Building a vacuum tube amplifier can be expensive. Transformers and power output tubes can be especially pricey. For the audiophile working on a home system, referring to sound pressure levels for loudness can save you money and save your hearing.

Consider building a lower-power system. Components such as transformers and output tubes used in lower-power amplifiers are significantly less expensive. Besides costing less, lower-power transformers are physically smaller, requiring less chassis space. Tube amplifiers with lower-power output draw less current and have lower electrical utility costs.

Amplifier power output does not affect sound. An amplifier with 10 watts of output power will sound the same as a 100-watt amplifier. The difference is loudness. Rather than using amplifier output watts as a gauge for loudness, use sound pressure levels (SPL).

When considering sound pressure levels, take into consideration how your ears respond to them. Using very low SPLs of around 40 to 50 dB as reference, to double the loudness you need to increase amplifier power four times. As you increase amplifier power, your ears will start to compress the sound in response to the increased SPL. At higher sound pressure levels, it will require eight times the amplifier power to double the apparent loudness. In the average living room, depending on the efficiency of the speakers, a comfortable background listening level only requires around a half-watt of amplifier power.

The following is a comparison of everyday sound pressure levels based on average published values.

Sound pressure levels above 85 dB are high enough to cause some hearing loss, depending on exposure time. The higher the SPL, the greater the chance of hearing loss. At an SPL of 100 dB, you really should be wearing ear protection. At 120 dB without ear protection, the chance of hearing damage is high, even for short periods of exposure. At 140 dB, ear damage will most certainly happen without ear protection. If you would wear ear protection out in the environment, then why would you want such a high SPL in your listening room? Unless, of course, you plan to wear ear protection.

The sensitivity of speakers is usually specified as "so many dB @ 1W/1m" (1 watt at 1 meter) from the speaker. For example, 93 dB @ 1W/1m. Higher sensitivity speakers are more efficient and require fewer amplifier watts for a specific sound pressure level. A pair of speakers with a sensitivity of 96 dB @ 1W/1m requires less amplifier power than speakers with a sensitivity of 93 dB @ 1W/1m. Assembling an audio system using 85 dB as a reference for maximum SPL will provide substantial loudness. If paired with the right speakers, 12 amplifier watts can provide an SPL of 85 dB 20 feet from the speakers.

On crownaudio.com, you can use the 'Amplifier Power Required' calculator to determine amplifier power output. The primary factors are the desired SPL level at listening distance from the speakers and speaker sensitivity. Enter the listener's distance from the source (speaker) in meters. For 10 feet from speakers, enter 3.05 meters; if 20 feet, then enter 6.1 meters. Enter the desired SPL level in dB at the listener distance, for example, 85. Enter the sensitivity of the speakers, for example, 93 (93 dB @ 1W/1m). For amplifier headroom, enter 3 dB to accommodate audio peaks. Entering those parameters, you should find that for 10 feet from speakers, it only requires 3 watts of amplifier power. For 20 feet, 12 watts of amplifier power.

For music, a normal SPL listening level is around 70 dB; 80 dB is loud. Sound pressure levels above 85 dB can cause hearing loss, especially with long-term exposure. A home audio system designed to produce an SPL of 80 dB to 85 dB at ten feet (or twenty feet in larger rooms) from the speakers should be sufficiently loud. Over time, you will save money if you pay a little more for more efficient speakers and drive them with a lower-wattage amplifier. Besides the savings in amplifier costs, lower-power amplifiers draw less current, which means lower electric power costs. Vacuum tubes used in lower-power amplifiers are less expensive, a savings each time you replace tubes.

Loudspeaker distortion increases at higher volume levels. At lower listening levels, the average speaker system has one to two percent distortion. Besides placing more pressure on your ears, higher amplifier levels will push speakers higher into distortion. From an audiophile point of view, keeping the SPL level within reason equates to a better, cleaner listening experience.

More information:
Noise-Induced Hearing Loss
Hearing loss and music
Loud Noise Hearing Loss
Speaker Distortion