Speaker sensitivity

[Phil Y]

A quick question. I built my speakers myself(not a kit) so i do not know what sensitivity they are. The bass units are Audax HM210CO rated at 90dBW/m and the treble units are Scanspeek D2008A 87dBW/m. They are in a 40 litre bass reflex box. As most of the sound output is coming from the bass unit, can i just assume that the whole system efficency is 90dB like the bass unit or is it more complicated than that? It is not a problem at the moment as they go plenty loud enough for me from 10 watts(Stereo 20 for many years but now KEL84) but if i ever replace them i would like to know as it is a guide when looking for new ones.

Regards, Phil.

[andrew ivimey]

Haven't a clue!

You made them yourself, what is your crossover? That is likely to dampen your sensitivity some.

[petercom]

Originally Posted by Phil Y

A quick question. I built my speakers myself(not a kit) so i do not know what sensitivity they are. The bass units are Audax HM210CO rated at 90dBW/m and the treble units are Scanspeek D2008A 87dBW/m. They are in a 40 litre bass reflex box. As most of the sound output is coming from the bass unit, can i just assume that the whole system efficency is 90dB like the bass unit or is it more complicated than that? It is not a problem at the moment as they go plenty loud enough for me from 10 watts(Stereo 20 for many years but now KEL84) but if i ever replace them i would like to know as it is a guide when looking for new ones.

Regards, Phil.

The bass unit may well be specified at 90dB at 1kHz but the output at lower frequencies would be much lower than this. Most bass units have a rising response above 600Hz, probably peaking at 2-3dB in the 1kHz region. So, if the unit was mounted on a very wide baffle or in the wall we could expect the bass sensitivity to be around 87 -88dB.

However the bass unit on a baffle radiates into a hemisphere in front of the baffle until the wavelength becomes long enough to diffract round the baffle. Below this frequency the bass unit will be radiating into a sphere and will therefore apparently lose a further 6dB sensitivity. This is called the baffle step. Under anechoic conditions the low frequency sensitivity might be in the 81-82dB range as a result.

However a speaker in a room will gain from reflections from the floor and, to a certain extent, the walls which will help elevate the bass level. For a free standing speaker this 'room gain' is usually around 3dB SPL. In the crossover a large value coil in series with the bass unit will equalise the midrange output so that it matches the low frequency system level. Thus the system sensitivity will lie around 84-85dB in actuality.

You can check this by looking at the crossover. If there is an attenuating resistor in series with the treble unit, and there usually is, it will be there to reduce the treble level from 87dB quoted sensitivity down to the 85dB of the bass unit.

[Phil Y]

The crossover is second order. It was produced by Wilmslow Audio. If you bung some money at them, they will put your driver details into their computer program and produce a design for you. The design they produced has no series resistor on the tweeter but i have fitted a 1 ohm as i found it slightly bright (as i do most comercial speakers). I know logic would say it would need some fine tuning, but it sounds pretty good to me and as i don't have the equipment to measure it properly, i'm leaving well alone. From what you say then, the figure for the system is would be about 87dB.