Measuring Frequency Response

jake3 · « **on:** 30 Oct 2004, 02:44 am »

Would anyone be kind enough to point me to a blurb on measuring frequency response. I have the tools, now I just need to know how to use them properly (signal generator, spl meter, new home made 2 ways that need testing, etc.)
Im feeding the signal (at 2.83v) to my speakers through my normal hi-fi setup, with the multimeter in parallel, one channel at a time etc. but is there more? When I set the amp to 0db its reading almost 110db on the spl meter. Do my speakers really have that good a frequency response?

Any thoughts welcome.
Cheers in advance!

WerTicus · « **Reply #1 on:** 30 Oct 2004, 05:18 am »

110db is loud for two ways.

i dont know how to test properly though sorry.

ooheadsoo · « **Reply #2 on:** 30 Oct 2004, 06:22 am »

You should try madisound or diyaudio.

MaxCast · « **Reply #3 on:** 30 Oct 2004, 12:18 pm »

If you want to test just the speakers you need to test them in a anechoic chamber. If you want to test them with your room (much more meaningful) all you need is a test cd and you dB meter.
Which do you want to do?

jake3 · « **Reply #4 on:** 30 Oct 2004, 12:56 pm »

A test cd is rendered superfluous due to my having a signal generator. I have also since realised that the ambient noise in the house has contributed to the 110db result.
Still, there arent many websites that go into the subject of testing speakers. I think once people construct their speaker they are blinded by love and do not consider testing necessary?
Is there no definitive site that deals with this subject in depth? Yes, I have books, but they are always rather ambiguous and American...(no offence intended to anybody).

JohnR · « **Reply #5 on:** 30 Oct 2004, 02:00 pm »

Hi Jake, with the equipment you have, you set the frequency of the signal generator and read the SPL on the meter to get the in-room frequency response. It will be tedious, though, because of the narrow peaks and nulls in the room response. A third-octave warble tone (as on the test CD suggested by MaxCast) is used to smooth things out.

If you want to measure the speaker, you need some software that gates the test signal in order to cut off reflected sound. You'll also need a mic and preamp connected to your computer. Speaker Workshop seems to be one of the popular free ones. You might like to read this:

http://www.audua.com/SW/Project/Main.htm

and

http://www.gti.net/wallin/audio/audua/audua.html

The ambient noise in your house won't be causing your 110dB result.

JohnR · « **Reply #6 on:** 30 Oct 2004, 02:03 pm »

Quote from: jake3

Im feeding the signal (at 2.83v) to my speakers through my normal hi-fi setup, with the multimeter in parallel, one channel at a time etc. but is there more? When I set the amp to 0db its reading almost 110db on the spl meter. Do my speakers really have that good a frequency response?

Are you measuring 2.83 volts at the speaker, or at the amp input?

Marbles · « **Reply #7 on:** 30 Oct 2004, 02:21 pm »

You should also be aware that some SPL meters aren't very acurate over the 20 hz to 20khz range. For example the Radio Shack one has a widely known correction chart.

Other less popular but more expensive ones might be better.

If you use the RS SPL meter let us know and I can post the correction chart.

ctviggen · « **Reply #8 on:** 30 Oct 2004, 03:46 pm »

The easiest thing to do is buy a program, as it'll do it all for you and produce graphs. Note, though, that you're not really testing the speaker; instead, you're testing the speaker and the effect of the room at a certain position. If you move the mic, you'll get a different result. I plan on using a program to test before and after I add room treatments. Here are a few programs:

www.etfacoustic.com
www.me-au.com/measure.html
www.purebits.com

ooheadsoo · « **Reply #9 on:** 30 Oct 2004, 06:40 pm »

Do you want the response of the speakers before room affects the sound? If so, I would highly recommend you hightail it over to madisound's forum to take a look and ask over there. Diyaudio as well. I don't know what you mean by "American" but there are plenty of non americans at diyaudio.

jake3 · « **Reply #10 on:** 31 Oct 2004, 07:54 am »

Thanks for the excellent responses all. Some very good points have been raised.

Quote

If you want to test just the speakers you need to test them in a anechoic chamber. If you want to test them with your room (much more meaningful) all you need is a test cd and you dB meter.
Which do you want to do?

Well, I guess both would be nice. Is an anechoic chamber absolutely necessary?

Quote

Are you measuring 2.83 volts at the speaker, or at the amp input?

At the amp input.

Quote

If you want to measure the speaker, you need some software that gates the test signal in order to cut off reflected sound.

Doesnt this need to be done in an anechoic chamber? See above.. Or is this the old way to do it, and the software allows one to avoid all of that? I was rather hoping to keep the computer out of it, as I spend too much time at it now. I shall absorb your links though.

Quote

You should also be aware that some SPL meters aren't very acurate over the 20 hz to 20khz range. For example the Radio Shack one has a widely known correction chart.

Other less popular but more expensive ones might be better.

If you use the RS SPL meter let us know and I can post the correction chart.

I am using a Digitech QM 1588. I have no idea of its accuracy.

Quote

I don't know what you mean by "American" but there are plenty of non americans at diyaudio.

Please dont misunderstand me. Americans are fine, I have American family. But Im a simple aussie and there always seems to be some vital (to me) info missing from texts. But with all this help I'm well on the way!

Thanks everybody. I'll read all the links provided, and may even try the Audua software, which I already have but havn't used.

By the way, my speakers are a simple ported 2-way, 15litres, Audax HM170C0 mid and Audax TW034X0 tweeter, first order crossover at 2900hz, with l-pad. I have to say, they sound exceptionally good, which is why I am so keen to test them, and see just what I did right!

Lost81 · « **Reply #11 on:** 31 Oct 2004, 08:22 am »

Quote from: jake3

TBy the way, my speakers are a simple ported 2-way, 15litres, Audax HM170C0 mid and Audax TW034X0 tweeter, first order crossover at 2900hz, with l-pad. I have to say, they sound exceptionally good, which is why I am so keen to test them, and see just what I did right! ...

By all means do it.
It is a very interesting experience.

Also, it is a good way (albeit tedious) to test if you had overdone your acoustic treatment for your room.

Chances are, you will find a severe dip at the crossover point.
And you will probably find out that you have a bass node somewhere, and maybe a bass bloom as well.

I just did it (twice) for my 2 systems.
The severe dips were at the crossover frequencies.
Rather interesting.
Other than a bass node, and a very slight bloom in the bass, it is quite linear. I discovered that the higher frequencies were a trifle too damped, so I removed 2 square feet of acoustic foam from the ceiling (at the midway point), and the slam / impact / sparkle is back. They were right, it is so easy to overdo damping and end up with a "dead room."

Sitting through the test CD 4 times is rather fatiguing on the ears.
I find it impossible to enjoy music for the rest of the evening after that.

-Lost81

JohnR · « **Reply #12 on:** 31 Oct 2004, 12:50 pm »

Quote from: jake3

Quote:
Are you measuring 2.83 volts at the speaker, or at the amp input?

At the amp input.

You need to do this measurement at the speaker. This voltage is to get 1 Watt into a nominal 8 ohm load ie power = V * V / R

Quote from: jake3

Quote:
If you want to measure the speaker, you need some software that gates the test signal in order to cut off reflected sound.

Doesnt this need to be done in an anechoic chamber?

The gating allows you to avoid having an anechoic chamber, above approx 200 Hz. Alternatively, you can measure outside, but anything that sound can reflect off will affect the measurement.