Let's Talk Frequency Response

[Peter P.]

When a manufacturer specifies a frequency response for a speaker, particularly the low end, are they referring to the natural roll off of the speaker or have they put a high pass filter on the woofer so it doesn't strain to reproduce frequencies below what the engineers designed it for? Does the approach vary based on how much the speaker costs? What does Klipsch do? Is it pretty obvious and easy to look at a crossover and tell the difference?

 

I've been wondering whether it would be beneficial to add a high pass filter to all my speakers' input terminals to filter out any frequencies below the speaker's low end cutoff.

[mattSER]

In my experience, the nicer models such as the RF-7, CF-4, and KLF-30 seem to have a specific high-pass filter used in conjunction with tuned ports to achieve the desired response. 

 

It's very clear on the RF-7 as it plays smoothly down to 32hz and then cuts off almost immediately, pointing to a sharp LF cutoff in the crossover.

 

A different technique is observable in the KLF-30. It seems to have a high-pass around 50hz with a very slow roll off combined with ports tuned to 30 or 35 hz in an attempt to flatten response down into the 30hz range.

 

Cheaper models, such as Synergy bookshelf models seem to only rely on natural roll off. You can play a 15hz test tone and watch the woofers pump like crazy with no audible results. 

[pzannucci]

No filtering should be used when doing frequency response analysis unless your speaker can not produce and will be damaged by lower frequencies.  Frequency sweeps should be at least 20hz-20khz.  Your response will be for the whole range but with smaller speakers or speakers which can not produce low frequencies, adding a high pass filter to keep the speaker from distorting when playing too loud out if it's (box) design range, may be beneficial and clean up the available frequency response.

 

Forcing a speaker to play below it's designed low frequency limits will cause distortion and possible destruction of the lower frequency units. 

[Chris A]

The problem is always with ported ("bass reflex") bass bins. Without some sort of woofer excursion protection, either electrically or mechanically within the woofer itself or by use of a passive radiator, the woofer can unload if fed high amplitude signals below its designed port frequency.  This is a typical situation when playing warped records on turntables/phono preamps without infrasonic filter protection.  It saps the amplifier channels of available power, introduces large amounts of modulation distortion, and can wind up damaging the woofer.

 

The use of high pass filters (either electrical, or mechanical via driver or box/porting design) always results in undesirable phase and group delay response shifts for the woofers. 

[garyrc]

11 hours ago, Chris A said:

The problem is always with ported ("bass reflex") bass bins. Without some sort of woofer excursion protection, either electrically or mechanically within the woofer itself or by use of a passive radiator, the woofer can unload if fed high amplitude signals below its designed port frequency.  This is a typical situation when playing warped records on turntables/phono preamps without infrasonic filter protection.  It saps the amplifier channels of available power, introduces large amounts of modulation distortion, and can wind up damaging the woofer.

 

The use of high pass filters (either electrical, or mechanical via driver or box/porting design) always results in undesirable phase and group delay response shifts for the woofers. 

 

Do speakers with horn loaded bass unload below their lowest good bass limit?

 

[Chris A]

Not if they're closed box (acoustic suspension) enclosures behind the driver...which most horn-loaded bass bins are.  They behave like closed box woofers, which typically have a 6 dB/octave drop in response below their closed box resonant frequency. 

 

Chris

[derrickdj1]

Chris I always thought the majority close speakers/sealed had a 12 db roll off or 2 order?  This is typical in a 2 way design.  I have only read about one speaker that used a 6 db roll off.  Please explain.

[Chris A]

That's correct - 12 dB/octave for sealed enclosures. Without losses, it's 6 dB/octave.

[Peter P.]

A lot of interesting input. Thanks!

 

The mention of the 12dB/octave for an acoustic suspension speaker interests me because I just built a simple first order, 6dB 120Hz high pass filter for my 6.5" woofer acoustic suspension speakers as I'm running a subwoofer. From what I've read, acoustic suspension designs have greater natural low end extension (6dB/octave) than ported or passive designs (12dB). My filter was just a novice attempt using the easiest method with available parts but I'm intrigued with trying a 12dB filter at the same cutoff frequency.

