It’s impossible to shop for audio products without confronting specifications—numbers included in web pages, brochures, and ads to lend scientific cred to marketing claims. In making their buying decisions, some shoppers rely heavily on specs, but many ignore them entirely. Considering that many marketing claims about sound quality are written by advertising pros who’ve never heard the products, it’s rational to seek out a more objective assessment. But while specs promise this, they rarely deliver.
As one of the few journalists who performs scientific measurements of audio gear and conducts blind listening panels to confirm the validity of those measurements, I can share a unique perspective on this topic. For this column, I’ll examine different categories of audio products and explain which specs are meaningful and trustworthy.
Decades of research show that frequency response is by far the most relevant spec for speakers. Frequency response tells us how consistent a speaker’s volume remains as the frequency of sound changes. It’s usually given as a plus/minus figure in decibels over a certain audio range—such as “40 Hz to 20 kHz, ±3 dB.” Unfortunately, how this spec was produced is almost never explained, so it’s rarely reliable.
What is reliable is when a manufacturer shows a chart of how the response changes with frequency. On this chart, you’re looking for a horizontal line that’s as flat as possible. If the line contains large peaks and valleys, the speaker won’t sound natural. On the left side of the chart, you can see how the speaker’s output decreases as the frequency goes down, which will tell you how well it can reproduce the 41 Hz low E note of a double bass—usually the deepest tone you’ll hear in an acoustic jazz recording. Extra points if the chart is labeled “CTA-2034,” because that’s the very latest standard.
With passive speakers (ones that don’t have an amp built in), you need to be concerned about two more specs: impedance and sensitivity. Impedance, measured in ohms, tells how much resistance to electrical current a speaker presents; the lower the impedance, the more current a speaker needs. Almost any amp can drive an 8- or 6-ohm speaker, but a 4-ohm speaker demands an amplifier rated into a 4-ohm load. Sensitivity tells you how loud a speaker will play with 1 watt of power; speakers with sensitivity above about 86 dB can be driven by most amps, but speakers with lower sensitivity will sound best with 80 watts of power or more. Speakers with sensitivity above about 92 dB are necessary if you have an amp rated at 20 watts or less.
With amplifiers, the most important spec is power, which tells you how much output the amp can produce from your speakers. Most amps for home use are rated at 30 to 100 watts. Every doubling of watts gets 3 dB more sound out of your speakers, so a speaker rated at 86 dB sensitivity with 1 watt will produce 89 dB with 2 watts, 92 dB with 4 watts, and so on. If your amp and speaker combination can get you over 100 dB, that’s enough for most jazz listening.
Amplifier power is rated in ohms, to match the impedance of your speaker. Most amps are rated into 8 or 6 ohms, and for most speakers, that’s fine. But if your speakers are current-hungry 4-ohm models, be sure to buy an amp rated to drive 4-ohm speakers—especially if you like to play your system loud.
As with speakers, frequency response is by far the most important spec of a headphone. Usually, it’s just specified as something like “10 Hz to 23 kHz,” which, as with speakers, is completely useless. Again, a chart is what you need. Headphone frequency response charts are notoriously difficult to interpret—Google “Harman curve” if you want to dig deeper—but in my opinion, the fact that a manufacturer publishes a headphone’s frequency response chart at all indicates competence and integrity.
Sensitivity is important to check with headphones. Most headphones can play loudly when connected to a tablet or computer, but some audiophile models need more power. If your headphones have sensitivity rated 95 dB or lower, it’s probably best to use them with a separate amp.
Becoming a real expert in audio specifications probably requires about as many hours as it takes to become a skilled pianist, so don’t worry if some of this seems overly technical. Just stick to the simple guidelines above and you’ll stand a stronger chance of getting audio gear that works well for you.
Extra: What’s a Decibel?
A decibel can specify electrical levels (like voltage) or sound levels. Decibels are logarithmic, so a 10-dB increase means 10 times the original amount. Thus, a trumpet blowing at 90 dB produces 10 times the sound pressure of a trumpet playing at 80 dB. But because our ears hear in a logarithmic fashion, a 10 dB increase sounds only twice as loud.