The frequency response of a speaker is one of the most basic and important measurements. It helps tell how well a speaker reproduces the bass, midrange, and treble sounds being fed into it. Frequency response will also tell you if the speaker is accentuating or diminishing any of those frequencies (i.e. maybe the speaker is playing the low sounds too loud relative to how it’s intended).
The unit of measurement of how many cycles a soundwave makes in one second is known as the hertz (Hz). Most people can hear sounds as low as 20Hz and as high as 20,000Hz (20kHz). Any speaker advertising a frequency response range beyond that may be overkill for what you need.
Assessing a Speaker’s Frequency Response
There are extremely precise technologies and best practices today to test the frequency response of a speaker by removing room reflections or other measurement artifacts from the resulting graph. The equipment, software, room setup, and knowledge of the test technician are critical to acquiring accurate measurements.
A frequency response graph, also known as a chart or curve, will show frequency measured in Hz on the X-axis, often 20 to 20kHz and amplitude measured in dB SPL on the Y-axis. The total range from the lowest dB to highest is best when the total Y scale is 50 - 60 dB.
A frequency response graph shows how a speaker is handling each of the individual frequencies put into it. The best theoretical graph of a full range speaker would be a flat line– indicating that the speaker played every frequency at exactly the same level. There is no perfect speaker and there are no perfectly flat frequency response graphs. Each speaker model has its own unique graph.
A graph of a subwoofer may look like a hill on the left side of the graph (where the lowest frequencies are) and then tail off at higher frequencies. This can be illustrated in the example below.
Wide frequency band (or full range) speakers may offer up a wide range of frequencies from a single driver, but you may not get much response in the lowest or highest frequencies.
A tweeter’s graph may resemble a plateau that rises as the frequency goes up–kicking in around a couple thousand Hertz–and then extending to 15,000 Hz or higher. Take a look at the graph below.
Some special-purpose speakers have nowhere near a flat frequency response graph, guitar speakers for example (see the image below). They can have output peaks between 2.5kHz and 4kHz to provide an edge to the sound quality, then fall off above that (which helps guitars keep from sounding “fizzy” on the highest chords). A guitar speaker doesn’t need to produce much sound below 100 Hz.
The speaker for a drive thru or ordering system kiosk, which mostly will be reproducing voices, better have a frequency response between 300 and 3000 Hz to provide good intelligibility.
The speaker for an alarm device may only really need to hit one frequency–so it’s frequency response graph could look like a steep peak in one narrow frequency band.
Getting On the Same Frequency
People often look at the frequency response range to decide the quality of a speaker. It’s important to know that your speaker manufacturer is providing data which is accurately measured and shown as a graphic. Often, if a speaker brags that it has a super smooth (or flat) response graph, be wary that the curve was drawn by the marketing department and not the engineers. To make a well-informed decision about which speakers to use in your audio system, it’s important to understand what the frequency response curve is telling you and that it was created under professional test conditions.