Applied Acoustics 30 (1990) 335-345

Technical Note

Rasti Measurements in Mosques in Amman, Jordan

A BSTRA CT

Field measurements in 30 mosques in Amman, Jordan are reported; the field measurements include RAST1, reverberation time, background noise lerel EDT and signal to noise ratio. RASTI values range between 0.39 and 0.59 where the articulation index ranges between poor and fair. There is not a sbzgle mosque in which the articulation index is good or excellent; despite the fact that these mosques are costly and their spiritual and functional aspects hat'e received special attention, the reverberation time is, in general high and, in one mosque it was around 27s at low frequency. In the Jordan University Mosque the reverberation time varies between 10 and 4 s. The poor acoustical performance of these mosques is mainly the result of little treatment inside and high background noise due to poor insulation of the building envelope against external noise.

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

Mosques, which are public buildings, are used for praying, speech, lecturing and similar activities. They have a special spiritual place in the Arabic and Islamic countries. Their functions have almost remained unchanged for the past 14 centuries and some of these early mosques are still functioning to- day. Alazhar mosque, in Cairo, Egypt is an example which was built as a mosque and university more than one thousand years ago, and is still in use. Activities in the mosque start early in the morning, approximately 1 h before sunrise, and cont inue until midnight; in fact some mosques are in use 24 h a

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

336 R. N. S. Hammad

day throughout the year. The mosque reflects not only architectural and structural theories but also represents the civilization which has existed during the course of history

This paper reports and discusses results of field measurements conducted in different mosques in the city of Amman, Jordan. It also evaluates the acoustical performance in those very important places using different sets of standards and techniques. Comparison of these results with similar specifica- tions are also given. Finally the paper offers different recommendations for improving the acoustical treatment of these places and means of remedying the existing problems.

2 DESCRIPTION

The mosque, in general, is a hall, rectangular or square in plan, with a dome located in the middle of the plan as shown in Fig. 1; the dome in some cases

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covers a large area of the hall and in a few more recent mosques the dome is eliminated. The height of the mosque ranges between 5 and 10 m and in some cases may reach more than 30m. It is usually located in the middle of residential areas with nearby noisy roads. Large windows and doors may be located on all walls, and they are open, in some cases, toward streets and courts. The floor is unfurnished, but carpeted. The internal surfaces are painted on cement plastering with little gypsum and wood decorations. Little attention is given to the acoustical characteristics of the materials used inside the mosques. This results in a long reverberation time and noticeable echo and other acoustical problems affecting their acoustical performance. There are more than 1500 mosques in Jordan, more than 40% of them located in Amman, and most of them built within the last 50 years.

3 FORMULATION OF THE PROBLEM

In this paper, different standards are used to evaluate the acoustical performance of these mosques. The acoustical parameters are well known and they are briefly outlined.

(i) Reverberation time (RT6o). Its value is given by different formulae I -a which were developed from Sabin's formula.

(ii) Articulation index. This index depends on the speech intelligibility inside a space and its value depends on the space volume, area, RT6o and the background noise. 4

(iii) Speech Transmission index (ST1) and Rapid speech transmission index (RASTI). RASTI is developed from STI, which takes into account the reverberation time, the background noise and the signal level. 6 This index can be calculated from the Modulation Transfer Function (MTF) which is given by

M(F) = { I + [JrcF(T/13.8)] t/2 [1 + 101 -s/m/lo]} - 1 (1)

where T is the reverberation time in seconds, F is the modulation frequency (between 0-4 Hz and 20 Hz), and SIN is the signal to noise ratio which is taken between + 15 and - 15 dB.

(S/N),pp = 1/182 ° o.a(g/N)app.F (2)

where

(S/N)app.F = 10 log Ira(f)/(1 -- re(f))] (3)

338 R.N.S. Hammad

T A B L E I

RASTI ranges Quality of acoustical encironment

0-0.3 Bad 0-3-0.45 Poor

0-45-0.6 Fair 0-6-0"75 Good

0-75-1'0 Excellent

STI is calculated from the following equation,

STI = [(S/N),nn + 15]/30 (4)

RASTI is determined by calculating the Modulation Reduction Index (MRI) for 500Hz and 2kHz. Table 1 shows the ranges given to RASTI to describe the subjective acoustical environment inside any enclosure and they are compared successfully with the Articulation Index and other subjective indices. 6 A RASTI instrument v is used to measure directly the value of RASTI in any location inside any space.

