At the risk of sounding fixated / stupid (please delete any if applicable).
The efficiency of a driver is expressed as a nominal or characteristic sensitivity i.e. 88db for 2.83volts at 1 metre.
If a driver has a given sensitivity,at what point in typical finished design does this occur?
To go back to Sretens comment, if I understand correctly, a Peerless HDS 6.5 woofer with a sensitivity of 88db/12.83volts/1m will exhibit an overall sensitivity of 82db when placed in a speaker box and is filtered in such a way as to obtain as maximally flat a response as possible.
Does this mean that the sensitivity of driver is basically only a guiding factor in selecting / matching drivers and that the figure is never really realised in a finished design?
MADINOZ
The efficiency of a driver is expressed as a nominal or characteristic sensitivity i.e. 88db for 2.83volts at 1 metre.
If a driver has a given sensitivity,at what point in typical finished design does this occur?
To go back to Sretens comment, if I understand correctly, a Peerless HDS 6.5 woofer with a sensitivity of 88db/12.83volts/1m will exhibit an overall sensitivity of 82db when placed in a speaker box and is filtered in such a way as to obtain as maximally flat a response as possible.
Does this mean that the sensitivity of driver is basically only a guiding factor in selecting / matching drivers and that the figure is never really realised in a finished design?
MADINOZ
Hi Madinoz,
For bass units basically true, real level is 6dB lower.
But many smaller speakers only implement say 3dB of BSC.
For midrange and treble its its fairly representative, though
usually treble units are picked to be somewhat higher and
padded back.
If the 82dB effective sensitivity of the Peerless seems low,
thats because it is. The driver is basically designed to be
used in pairs for 4 ohm and with full BSC 88dB/2.83V.
(Though as the pair are now 4 ohm really 85dB/W)
(Without BSC above baffle step a pair would be 94dB/2.83V 4ohm)
sreten.
For bass units basically true, real level is 6dB lower.
But many smaller speakers only implement say 3dB of BSC.
For midrange and treble its its fairly representative, though
usually treble units are picked to be somewhat higher and
padded back.
If the 82dB effective sensitivity of the Peerless seems low,
thats because it is. The driver is basically designed to be
used in pairs for 4 ohm and with full BSC 88dB/2.83V.
(Though as the pair are now 4 ohm really 85dB/W)
(Without BSC above baffle step a pair would be 94dB/2.83V 4ohm)
sreten.
"The midrange should have either a large-ish or small-ish baffle so that there isn't a step in sensitivity half-way through its frequency range" Ceramicman.
In an ideal world this would be fantastic make the midrange baffle small enough to remove BS effect from the range of frequencies you want it to reproduce. But do you know how small this would need to be!
I have excel w15's mounted in a square baffle with about 5mm spacing either side of the driver. You would need to cross over at about 400hz to completely remove baffle step in this cabinet from the equation. I have measured them in the position they are used and yes they are on top of a 60cm tall 30cm wide 60 deep bass box BUT they require about 3db baffle step comp starting at about 1600hz and ending at 400hz. This baffle could not get any smaller.
Anyone know of a tweet i can cross at 400hz??
With bass boxes what you said can be implemented, if you xover low enough from the mids!, I cross over at 150hz to the bass. This is after baffle step has occured, ie the bass is the theoretical 6dBdown throughout the pass band, so I just ramp up the bass by 6dB to compensate for this. In reality I only need about 3dB though the same as the baffle step applied to the w15's.
In an ideal world this would be fantastic make the midrange baffle small enough to remove BS effect from the range of frequencies you want it to reproduce. But do you know how small this would need to be!
I have excel w15's mounted in a square baffle with about 5mm spacing either side of the driver. You would need to cross over at about 400hz to completely remove baffle step in this cabinet from the equation. I have measured them in the position they are used and yes they are on top of a 60cm tall 30cm wide 60 deep bass box BUT they require about 3db baffle step comp starting at about 1600hz and ending at 400hz. This baffle could not get any smaller.
Anyone know of a tweet i can cross at 400hz??
