How does a digital SLM compute Leq and Lp ?

Thread view:

Enable EMail Alerts

Start New Thread

Thread rating:

Laurent - 29 Jan 2004 22:53 GMT

Hi,

I am using a Bruel & Kjaer 2238 SLM for some time and I would like to know
how some parameters are computed.

The displayed values are updated every second, whatever the time weighting.
So if for instance I am trying to get the Lp with a fast (125 ms) time
setting,
which of the 8 values than can be computed over this second will be
displayed ?

As for the Leq, how is it computed from these time samples ? Will each of
these
8 values be used and how ?

Thanks a lot for your help!

-Laurent

Reply to this Message

Noral Stewart - 30 Jan 2004 10:59 GMT

> Hi,
>
[quoted text clipped - 14 lines]
>
> -Laurent

I am not familiar with the specific B&K meter, but can take a good guess at
your answers. I probably displays the most recent available sample of the
fast sound level. You do lose a little insight trying to use a digital
display for fast sound level where a needle or analog display might show
more.

For the Leq, my guess is that this instrument follows the standard for the
true one second Leq rather than computing it based on the fast samples.
Thus, it actually computes the average of the total sound over a second and
displays it without using the fast time weighting samples. Some instruments
will compute the Leq from sampled time-weighted levels. These will perform
a logarithmic average of the samples displaying the result of the most
recent second it has been able to compute. The error is not usually
significant especially if fast weighting is used. With slow weighting, you
could see differences in the one-second Leq especially if the actual sound
is varying rapidly. Over longer periods the difference would not usually be
as significant.

Noral Stewart

Reply to this Message

Natural Science Forum / Physics / Acoustics / January 2004

How does a digital SLM compute Leq and Lp ?