# Radioshack SPL questions



## Sphinx (Feb 29, 2012)

I have an anolog RS SPL meter that I bought clearance because of discontinuation. I decided to try it out last night.

I set C weighting.
I set slow setting.
SPL meter is attached to tripod set to ear level at main listening position, mic pointed at television.
Used 1 minute 10 hz range sweeps from my laptop connected to my avr.
Recorded by hand the frequency response from 20 Hz to 100 Hz.

Am I doing this correctly?
I will look up the model number when I get home, but what is the appropriate correction tables to use for the analog SPL meter? I have seen different tables on different websites.

What is the best frequency and dB to use as a baseline? I used 80 db at 40 Hz, then measured all other frequencies from there.

Thanks for any and all input.


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## jdent02 (Jan 13, 2012)

Hi Sphinx,
One's probably as good as the other. The Radio Shack meter is well known for each unit having a slightly different response....... So even with the comp tables it won't be 100% accurate.

If I remember right they are fairly linear above 30 or 40Hz so if you're running an 40Hz tone it should be reading 'correct' within a dB or two.


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## Sphinx (Feb 29, 2012)

Thanks Jdent02

I was playing around tonight using the spreadsheet in the Downloads section and burnt the cd test tones there also. Here is the native in room response. Gain: 10oclock 20 Hz Tune LPF: 80 Hz. Mains: Small 80 Hz

I have jpeg of the graph from the spreadsheet. What is causing the major hump and dips? I will measure my room dimensions and get back to you.


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## jdent02 (Jan 13, 2012)

Hi Sphinx,
How familiar are you with acoustics? Specifically room modes? Figured I'd better ask so I don't rattle off stuff you already know


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## Sphinx (Feb 29, 2012)

I understand it in concept but it is always good to hear it again in a new way. I just don't have the math or calculator in front of me to calculate it. Distance between couch and sub (on opposite wall) is about 13-14 ft. The living room is about 25 ft wide with large opening to stairs and kitchen. Ceiling is trapazoidal shape between 8 ft and 11 ft tall.

What effect would the 20 hz tuning (1 of 3 ports plugged)? Why wouldn't the peak be at 20? It appears to be at 23.

The sub also has 25 hz and 16 hz tunings. Has anyone had experience with these?


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## jdent02 (Jan 13, 2012)

Basically room modes are where bass waves of a common frequency interact with each other and modulate themselves in some way. One wave is coming from the woofer, the other is reflecting off the wall (or floor, ceiling, etc...). When they interact, if they are in phase (the waves are the same shape), they amplify in volume. If they are out of phase, they cancel each other out. Different frequencies will interact at different locations, which is why you can hear the 'sound' of the bass change as you move around the room. From the looks of you graph you've got a big room mode at about 24Hz and a smaller one at 50Hz, and then a slight room null in the 30's. You can get rid of the boosted frequencies by either moving the sub or using an EQ to reduce it. Just keep in mind that if you move the sub you might end up with diferent frequencies being boosted or cancelled. Because bass waves are so long in length, it can be really tricky to find the right spot for your sub. I tried several places in my room and the 'best' option still had a sharp room null at 45Hz.

As far as the tuning mode, that mainly determines what the frequency response of your sub will be, not necessarily where it will have peak output. Since you have that 24Hz room mode, bass sounds a lot louder there than it otherwise would. Changing the tuning point to 20Hz or even 16Hz won't get rid of the room mode, but being that the sub has less output in the mid 20Hz range with either of those modes, it probably won't be as loud.

Just for some comparison (sp?), with no EQ'ing and in 20Hz tuning my PC-Plus had a pretty noticeable boost in the 22-27Hz range, and it dropped off like a rock under 20Hz. Then I tried it in 16Hz mode. That reduced the bump and gave me a lot more energy under 20Hz. The remaining mid 20Hz bump I filtered out with a BFD.

