On Sun, 23 Nov 2003 02:44:43 -0800, "The Flash" wrote:

I have had no luck in measuring speaker response at low frequency's, I can
measure from ~60Hz to 20Khz with near 100% repeatability (How accurate is
questionable)

I have questioned a number of speaker builders and a couple of companies
that 'tune' speakers the answers given on what they measure with and how the
do the tests give serious rise to the claims at low frequency.

Almost any speaker system sold today claims 20Hz to 20Kz response yet this
is so far from the truth I cannot understand how they dare claim such
figures.

I have yet to find ANY speaker system using an 8 inch driver that has the
ability to produce 40hz or lower frequencys as they all have fallen so far
down in output level as to be useless.


How do you measure the responese at 20Hz?, I have tested a few speakers and
basically I cannot get accurate (repeatable) data at much below 60Hz, even
using a borrowed shure KSM141 stereo pair in omnidirectional gives vague
results ($3000 for the mic's!) I have come to the conclusion that you are
really measuring air displacement at anything under 30Hz and a flat panel
with a transducer is the only repeatable method of measuring the output, how
one calibrates said device is open for discussion.

(Oh one firm told me that they use a laser to measure the low frequency of
their speakers, check this out for novel! They place a small piece of
reflective foil on the base driver, and shine a laser beam on it, they then
apply signal and measure via 'laser' the deflection, Also they place a
passive radiator 1 meter infront of the driver unit and use the same method
to measure its deflection.

The apply a 'correction factor' and the produce the frequency response data
(company builds very expensive car and home audio subs!)






I thought up my own method for eliminating the effect of the room on
bass measurement (although, no doubt it has been done before). The
main feature is to place the (pressure) microphone within the speaker
and to compensate by 12 dB/oct.

Normally I use a MLS and filter out higher frequencies, say above
100Hz, with a brick wall FIR filter using Coooledit. The 12 dB/oct
compensation is also done in a similar way, either to the input MLS
or to the output recording. This sort of pre-filtering can give a
large increase to the signal to noise of the meaasurement.

There must be some restrictions on the accuracy of this method but
the only one I can think off at the moment is that the wavelength must
be large compared to the internal dimensions of the speaker. The
meaasurement includes the effect of any port and of the enclosure
flexing.

There is probably an assumption that the air is compressed
adiabatically. The pressures involved are, of course, small if the mic
is not to be overloaded.