EDIT - this got moved to the full range forum, but I don't think that was appropriate - see text in bold.
I have four 4" drivers that should be identical. One is faulty with a weak magnet (0.6 Qts), and 3 are good (slightly stronger than spec, averaging 0.38 Qts), which makes for interesting comparisons.
Other than a 2dB difference in sensitivity, their FR plots are nearly identical - the same HF extension and even the same anomalies (when horn loaded or in free air).
I think there are two lessons I can learn from this:
1) Since even a defective driver can get >100dB/watt, I should probably pay more attention to stuff like distortion levels and motor design, and less to Olde Horn Lore about magnet strength & low Qts.*
2) One bit of mystique I've read about Lowther and similar drivers, that ultra powerful magnets = better HF extension, seems to be utter twaddle.
The 3 measurements (not calibrated) in the attachment were taken with the same settings, same horn, mic at the horn mouth - there may have been mild movement as I swapped drivers, but I tried to keep the mic position consistent. No eq. Mild (24/octave) smoothing.
Purple: 4" normal P Audio driver. Nominally 90dB; the horn boost is substantial.
Pale grey: faulty 4" P Audio driver.
Black: JBL 2445 (with mystery diaphragm). This is nominally 111dB. Based on that, the good cone driver should be ~105dB and the faulty cone driver ~103dB/1w/1m from 300-1400Hz (these are nominally 16ohm vs. 8ohm drivers, so -5dB sensitivity should become -8dB when guesstimating relative efficiency).
*...so (sort of) putting my money where my mouth is, I've ordered some Scanspeak drivers, the 15M/4624G00 to try. I've had my eye on these for a while, hoping they'll give some improvement vs. the P Audio SN4-60F:
-slightly fancier magnet system
-distortion looks great
Nice curves... - Scan-Speak 15M/4624G - Techtalk Speaker Building, Audio, Video Discussion Forum
-Scanspeak has a better reputation for unit consistency & build quality
-the 15M frame is more open (the SN4-60F is very closed in, with a 100mm magnet close to a 100mm cone!)
-the 15M has slightly more efficiency & displacement
-easier tweaking: I could build a phase plug for either driver, but the 15M gives an easy option - I could affix something like the "simple Phase Plug" in this link to the stock 'bullet' phase plug.
Phase Plugs
I ordered 3 drivers in case a DIY phase plug or other tests lead to disaster 🙂
I have four 4" drivers that should be identical. One is faulty with a weak magnet (0.6 Qts), and 3 are good (slightly stronger than spec, averaging 0.38 Qts), which makes for interesting comparisons.
Other than a 2dB difference in sensitivity, their FR plots are nearly identical - the same HF extension and even the same anomalies (when horn loaded or in free air).
I think there are two lessons I can learn from this:
1) Since even a defective driver can get >100dB/watt, I should probably pay more attention to stuff like distortion levels and motor design, and less to Olde Horn Lore about magnet strength & low Qts.*
2) One bit of mystique I've read about Lowther and similar drivers, that ultra powerful magnets = better HF extension, seems to be utter twaddle.
The 3 measurements (not calibrated) in the attachment were taken with the same settings, same horn, mic at the horn mouth - there may have been mild movement as I swapped drivers, but I tried to keep the mic position consistent. No eq. Mild (24/octave) smoothing.
Purple: 4" normal P Audio driver. Nominally 90dB; the horn boost is substantial.
Pale grey: faulty 4" P Audio driver.
Black: JBL 2445 (with mystery diaphragm). This is nominally 111dB. Based on that, the good cone driver should be ~105dB and the faulty cone driver ~103dB/1w/1m from 300-1400Hz (these are nominally 16ohm vs. 8ohm drivers, so -5dB sensitivity should become -8dB when guesstimating relative efficiency).
*...so (sort of) putting my money where my mouth is, I've ordered some Scanspeak drivers, the 15M/4624G00 to try. I've had my eye on these for a while, hoping they'll give some improvement vs. the P Audio SN4-60F:
-slightly fancier magnet system
-distortion looks great
Nice curves... - Scan-Speak 15M/4624G - Techtalk Speaker Building, Audio, Video Discussion Forum
-Scanspeak has a better reputation for unit consistency & build quality
-the 15M frame is more open (the SN4-60F is very closed in, with a 100mm magnet close to a 100mm cone!)
-the 15M has slightly more efficiency & displacement
-easier tweaking: I could build a phase plug for either driver, but the 15M gives an easy option - I could affix something like the "simple Phase Plug" in this link to the stock 'bullet' phase plug.
Phase Plugs
I ordered 3 drivers in case a DIY phase plug or other tests lead to disaster 🙂
Attachments
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You would need to get the simulated Q on your horn as total Q is dependent on the alignment. I suspect that value to be much closer between units regardless of free air Qms.
I'm not sure I understand what you saying.
If the value will always end up about the same, why bother measuring it?
If the value will always end up about the same, why bother measuring it?
in the aqua trace below, there's a 250Hz flare rectangular tractrix with old JBL alnico LE5 - its present qts in free air is near 2. I used to use that horn/LE5 on top of Klipschorns and then with Bruce Edgar's 100Hz midbass horn/ JBL 2220H (with around 5dB attenuation of the midbass horn) I don't know how the LE5's parameters may have changed in the last ~25 years. The other midhorn is an Altec 511 with University Sound SAHF from the 1950's
Yes and no since it depends on the needs of the app. In the beginning, extremely high acoustical, electrical, mechanical system efficiency and speech intelligibility were the goals, so matching impedance amps were developed, which in turn doubles Re, Qes and nearly Qts, hence the need for powerful motors, but in T/S parlance they were actually medium to [very] high Qt systems depending on the app.
As amp power increased and different amp topologies were developed, moving amp DF ever higher till it had no impact on driver specs for a long time now; Re was lowered, Qes, Qts was increased to compensate plus some juggling with compliance specs [Vas], so they could work well enough in existing systems when coupled to upgraded amps or at least EQ'd with tone controls.
Nowadays, we only need low Qt drivers for making small and/or high efficiency speakers or very wide BW compression horns.
GM
edit:
It's true, just that the pioneers meant amp/box/horn loading HF capability, i.e. how high its upper mass corner [Fhm = 2*Fs/Qes'] is in today's T/S parlance where Qes' = Qes + any added series resistance [Rs]: HiFi Loudspeaker Design
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The Q and the high frequency drop works if the cone works as a totaly rigid body. If there is cone breakup that link flies out the window.
This is where I'm at. I don't have the $ or inclination to build 1930's amps.
I'm not really into high power though, I use a NAD 3020i on my horns which is a 20watt amp. It has a "low level" button, which reduces output (lowers gain?) by about 20dB. I have this on almost all of the time, effectively making it a 1 watt amp.
I've done the opposite for my very wide BW application - moved from using 14kg JBL compression drivers to 140g FR drivers 🙂
The CD is much louder in the midband, but after applying EQ, for use from 500Hz - 10kHz, they end up pretty close.
The SN4-60F that I tested is a low excursion 4.5" driver with a fairly stiff cone. Where would you say the breakup begins?
I agree. Most of the old papers were guidelines and even have a disclaimer as such. I’m always amazed at how well Horn loading works for drivers that are not “supposed” to be horn loaded.
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