Wow, I'm waving my geek flag high............:blink:


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## Wayne A. Pflughaupt (Apr 13, 2006)

Hey Sphinx,

You’ve got a 30 dB swing between your worst peak and trough. That’s more than can be reasonably equalized. I’d suggest trying some different locations to smooth things out.

Regards,
Wayne


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## Sphinx (Feb 29, 2012)

Thanks for the response Wayne.

Unfortunately, as this is our living room, my options are severly limited (3 possible) I will test the other two again this weekend. Initial selection was picked for convenience and protection (18mo hands and toys).

The sub has 3 tuning options, so I will try and run the other two options also and see if that helps. Maybe the max output 25 Hz tune will help or max extension 16 Hz tune.

I just have a budget Onkyo which only has Audyssey 2eq, but I will run that also to see how effect it is at leveling things out.

Wayne,

What is the most I can resonably expect to EQ out with say one of the <200 parametric EQs out there?


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## Wayne A. Pflughaupt (Apr 13, 2006)

> The sub has 3 tuning options, so I will try and run the other two options also and see if that helps.


 The tuning options will only affect extension. Won’t do a thing for your peaks and troughs.



> What is the most I can resonably expect to EQ out with say one of the <200 parametric EQs out there?


The price of the equalizer has little relevance to its effectiveness. Typically, you can’t realistically equalize more than a 12-15 dB variance from flat response (cutting vs. boosting makes no difference). IOW, 12-15 dB max between the worst peak and trough. So, you can apply a ~12 dB cut to a single big peak, if that’s what your particular room has, or boost a big 12 dB trough, or a combination of boosts and cuts (if you have peaks and troughs like you do) that total no more than ~12-15 dB between the extremes (e.g. worst peak that needs a 5 dB cut and worst trough needs a 8 dB boost). Naturally, you can apply other filters if they are needed, but the total “distance” between the extremes is what you’re concerned mainly with. That's why it's best to find the location with the best response before you start equalizing.

The reason is that equalizing taxes the sub’s headroom – boosting or cutting, makes no difference – and few subs can handle more than that amount of EQ before their usable output is reduced to – well, unusable. 

This is just a rule of thumb, of course. How much EQ you can apply ultimately depends on the capabilities of your sub system. You couldn’t reasonably EQ the typical HTIB sub even as much as mentioned above. Or, if you have four 18” monsters backed by 6000 watts of amplification in a 12’ x 12’ room - well something like that you could equalize to your heart’s content.

Hope this makes sense.

Regards,
Wayne


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## Sphinx (Feb 29, 2012)

Correct me if I am misunderstanding this:

The sub is currently in 20 Hz tune (One Plug). I thought that a ported sub at tuning has a boost in FR at tuning frequency. If I am set at 20 Hz, would some of the peak at 22-23 be attributed to the tune? If I were to switch to the 16 Hz tune (two Plugs) would that reduce some of the peak? This is just a hypothesis. I understand that the room has modes and nulls that I can't get around with current location.

I thought I read that Audyssey 2eq does not equalize bass whatsoever. I will research this more, but is someone knows definitively, please speak up.

Based on your suggestion. Would it be better to take ~8 off of peak and +4 to trough? Is this what you meant by that cutting one frequency too severely (more than 10-12 dB) could be detrimental to that frequency?


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## Sphinx (Feb 29, 2012)

I measured the sub in its three tunings at its current location. Afterwards I was demoing Hulk sonic cannon scene. It sounded great, then I told my wife to watch how different it sounded when I turned the sub down. I went into the AVR and trimmed the sub to -10 dB and the sub essentially turned off? When I turned the trim back to +3 dB, the sub didn't come back. All wires still connected, sub still on. I didn't touch the sub at all?

Any thoughts?

When the kids nap next, I will try location two hopefully.


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## Sphinx (Feb 29, 2012)

Location 2 and location 2 Sideways. This location was flatter but caused 3 different resonances in other rooms - dishes in kitchen, entryway light, plumbing in wall.

Location 3 to come later, possibly tomorrow.


