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Altec 950-8A cabinet design question
Altec 950-8A cabinet design question
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Old 20th May 2020, 03:22 PM   #1
Grote_spuit is offline Grote_spuit  Netherlands
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Default Altec 950-8A cabinet design question

Hi,

I was lucky to get my hands on a pair of Altec PS-95 speakers. These cabinets use the 15 inch Altec 950-8A coaxial drivers. I really like how they sound except they lack bass. In these closed cabinets there is not much bass below 100Hz and I would like to build a bigger cabinet for them to improve that.

I am totally new to this. I have tried to add the parameters of the Altec 950-8a to WinISD but I am not sure if what I did was correct and if WinISD is the right program for this. Can somebody please confirm that a 300 liter cabinet like WinISD seems to suggest may result in a better extended bass or are is there anything in the parameters that suggests that it's not a good idea to try to go so low with these coaxials?

I ask this because the folder says that this coaxial speaker is specifically designed to work in a 4 cubic feet cabinet. Does that mean that it is not possible to get more bass out of these drivers although WinISD suggests otherwise? Any other cabinet idea's for these drivers? I also loooked at the Altec Milestone 950 cabinets but they are designed for the Altec 950-8B drivers and my 950-8A drivers have different parameters.

Any help is very much appreciated!
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Old 20th May 2020, 04:17 PM   #2
weltersys is online now weltersys  United States
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Quote:
Originally Posted by Grote_spuit View Post
1) In these closed cabinets there is not much bass below 100Hz and I would like to build a bigger cabinet for them to improve that.

2)Can somebody please confirm that a 300 liter cabinet like WinISD seems to suggest may result in a better extended bass or are is there anything in the parameters that suggests that it's not a good idea to try to go so low with these coaxials?

3)I ask this because the folder says that this coaxial speaker is specifically designed to work in a 4 cubic feet cabinet. Does that mean that it is not possible to get more bass out of these drivers although WinISD suggests otherwise?
1) The specifications for the 4 cubic foot 5197-X indicate it is ported, not sealed. A sealed cabinet of that size will definitely roll off bass.
2) For a low tuning, that size seems reasonable.
3) A 4 cubic foot cabinet with a tuning (Fb) of around 55 Hz from the perforated 15.8 square inch perforated port results in a fairly flat alignment to 70 Hz, rolling off below. You could use a lower Fb (longer port) in a 4 cubic foot cabinet and extend low frequency response, though it will roll off steeper than the larger cabinet above Fb.

Art
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Old 20th May 2020, 05:29 PM   #3
Grote_spuit is offline Grote_spuit  Netherlands
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@ Weltersys: Thanks a lot for your help!

The closed PS-95 cabinets that I use right now are about the same size as the recommended 5197-X cabinets and they roll of bass for sure. As the 5197 cabinets are ported they might have a fairly alignment to 70Hz but i'd rather go even lower then that because I don't want to add a subwoofer. The coaxials will be used for music only so if I can get the bass down to around 40Hz then that would be great.

I am glad that you did not see any problems with building big cabinets for them. Very curious to see how they will perform in 300 liter ported cabinets. Hope they will sound as big and impressive as their size will be :-)
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Old 20th May 2020, 06:13 PM   #4
weltersys is online now weltersys  United States
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Quote:
Originally Posted by Grote_spuit View Post
I am glad that you did not see any problems with building big cabinets for them. Very curious to see how they will perform in 300 liter ported cabinets. Hope they will sound as big and impressive as their size will be :-)
Not to dissuade you from the larger cabinet, but it would be worth trying/modeling an external port with your present cabinet volume.
An Fb around 40 Hz would get you most of the output of what the larger cabinet would give, and only require installation of a chunk of PVC and an elbow on the back of the cabinet.
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Old 21st May 2020, 11:35 PM   #5
GM is offline GM  United States
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Greets!

WinISD Pro and its more recent updates is plenty good enough for basic T/S alignments, but without loading the program can't critique your efforts , though IME all the true vintage spec drivers sound best overall when Vb = Vas, Fb = Fs and some form of EQ is used to fine tune it to the room [traditionally a high output impedance tube amp with variable DF tone controls].

Note too that in general, as system output impedance lowered, Altec often altered Fs, Vas, Qts specs to make them a 'drop-in' replacement for PP, SS amps, which in turn meant that tuning [Fb] tended to be closer to ~1.56x Fs than at Fs to better protect them with the higher power amps then coming into vogue.

Regardless, loaded Hornresp with the 8A's original OEM specs [+ 0.5 ohm for wiring] using the Margolis Small T/S max flat alignment calcs = ~172.4 L/40.72 Hz Fb Vs a 'classic' BR's best overall balance between size, bandwidth [BW] Vb = Vas/1.44 = 285.4 L tuned to Fs, which is usually a better choice overall when the speakers are near/at a wall boundary and especially if corner loaded and/or driven with a high output impedance amp.

