Definition and Explanation of Decibels (dB) in Home Theater

What the decibel scale is and why it is important

Decibels (dB) are a unit for measuring sound. Since sound reproduction is critical for the home theater experience, it's important to understand the meaning of a dB in music.

Decibels are also used to measure the power of electrical signals. This article pertains specifically to the measurement of sound.

What Is a dB in Music?

A decibel, designated by the letters dB, is a logarithmic scale of loudness. Our ears detect changes in volume in a non-linear fashion. Sound loudness, or volume, is determined by a multitude of factors. These include the amount of air that reaches the ear and the distance of our ears from the source.

A signal strength meter showing input power and output decibels
RonPeigl / Getty Images

The Decibel Scale

To quantify how loud sounds are, the decibel scale was created. A difference of 1dB is perceived as a minimum change in volume. A difference of 3dB is a moderate change, and a difference of 10dB is perceived by the listener as a doubling of volume.

The threshold of hearing is 0dB. Here are some examples of common sounds and where they typically fall on the decibel scale:

  • Whisper: 15-25 dB
  • Background noise: 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

For amplifiers, decibels are a measurement of how much power it takes to produce a specific sound output level. For one amplifier or receiver to be twice as loud as another, you need 10 times more wattage output, so a receiver with 100WPC is capable of twice the volume level of a 10WPC amp. A receiver with 100WPC needs to be 1,000WPC to be twice as loud.

Decibels are also used in relation to the sound output of loudspeakers and subwoofers at specific frequencies and volume levels. A speaker may have the ability to output a frequency range of 20Hz-20kHz, but at frequencies lower than 80 Hz, the sound output level (volume) may be -3dB less. This is because more power output is required at lower frequencies to produce the same volume level.

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

For video projectors, the decibel scale is used to measure how much sound is produced by the cooling fan. A video projector with a fan noise rating of 20dB or less is considered very quiet.

How to Measure Decibels

One way that decibels can be measured is with a portable sound meter. Examples include the Reed Instruments Sound Meter, the BAFX Products Basic Sound Meter, and the Extech 407730 Sound Meter. There are also sound meter apps that work with the microphone in many smartphones.

Most home theater receivers have built-in test tone generators that you can use to determine the generated decibel level for each speaker. When all your speakers register the same decibel level at a given volume level, your sound listening experience will be balanced.

Measuring Decibels Without a Sound Meter

Many home theater receivers have an automatic speaker/room correction setup system that doesn't require the use of a separate sound meter. A microphone is provided that plugs into the front of the receiver. The receiver sends out test tones to each speaker, which is picked up by the microphone and sent back to the receiver.

The receiver then determines how many speakers there are, the distance of each speaker from the listening position, and the size of each speaker. Using that information, it calculates the optimum speaker level relationship between the speakers (and subwoofer) along with the best crossover point between the speakers and the subwoofer.

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