Aguilar DB112 stuffed with foam????

[Ray]

I happened to look into the ports of one of my recently acquired DB112s and was suprised to see a load of foam in there!

Now, I don't know much about the science of building speaker cabs but wouldn't all that foam mess with the internal dimensions and tuning of the cab? I've never known a bass cab to have foam inside. Is it supposed to be there?

Where's Alex when you need him?

[Musicman20]

Yep, my DB212s are full of yellow 'padding' inside. I did the same, looked in the port.

Its usual.

There must be some reason for it.....not sure what.

[alexclaber]

Yes, it should be there. A well built bass cab will be internally lined with some kind of sound absorbent material to stop midrange from the backwave being reflected and messing up the sound.

Alex

[Ray]

Ah ok. Thanks guys.

[quote name='alexclaber' post='621297' date='Oct 9 2009, 08:57 AM']... to stop midrange from the backwave being reflected ...[/quote]
I think I know what you mean but would you mind explaining this in a little more detail please. Thanks.

[GreeneKing]

[quote name='alexclaber' post='621297' date='Oct 9 2009, 08:57 AM']A well built bass cab will be internally lined with some kind of sound absorbent material to stop midrange from the backwave being reflected and messing up the sound.

Alex[/quote]

I take that as a compliment to Aguilar

[Musicman20]

[quote name='GreeneKing' post='621311' date='Oct 9 2009, 09:13 AM']I take that as a compliment to Aguilar [/quote]

Me too. In fact, the inside build on these cabs is great. Im no cab builder, but it looks extremely well made.

[alexclaber]

[quote name='Ray' post='621309' date='Oct 9 2009, 09:11 AM']I think I know what you mean but would you mind explaining this in a little more detail please. Thanks.[/quote]

You know those gigs that have happened at Battersea Power Station, where the sound was always terrible because you'd hear the sound from the PA and then a moment later hear the same sound echoing off one of the power station walls? Just like that but on a smaller scale. Because it's on a smaller scale instead of hearing discrete echoes you end up with out of phase signals mixing, which means you gets lots of sharp peaks and troughs in the response as some frequencies add but some cancel, which causes uneven midrange and treble response and a blurrier sound.

Alex

[Ray]

[quote name='alexclaber' post='621320' date='Oct 9 2009, 09:21 AM']You know those gigs that have happened at Battersea Power Station, where the sound was always terrible because you'd hear the sound from the PA and then a moment later hear the same sound echoing off one of the power station walls? Just like that but on a smaller scale. Because it's on a smaller scale instead of hearing discrete echoes you end up with out of phase signals mixing, which means you gets lots of sharp peaks and troughs in the response as some frequencies add but some cancel, which causes uneven midrange and treble response and a blurrier sound.

Alex[/quote]
Ah ha, I understand perfectly. Thanks Alex.

[Bill Fitzmaurice]

[quote name='Ray' post='621295' date='Oct 9 2009, 03:54 AM']I've never known a bass cab to have foam inside.[/quote]Cabs that you may have seen without damping should be considered defective. There are two reasons for not fully lining a cabinet. One is that the manufacturer doesn't know any better, the other is that the manufacturer does know better but prefers not to go to the expense.

[Mr. Foxen]

Does packing the cab with stuff (not just covering the walls) lower the speed of sound and make the cab respond as if it is slightly larger also?

[alexclaber]

[quote name='Mr. Foxen' post='621597' date='Oct 9 2009, 01:50 PM']Does packing the cab with stuff (not just covering the walls) lower the speed of sound and make the cab respond as if it is slightly larger also?[/quote]

No, that's an old idea that has since been proven to be incorrect.

Alex

[Mr. Foxen]

[quote name='alexclaber' post='621605' date='Oct 9 2009, 01:59 PM']No, that's an old idea that has since been proven to be incorrect.

Alex[/quote]

Got a citation on that? Wanna present it to the speaker designer at AVI, who told me it, as well as other stuff I know not to be true (my amp wasn't worth fixing because it didn't have 'Marshall' written on it.

