## Neutralizing Echoes in Music Room

This paper is a continuation of acoustic article writing # 1 – Measuring Resonance of the Hearing Room.

After obtaining the ideal dimension of the hearing space the next step is to measure the echo of the hearing room and neutralize the hearing echo.
Each hearing room has an echo and an echo in one room different from the other. For the hearing room we need a controlled echo is not an echo that is absorbed. A controlled echo will give the feel of a good live music room. The space echo has the magnitude: (1) The echo time with the second unit and (2) the reflected / absorbing frequency with the Hertz unit.

### 1. ECHOTIME

The ideal echo time for hearing space with a volume of 300 cubic feet is 0.9 seconds and 20,000 cubic feet of 1.4 seconds.
If your hearing space is 10,150 cubic feet then its ideal time of echoes is 1.15 seconds. But the fact is when the echo is 1.7 seconds then you need sound absorbing material. And vice versa if the echo time is only 0.7 seconds then the space is a lot of sound absorbing material.

There are various methods of measuring the echo time but the most commonly used is RT 60. RT stands for Reverberation Time and 60 represents 60dB. RT 60 has the definition: the time required sound to weaken as much as 60dB.

### 2. ECHO FREQUENCY

Each material has different absorbing and reflecting characters for different frequencies. For example the material of cement tends to reflect high notes and for low tones in continue. While carpets tend to absorb high notes and continue low tones. Often I see people making studio space or audio space by installing carpets on the floor and walls. Space like this tends to give a “boomy” sound effect with poor sound detail.

The table below is the coefficient of absorption / reflection at different frequencies

 125 Hz 250 Hz 500 Hz 1 KHz 2 KHz 4 KHz Permukaan Semen 0.01 0.01 0.015 0.02 0.02 0.02 Permukaan Karpet 0.08 0.24 0.57 0.69 0.71 0.73 Permukaan Dinding 0.29 0.1 0.05 0.04 0.07 0.09 Permukaan Fiberglass 0.01 0.21 0.68 0.85 0.9 0.97 Permukaan Kayu 0.28 0.22 0.17 0.09 0.1 0.11

After the table absorption coefficient / reflection you also need the following formula:

(0,049) Volume
RT60 = ———————————————————–
Surface Areax Absorption Coefficient/reflection

With the formula and coefficient of sabins then you can measure the frequency of your hearing echo.
Here is an example of calculating the frequency of the hearing echo.
If we have a hearing room with dimensions: Length of 21.8 feet; The width of 15.8 feet and the height of 7.9 feet then we the volume of the hearing space is 2721.076.

### A. BEFORE ACOUSTIC TREATMENT

Before the treatment of the hearing room has the following surface composition: Concrete floor, cement wall and ceiling are also cement. We can see the calculation table and the frequency graph of the echo as follows:

 Material Luas Sq ft 125 Hz a Sa 250 Hz a Sa 500 Hz a Sa 1 kHz a Sa 2 kHz a Sa 4 kHz a Sa Lantai Beton 344.44 0.01 3.4444 0.015 5.1666 0.015 5.1666 0.02 6.8888 0.02 6.8888 0.02 6.8888 Dinding 940 0.29 272.6 0.1 94 0.05 47 0.04 37.6 0.07 65.8 0.09 84.6 Total serap  sabins 276.0444 97.4444 52.1666 44.4888 72.6888 91.4888 Waktu gema (detik) 0.48301 1.36829 2.55590 2.99699 1.83429 1.45736

A1.Table of hearing calculation before treatment

A2. Frequency graph of echo before treatment

### B. AFTER THE ACOUSTIC TREATMENT

Once analyzed and carefully calculated then selected the acoustic material with the composition: Carpets, layered walls with fiberglass content. So we can see the difference by looking at the calculations and graphs below:

 Material Sq ft 125 Hz a Sa 250 Hz a Sa 500 Hz a Sa 1 kHz a Sa 2 kHz a Sa 4 kHz a Sa Carpet w/40-ounce felt 345 0.08 27.6 0.24 82.8 0.57 196.65 0.69 238.05 0.71 244.95 0.73 251.85 1/2″ drywall 840 0.29 243.6 0.1 84 0.05 42 0.04 33.6 0.07 58.8 0.09 75.6 1/2″ drywall covered in 1″ Fiberglass 100 0.29 29 0.1 10 0 0 0 0 1″ Fiberglass 100 0.01 1 0.21 21 0.68 68 0.85 85 0.9 90 0.97 97 Total Absorption, sabins 301.2 197.8 306.65 356.65 393.75 424.45 Reverberation Time, seconds 0.442672 0.674078 0.434804 0.373848 0.338623 0.314131

B1.Table of hearing calculation after treatment

B2. The frequency graph of echoes after treatment

From the example above problem we see that after treatment the frequency of space echoes become more flat (flat).
Thus the article on the calculation of the hearing echo and how to neutralize it.
Hopefully useful.

Regards
Herwin Gunawan

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