Understanding Frequency Response

Frequency response is a fundamental audio characteristic – here’s why it matters

What we call sound is caused by variations in air pressure. This is like the motion of ocean waves, except that instead of having crests and troughs of water, we have crests and troughs of air pressure. But not all sounds are alike. Some are bassy, some are shrill; some are loud, like thunder, and some are soft, like rain falling.

We can classify these sounds by their level (commonly referred to as volume), and their frequency. Frequency measures the changes in air pressure. If these pressure changes occur many thousands of times in a single second, that’s considered a high-frequency sound. If the air pressure changes occur at a slower rate—say, only 40 or 50 in a second—that’s a comparatively low-frequency sound.

Because of the wave-like motion of sound, each “wave” (the crest and trough) is called a cycle. We measure frequency by counting how many cycles occur in a single second; this gives us a figure in cycles per second. Several years ago, the term “cycles per second” was replaced with the single word Hertz (abbreviated Hz), to commemorate Heinrich Rudolph Hertz (1857 – 1894), a scientist who contributed much to the subject we’re discussing. So, whereas it is correct to refer to a “100 cycles per second” tone, we would more likely call it a “100 Hertz” tone. For expressing higher frequencies, we can use the term kiloHertz (abbreviated kHz), which stands for 1000 Hz. Thus, a 1000 Hz tone has the same frequency as a 1 kHz tone.

Level defines a sound’s loudness or softness. So, if we know the frequency and level of a sound, we have a rough idea of the type of sound we’re talking about (in practice, though, sounds are complex, and can be the sum total of numerous frequencies at numerous levels).

Now that we’ve defined our terms, let’s turn to the subject of frequency response.

Frequency response is a characteristic we usually associate with a particular piece of audio equipment. Since everyone has a set of ears, that’s a pretty universal piece of audio equipment to examine first. Our ears respond to frequencies over about a 10 octave total range, from approximately 20 Hz to 20 kHz; but unfortunately, these figures only hold true for the ears of a relatively healthy youngster. As we get older, our ears lose their ability to respond to high frequency sounds. So, at a very advanced age we could have a response that only extends up to 5 or 6 kHz. (Our ears are also less sensitive to lower frequencies at low volume levels; however, at high volume levels the response of the ear becomes more uniform.)

Since most ears respond differently to different frequencies, this is an example of an uneven frequency response.

Now let’s take this concept a step further. If the ear was a perfect listening machine, then if a sound source (loudspeaker or whatever) produced tones from 20 Hz to 20 kHz at exactly the same level, then our ears would respond equally to these tones; the high frequency ones would sound just as loud as the low frequency ones. This would be an example of flat response—i.e., the response would be even throughout the audible frequency range.

But as already noted, there’s a tendency for the ear to lose treble response as it ages, which means there’s a deviation from flat response. Also, the aging effect is not the only problem we have with our (nonetheless marvelous) hearing mechanism. The ear also has a different frequency response at different sound levels. At fairly low listening levels, the ear responds less to very high, and very low, frequencies. On the other hand, at high listening levels the ear’s response is much flatter, although it’s still not flat. So much for the problems inherent in our hearing! It would be helpful if these were the only problems we had to deal with, but unfortunately, there are also other trouble spots in the audio signal chain.

The top image shows a frequency response curve that’s very close to a flat response. The image below is typical for moderately-priced loudspeakers – note the frequency response peaks and dips.

A speaker never has a flat frequency response. No matter how much you spend, every speaker will deviate from an ideal response. For example, at very high frequencies a loudspeaker has to create very fast variations in air pressure; but the mass of the speaker’s cone, friction, and other error sources make totally accurate high frequency reproduction difficult. At the other end of the audio range, reproducing low notes at high volume levels requires moving large amounts of air. Even a 15″ speaker can have trouble moving enough air to generate massive air pressure changes.

A typical loudspeaker will have a frequency response that rolls off towards both the extreme high and low ends. And that’s not all: resonances (response anomalies) in the speaker and speaker enclosure itself can cause deviations in the midrange response. To complicate matters even further, the room in which you are listening to the speaker will also alter the sound. A room with many hard surfaces (concrete, glass, etc.) will bounce high frequencies around and make them appear more prominent, while a thickly carpeted room will absorb many of the high frequencies.

And we’re not done yet! Microphones and headphones also introduce their own deviations. Amplifiers don’t have perfect frequency responses either, but compared to our ears (or loudspeakers), they are very good. Many amplifiers can reproduce tones from 20 Hz to 20 kHz, or even 100 kHz, with ruler-flat response. Generally, the amp will not be the weak link in an audio system.

We’re reaching the moral of the story: with so many variables between the sound source and the listener, we have to do something to keep the chaos to a minimum. So, we strive for audio systems with the flattest possible frequency response. Then, the only variables left are the ears and listener’s acoustic environment, and the listener can compensate for those issues at least partially with tone controls.

Professional recording studios spend a small fortune on monitor speakers, acoustic treatment, microphones, and more to ensure as flat a frequency response as possible in the recordings they produce. If that recording plays through a listening system with a flat frequency response, then the recording will be heard as the studio intended. But if the studio’s loudspeakers exaggerate the high frequencies, then the highs won’t be mixed properly, because the speakers will make you think the highs are more prominent than they are. So, if played back over a system with flat response that doesn’t exaggerate the highs, the high frequencies will sound deficient. For this whole process to work properly, systems need to have flat frequency response, both at the recording and playback ends.

Since it’s impossible for all systems to have a flat frequency response, it’s important to create music that sounds good on a variety of systems. Recording studios will often have small, imperfect “real-world” speakers right next to their standard, high quality studio monitors, thus making it easier to create a recording that sounds acceptable over both types of speakers. This may require a slight compromise—for example, the sound might be a shade too bright on the good speakers and not quite bright enough on the real-world speakers. Nonetheless, this is a better option than having the correct sound on one system, but sound that’s way off on a different system.

So the bottom line is now you understand what frequency response is, why a flat frequency response is the standard to which recordings aspire, and why this helps hit the sonic “sweet spot” over as wide a range of systems as possible. For more real-world applications of frequency response, as well as how to measure it, please see the article about the decibel.

Feature image photo credit: Ingo Schulz