OEM specs: Altec Thiele-Small Parameters

Margolis-Small's HP 67/97 & 41C calculator program: AES E-Library >> Personal Calculator Programs for Approximate Vented-Box and Closed-Box Loudspeaker System Design

Vb = 20*Vas*Qts'^3.3

Fb = 0.42*Fs*Qts'^-0.96

IMPORT HR files to view/tweak/whatever:

GM
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File Type: txt 950-8A_TS_BR.txt (1,023 Bytes, 3 views)
File Type: txt 950-8A_ClassicBR.txt (1,018 Bytes, 6 views)
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Old 22nd May 2020, 03:02 AM   #6
Grote_spuit is offline Grote_spuit  Netherlands
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Quote:
Originally Posted by weltersys View Post
Not to dissuade you from the larger cabinet, but it would be worth trying/modeling an external port with your present cabinet volume.
An Fb around 40 Hz would get you most of the output of what the larger cabinet would give, and only require installation of a chunk of PVC and an elbow on the back of the cabinet.

You are right, that is at least worth the try. But what diameter would the PVC pipe need to be and how long? Just a matter of experimenting? The PS-95 cabinets are not in a great shape so building new (and better looking) cabs was already my plan. But as building new cabinets will have to wait for at leas two more weeks, I am eager to try this to get some more extended bass output right away. Thanks for this idea.


@GM: Wow, that is some very usable information! Like I said I am all new to this. In the past I have build several Fostex backloaded horn cabinets that they recommend for their drivers, so building cabinets will not be much of a problem I guess. I have no experience with designing cabinets for specific drivers myself though so your help is much appreciated.

If I understand you correctly, you are saying that vintage spec drivers usually sound best in a cabinet with internal volume around Vas? The Vas for the 950-8A is 410 liter.

Quote:
a 'classic' BR's best overall balance between size, bandwidth [BW] Vb = Vas/1.44 = 285.4 L tuned to Fs, which is usually a better choice overall when the speakers are near/at a wall boundary and especially if corner loaded and/or driven with a high output impedance amp.
My speakers will be placed close to the rear wall so I'd better go for the Vb=285.4L then? That would make them a bit more living room friendly compared to 410 liter monsters
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Old 22nd May 2020, 04:28 PM   #7
weltersys is online now weltersys  United States
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Quote:
Originally Posted by Grote_spuit View Post
1)You are right, that is at least worth the try. But what diameter would the PVC pipe need to be and how long? Just a matter of experimenting?
2)If I understand you correctly, you are saying that vintage spec drivers usually sound best in a cabinet with internal volume around Vas? The Vas for the 950-8A is 410 liter.
3)My speakers will be placed close to the rear wall so I'd better go for the Vb=285.4L then?
1)Your driver does not have huge displacement, so the port does not have to be super large to avoid chuffing (too much vent velocity), around 5" diameter duct would be OK. Port area as large as the cone area could be used, but would end up with a very long port.
The larger the port diameter, and the lower the tuning, the longer it needs to be.
For your small box, probably shoot for around 40 Hz Fb, much lower and the bass will roll off too much to be of much use.
There are lots of port calculators on line, here's one:

Port Length Calculator

2)Hoffman's Iron Law- Loud, Low, Small- pick two. Going from 410L to half that would be something like 3dB difference at Fb if Fb is equal to Fs.

3) The placement in the room can have far more effect on the bass response than nearly anything you do with the box, but that is another discussion.

Art
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Old 23rd May 2020, 08:07 AM   #8
GM is offline GM  United States
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You're welcome!

Yes, high eff./Vas, low Qts drivers in Vas size cabs [over-damped, i.e. allowed to 'breathe'] seem to track what some folks call 'inner detail'/'effortless sound' better when tuned to Fs and even below in some cases [extended bass shelf/EBS alignments] where group delay [AKA timing error, the 'bugaboo' of vented box distortion] can be as low in the audible pass band as a normally superior transient perfect [0.5 Qtc] sealed alignment. The trade-off of course is reduced power handling, i.e. in audio design, one is ALWAYS trading efficiency for gain bandwidth [BW].

Historically, I always recommend Vb = Vas, but most folks don't/can't tolerate such big cabs, so the pioneer's Vas/1.44 is a good one and if still too big, then make it as big as you can afford/tolerate, tune it to what sounds best overall in room and performance 'is what it is'.

Note that if driven with a high output impedance amp, then a good plan to always tune to Fs regardless of cab size.

GM
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Old 23rd May 2020, 07:10 PM   #9
waxx is offline waxx  Belgium
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This driver won't give much bass when you give it more than a few watt as the xmax is very limited. But it's high efficient so that won't matter much in home use.

I come to a 220L cabinet with a 13cm diameter circular hole in the box (port of 18mm) when using 18mm wood. That gives a tuning to 40hz and is good for a response to that for maybe 3w with bass, after that it goes in overexcursion to much to give bass (and it will cut of). 2W gives already an SPL of 100dB per cabinet (103dB for a pair) wich should be more than you will use in an average home...
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