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## Wayne A. Pflughaupt (Apr 13, 2006)

Sphinx said:


> Correct me if I am misunderstanding this:
> 
> The sub is currently in 20 Hz tune (One Plug). I thought that a ported sub at tuning has a boost in FR at tuning frequency. If I am set at 20 Hz, would some of the peak at 22-23 be attributed to the tune? If I were to switch to the 16 Hz tune (two Plugs) would that reduce some of the peak?


I don’t know enough about the ins and outs of port plugging to say for sure, but I can’t see how the open-port position would single-handedly generate a 15 dB peak



> Based on your suggestion. Would it be better to take ~8 off of peak and +4 to trough? Is this what you meant by that cutting one frequency too severely (more than 10-12 dB) could be detrimental to that frequency?


 Equalizing is not “detrimental” to the frequencies in question. 

Equalizer filters don’t affect a single frequency, they affect a _range_ of frequencies depending on how wide a path they cut (i.e. the filter’s “bandwidth” or “Q”). The graph below shows the electrical response of an EQ filter centered at 50 Hz. As you can see, frequencies between 30 Hz and 80 Hz are affected by this filter. 










Behringer FBQ2496 1/3-octave filter​

The “detrimental” component of severe cuts – or boosts - merely has to do with the amount of headroom it “costs”. You can see more about that  phrase , if you’re interested.

**************

So far your next-to-last graph looks best; reducing the 30 dB peak/trough differential to 20 dB.










You might try equalizing it with a -9 dB cut at 22 Hz and a 10 dB boost at 31.5 Hz and see what happens. If you can get through some demanding scenes (like the one you mentioned in Post #11) without the sub bottoming out, you’re okay.

Something to keep in mind though – a lot of high-performance subs these days have built-in limiters to keep their drivers from bottoming out. So instead of a rude noise when the speaker hits its excursion limits, you might find instead a “flattening,” or reduced low freq impact. (It’s probably easiest to judge this by listening to the sub only, with all the main speakers unplugged.) Whether or not that’s an acceptable trade-off to totally eliminating the peak and trough is something only you can decide. If you experience unacceptable limiting, an good compromise might be to reduce the gain of each of the two filters a few dB (say, -6 at 22 Hz and +7 at 31. Hz). This would still substantially reduce the two problem areas while leaving a bit more headroom.

Regards,
Wayne


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## Sphinx (Feb 29, 2012)

Thanks Wayne,

I didn't mean the whole peak was due to the tuning, just a portion of it. It appears this is true as the 16Hz tune generally has less range.

I will read the thread you linked. I want to learn more about the ins and outs. I do not have any parametric equalization currently. Once I know more about them, I might start looking for a used BFD or SMS or something. I don't have a ton of cash, so I am not sure if I will make it up to the SMS range.

I did move the sub to location 3 last night, which is nearfield. Replaced end table and shakes the couch a lot. It is very visceral. The sonic cannon scene was so much better there. The sub is playing relatively louder because it is 3 ft away instead of 12 ft. The FR isn't really any better, it is closer to location 1 than location 2's FR plot. But like I said, it is much more enjoyable.

I will post the FR later today.

Thank you for all your patience and helpful explanations and responses.


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## jdent02 (Jan 13, 2012)

Hi Sphinx,
Nice to see some of the peaks are getting smoothed out.

Wayne's right though that boosting a frequency in an EQ can rapidly eat up your headroom and lead to overloading the sub. SVS does have limiters in the amps but they are not that aggressive, and you can still bottom them out by boosting a frequency too much and playing the right material (found this out the hard way).


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## caper26 (Sep 10, 2010)

jdent02 said:


> Hi Sphinx,
> Nice to see some of the peaks are getting smoothed out.
> 
> Wayne's right though that boosting a frequency in an EQ can rapidly eat up your headroom and lead to overloading the sub. SVS does have limiters in the amps but they are not that aggressive, and you can still bottom them out by boosting a frequency too much and playing the right material (found this out the hard way).