[geoffbyrne]

So, if I want to put some padding round the inside of my recent cab, what should I use and is it available from B&Q?



G.

[alexclaber]

[quote name='Mr. Foxen' post='621618' date='Oct 9 2009, 02:07 PM']Got a citation on that? Wanna present it to the speaker designer at AVI, who told me it, as well as other stuff I know not to be true (my amp wasn't worth fixing because it didn't have 'Marshall' written on it.[/quote]

My understanding is that in a closed box the loss of volume by filling with stuffing loses you as much or more internal space than the very very marginal decrease in the wave speed. In a transmission line the stuffing makes the line appear very marginally longer - but the main function is to damp the overtones of the 1/4 wave resonance. In a ported box stuffing damps the port response killing the lows. However the driver does perceive a different box size to what measurement suggests but this is due to losses in the system.

Alex

[alexclaber]

[quote name='geoffbyrne' post='621629' date='Oct 9 2009, 02:14 PM']So, if I want to put some padding round the inside of my recent cab, what should I use and is it available from B&Q?[/quote]

Get a roll of loft insulation and use what's left over to keep your house warm!

Alex

[stevie]

[quote name='alexclaber' post='621646' date='Oct 9 2009, 02:22 PM']Get a roll of loft insulation and use what's left over to keep your house warm![/quote]
Fibreglass works really well. I'm not sure of the health issues in a ported cab, although I have a JBL cab here with fibreglass in and I've not noticed any problems.

You can get BAF wadding and acoustic foam from specialist suppliers like Wilmslow Audio or on eBay. I've found that foam at the the long end and BAF along the side walls works best, as foam can kill the bass if you're not careful. I've also heard of people getting good results from furniture stuffing, although I'm not sure exactly what they used. Make sure to keep the wadding well away (6-inches, say) from the ports.

This is well worth doing and you'll hear the difference.

[alexclaber]

[quote name='stevie' post='621672' date='Oct 9 2009, 02:41 PM']This is well worth doing and you'll hear the difference.[/quote]

+1

I've had at least one basschatter pm me for advice on this and been very happy with the end results!

Alex

[Mr. Foxen]

So if I fill my sealed cab with [url="http://en.wikipedia.org/wiki/Tungsten_hexafluoride"]tungsten hexaflouride[/url] I'd get the lower response of a larger cabinet. But I'd have to keep it above 17 degrees or it would liquefy and my cab would implode. Maybe Sulphur hexaflouride would be more sensible. It does the same trick on your lungs:

[Bill Fitzmaurice]

[quote name='Mr. Foxen' post='621597' date='Oct 9 2009, 08:50 AM']Does packing the cab with stuff (not just covering the walls) lower the speed of sound and make the cab respond as if it is slightly larger also?[/quote]No. Packing a sealed cab will lower the box Q to tame a midbass peak. Contrary to popular belief it will not give the same result as a larger box. Ported cabs should be fully lined, to no more than a 2" thickness, but not stuffed.

[stevie]

[quote name='Bill Fitzmaurice' post='621735' date='Oct 9 2009, 03:38 PM']No. Packing a sealed cab will lower the box Q to tame a midbass peak. Contrary to popular belief it will not give the same result as a larger box.[/quote]
Mr Small ([i]the[/i] Mr Small) disagrees. In fact, he disagreed in 1972 in his AES paper called "Closed-Box Loudspeaker Systems" when he said:

"Many closed boxes contain filling material which helps to damp standing waves within the enclosure at frequencies in the upper piston range and higher. If the filling material is chosen for low density but high specific heat, the air compression inside the enclosure tends to be partly or completely altered from adiabatic to isothermal. This increases the effective compliance of the enclosure, which is equivalent to increasing the size of the unfilled enclosure. The maximum theoretical increase in compliance is 40%, but using practical materials the actual increase is probably never more than about 25%. An ancillary effect is an increase in the total moving mass of the system. Depending on the initial diaphragm mass and the conditions of filling, the mass increase may vary from negligible proportions to as much as 20%"

It doesn't apply to a ported cab, though.

