This is extracted from my earlier question, which perhaps I didn't express clearly enough. If you buy a bare driver - Kef B200 for example - it has a frequency range in its spec. How is that measured? Is there a standard 'box' for all drivers? Or do they measure the cones ability to 'flap' at lower frequencies?
And we talk about 'bass extension' - there is no doubt you can hear that in a transmission line cab - but does the cabinet loading extend the ability of a driver to reproduce low frequencies beyond its rating as a bare driver?
And we talk about 'bass extension' - there is no doubt you can hear that in a transmission line cab - but does the cabinet loading extend the ability of a driver to reproduce low frequencies beyond its rating as a bare driver?
There are a number of Industry standards for measuring loudspeakers.
Two popular ones are the IEC and JIS industry standards.http://www.diy-audio.narod.ru/litr/Measuring_the_true_acoustical_response_of_loudspeakers.pdf
The IEC standard baffle is 165cm x 135cm in size.
Cabinets extend the low frequency capabilities of a driver in two main ways:
Two popular ones are the IEC and JIS industry standards.http://www.diy-audio.narod.ru/litr/Measuring_the_true_acoustical_response_of_loudspeakers.pdf
The IEC standard baffle is 165cm x 135cm in size.
Cabinets extend the low frequency capabilities of a driver in two main ways:
- By preventing the rear wave from cancelling the front wave (e.g. large open baffle and sealed box)
- By reversing the phase of the rear wave so that it reinforces the front wave at certain low bass frequencies (e.g. ported enclosure)
I'd say it's measured in whatever way would lead to the manufacturer selling the most units. I think there's a universal problem of there not being a one size fits all standard - for an 8" driver, everyone does this. Some may use a very large OB that's completely impractical for actual use.
Of course, that's what cabinets are for, besides being a convenient way to carry relatively fragile speakers around without damage. I suspect when you coerce a driver to go lower than spec by some acoustic trick, you risk IM distortion at lower overall levels - because, as you say, the cone is flapping around more to excite the cabinet's trick for the additional acoustic output.
Could be wrong about that; my thinking is based on you dont get something for nothing as a consequence.
The bare driver has a fundamental frequency of vibration or resonant frequency, fo.
Looking at it simply, when placed in a sealed box the resonant frequency rises higher than fo and the bass response begins to fall off below that frequency.
A ported enclosure works with the resonant frequency of the driver to extend the bass response to a lower frequency.
I'm not sure if we're looking for a simple or a highly technical explanation here, but at least the discusion is underway! 🙂
Thank you - I assume this is an open baffle? And measurements are made with a standard microphone?
I started thinking about this as I have acquired a pair of home made Decca Corner horns. The original driver from 1949 seems to have been lost in the mists of time, but later recommendations were Wharfedale twin axiom and Lowther PM6. Mine have been replaced with Kef B200/T27s, which is fine, but I was toying with the idea of fitting a single driver - possibly the Tannoy 8" DC. It is rated down to 40Hz and Decca's claim for the horn loading is 30Hz.
That's when it occurred to me, that after all these years, I had never wondered how they measured the range of a driver outside of its cabinet.😕
The IEC standard baffle is an open baffle (see the link). I don't think there is one standard microphone, but the microphone will be of a high standard, particularly in terms of having a frequency response that smoothly covers the entire 20–20,000 Hz audible range. (Ordinary microphones, e.g. for vocal use, intentionally attenuate low frequencies in order to reduce rumble or intentionally attenuate higher frequencies to reduce unwanted high-frequency noise.)
Your corner horns sound interesting. Have you described them elsewhere on this forum? If so, I seem to have missed them.
By the way, it was Goodmans that made the Twin Axiom, not Wharfedale. 🙂
Your corner horns sound interesting. Have you described them elsewhere on this forum? If so, I seem to have missed them.
By the way, it was Goodmans that made the Twin Axiom, not Wharfedale. 🙂
Galu said:
So they did 😱. I should know - I have one in my strange but oddly lovable Jabez Gough reflecting loudspeaker!!
I haven't posted about the Deccas, other than my first about driver selection. I have a bit of history. Back in the 70s a friend was trying to build a pair and he never quite got there. In the end he gave the incomplete speakers to me and I turned them upside down and inserted a kind of transmission line labyrinth (vent where the driver should have been). They made some serious noise!
Now I have grown up (!) I developed a yearning to hear what the real thing should have sounded like....and I am very impressed. Obviously they were not designed with stereo reproduction in mind, but they don't half do it well. That dispersed 'concert hall' sound results in a wide stereo image and 'presence' that is really something special.
Now I have grown up (!) I developed a yearning to hear what the real thing should have sounded like....and I am very impressed. Obviously they were not designed with stereo reproduction in mind, but they don't half do it well. That dispersed 'concert hall' sound results in a wide stereo image and 'presence' that is really something special.
As already noted there's various standards that date from the 1930s plus the plane wave tube [PWT] horn driver standard.
Yes, the basic standard is the pioneer's that was much later fine tuned as T/S box filter theory defining where it loads to the driver's upper [Fhm]/lower [Flc] mass corners [+/-] BW, i.e. rolls off from its acceleration to mass controlled BW:
Fhm = 2*Fs/Qts'
Flc = Fs*Qts'/2
Qts' = 2*Fs/Fhm
Qts' = Qts + any added series resistance [Rs]: HiFi Loudspeaker Design
From this we see that the lower the Qts', the wider its total gain BW with the late Bagend ELF sub an example where only the BW below Fs is used: http://web.archive.org/web/20120905214418/http://www.bagend.com/bagend/downloads/elfman12.pdf
DIY version: Sub-Woofer Controller
Normally though, not a good plan tuning < a half octave [0.707x] below Fs unless driver power handling/room gain meets the needs of the app.
GM