What Is the Sound Reduction Index?

Mary McMahon
Mary McMahon

The sound reduction index is a reflection of the acoustic insulating properties provided by a material. Several standardized methods can be used to develop a sound reduction index that will be readily understood by observers anywhere in the world. Companies test products designed for acoustic insulation before sale, and label the packaging so buyers understand what kind of insulation is available. This can be important for purchasing decisions, as people want to select acoustic insulators that will meet their needs.

Acoustic insulation is rated by its sound reduction index.
Acoustic insulation is rated by its sound reduction index.

One simple method to test products is to use two rooms with an opening between them. Technicians can place a sample of the test material in the opening and use a noise source to produce standardized noise. Tools in both rooms measure decibel levels, frequencies, reverberation, and other characteristics of the sound. The researchers can compare the data between the room with the noise source and the target room to determine how much sound reduction the material offers. This allows them to create an index to provide information about the insulating properties of the test material.

Concerns surrounding acoustic insulation can vary. In a setting like a radio booth, operators want no outside noise to penetrate, because it could interfere with the quality of the broadcast or recording. Materials with a very high sound reduction index are necessary, and they should have a broad scope, limiting not only most decibel levels but also a range of frequencies. This will prevent all but the loudest of noises in all frequencies from penetrating the booth.

For other settings, the sound reduction index may concentrate more on frequencies than decibel levels. Using noise filtering, for instance, it may be possible to cut out specific frequencies from recordings and other activities, and thus it is acceptable if they bleed into an environment. For settings like freeway barriers, vibration may also be a cause for concern, and it is necessary to develop a material that resists noise as well as vibrations from heavy cars and trucks.

The use of standardized techniques and measurements for sound reduction index values is important. A material with a standardized rating can be used in any setting around the world. A sound technician will get the necessary information about its properties from the packaging and can decide if it will be appropriate for a project. When technicians use methods that are not standardized, it can create problems, as only people familiar with those methods will understand what the index actually means.

Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a wiseGEEK researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

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Discussion Comments


What kind of sound is used for measuring the sound reduction index?


@allenJo - If you want the best in sound control you will probably need something more than pillows. You may need to resort to caulking your walls to eliminate every possible place where sound can enter through.

This is the more extreme option but it’s one that works, from what I understand. Of course, this is in addition to other steps you would take, like acoustic insulation.


@Mammmood - I would hate to live near a busy road that made a lot of noise. Several years ago in our town the city put up sound barriers along the busiest road in town.

These were sound absorbing panels that were put up for about a quarter of a mile along the noisiest part of the road. The panels were stacked pretty high. There were people living on the other side of the panels and I am sure they wanted their peace and quiet.

I know we have noise ordinances in our city but I don’t know if road traffic is subject to the ordinances as such. All I know is that we voted on it and the people unanimously agreed that the panels needed to be there. Can you imagine trying to get some sleep with cars blaring all evening long?


@allenJo - Yeah, that’s OK for a home production but not for something professional. You would need precise measurements. I don’t do any professional production myself but I do try to clean up the sound in post production.

I have a free sound editor that I use for this purpose. What I do is I isolate the section of the audio that has “noise” and then create a profile of that clip. I then use the “Eliminate Noise” menu option from my audio editor, using that profile as my sample.

The program will search throughout the whole program looking for segments of audio that have that particular noise and then delete it. It works wonders.

It’s better if you can eliminate the noise before you do your shoot, but post production has been a life saver in many cases.


You don’t necessarily need to know about the particular sound index of your materials to do a good job of soundproofing. Just use some stuff that you know will absorb sound waves.

For example, I recently had to do some audio recording for a voice over at home. This was not a professional voice over but something for a home video. So I took my microphone, locked myself in my closet and then padded the closet with pillows all around.

That was a tip I got from somebody. It made a big difference in the final audio sound, and I had done before and after tests just to see if it would work.

I am sure that there are specific sound indexes for the pillows. I don’t know what they are but I do know that they are highly recommended for soundproofing a wall.

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