From: SFPhysics@aol.com
Date: Wed Jan 15 2003 - 02:02:23 PST
From: SFPhysics@aol.com Message-ID: <65.72675e5.2b568baf@aol.com> Date: Wed, 15 Jan 2003 05:02:23 EST Subject: Decibel Ratings on Tuners
Subject: Decibel ratings on tuners
From: "Nathania Chaney Aiello" <nathania@attbi.com>
Date: Tue, 14 Jan 2003 17:49:45 -0800
I have been teaching sound to my AP Physics students and discussing that
loudness is a perception based on the intensity of the wave. I indicated
that the decibel scale was designed so that 0 was the threshold of human
hearing and positive numbers were used for sound that humans (on average)
could hear. So today, a sharp kid brings in the specs for his stereo
receiver and it has a range of "-60 to 0 to 18 dB, in steps of 1 dB." How
is this possible? What is this 0 based on and how can it be reading negative
numbers? Thanks in advance
Nathania Chaney Aiello
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Greetings Nathania:
How the decibel is used is the confusing factor you are encountering.
The decibel (one-tenth of a Bell), named after Alexander Graham Bell, is a
unit of acoustical power equivalent to 20 times a ratio where the reference
energy is 0.0002 dynes/cm^2. A full Bell would be very loud so the unit that
most people use is called the deciBel or dB for short, now more commonly seen
as db. On the stereo tuner amplifier the term is used to show where the
output is in relation to the maximum power rating. The steps are those of 1
dB because the single dB is generally thought of as the smallest unit of
power *change* that a good human ear can detect. The modern normal ear,
exposed to huge quantities of noise in this modern world, is good for
detecting a change of about 3 dB which is a doubling or halving of the
acoustical power. Where the negative numbers come in is where the ratio goes
from 1:1 to less than unity. It can vary from amplifier to amplifier
depending on the total rated RMS (root means square) output. Did I mention
that the measurements can get complicated and subjective from one amplifier
to another?
Let us look at a hypothetical amplifier that can put out just 10 Watts of RMS
power into a single speaker load. If the front panel shows +3 dB for full
range of 10 Wrms then the 0 dB setting would be at just 5 Watts output. At a
-3 dB the output would be just 2.5 Watts. At -6 dB you would have 1.25
Watts. Now most speakers will knock you out of the house at just 2 Watts
because the energy conversion is very efficient. Having dB settings on
amplifiers is more of an affectation than a realistic number because no two
speaker models have the same conversion efficiency. But, since people like
numbers, there the numbers are and they provide a reference for whether you
want to break the lease or relax with a glass of Dom '57.
The only way to really trust the numbers on the amplifier would be to use an
audio Wattmeter or a dBm (deciBelmilliWatt) meter. These are referenced
against a standard power output into a standard load.
0 dB when used as a human measurement of sound intensity *is* the threshold
of hearing and is ambiguous by large part with dependence on the person doing
the hearing, it is like listening to an ant cross a sheet of paper. Then the
scale picks up with normal breathing at 10 dB, two people talking at 60, busy
traffic at 70, a 747 taking off from 100 meters at 110, the space shuttle
lifting off from 1000 meters at 140, etc. When used on the human sound
intensity scale, dB are far from accurate or even useful since the zero is
set subjectively.
As for the dB itself, the formula is dB = 10 log base 10 of ratio p1/p2 so it
is a dimensionless number and cannot be used the way many consumer products
display it on the front panel.
There, have I confused the issue beyond all redemption?
Best wishes to all,
Al Sefl
Sitting here nursing my Glenlivet on the rocks so all the above information
is suspect... better research it for yourself...
This archive was generated by hypermail 2.1.3 : Mon Aug 04 2003 - 16:18:11 PDT