Common Misconceptions: Specifications - Frequency Response
Most people believe that the specifications of a piece of audio equipment define how well it can reproduce sound. That would be accurate to a certain extent, but it is not necessarily true. Most consumers read the labels on the product, and they assume the quality of sound reproduction based on a certain rating listed. The most cited specification of audio is the frequency response. It essentially tells a user how many frequencies an audio product can reproduce. Human hearing is regarded to be from 20Hz, very low bass tones, to 20,000 Hz, the highest treble. Based on that logic, most consumers assume that a speaker (or pair of headphones) will sound lifelike if it's frequency response is able to cover that whole range. Realistically, on the other hand, the frequency response is not the best way to gauge the sound quality produced through the audio equipment.
Paul Dicomo, Marketing Manager of Polk Audio, explains frequency response in a way such that the common person can understand:
" You don't want the speaker to change the "mix" of tones; that would ruin the timbre of voices and instruments, making them sound unnatural. Ideally, you want the sounds that are on the recording to be reproduced as they were recorded, without the speaker changing the sound. To say it another way: if you made a recording of all the audible tones at the same volume and played that recording through a speaker, you'd want all the audible tones to come out at the same volume. In fact, that's one way of measuring speakers. A signal that's comprised of all frequencies at equal volume is fed into a speaker that sits in a room with no reflective surfaces. A calibrated microphone is placed in front of the speaker and feeds the speaker's output into a machine that plots the frequency vs. amplitude as shown in Figure A.
Paul Dicomo, Marketing Manager of Polk Audio, explains frequency response in a way such that the common person can understand:
" You don't want the speaker to change the "mix" of tones; that would ruin the timbre of voices and instruments, making them sound unnatural. Ideally, you want the sounds that are on the recording to be reproduced as they were recorded, without the speaker changing the sound. To say it another way: if you made a recording of all the audible tones at the same volume and played that recording through a speaker, you'd want all the audible tones to come out at the same volume. In fact, that's one way of measuring speakers. A signal that's comprised of all frequencies at equal volume is fed into a speaker that sits in a room with no reflective surfaces. A calibrated microphone is placed in front of the speaker and feeds the speaker's output into a machine that plots the frequency vs. amplitude as shown in Figure A.
Now take a look at the graph in Figure B. That's the frequency response of the Erehwon Model 10, with drivers and tweeters made of pure Unobtainium ("Half the carbs, all the sound!"). The flat line on the graph indicates that the speaker is "flat"; it reproduces all the musically relevant tones at the same volume. That doesn't mean that a "flat" speaker will play all recorded sounds at the same volume -- bear with me here -- it means that it will treat all sounds equally; it won't impose its will on the music but will allow you to hear the music as it was recorded. Flat is good. Flat response means that the speaker reproduces sound accurately.
Too bad that the Erehwon Model 10 doesn't really exist, and neither does Unobtainium. Today's technologies allow speaker designers to get closer to the "flat" ideal than ever before, but they still fall far short of "perfection." So if a frequency range spec is not adequate, what is?
Frequency Response in Context
A big improvement would be a frequency response number that also includes the amplitude tolerance, expressed as "XHz-YkHz +/- 3dB." This tells you that the amplitude of the speaker's response relative to frequency does not deviate more than 3 Decibels from the center line. The "plus or minus 3dB" spec is regarded as a standard of sorts. The theory is that 3dB differences are "just perceptible," so a speaker whose response curve lies within that tolerance window is a reasonably accurate speaker. Let's see if that idea holds water.
Take a look at Figure C. This speaker has response that can be specified as 20Hz-20kHz +/- 3dB. Take a look at Figure D; it, too, can have the exact same specification as Speaker C! Do you think they will sound similar? NOT! They won't sound even remotely like one another. Speaker C will have "one note" bass and will make voices and other instruments sound unnatural, but Speaker D will sound smooth and more natural.
If I had to choose strictly by the response curves, I'd choose speaker D because its amplitude variations are smoother and gentler. In contrast, speaker C's amplitude variations are more extreme and "spikey." Experience has shown speaker designers that those rapid changes in response produce a sound that is more fatiguing, less pleasing and subjectively less accurate.
Now look at the response of the speaker in Figure E. This speaker exhibits a smooth response curve with low amplitude variations so you'd expect a fairly natural sound; however, the bandwidth of these errors is very broad, and experience has shown us that even low volume variations are audible if they cover a broad range of frequencies. In this case, Speaker E would have rich bass, prominent treble and be somewhat recessed or "laid back" in the midrange. Audiophiles call this "The Smile Curve." It's not the desirable trait it sounds like but it's a very "sellable" trait to naive buyers.
My Response to Frequency
Now that you know the importance (and limitations) of amplitude variations in frequency response graphs, you might ask: "does the frequency range tell us anything at all?" Yes, it does. As long as you know the amplitude tolerance (+/- 3dB), the frequency response range or width tells you how high or low the speaker goes. A speaker rated as 20Hz - 25kHz +/- 3dB will play lower bass and higher treble sounds than a speaker that measures 40Hz - 20kHz +/- 3dB. I wouldn't bet money that it would be the better, more enjoyable speaker, but at least I'd know something of value.
And now that you know how to interpret these numbers, you're ready to run right out and buy a speaker just by looking at the response curve, right? I wouldn't recommend it. Despite many advances in technology over the past 20 years, frequency response measurement is an imperfect science. The same speaker measured by two different labs may yield different response graphs. And some companies just plain cheat when they publish response curves. If it looks hand drawn, it probably was. ( Yes, the graphs were hand drawn for illustration purposes.) "
Source: [eCoustics.com - Understanding Frequency Response]
And now that you know how to interpret these numbers, you're ready to run right out and buy a speaker just by looking at the response curve, right? I wouldn't recommend it. Despite many advances in technology over the past 20 years, frequency response measurement is an imperfect science. The same speaker measured by two different labs may yield different response graphs. And some companies just plain cheat when they publish response curves. If it looks hand drawn, it probably was. ( Yes, the graphs were hand drawn for illustration purposes.) "
Source: [eCoustics.com - Understanding Frequency Response]