SPL vs Frequency Graphs
Well earlier I put up a note in a thread saying if someone needed an explanation on what a SPL vs Frequency graph means and how to read one I got a couple replies so I figured instead of answering a few PMs I would just create a thread. If there are any mistakes please correct me as I am in no way a expert, but I feel like I understand the subject that seems to come up quiet often and that I should be able to express myself well enough for others to understand.
This will probably include some information that most posters will find common knowledge, but my hope is that someone who has never purchased a speaker before will be able to read and understand this.
Here is a picture of a graph I am talking about:
As you can see the X axis has frequency and Y axis has Decibel (dB). The frequency corresponds to the wavelength, which basically tells you the pitch of the note so 20,000hz is a very high pitch tone while 20 is very low (Human hearing is generally 20hz-20khz but lower can be felt). The decibel rating refers to the loudness of the sound and is on a logarithmic scale which means 20dB is not twice as loud as 10dB and a change of approximately 6dB is twice as loud or half as quiet depending the direction.
So what does this all tell us?
Well many reputable speaker manufacturers will give you a frequency response like this one: "Frequency Response: 55 Hz to 20 KHz (± 3 dB)." This lets us know that from the low end of the spectrum of 55Hz all the way up to 20KHz this speaker will play a frequency sweep (a tone that starts at one frequency and plays frequencies in between up to its limit or vica versa) without having a loudness difference of 3 decibels in either the positive or negative direction. Imagine a speaker playing at 90dB with this example on the whole given it will be no be within the range of 87-93dB.
Why does this matter?
This is important because a speaker that is not linear in the range needed could sound bad. Here is an example: Say you are listening to a CD and two people are singing. One is a man who is a bass with a very low voice and the other is a woman with a higher voice. Now say your speakers aren't very linear across the spectrum. This could cause the man's voice could be quieter or louder than the woman’s which just wouldn't sound right! While some smaller differences might not be audible a difference of 6dB could make one of the singers sound like a whisper compared to the other.
Some people are more susceptible to noticing these differences in loudness compared to others, but it has been shown that the smallest audible changes varies with the frequency but at about 1KHz it is 3 dB and at 35Hz is 9dB.
The human vocal range is quiet broad running from about 60Hz to 1600Hz that is coincidentally the most sensitive range of human hearing. Instruments play similar frequencies but are able to go both lower and higher depending on the type.
Acoustics:
Room acoustics will play an important role in how a speaker acts in your specific room. In general manufactures use an
anechoic chamber that will act completely different from your room! Proper placement of room acoustics will help you get a better response. Remember this if you are purchasing speakers room acoustics can help you as much as if not more than speakers so budget properly.
Acoustics do affect all frequencies but in different ways. With the high end you are correct the waves will reflect off walls and can cause a "harsh" sound or muddy the sound up, but this is not the same as what happens with lower frequencies. When one hears muddied dialog or a harsh sound this is from the same signal produced by a loud speaker hitting various reflection points then being reflected towards one ears at slightly different time. The brain has trouble separating all the information and it becomes muddied.
Room nodes (specifically how sound waves interact with a given room) play more a role in completely canceling the waves out or making them double up on themselves with the lower frequencies that can cause more nulls and peaks in specific response graphs.
Different frequencies respond differently to rooms due to wavelength size.
Other concerns/Information?
Frequency response isn't the only thing to look at when comparing speakers. Be sure to look at all other specs, but more importantly audition speakers, as many as you can, and if they sound good to you then they are what you want! (Again remember the showroom will sound different than yours)
Also, there are two types of frequency response, on axis and off. Most measurements you will see are on axis responses that are important, but nearly as important are off axis frequency responses. These are taken at a certain angle from the driver, but on the same plane. This is important if you are going to be listening to these speakers from across the room due to that important concept, room acoustics.
There are a variety of factors that cause differing plots in speakers from crossover design, driver selection, to cabinet design/bracing and much more. Pretty much anything and everything from the speaker or room can affect how linear its response is.
One last note on speaker linearity: This primer was written on a speakers frequency response with the idea that a linear response is more desirable that not. Speaker linearity is another term that refers to a speaker’s ability to increasingly powerful signals by generating louder tones proportionally to signal strength. Speakers differ in their ability to play linearly both on the upper and lower ends. A large linear response is desirable in this case as it makes for a more real performance as it will create the illusion of a musician being in your room rather than in the speaker itself.
Please let me know what you guys think and feel free to add to this or correct me! I am sure I left something out/made some mistakes.
Hope this helps and if you need more explanation let me know by posting or through PM and I will get back to you ASAP.
Lastly, thanks to Swerd for his corrections/additions.
