Measuring Sound Levels — Decibels (dB) in Home Theater

What the decibel scale is and why it is important

A signal strength meter showing input power and output decibels

RonPeigl / Getty Images

Two of our most important senses is the ability to see and hear. With our ears, we can detect sound changes from the softest whisper to the loudest thunderclap.

How We Hear

However, in addition to the ability to hear, is the way we hear.

Sound (which are waves that move through the air, water, or another compatible medium) reaches the outer portion of our ears, which funnels it through the ear canal to the eardrum. 

What Determines the Loudness of Sound

How loud a sound is is determined by a multitude of factors, which include a combination of the amount of air that reaches the ear from the originator of the sound, and the distance of our ears from the origination point of the sound.

The Decibel Scale

To interpret the sound receiving process, a scale, known as decibels, was created.

Our ears detect changes in volume in a non-linear fashion. A decibel is a logarithmic scale of loudness. A difference of 1 decibel is perceived as a minimum change in volume, 3 decibels is a moderate change, and 10 decibels is perceived by the listener as a doubling of volume. Decibels are designated by the letters: dB.

0 dB is the threshold of hearing — Other examples include:

  • Whisper: 15-25 dB
  • Background noise: about 35 dB
  • Normal home or office background: 40-60 dB
  • Normal speaking voice: 65-70 dB
  • Orchestral climax: 105 dB
  • Live Rock music: 120 dB+
  • Pain Threshold: 130 dB
  • Jet aircraft: 140-180 dB

How the Decibel Scale Is Applied

The decibel scale is applied to the home theater environment in the following fashion:

For amplifiers, decibels reflect a measurement of how much power it takes to produce a specific sound output level. However, there is an interesting thing to point out.

For one amplifier or receiver to be twice as loud as another, you need 10 times more wattage output. A Receiver with 100 WPC is capable of twice the volume level of a 10 WPC amp. A receiver with 100 WPC needs to be 1,000 WPC to be twice as loud.

In a more precise application, decibels are also used in relation to the sound output capability of loudspeakers and subwoofers at specific frequencies, at specific volume levels. For example, a speaker may have the ability to output a frequency range of 20Hz to 20kHz, but at frequencies lower than 80 Hz, the sound output level may be -3dB down in terms of volume output. This is because more power output is required at lower frequencies to produce the same volume level.

Also, the dB scale is applied to the sound level output capability of a specific speaker when fed a tone carried by one watt of power.

For example, a speaker that can produce 90 dB or higher of sound output when fed a one-watt audio signal is considered to have good sensitivity.

However, just because a speaker has good sensitivity does not automatically determine if it is a "good" speaker. A speaker that needs more power to produce sound just indicates the amount of power required for the speaker to produced audible sound. Other factors, including frequency response, distortion, power handling, and speaker construction, are also important.

In addition, for video projectors, the decibel scale is also used to measure how much sound is produced by the cooling fan. For example, if a video projector has a fan noise rating of 20dB or less, that is considered very quiet. Unless you are sitting close, you should not be able to hear the fan — and if you do, it should not be distracting.

How to Measure Decibels

Now that you have an idea what decibels are and how they factor into the music and home theater listening experience, the question is "How can you measure them?".

For consumers, one way that decibels can be measured is by using a portable sound meter (similar to the one shown in the above photo attached to this article.

Since most home theater receivers have built-in test tone generators, you can use those tones to determine the generated decibel level for each speaker at a given volume level setting. Once you determine the decibel level generated by each speaker, you can then adjust your individual speaker volume levels so that the entire speaker system matches. When all your speakers register the same decibel level at a given volume level, then your sound listening experience will be balanced.

Examples of Sound Meters include:

It must be pointed out that decibels are only one measure of how sound is produced and reproduced in home entertainment.