Sound System Design Research Report

Hadi Sumoro

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Contents

 

1. Summary                                                                                                               2

2. Statement of the Design Project                                                                             3

3. Analysis of the Existing Sound System                                                                    4

         A. List of Equipments and Settings                                                                    4

         B. Description and Analysis of the Installed Sound System                                 4

         C. Calculation                                                                                                   6

4. New Sound System Design Proposal                                                                     7

         A. Overview                                                                                                     7

         B. Room Data                                                                                                  7

         C. Calculation                                                                                                   7

         D. Analysis                                                                                                       11

         E. Conclusion and Installation                                                                           12

5. Documentation                                                                                                      13

 

 

 

 

 

 

 

 

I. Summary

 

As a Peabody student, I’ve experienced a class held in the Norman auditorium, in the Bloomberg building, or the Physics and Astronomy building of the Johns Hopkins University. The sound system is good for everyday lecture, but there are some spots that don’t sounding good in the hall, especially in the middle to front area. This research is made to make a hypothesis of the hall’s sound system misalignment and suggest a cheaper but better sound system for the Norman auditorium.

II. Statement of the Design Project

 

The Norman auditorium is equipped with 2 sound systems. The music/program is being handled with a type 4A system, and the lecture is being handled with the ceiling distributed loudspeakers. The hypothesis of the wrong installation of the sound systems and a new design of a sound system in this auditorium is presented in this research report.

III. Analysis of the Existing Sound System

 

A.     List of equipments and settings

Distributed ceiling speakers: JBL 2105H

   Sensitivity: 94 dB, 1W, 3ft

   Impedance: 8ohms

   Diameter: 4.5” cone driver

Power: 50 watts

   Frequency range: 300Hz - 15 KHz

   Coverage Angle: 110

 

Voice compressor: Urei 1176LN (low threshold, 20:1 ratio)

Program compressor: Urei 1178 (medium threshold, 8:1 ratio)

Equalizer: Rane PE15

Power amplifier to the distributed ceiling speaker: Crest Audio 1501A

Power amplifier to the program speaker: Crest Audio CC151

Wireless mic receiver: Telex FMR-2 and Shure ULX wireless

Automatic mixer: JBL 7510B

Program mixer: Urei 1620

Podium monitor: Soundolier MVXA-195

Note: Both compressors are set to medium-fast attack and medium release

 

B.      Description and analysis  of the installed sound system

The auditorium is equipped with 2 sound systems. The distributed ceiling sound system which is used for the lecture, and stereo loudspeakers which are used for program/music use.

 

This hall is used primarily for lecture, the music/program speakers aren’t used a lot. There isn’t any specification document for these homemade speakers. The frequency range of the speaker is wide and the sound pressure level distribution can be considered good from the very back row up to the middle row. The front rows don’t get good coverage from the speaker position and aiming angle (see documentation chapter). To get a good coverage in this hall with two speakers hung in the front, the vertical angle of the loudspeakers must be very wide, up to 80. It is recommended to put 2 pair of stereo speakers to cover the back-middle area and the middle-front area to fix the coverage uniformity.

The distributed ceiling system can be perfect for this lecture hall, but there’s a misalignment on the system. There are 5 rows of the speakers, and each row contains 7 speakers. The spacing for each speaker (in a line/row) can be rounded up around 8.2ft. The back ceiling height is 10’6”, and it is 21’ at the front. At the back, each loudspeaker projects a coverage radius of 9.2ft (measured in sitting position), this gives us a center to center layout pattern, which will deliver a perfect level for the listener from the middle to the back rows. However, at the front where the ceiling height reaches 21’, the coverage radius of the loudspeakers is reaching 24’ (calculation is explained below).

It is difficult to listen and understand spoken words from the middle to the front area because of the comb filtering and the need of more gain of the front area ceiling loudspeakers. This can be fixed by putting more gain (adding an additional amplifier) to the 1^st-3^rd row of the loudspeakers and disabling some of them to reduce the comb filtering caused by the overlap coverage.

C.     Calculation

The coverage area radius of the ceiling loudspeaker is determined by:

R = (h-l)tan

h = floor to ceiling height

l = 4ft for sitting position

 = the driver coverage angle (110 degrees for JBL 2105H)

Taking h equals 10’6” at the back and 21’ at the front, the coverage radius area at the back is 9.2ft and 24.3ft at the very front row.

IV. New Sound System Design Proposal

 

A.     Overview

The room is considered wide but it’s not long. Delay can be disregarded because the room length doesn’t exceed 30 feet. The problem is more into the width of the room and calculating the right aiming angle/position for the loudspeakers. The cheaper way to provide a sound system in this hall (compared to the distributed ceiling and the stereo loudspeaker) is a central cluster system (with four speakers). Since the hall is mainly used for lecture, the music/program can be played in mono for a certain occasion.

 

B.      Room data

Volume: 47019.3	EAD = 4 ft
Room Constant: 2445.08	D= 1 ft
Surface area: 9198.14	D= 36 ft
= 0.21	D= 12 ft
Ambient noise level: 55 dB (A)	T= 1.2 s

Note: the calculation page for these calculations is not included in this report.

 

C.     Calculation

The calculations are attached on the following pages.

1.      Determining the number and coverage angle of the loudspeakers needed

2.      Determining the PAG and NAG

3.      Determining the power needed (EPR)

 

 

 

 

 

 

 

 

D.     Analysis

As we can see from the calculation page 1, the horizontal coverage angle needed is 84.36-120and 80vertical coverage angle for the whole seating area. This shows that we need 2 speakers to cover the front and the back area, which has 40vertical coverage angle each and 2 speakers to cover the left and right seating area. The two bottom speakers (those are covering the front-middle rows) need 60horizontal coverage and the two top speakers (which are covering the back-middle rows) need 50horizontal coverage. In summary, we need 2 speakers with 40vertical coverage angle and 60 horizontal coverage angle to cover the front-middle seating area, and 2 speakers with 40vertical coverage angle and 50horizontal coverage angle to cover the back-middle seating area.

The minimum directivity of the speaker can be calculated to get a minimum 15% AL. Q =

Q = 10

Q =2

From calculation page one and two, the speakers needed for this auditorium are two 12” single cone driver speakers for the top, and two 8” single cone driver speakers for the bottom.

E.     Conclusion and Installation

By taking the middle acoustics diffuser panel that is already installed in the auditorium and substituting the middle part with a central cluster system, the sound system proposed will work more efficiently and perform better performance (and coverage uniformity) to the audience this auditorium.

The equipments needed for the new sound system are listed below:

·        two 12” single cone driver speakers with 40vertical coverage angle and 50 horizontal coverage angle, and sensitivity 90dB, 1W, 1m

·        two 8” single cone driver speakers with 40vertical coverage angle and 60 horizontal coverage angle, and sensitivity 90dB, 1W, 1m.

·        two stereo power amplifiers, one 250W stereo power amplifiers for the 12” speakers to cover the back-middle seating area, and one 200W stereo power amplifiers for the 8” speakers to cover the front-middle seating area.

Since the auditorium is equipped with a stereo Crest Audio power amplifier CC151, 500W, we can arrange those four loudspeakers connection to be fed by this amplifier. This will reduce the cost of the new sound system.