Applied Acoustics 28 (1989) 247-251

Short Leq: A New Acoustic Measuring Technique

Alan D. Wallis

Cirrus Research Limited, Acoustic House, Bridlington Road, Hunmanby, North Yorkshire, YOI4 0PH, UK

(Received 15 October 1988; accepted 4 July 1989)

A BSTRA CT

Using the technique of 'Short Leq', raw acoustic data can be stored on a computer disk and used to recreate any acoustic situation, where the actual peak value of the signal is not involved. The technique involves the use of a fieM portable unit which acquires and stores the raw data, which is subsequently transferred to a desktop computer for actual measurement.

I N T R O D U C T I O N

The technique of 'Short Leo' has its origins in one of the oldest methods of acoustic measurement, 'outbox' processing, as for example where a tape recorder is used to record the actual noise for replay in the laboratory. The disadvantages of this method are well known and include the limited measuring time, the low accuracy of the overall system and the deterioration of the raw data as the tape is constantly replayed.

'Short Leo' is a new method of outbox processing. It was devised as a method of compressing the data, ensuring its integrity yet still storing a true representation of the original noise. The method linearly integrates the sound level over a short period, typically under 1 s. This Leo is stored and a further L~ taken, with no gap between them, continuing with successive Leo values for the duration of the whole measuring period. The advantage of the method is that the Leq is a true integral of the energy and thus accurately describes it for all statistical purposes.

247 Applied Acoustics 0003-682X/90/$03.50 O 1990 Elsevier Science Publishers Ltd, England. Printed in Great Britain

248 Alan D. Wallis

THE SHORT L,q CONCEPT

While the original reasoning behind the concept was to store data in a compressed form, it is obvious that having stored the data on an external computer, the computer can make the actual measurements. Thus, the Leq meter has the status of a simple, 'dumb' acquisition unit. If this acquisition unit has adequate dynamic range, the whole of the measuring task during acquisition is to place the Leq meter in the correct place, to keep it secure and to ensure an adequate power supply. Natural ly the L=q meter can perform other background tasks, such as giving conventional L,q or sound level readings, etc., but these tasks are incidental to the main acquisition function. In use, the data is stored in the non-volatile memory of the sound level meter until required for use. There is no limit to the time that it can be so stored and thus there is no need to have a computer on site.

Sometime later, the data is transferred to a desktop computer and stored on the disk of the computer. While the process is called 'transfer', it is really a 'copy' process. The original data is not modified in any way by the transfer to the computer and the original data is still intact inside the meter. Now, identical copies of the raw data can be made and distributed for processing on any suitable computer anywhere in the world; no longer does data have to be processed where it is acquired. For teaching purposes, the new method has great attractions as the actual measurement is performed at the keyboard and not on site. Thus different noise descriptors can be readily compared using real data without the need for students to work out of the laboratory.

A consequence of separating the acquisition and measurement is that no measurement decisions need be taken at acquisition time. With conventional 'inbox' units the wrong selection of acquisition parameters, either by the use of the wrong program module or simple control mis-setting, means that the noise must be measured again. If it is a 'once only' event, a second chance may not occur; the nightmare of the wedding photographer. With Short L,q, everything is captured within the practical range of the microphone.

DP37 PROTOCOL A N D C O D I N G

Providing the data is written in a s tandard format, any program will be able to make measurements from any data disk. This standardization of data was one of the first problems addressed and a format, called the DP37 protocol, was agreed by several producers. This defines not only the form of the data, but also the method used in its transfer. It was decided that a 0"1 dB resolution from - 1 0 to + 190dB was needed, which would

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require 2000 data points or 11 bits; thus a 16 bit word was decided upon, leaving 5 bits not used for the actual Le, value. As IEC 804 demands that any overload of the system be flagged, an overload flag is added to each data word using the 12th bit. The remaining 4 bits, giving 16 possibilities, are then left for data coding which is used to describe the Lcq in that word. This is a new concept which opens up totally new measurement possibilities. The code can be added during acquisition by the L,~ meter itself, or it can be added after acquisition at the computer keyboard. Once a code has been added to each L~q word, thus coding the noise source, each source can be measured separately. Thus all the L~q coded 'Renault', 'Ford' or 'Rover' can be compared and any parameter of these sources measured. For example, noise events rising out of a background can be identified by their code and measured independently of the rest of the data. The concept behind the software format has been described 1 and thus academic institutions can readily write software for teaching special aspects of noise measurement, without breaching any copyright.

M E A S U R E M E N T POSSIBILITIES

At the elementary level the computer can simulate the trace from a classic level recorder. Figure 1 shows a portion of a trace from a measurement of a television reporter doing an interview. Superimposed is the same data, plotting 2 s combined Le~ values. For many uses, this gives more idea of the time history. It is clear that this changing of the time base would be impossible with classic techniques, where the data interval has to be fixed at acquisition time.

In Europe, L90 is usually used as the background, while in New Zealand L9s is preferred. With Short LcQ, this is of no importance and the index required is only decided on replay. Any index over any time inside the

1 0 0

9 0 .

0 0 .

7 0 .

6 0 .

5 0 .

4 0 .

5 0 .

2 0

6

dDR Leq

i i i

@ 6 6

Fig. I. Mimicing a level recorder.

250 Alan D. Wallis

l o 0 dDA Leq

9 0 _

7 0 , , n r

5 0

Fig. 2. lOs and lOmin superimposed time history.

acquisition period can be produced. For example, if it is decided that a new descriptor would better describe the effect of a particular noise source, it is a simple matter to present the data in a way which will check this. For teaching acoustics this is invaluable.

Long periods of measurement are possible and Fig. 2 shows part of a 2 h plot with an elementary period of~th s plotted on a 10 s basis. Superimposed is the same data plotted on a 10min period. The loss of data is alarming. With a conventional 'inbox' unit taking 10min L,q values it is clear that individual events will be lost, When acquiring, up to 30 h of data storage is possible. A full description 2 of the type of instrument used to acquire these files shows the basic simplicity of the practical realisation of Short L~q.

'PROPER' A N D 'PARTIAL' L~q

There are two ways of describing the basic energy of an event: 'Proper' L,~ or 'Partial' Leq. These are defined as the energy expressed over the event time or the whole measuring time, respectively. Either can be plotted. In rough terms 'Proper' Leq is the actual L,~ of a noise source while 'Partial' L~q is the contribution of that source to the total environment.

Each source contribution is given as though no other source was present. This means 'What if?' predictions can be made. By using data coding and the two L,q types together, the contribution or effect of various noise sources can be very accurately described. Using coding, the L,q or SEL of every source can be found, as can the L n series for any portion of the acquisition time. Not only can this be done with one Leq meter, but two can be used at the same time to obtain difference and comparative measures, something no other system can do.

With classical measuring techniques it was not possible to estimate or teach the value of a new method without the actual equipment present and

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usable by each student. Today, using computers, the acoustic techniques can be demonstrated and explained without the need to have the actual meter present; the software being able to describe the process and to make measurements on data previously recorded in real situations, thus the technique as taught in the classroom directly relates to its use in the real world.

REFERENCES

1. Wallis, A. D. & Holding, J. M., A method of generating Short L,q. Proc. lnternoise, Hawaii, USA, Dec. 1984. Noise Control Foundation, Ploughkeepsie, New York, USA, 1984, pp. 1039-41.

2. Wallis, A. & Luquet, P. The use of personal computers in acoustic metrology. Proc. Internoise, Beijing, China, Sept. 1987. Acoustical Society of China, 1987. pp. 1207-11.