喇叭怎麼測量？

http://en.wikipedia.org/wiki/Loudspeaker_measurement

最標準的當然是無響室中，但wikipedia這篇也寫著，

Most anechoic chambers are not designed for accurate measurement down to 20 Hz.

沒幾間無響室能搞定20Hz勒...

無響室很貴，那換個方式，搬到戶外去，嘿，這就只剩下地板，
那，地板的效應呢？
這我之前寫過一篇 http://records2ear.blogspot.tw/2012/03/blog-post_13.html

當然這也是離地板越遠越好，一般喇叭的高度總是會有明顯的中頻梳形濾波效應，
wikipedia這篇寫著

Raising both speaker and microphone on poles has been used as a way of reducing ground effect, and some speaker manufacturers specify a height of 50 feet (15 m) in their measurements.

拉高到15m夠高了吧？這等於22Hz的波長囉～地板效應夠低了
搭一間30m立方的大棚子應該比無響室便宜很多喔...

這還有個變形，讓喇叭躺著，朝上發聲，這叫half-space
地板效應又會更少，這招可用來測中頻，更低頻（越低的頻率越能繞射到喇叭後面），不過這有些部分等於讓喇叭貼著牆，低頻一定增加，

Speakers that are equalised to give a flat ‘free-space’ response, will always sound very bass-heavy indoors, which is why monitor speakers tend to incorporate ‘half-space’, and ‘quarter-space’ (for corner use) settings which bring in attenuation below about 400 Hz.

但這不是比較接近一般在室內空間使用的狀況嗎？似乎更實用吧？
徹底點，喇叭埋進土裡，更是精準的half-space，但這實用價值較低...

換個方式，近距離測量，這能壓低空間效應，
但各單體離測量的時間差就與實際上坐在遠處聽不同，此外也有高音指向性問題得考量，
因此不適於測整體喇叭，僅適用於分別測各音路。

microphone該擺多近？
當然是比間接音行程近很多才能壓低空間效應，
例如若低音單體中心離地高70cm，地板造成的間接音與直接音的行程差至少多70cm，若測量位置在單體前70cm，那麼間接音量至少會低6dB；
若減半到35cm，那麼間接音行程為四倍，那就低了12dB。
這麼說，貼著單體不就最好？
嘿...這又是另一回事，microphone會受到波前效應影響，這種測法得用另一種機私測。

 Fast Fourier Transform, or FFT, measuring equipment was introduced in order to measure the delayed output from speakers and display it as a time vs. frequency waterfall plot or spectrogram plot. Initially analysis was performed using impulse response testing, but this 'spike' suffers from having very low energy content if the stimulus is to remain within the peak ability of the speaker. Later equipment uses correlation on other stimulus such as a Maximum length sequence analyser or MLSSA. Using multiple sine wave tones as a stimulus signal and analyzing the resultant output, Spectral Contamination testing provides a measure of a loudspeakers 'self-noise' distortion component. This 'picket fence' type of signal can be optimized for any frequency range, and the results correlate exceptionally well with sound quality listening tests.

這段討論的則是送啥訊號進去，以及用啥方式分析訊號，
當然這前提是，你打算看哪種問題？
不同的問題得用不同的訊號，不同的手法分析，
所以說呢，別再繼續說啥看頻譜沒用這種話了，真是句廢話哪...

就說一般人比較有概念的振幅頻率響應，我已經不知在幾位樂迷家中看著剛測出來的響應圖直接驗證呈現的聲音特質完全符合囉～