 

I figured manufacturers would install the high pass filter on the woofers to protect them from excessive excursion and possible damage, as well as distortion control. But they cost money, so I could imagine some companies leaving them off to meet a price point. It sure would be nice to know when you read the spec sheet, though.

[JohnA]

On 11/27/2016 at 11:37 PM, Peter P. said:

When a manufacturer specifies a frequency response for a speaker, particularly the low end, are they referring to the natural roll off of the speaker or have they put a high pass filter on the woofer so it doesn't strain to reproduce frequencies below what the engineers designed it for? Does the approach vary based on how much the speaker costs? What does Klipsch do? Is it pretty obvious and easy to look at a crossover and tell the difference?

 

I've been wondering whether it would be beneficial to add a high pass filter to all my speakers' input terminals to filter out any frequencies below the speaker's low end cutoff.

 

Pro audio speakers are often specified with a frequency response and an f3 (3dB down point) or f10 (10dB down point); i.e. 50 - 15k, f10 40 Hz.  Home speakers are undoubtedly tested the same way, but the full spec is not stated.  I have also seen speakers spec'd at 20 - 20k, but when/if you see a response curve, it might be down 40dB at 20 Hz and 20k Hz. Without a specified fx frequency, you cannot know what the lower response of the woofer is. 

 

Since any filter affect frequencies well above it's -3dB point, I am sure no manufacturer inserts a high-pass filter when testing a speaker's low bass response. 

 

You are expected to care for your speakers and not overdrive them. 

 

I have never seen a home speaker with a high pass filter to protect the woofer from notes below its designed low end response.  It is *sometimes* done in pro audio subwoofers.  Many home subwoofers also have electronic EQ, limiters and high-pass filters built into the amp.  I would not add an outboard high pass filter to any speaker's input terminals.  You would need to calculate and then test the interaction between the added bass filter and speaker's crossover network.  It is far better to use an electronic crossover between your amp and pre-amp, or the built in high-pass in the pre-amp/processor/receiver, if it has one, and feed the subwoofer from the subwoofer outputs.  Almost all subwoofers have an adjustable high pass for the mains, as well.  You can use that, too. 

[russ69]

In most cases my understanding is that the woofer response just rolls off. The curve is pretty steep. I'm sure the designers would let the speaker go down to 0 hz if it still had some response down there, but they don't, they roll off.  I'm not sure how you would over drive a speaker that is 20 db down unless you have the gain set to crazy levels. 

[pzannucci]

If you look at some woofers, they are unloaded and flapping around at very low frequencies.  No appreciable output so 20db down isn't hard to do.  Keep doing that and the woofer will destroy itself.

[russ69]

15 hours ago, pzannucci said:

If you look at some woofers, they are unloaded and flapping around at very low frequencies.  No appreciable output so 20db down isn't hard to do.  Keep doing that and the woofer will destroy itself.

My point is that to get any output at low frequencies you would be adding a lot of gain, much louder than normal levels but you are right the woofer can only take so much and it is asking for a lot of power that you may or may not have. . 

[pzannucci]

Actually it doesn't take all that much power, with no gain, to get the woofers flapping around with a typical amplifier and it's normal power rating.  Once you go beyond mechanical xmax you get in trouble and on a lot of high efficiency woofers, xmax is quite low since part of the efficiency is to have as much of the voice coil in the highest saturation of the magnetic field.

 

russ69,

Your point is correct though more times than you would think, isn't the cause.

[Axz Hout]

On 11/29/2016 at 10:41 AM, John Albright said:

 

 

 

You are expected to care for your speakers and not overdrive them.

 

 

T

[avguytx]

In the almost 30 years I spent in home a/v retail and as a manufacturers rep, I don't recall any brand of home speakers that used a passive high pass filter on the woofer side. It's either a person has common sense with the "throttle" or they don't.