Assessing the sound level
Most sounds that you’re going to measure fluctuate in level. That immediately leaves you with two problems – how to measure these variations as accurately as possible, and how to be able to end up stating that the sound pressure level turned out to be, say 58 dB in a given situation?
The practical world is filled with trade-offs – you can’t always get it the way it ideally should be.
Instead you will have to look for quality and features sufficient to match the purpose, i.e. the errors or shortcomings introduced by the method or technology used should have as little significance as feasible for practical, economical or technical reasons.
Consider a Sound Level Meter with an analogue display (a deflecting needle or a bargraph). If the sound level fluctuates too rapidly, the needle or bargraph change so erratically that it is impossible to get a meaningful reading. On the other hand, if we introduce a damping of the needle deflection to slow down its movements and thus make it produce more meaningful results (to us, that is), we run the risk of missing rapid changes in the sound level.
Obviously, if two Sound Level Meters have different damping of the needle deflection, they will not give identical readings of the sound level when exposed to the same sound field. To circumvent this problem, standardised detector response times have been introduced.
The Sound Level Meter deflection must be fast enough to follow the fluctuations in the sound itself, yet slow enough to enable a read-out of the level:
- Meter deflection due to damping
- The signal itself
- If the deflection is too slow, peaks like this may pass totally unnoticed
In order to be able to compare measurements made with different Sound Level Meters, the meters must have the same amount of damping of the meter deflection. The damping is called time constant.
Detector Response Time
A Sound Level Meter is always equipped with a detector. The purpose of the detector is to convert the measured sound pressure to a sound pressure level (a number of decibels above the threshold of hearing – which is 20 µPa). Two detector response times have been standardised. These are F (for Fast) and S (for Slow).
By detector response time we mean how rapidly the detector output signal changes for a sudden change in the detector input signal. The correct term for detector response time is time constant.
If the detector input signal changes suddenly, the time constant expresses the time it takes for the detector output signal to reach 63 % of its final value.
The F has a time constant of 125 milliseconds and provides a fast reacting display response enabling us to follow and measure not too rapidly fluctuating sound levels.
The S time constant, on the other hand, has been set to be eight times as slow – viz. one second. This will help to average out the display fluctuations on an instrument with a needle or bargraph, and make readings possible in situations where the F time constant setting would produce fluctuations impossible to read.
TIP: here is a way to circumvent this problem of display fluctuations. Most modern Sound Level Meters have digital displays where the sound level is presented as figures. These figures are typically updated once per second and indicate the sound level at the moment of sampling with the selected time constant.
RMS, Impulse and Peak
A term you will encounter frequently when measuring sound, is the RMS, or the root mean square, value. The RMS value is a special kind of mathematical average value which is directly related to the energy contents of the sound.
The energy contents of the sound is a fundamental part of hearing impair risk assessments. However, if the sound to be measured consists of impulses or contains a high proportion of impact noise, measuring RMS values with F or S time constant will not give results correlating very well with the perceived noise level.
To cope with this, a third time constant called I (for impulse) has been developed. The time constant of I is 35 milliseconds, which is sufficiently short to permit detection and display of transient (rapidly changing) noise in a way resembling the human perception of sound. To enable convenient read-out the decay-time for I is 1.5 seconds.
The perceived loudness is a function of the frequency and the sound level, but also a function of the sound duration. Sounds of short duration are perceived to be of a lower level than steady continuous sound of the same level.
The risk of hearing impair is in general not coupled to the perceived loudness. Therefore, precision Sound Level Meters like the Nor118 often include a circuit to measure the peak value of the sound.
TIP: What Is the RMS? The mains (line) voltage in your country will typically be 240 or 110 VAC (Alternating Current) with a frequency of 50 or 60 Hz. This AC voltage represents energy when you use it for illumination etc. How much depends on the amount of current you draw from the mains. But even DC voltage may be used for this, i.e. the energy term is applicable for this as well. The RMS term expresses what value an existing AC-signal should have – had it been a DC-signal – to develop the same amount of energy as the DC-signal would for the given configuration.
Energy Parameters – the Leq and the SEL
Sound is a form of energy. The risk of hearing impair depends not only on the level of the sound, but also on the amount of sound energy entering the ear. For a given sound level the amount of energy entering the ear is directly proportional to the duration of exposure.
Therefore, to assess the hearing damage potential of a sound environment, both the level and the duration must be taken into account. If the level is high enough, however, the duration will be irrelevant – the hearing impair will occur almost instantly.
To better understand the significance of the measured levels, we are always looking for data reduction. If we manage to boil down the data to one or a few numbers, without sacrificing the hearing damage potential or – if found to be a lower sound level – the degree of annoyance that they represent, we have succeeded.
We do that by introducing the equivalent continuous level or the Leq. The Leq is the constant level which has the same energy and consequently the same long term hearing damage potential as the actual varying, measured sound level. This will hold true if we for a moment disregard peak effects which may damage your hearing instantly.
Definition: Mathematically the Leq is defined as:
the average value with respect to time.
TIP: To understand the difference between RMS and the Leq consider the following: Although they both express an equivalent constant signal containing the same amount of energy as the actual time-varying signal itself, they cannot be substituted. The Leq expresses the linear energy average, while the RMS value expresses a weighted average where more recent events have more weight than older events.
NOTE: A few Words more on Annotation: LA means the A-weighted sound pressure level, LAeq, T means the A-weighted Leq, LAeq, T, I means the Impulse- & A-weighted Leq, LAE means the A-weighted SEL. The term dB(SPL) is often used to keep the “sound decibels” apart from “other decibels”.
Fig. shows the relationship between the SEL, the SPL and the Leq. The Leq is the constant level needed to produce the same amount of energy as the actual varying sound (the SPL).
The SEL is the Leq normalised to 1 second. It is what the Leq would be if everything took place during 1 second.
