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Acoustic Reverberation Control – Acoustical Measurements In Small Rooms

Dennis Foley June 25, 2014 No Comments
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If you are in pursuit of reverberation control, there are some important measurements and definitions you must first be aware of if you want to find the proper application and eventual acoustical treatment for your room or studio. Lets examine some of these measurements and try and place some real world meaning to them, so that we can understand what works for acoustical treatment and what does not. Our first measurement is reverberation times.

Acoustic Measurements You Need To Take In Small Rooms

If you remember from our past articles, we discussed direct and reflected energy within our small rooms. Direct energy is that straight line energy from our loudspeakers. All other energy heard or felt at the listening position is reflected energy or “room sound”. Reverberation takes this definition and expands upon it. Reverberation times are defined in the time domain and they are the amount of time it takes for a inaudible decay from the original direct sound. It is the time difference between the direct and last life or audibility of the reflection time of the last wave or ray. This number is called RT60.

Direct Or Straight Line Energy

Direct Or Straight Line Energy

RT 60

RT60 is a number that illustrates that difference we just discussed above between direct and reflected, for the reflections only to decay to 60 dB. The scale one uses for this number, can be a single value or you can also measure it by applying a wide band signal. That scale can go from 20 Hz. – 20 Khz. A more typical and narrowing of this frequency range on the scale is to use octave bands such as 1/3 octave, 1/6, 1/12, 1/24… The RT60 numbers derived from a single number versus octave band scaling are usually different. Also, a more narrow band approach is frequency dependent meaning the number will differ depending on frequency band measured.

Acoustic Signature

The reverberation time, number no matter what the scale used, is part of the acoustical signature of the room. In order to correctly measure the difference between the direct and reflected energy decay rates, we need to excite the room with energy but we must make that excitement and distribution of that energy uniform and consistent throughout the room. You must also also work from the assumption that the room is an empty room free of anything that has an absorption coefficient attached to it.

Lateral Room Reflections

Lateral Room Reflections

Diffuse Sound Field

We now have our empty room with an energy source that is omnidirectional and all the internal room volume is filled with equally distributed energy/reflections, therefore you must now assume a diffuse sound field in order for all of this to occur. When testing equipment is brought in, do not use sources such as a hi-fi speaker that has by design (to minimize room effect due to reflections) a more narrower energy projection of sound energy into the room. You must use caution when dealing with the speaker/room interaction and not introduce any more distortion into the deal. There is already enough.

Use Multiple Measurements

The proper way to achieve good results is to use not just one measurement or just one value. Lets take a series of measurements at different frequency bands and then lets also measure the rate of decay over a series of predetermined sound pressure levels. Do not forget to focus particular attention to the mid range frequencies. Reverberation times are super critical when it comes to middle range frequencies. It is critical that you preserve the timbre and the spectral balance of any frequency in these middle bands where our vocals lie. This clarity provides us with the emotional connectivity to our music. Absolutely, can not mess with that.

30 Hz. Friendly Room Size

30 Hz. Friendly Room Size

Middle Frequencies / Reverberation Times

The middle frequency reverberation times is an area that must be focused upon. You need to look at the middle frequency reverberation times and how these times relate to middle frequencies. This factor is critical because you do not want to alter any spectral balance or content of the original music signal. This is an area where our vocals lie and care must be taken not to modify this frequency range with excessive reverberation times.

Direct And Reflected Energy

Our final sonic experience within our room is composed of both direct and reflected energy. Our room is a space where we have reflected (room sound) and direct energy from our speakers. Together they produce the sonic experience we define within the room. The sound within the room is a balance of direct, early and late reflections that together combine with the acoustics of the room boundaries. Unfortunately, RT 60 measurements will not assist us with this but it does provide certain ranges of reverb times that allow for all of this energy to produce a quality sound field.

In Summary

So I hope that helps you in pursuit of true reverberation control in your room. If you have any questions at any time I am always on hand to help answer them. Leave them in the comments section or email me at info@acousticfields.com. If you would like to learn more about room acoustics please sign up for my free videos and ebook by joining the mailing list here. I send room tuning tips and things for you to test in your room every Wednesday. They are easy to follow and really help you enjoy more of your music.

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Dennis Foley

I am an acoustic engineer with over 30 years’ experience in the business. My technology has been used in Electric Lady Land Studios, Sony Music of New York, Cello Music and Films founded by Mark Levinson, and Saltmines Studios in Mesa, Arizona, along with hundreds of others.

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