TS parameters - Volt B2500.1 bass driver

[Simtu]

I have started to design a pair of 3 way active speakers using a pair of Volt b2500.1 woofers that I bought in the late 90s.

I plan to use these with a pair of pmc 3" soft dome midrange drivers and some scanspeak r2904/700000 tweeters that I also acquired a long time ago. A pair of Hypex fusion fa253 plate amps will power them. Given the low compliance of these drivers, I plan to use a bass reflex design.

Incidently, these midrange drivers are manufactured in-house by pmc and not supplied by Volt as is often stated. These are crossed over at 380hz (as atc) by pmc, so are much closer in spec to the atc sm75-150s than the Volt vm752s.

I have various datasheets for the B2500.1s which date from 1997 through to 2014. These give fs as being between 25hz and 39hz, depending on vintage. Obviously, there have been changes made over the years.

As such, I have been trying to measure the ts parameters for myself. Following the advice given by Vance Dickason (Design Cookbook) and by Rod Elliott (ESP), I have broken the drivers in for several hours at a low frequency, then taken measurements using the current source method (using a 10ohm series resistor).

At small signal levels (<1v), I measure fs at around 50hz. When I up the signal to 6 or 7 volts, this drops to around 39/40hz. (Vance Dickason states that fs should increase with a rise in signal??). Either way, it's a country mile away from the 25hz that the 1997 datasheet is giving.

I don't know what I'm doing wrong or why my measurements are so far away from those given by Volt. These are £400 drivers at current prices, so are presumably manufactured with some care. Why are measurements taken at low signal levels, which are not representative of real listening conditions, when clearly this has a bearing on the enclosure tuning?

Any ideas?

 

[GM]

This may help

Otherwise I can only speak from ancient history when I toured the Altec plant in '68 re quality assurance (QA) in that they only measured efficiency to a low tolerance since it's the sum? total of Fs, Vas, Qes (scroll down), so recommend calculating/comparing eff. and not worry about individual specs tolerances too much.

 

[markbakk]

Drivers age. Spiders stiffen up, surrounds break down. That all likely happens in 26 years.

 

[Simtu]

I've kept these drivers boxed, wrapped and in a wardrobe since I bought them. I can't believe that the suspension could stiffen up that much. Adding to which, I've been running them with a 20hz signal for 24 hours or so.

If the 1997 datasheet is correct, these should have an fs of 25hz. Had I built these speakers back then, I'd have tuned the cabinet accordingly. Listening to the same speakers now, wouldn't there be a great big dip in the response curve?

I understand that designing a loudspeaker enclosure is not a wholly predictable process but if the variables can change this much, how can anybody design good sounding speakers that stay that way for the long term?

 

[Shaun]

I have just ordered myself a pair of these. When I receive them I'll run them in and do my own T/S measurements. T/S parameters should be measured using low level signals.

I don't believe that these woofers are meant to play very low and I'm prepared to sacrifice low end extension for dynamic range and use a subwoofer with it if need be.

I would like to compare notes with other users of this woofer.

 

[GM]

I have started to design a pair of 3 way active speakers using a pair of Volt b2500.1 woofers that I bought in the late 90s.

I have various datasheets for the B2500.1s which date from 1997 through to 2014. These give fs as being between 25hz and 39hz, depending on vintage. Obviously, there have been changes made over the years.

At small signal levels (<1v), I measure fs at around 50hz. When I up the signal to 6 or 7 volts, this drops to around 39/40hz. (Vance Dickason states that fs should increase with a rise in signal??). Either way, it's a country mile away from the 25hz that the 1997 datasheet is giving.

How do you know there ever was any 25 Hz units for consumer sale and/or intentionally sold 'B' stock same as Altec and presumably others did when economic times such as occurred in your country ~demanded budget cuts?

 

[GM]

I can't believe that the suspension could stiffen up that much.

If the 1997 datasheet is correct, these should have an fs of 25hz. Had I built these speakers back then, I'd have tuned the cabinet accordingly. Listening to the same speakers now, wouldn't there be a great big dip in the response curve?

I can, especially since it has two spiders full of starch to loosen up; I've had a pair of NIB Altec 421-8H since the mid '70s and didn't take them out of the box till the mid '90s and wasn't surprised that they were locked up, so for several days set them out on my concrete walkway in 'Hotlanta's' noonday Sun to return the cloth surround's goop to a uniform liquid gel and once cooled it was obvious that Vas was even higher than OEM/measured, so typical for this type surround.

Regardless, no clue what the Volt's surround's material is, though being designed for studio monitors I'd only try with an cooking oven.

All that said, based on your being able to drop it to a steady 39/40 Hz where they stalled out, I'm inclined to believe this is these driver's correct OEM specs.

Correct, though how big depends on the specs and could only find the 39 Hz ones in a quick search.

 

[Simtu]

Thanks for the replies so far.

Shaun - good to know someone else is beginning a project using these drivers. Hopefully we can compare notes as we go!

I'm going to leave them breaking in for a few days and try again.

I don't understand how fs is dropping as I increase voltage. This is the exact opposite of how Vance Dickason says it should behave.

What's the point of designing loudspeakers based on ts parameters that have been taken at small signal conditions, when these are not representative of normal listening conditions?

Fs may be 49hz at barely audible levels, but as soon as I turn the volume up, the enclosure will no longer be tuned correctly.

 

[Shaun]

I'm not an expert, but ChatGPT has an opinion:

ChatGPT


In loudspeaker enclosure design, the signal level affects the box tuning by changing the effective volume of the enclosure, which in turn changes the tuning frequency of the box.

The tuning frequency of a loudspeaker enclosure is determined by its physical dimensions and the properties of the driver(s) used in the enclosure. The box tuning frequency is the frequency at which the enclosure and driver(s) resonate, resulting in a peak in the frequency response of the system. This peak is often used to enhance the bass response of the loudspeaker.

The effective volume of the enclosure can change with the signal level due to several factors. First, as the driver moves in and out in response to the signal, it can cause changes in the air pressure inside the enclosure, which can change the effective volume. Second, at high signal levels, the driver may reach its maximum excursion limit, causing distortion and reducing the effective volume of the enclosure. Third, at low signal levels, the driver may not be able to generate enough sound pressure to fully excite the enclosure, again changing the effective volume.

Therefore, it is important to design the enclosure and choose the driver(s) carefully to ensure that the box tuning is appropriate for the intended signal level range, and to test the system at different signal levels to ensure that the performance remains consistent.