace: dipole comb filter




Michael

[sendler]

Excellent work! One pretty picture equals a thousand words including the sketchup shots. They make the layout instantly understandable. Which of the graphs, red or blue, will be more representative of the anechoic far field response? How much does the dual ripol shake? Is the dipole distance "d" the same for a three port, vibration canceling dual ripol of the same footprint as the four port dual ripol?

[Zero Cool]

The Tymphany LAT subs work in a similar fashion to a AMT

https://www.madisound.com/store/prod...oducts_id=8311

[DQ828]

Originally Posted by mige0

Below for reference a simple H-baffle subwoofer

Chassis is a 15” with Fs = 40Hz and Qts = 0.4
(Selenium 15PW5)


The H-frame subwoofer is roughly 40x40x80cm (WxHxD)


This is how it roughly looks like (top opened):







This is how it measures:
RED Trace: close mouth
BLUE Trace: close mouth measurement “dipole inverse-EQed”
GEEN Trace: dipole comb filter






and this is how AJ-Horn and Hornresp simulate:








Michael

I'm amazed your Dipole Peak is so so high when I calculate the peak for a 80cm H frame it is 160Hz yours appears to be over 200hz?

David