4 FIELD MEASUREMENTS

A Speech Transmitter, B&K type 4225, is used to simulate the human voice at 500 Hz and 2 kHz; and a Speech Receiver, B&K type 4419, is used to pick up the signal and calculate RASTI in any required location. It also measures the early decay time, EDT, Speech Transmission Index, STI, L,q and the background noise.

Some 30 mosques were chosen from Amman representing different styles and forms and special architectural characteristics. RASTI values were measured in several places inside the mosque, at least in six different positions, while they were unoccupied. Other acoustical terms were also measured such as early decay time, Speech Transmission Index and S/N, at 500 Hz and 2 kHz. The reverberation time RT6o using a Building Acoustic Analyzer, B&K type 4418, and a sound source, B&K type 4224 were also measured. The signal was received by a ½ in microphone B & K type 4265 and a rotating microphone boom, B&K type 3923, to average the signal.

5 RESULTS AND DISCUSSION

The relation between mean, minimum and maximum values of RASTI for the different mosques in Amman is shown in Fig. 2. The mean values range

R A S T I measurements in mosques in Amman, Jordan 339

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between 0-39 and 0.59; where the articulation ranged between poor and fair. There is not a single mosque (except King Abdullah Mosque) at which the mean RASTI value is equal to or greater than 0-6, i.e. the articulation is good or excellent. This is evidence that the acoustical performance was not considered in the design or even at a later stage. This is despite the fact that these buildings were built within the last 50 years and the cost of one mosque may be several hundreds of thousands of dollars. In fact the cost of a single minaret is, in some cases, more than $150000 and most of these mosques have at least one. The values of RASTI plotted versus S/N, and early decay time (EDT) are shown in Fig. 3. The values of SIN lie between - 6 and 10 while EDT values are in general greater than 0"5 s. This clearly indicates that RASTI values are mainly dependent on EDT, which is the result of the absorption and volume. It is possible to improve the acoustical performance of an enclosure by decreasing EDT, i.e. acoustic treatment, but improving it by increasing SIN only is too difficult. When EDT is greater than or equal to 0-5, RASTI is less than or equal to 0"7, i.e. the articulation index is good but never excellent. It is clear that the reason for the poor

R A S T I measurements in mosques in Amman, Jordan 341

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performance of these spaces can be attributed to both SIN (high background noise) and EDT (lack of absorption treatments and volume); but is mainly the result of the latter..

The reverberation time versus frequency in some mosques is given in Fig. 4. In general, RT6o measured is greater than 3 s at frequencies less than 1 kHz. Its measured value in some mosques is greater than 10 s; compared with the recommended RT for lecture rooms, approximately 1 s, it is clear that the RT is very high and again the poor performances of such mosques are mainly the result of the reverberant field. The reverberation time of King Abdullah mosque, which was recently built, and before any treatment is

342 R. N. S. Hamrnad

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shown in Fig. 5; it varies between 27 s at 100 Hz and 4 s at 8 kHz. These values are very high and it is almost impossible to achieve an',' intelligible conversation inside the mosque even within a few meters spacing. However this mosque attracted much attention in the later stages and the reverberation time was reduced to approximately 2 s and the RASTI value increased to approximately 0-8. Measurements of the articulation index shows that values are between very good and excellent. A 40% perforated plywood dome was added to the concrete one with 50 mm mineral wool and 100 mm air gap between the plywood and the concrete. This has the effect of reducing the reflection of sound waves from the dome and the focusing and redirection of outside noise inside the mosque. The internal walls, up to 3 m height, which consist of marble, 1-40 m, and decorative wood (mashrabia) were designed with a perforation with 50 mm mineral wool treatment and 100 mm air gap behind the mashrabia. The carpet is heavy with 10 mm felt beneath (measured absorption coefficient 0.5 at mid-frequencies). A more detailed description of measurements and construction of this mosque are given elsewhere. 8 Acoustical treatment of this mosque cost the owner some