[Bill Fitzmaurice]

[quote name='stevie' post='621765' date='Oct 9 2009, 11:17 AM']Mr Small ([i]the[/i] Mr Small) disagrees. In fact, he disagreed in 1972 in his AES paper called "Closed-Box Loudspeaker Systems" when he said:

"Many closed boxes contain filling material which helps to damp standing waves within the enclosure at frequencies in the upper piston range and higher. If the filling material is chosen for low density but high specific heat, the air compression inside the enclosure tends to be partly or completely altered from adiabatic to isothermal. This increases the effective compliance of the enclosure, which is equivalent to increasing the size of the unfilled enclosure. The maximum theoretical increase in compliance is 40%, but using practical materials the actual increase is probably never more than about 25%. An ancillary effect is an increase in the total moving mass of the system. Depending on the initial diaphragm mass and the conditions of filling, the mass increase may vary from negligible proportions to as much as 20%"

It doesn't apply to a ported cab, though.[/quote]Mr. Small disagreed in 1972. In the intervening three plus decades the above explanation of how stuffing works has been found to be incorrect. As recently as three years ago the esteemed Vance Dickason's 'Loudspeaker Design Cookbook' still espoused this debunked theory, but in the latest edition he too has come on board with what is now recognized by the AES as to the true function of stuffing. How stuffing affects the cabinet Q and impedance can be clearly and easily seen with current loudspeaker modeling software; the accuracy of the software has been confirmed with actual measured results.

[Balcro]

[quote name='Bill Fitzmaurice' post='621593' date='Oct 9 2009, 01:47 PM']Cabs that you may have seen without damping should be considered defective. There are two reasons for not fully lining a cabinet. One is that the manufacturer doesn't know any better, the other is that the manufacturer does know better but prefers not to go to the expense. :angry:[/quote]

Having both eyeballed and stuffed my arm up the port of and old Trace-Elliot 2x12 (a la James Herriott) in Cash Converters earlier this morning, I'm pleased to hear you say that.

Could the manufacturers also do this to deliberately give the speakers a rougher sounding edge?

Balcro.

[Bill Fitzmaurice]

[quote name='Balcro' post='621785' date='Oct 9 2009, 11:44 AM']Could the manufacturers also do this to deliberately give the speakers a rougher sounding edge?

Balcro.[/quote]One could make that case, but the more likely explanation is that they don't want to add to the build cost of the cab by properly lining it. They'd feel quite safe in so doing based on the facts that most users aren't aware that the cab should be lined, and those who do aren't likely to examine the inside of the cab before buying one.

[stevie]

[quote name='Bill Fitzmaurice' post='621782' date='Oct 9 2009, 04:41 PM']Mr. Small disagreed in 1972. In the intervening three plus decades the above explanation of how stuffing works has been found to be incorrect. As recently as three years ago the esteemed Vance Dickason's 'Loudspeaker Design Cookbook' still espoused this debunked theory, but in the latest edition he too has come on board with what is now recognized by the AES as to the true function of stuffing. How stuffing affects the cabinet Q and impedance can be clearly and easily seen with current loudspeaker modeling software; the accuracy of the software has been confirmed with actual measured results.[/quote]
His explanation about how stuffing works may be incorrect (although I think if you slag off Richard Small the least you could do is provide some supporting references) but the practical consequences of stuffing a sealed box are the same – you increase the effective box size! Call it increasing the compliance of the enclosure or lowering the system Q - these are both things that happen when you increase the size of the box.

[stevie]

[quote name='Bill Fitzmaurice' post='621842' date='Oct 9 2009, 05:28 PM']One could make that case, but the more likely explanation is that they don't want to add to the build cost of the cab by properly lining it. They'd feel quite safe in so doing based on the facts that most users aren't aware that the cab should be lined, and those who do aren't likely to examine the inside of the cab before buying one.[/quote]
I feel compelled to agree with Bill here.