With bass boxes what you said can be implemented, if you xover low enough from the mids!, I cross over at 150hz to the bass. This is after baffle step has occured, ie the bass is the theoretical 6dBdown throughout the pass band, so I just ramp up the bass by 6dB to compensate for this. In reality I only need about 3dB though the same as the baffle step applied to the w15's.
So the obvious thing to do is make the baffle big enough
to prevent a step in the midrtange response, then by
defintion the step occurs ithe bass region.
IMO this is one reason why many simple 3 ways are flawed.
The bass unit c/o is chosen for the mid c/o frequency, whilst
it should be chosen to implement BSC as well.
Another reason most 3 ways are flawed is the bass unit
sensitivity needs to be 6dB more than mid and treble.
sreten.
to prevent a step in the midrtange response, then by
defintion the step occurs ithe bass region.
IMO this is one reason why many simple 3 ways are flawed.
The bass unit c/o is chosen for the mid c/o frequency, whilst
it should be chosen to implement BSC as well.
Another reason most 3 ways are flawed is the bass unit
sensitivity needs to be 6dB more than mid and treble.
sreten.
rabbitz said:
Firstly, baffle step is no BS and the effect of compensation is very worthwhile.
Tried BSC via a passive crossover and reduced the SPL too much and the speaker lost some "life".
The woofer at the back was much more successful but still have some fine tuning to do.
Looks like it's going to be a sealed 5" Peerless HDS (still looking at other drivers) at the back to go with the 6½ Peerless CSX on the front.
*F3=120Hz and 1st order xo at 600Hz
*Room modes give me enough lift from about 90Hz down
*If more required I can use a BR enclosure with F3=70Hz
*Pad the 5" HDS down due to it's higher sensitivity and proximity of the rear wall, about 250mm (got to locate that formula for padding with a single resistor... not an L-pad)
I was surprised that the rear woofer didn't cause imaging problems with the reflected sound from the wall. Why is it so?
Does the rear driver have to be in line with the front driver, i.e. same vertical height..... I imagine so?
rabbitz said:
In a word no. Just needs to be fairly close to other driver.
It could be incorporated into a stand for the speaker,
with the drive unit placed near the top of the stand.
Another (simpler) option is line level eq :
http://sound.westhost.com/bafflestep.htm
Without preamp - power amp links it could be incorporated
into the power amps feedback loop, by increasing gain at
low frequencies.
sreten.That baffle is loss due due the change in radiation resistance is quite well documented. It is documented quite well by Roy Allison in his patents and other writings. The change in radiation resistance causes a 3db loss in the driver's POWER response. This is not the same as amplitude frequency response, which changes by 6db. The POWER output of the driver is actually reduced. The reason it might not show up as a 6db amplitude drop is because at the low frequencies we're talking about, the floor, nearby walls, and or ceiling come into play, at various frequencies, causing room gain to the power response (or dip, if the stars align properly), that can more than offset the 3db drop in powe due to baffle loss.
rabbitz said:
Plan "B"...... going with the 6½ Peerless CSX on the back as well.
Less hassles and sounds better. The thing that I wasn't expecting is that the overall sensitivity is the same, which is great.... so it's only doing the BSC (in the same volume as the single driver, with the same port length). The F3 has gone up only a few Hz but I can muck around with volumes and ports later.
Thought I might have to redo the TM xo...... phew!
The port?
Here's one to ponder over.
In a bass reflex box I've done the BSC by using a driver at the back which mirrors the roll-off on the driver at the front with the driver F3 being about 90Hz. What happens to the port as it doesn't get any benefit of the BSC or do you have to have ports front and back as well?
Whoops, forgot about room modes which is giving a boost of 2-5dB below 90Hz, so it's providing the extra.
Here's one to ponder over.
In a bass reflex box I've done the BSC by using a driver at the back which mirrors the roll-off on the driver at the front with the driver F3 being about 90Hz. What happens to the port as it doesn't get any benefit of the BSC or do you have to have ports front and back as well?
Whoops, forgot about room modes which is giving a boost of 2-5dB below 90Hz, so it's providing the extra.
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