Did you get that loud "POP" sound like 2 pieces of wood slamming together, or like a hammer hitting the cabinet from inside the sub??


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## jdent02 (Jan 13, 2012)

Hammer inside the enclosure. I'd been looking for a good way to describe it......


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## caper26 (Sep 10, 2010)

I hear ya. I am having that problem with transients (gunshots, engine backfiring, etc...)


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## Sphinx (Feb 29, 2012)

I have had the sub make a sound like that before. I describe it as a metal CLANK. Happens on quick loud events. For example, Iron Man 2 opening where he accelerates fast, or, even better, random movie with car accident that made the sub squeal.

I will try and take measurements of location 3 tonight. In this location the sub is adjacent to the couch, in what I would call a partial corner loading - there is a half wall, full wall, couch on the three sides. This can make the sub boomy at times but also allows the sub to really shake all of the listening positions like no other location yet.


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## caper26 (Sep 10, 2010)

Sphinx said:


> Happens on quick loud events.


Thats what I meant when I said "transients" . The noise is this: the woofer travelling back into the basket too far, and the voice coil hits the magnet. It happens after the woofer is asked to perform beyond Xmax I think. The transients require a fast acceleration of the woofer. What puzzles me is that these transients in the movie don't seem all that "powerful" either. I wish there was more info out there on this issue.


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## jdent02 (Jan 13, 2012)

Transients are a funny thing. My sub bottomed on the ring thumps from rotk when frodo is in mount doom. But it only happened when the bass levels went into the red at 20 Hz (one or two of the thumps). Strangely it handles war of the worlds and cloverfield just fine, and both of those have plenty of output at the same frequencies. I think it had something to do with me adding a 7db gain at 45 Hz to remove a room null. Once I dialed that boost back to 4db the problem vanished, at least at my normal listening levels. I can still bottom it if I turn it up really loud. Of course that same scene bottomed out the subs in the theater I saw it in too, so it's a pretty serious transient no matter how you look at it.


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## SAC (Dec 3, 2009)

jdent02 said:


> ..... I think it had something to do with me adding a 7db gain at 45 Hz _to remove a room null. _ ...



You can EQ peaks (Modal and in the direct signal) and dips in the direct signal, but you cannot remove nulls with EQ.

Think about what is happening physically in a null and then ask yourself how you are elevating the gain of a signal whose energy is effectively 'gone' (negated by its return to ambient pressure).


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## caper26 (Sep 10, 2010)

I agree. All that you are doing with EQ on a null is increasing the wave amplitude before it reaches the cancellation point... kinda like 3-3 = 0; then you increase the waves to get 6-6=0...


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## jdent02 (Jan 13, 2012)

It wasn't a null so much as a dip. It did respond to eq, I just boosted it too much.


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## Wayne A. Pflughaupt (Apr 13, 2006)

How to tell a null from a dip or trough:

http://www.hometheatershack.com/forums/audio-processing/55463-audyssey-bass-nulls.html#post501694

Regards,
Wayne


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## Sphinx (Feb 29, 2012)

I took preliminary measurements of location 3 a few nights ago. I was going to run all three tunings, but did not get around to it. So here is just the one sweep that I did run.

If I decide I want to EQ this sub to better dial it in to the room, what equipment do you recommend. I have seen miniDSP and BFD around on forums and those could be in my price range. Is there a significant quality jump going up to something like the SMS-1? What other options do I have for eqing? My onkyo just has 2eq.

My apologies that the title on the graph is wrong. This is location 3, not 2S


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## Wayne A. Pflughaupt (Apr 13, 2006)

Either the mini-DSP or BFD will work fine. For other equalizer options, see the “BFD Alternatives” sticky thread.

If you’re using REW or the manual graph-plotting method, the SMS-1 really offers no advantage, other than being faster.

Regards,
Wayne


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## jdent02 (Jan 13, 2012)

+1 for the bfd. Nulls and dips aside it really helped my setup.


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