RASTI measurements in mosques in Amman, Jordan 343

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$0-5 million; which is equivalent to the cost of the shell construction. The reverberation time of the University mosque (unoccupied) is shown in Fig. 6, it varies between 10 and 2 s in the frequency range 100 Hz to 4 kHz. The worshippers always experience dissatisfaction with the articulation of the speaker inside the mosque. The University paid great attention to solving the problem by increasing the S I N ratio; but the articulation of the speech is still poor due to the fact that the location of the mosque is near a busy street with more than 3000 vehicles (on the average) during the day (6-24 h) at speeds of more than 60 km/h. 9 Its envelope skin has many windows and doors, some of them opened toward the busy street.

The background noise level inside the mosques were also measured and its value is in general, high, varying between 50 and 70 dBA. This is due to the fact that they are located in the vicinity of busy roads in the city and the poor insulation of the building envelope due to the large number of windows and doors which are kept open for ventilation during services. In some cases the background noise level inside the mosque is higher than outside. This is due to windows located in the neck of the dome which collects the outside noise from 360 ° and redirects it inside the mosque.

344 R. N. S. Hammad

6 C O N C L U S I O N

These field measurements indicate clearly that little at tent ion has been given to the acoustic design of mosques in Amman , and their acoustic performance is in general poor; despite the fact that these public buildings have been built recently at great expense and have special visual and spiritual importance in Arabic and Islamic cities. Two goals should be recognized by architects to overcome such poor performance: the outside noise should be isolated and the amount of absorpt ion inside the hall should be increased. The first goal can be achieved by opening doors and windows towards courtyards, similar to those in old mosques, such as Alazhar mosque in Cairo or /and by increasing the insulation of the building envelope (small windows with double glazing). Other difficulties can be overcome by employing consultants specializing in architectural and building acoustics.

A C K N O W L E D G E M E N T

The au thor wishes to recognize the Deanship of Scientific Research in the university of Jordan for their suppor t and encouragement .

R E F E R E N C E S

1. Sabin, P. E., Acoustics and Architecture. McGraw-Hill, New York, London, 1932, pp. 60-70.

2. Eyring, C. F., Reverberation time in dead rooms. J. Acoust. Soc. Amer., 1 (1929) 217-41.

3. Fitzroy, D., Reverberation formula which seems to be more accurate with non- uniform distribution of absorption, J. Acoust. Soc. Amer., 31 (1959) 893-7.

4. Kryter, K. D., Methods for calculation and use of articulation index, J. Acoust. Soc. Amer., 34 (1962) 1689-97.

5. Houtgast, J., Steeneken, H. J. M. & Plomp, R., Predicing speech intelligibility in rooms from the modulation transfer function: general room acoustics. Acustica, 46 (1980) 60-72.

6. Houtgast, J. & Steeneken, H. J. M., A multi-language evaluation of the RASTI method for estimating speed intelligibility in auditoria. Acustica, 54 (1984) 186-94.

7. Bruel and Kjaer, Instruction Manual: Speech Transmission Meter Type 3361. B&K, Denmark, 1986.

8. Hammad, R. N. S. & Abdelazeez, M. K., .The acoustical design of King Abdullah Mosque, Amman, Jordan: suggestions and results (in Arabic). Dirasat (Jordan University Journal) 16(5) (1989) 24-39.

RASTI measurements in mosques in Amman, Jordan 345

9. Hammad, R. N. S. & Abdelazeez, M. K., Measurements and analysis of traffic noise in Amman, Jordan and its effect. Appl. Acoust., 21 (1987) 309-20.

10. Cremer, L. & Muller, H. A., Principles and Applications of Room Acoustics. Applied Science Publishers, London, 1982.

R. N. S. Hammad Architectural Department, Faculty o f Engineering and Technology, University o f Jordan, Amman, Jordan

(Received 19 September 1989; revised version received 10 January 1990; accepted 25 January 1990)